[From the U.S. Government Printing Office, www.gpo.gov]



      Journal of Coastal Research    f    13          4          1064-1085    I    Fort Lauderdale, Florida          Fall 1997



Monitoring the Coastal Environment; Part III:

Geophysical and Research Methods

Andrew Morangt, Robert Larson: and Laurel Gormanï¿½

tUS. Army Engineer               tU.S. Army Engineer              ï¿½U.S. Army Engineer
 Waterways Experiment             Waterways Expreriment            Waterways Experiment
  Station                          Station                          Station
Coastal Engineering Research   Geotechnical Laboratory            Information Technology
  Center                         Vicksburg, MS 39180, U.S.A.        Laboratory
3909 Halls Ferry Road                                             3909 Halls Ferry Road
Vicksburg, MS 39180, U.S.A.                                       Vicksburg, MS 39180, U.S.A.

                      ABSTRACT
                      MORANG, A.; LARSON, R., and GORMAN, L., 1997. Monitoring the coastal environment; Part III; Geophysical and
                      research methods. Journal of Coastal Research, 13(4), 1064-1085. Fort Lauderdale (Florida), ISSN 0749-0208.

                      Acoustic and electromagnetic geophysical methods are widely used in coastal studies for determining water depth,
                      identifying bottom sediment type and surficial features, locating man-made objects and hazards, and studying sub-
                      bottom geology and structure. Acoustic methods are broadly divided into categories: 1. high-frequency systems such
                      as echo-sounders whose main purpose is to determine water depth (minimal seafloor penetration); 2. lower-frequency
                      systems with the ability to penetrate bottom sediments.
                        Coastal hydrographic surveys must be conducted by qualified personnel with meticulous quality-control procedures.
                      The maximum practicable achievable accuracy for coastal surveys using echo sounders is about + 0.5 ft (0.15 m).
                        High-resolution seismic surveys are used for engineering and for beach fill studies. The thinnest bed or layer that
                      can be detected is about k/4, where X is the wavelength of the acoustic source.

                      ADDITIONAL INDEX WORDS: Depth sounding sub-bottom profiling, side-scan sonar, ground penetrating radar, hy-
                      drogrphic surveying, acoustic impedences, high-resolution seismic.



                    -NTRODUCTION                                    but are not common in reconnaissance coastal studies and
                                                                  therefore will not be discussed in this paper. Geophysics is
  This is the third paper in a series of four describing practical   defore  as the "scud  in the er.  Geophysical
procedures for monitoring coastal processes and collecting geo-   dethof           te  and by           1984).    physical
logic, sedimentary, hydrographic, and hydraulic data in the
                                                                  are a form of remote sensing in that a researcher uses a tool
coastal zone. This paper concentrates on geophysical survey   fo remote   inge thesea rcor use                  a tool
methods and analyses. We emphasize basic procedures and de-          ima depiction or the st rata below.  a relt
scnptions of some of the underlying geophysical relationships.           is  acoustic thedace  of a   model  sed                 on
                                                                  varying acoustic impedances of air, water, sediment, and
  Some geophysical methods, such as subbottom profiler data          rock. The model, which must be interpreted; is based on nu-
recorded on analog paper records, may be considered old-             merous assumptions, and the user must always remember
fashioned, but they are still widely used for many engineering       that the real earth may be very different than the model
and geological studies. The state-of-the-art is changing rap-        printed on paper or displayed on his monitor. This warning
idly, and modern digital systems are proving to be remark-           notwithstanding, geophysical (particularly acoustic) methods
ably powerful tools. However, for many studies, simple tech-         have proven to be extremely powerful tools in numerous
niques are adequate and many researchers and contractors             coastal applications, including:
are still using traditional analog systems.                          * Determining water depth (hydrographic surveys)
                     BACKGROUND                                     * Imaging the sea bottom to identify surficial sediments,
                                                                     measure bottom features such as ripples, and locate man-
  Geophysical survey techniques, involving the use of acous-           made structures and debris
tic transmission, receiving, and measuring instruments and           0 Measuring the thickness of strata to locate suitable quan-
high quality positioning systems on survey boats, are widely            tities of sand for beach renonrishment
used for gathering subsurface geological and geotechnical            * Mapping gas pockets, rock outcrops, and other geological
data in coastal environments. Other methods, such as mag-              hazards
netic, gravity, and electrical resistivity, are used in special-     * Identifying coral and other biologically sensitive areas
ized engineering applications (GRIFFITHS and KING, 1981),              Echo sounders or depth-sounders,' side-scan sonar, and

96026-11I received 22 February 1996; accepted in revision 10 April    I     Often called Fathometer, although this is a Raytheon trade name
1996.                                                                and not a generic term.






                                        Monitoring Coastal Environments: Geophysical Methods                                   1065


Table 1. Summary of acoustic survey systems.

      Acoustic System           Frequency (kHz)                                       Purpose
Sea floor and water column
 Echo sounder (single beam)     12-200            Measure water depth for bathymetric mapping
 Echo sounder (multi-beam)      75-455            Map sea floor topography and structures
 Water column bubble detector
 (tuned transducer)              3-12             Detect bubble clusters, fish, flora, debris in water column
 Side-scan sonar                38-455            Map sea floor topography, sediment type, texture, outcrops, man-made debris, structures
Subbottom profilers
 Tuned transducers              3.5-7.0           High-resolution subbottom penetration
Electromechanical:
 Acoustipulseï¿½                  0.8-5.0           Bottom penetration to -30 m
 Uniboomï¿½                       0.4-14            15-30 cm resolution with 30-60 m penetration
 Bubble Pulser                  -0.4              Similar to Uniboomï¿½
Sparker:
 Standard                       50-5,000 Hz       Use in salt water (minimum 20%o), penetration to 1,000 m
 Optically stacked              (same)            Improved horizontal resolution
 Fast-firing 4 KJ & 10 KJ       (same)            Improved horizontal and vertical resolution
 De-bubbled, de-reverberated    (same)            Superior resolution, gas-charged sediment detection
 Multichannel digital           (same)            Computer processing to improve resolution, reduce noise
From SIECK and SELF (1977), EG&Gï¿½, Datasonicsï¿½, Resonï¿½, and other company literature



subbottom profilers are three classes of equipment used to                      HYDROGRAPHIC  (WATER DEPTH
collect geophysical data in marine exploration programs. All                        MEASUREMENT) SYSTEMS2
three are acoustic systems that initiate the propagation of          Importance of Surveys
sound pulses in the water and measure the lapsed time be-
tween the initiation of the pulse and the arrival of return            Hydrographic charting has always been of critical concern
signals reflected from various target features on or beneath         for navigation. As foreign trade becomes increasingly impor-
the seafloor. Single-beam acoustic depth-sounders are used           tant in the world economy, many harbors are being improved
for bathymetric surveys. Multi-beam echo sounders are im-            to handle larger deep-draft merchant vessels. As a result,
provements on the traditional single beam systems, allowing          many coastal inlets are being deepened to accommodate larg-
          very detailed imaging of underwater structures and topog-  er vessels, and the measurement of water depth remains vital
very detailed imaging of underwater structures and topog-
                                                                   to navigation safety. Bathymetric surveys are also required
raphy. Side-scan sonar provides an image of the aerial dis-
                                                                   for most coastal geology and geomorphology studies. Water
tribution of sediment, surface bed forms, and large features         for most  coastal geology and geomorphology studies. Water
                                                                   depths are measured by both direct contact procedures and
such as shoals and channels. It can thus be helpful in map-
ping directions of sediment transport. Subbottom profilers, as de-
the name implies, are used to examine the stratigraphy below
the seafloor. Table 1 lists frequencies of common acoustic geo-      Direct Elevation Measurements (Lead Lines and
physical tools.                                                      Sounding Poles)
  Ground-penetrating radar (GPR) uses electromagnetic en-
ergy to image subbottom sediments. Radiowave energy is                 Before the mid-1930's, most hydrographic data was collect-
transmitted through the sediment and reflects from materis           ed with lead lines or sounding disks, a labor-intensive, slow
transmitted through the sediment and reflects from materi-
als as a function of variations in dielectric constants and elec-    procedure (SALowi, 1964). In the Great Lakes, soundings
                                                                   were sometimes made through winter ice to eliminate the
trical resistivity. The main limitation of GPR is that it must       problems of seiching and other water-level oscillations. 
                                                                   problems of seiching and other water-level oscillations. Lead
be used in freshwater environments or in barrier locations
                                                                   lines and sounding poles are still used in shallow locations
where salinity is very low.                                          where electronic echo sounders might produce erroneous re-
  A single geophysical method rarely provides enough infor-          suits, such as near rock structures or bulkheads that cause
mation about subsurface conditions to be used without sedi-          strong side echoes or in areas of dense vegetation. A sounding
ment samples or additional data from other geophysical               pole is the most accurate hydrographic measuring device in
methods. Each geophysical technique typically responds to            shallow water. Procedures for using lead lines and poles in
different physical characteristics of earth materials, and cor-      shallow water are described in HEADQUARTERS, U.S. ARMY
relation of data from several methods provides the most              CORPS OF ENGINEERS (USACE) (1994); equipment specifi-
meaningful results. All geophysical methods rely heavily on
experienced operators and analysts. Inexperienced users
                                                                     2 Lead reference for this section is the Corps of Engineers' Engi-
should seek help both in contracting for surveys and in in-          neer Manual "Hydrographic Surveying" (HEADQUARTERS, USACE
terpreting records.                                                  1994).



                                            Journal of Coastal Research, Vol. 13, No. 4, 1997





1066                                              Morang, Larson and Gorman


cations are listed in the National Oceanic and Atmospheric       Table 2. Maximum allowable errors for hydrographic surveys.
Administration (NOAA) Hydrographic Manual (UMBACH,
1976). Note that sleds, commonly used to collect shore-par-                                           Surey Classification
allel profiles, are a form of sounding pole. Beach profiling                                       1         2         3
concepts and equipment are discussed in Paper 4 of this se-                                     Contract   Project    Recon-
ries (GORMAN, MORANG, and LARSON, 1997, this edition).                     Type of Error        Payment  Condition  naissance
                                                                 Resultant two-dimensional one-sig-    3 m  6 m      100 m
Acoustic Depth-Sounders                                            ma RMS positional error not to
                                                                   exceed
  The development of acoustic, electronic survey instruments     Resultantvertical depthmeasure-  +0.152m  ï¿½0.305m  +0.457m
after World War I revolutionized river, lake, and offshore sur-    ment one-sigma standard error  (ï¿½0.5 ft)  (ï¿½1.0 ft)  (ï¿½1.5 ft)
                                                                   not to exceed
veying because large areas could be covered rapidly from mo-
torized vessels, allowing a survey to be completed before        From HEADQUARTERS, USACE (1994)
storm waves or currents might alter the seafloor. Acoustic
depth-sounders measure the elapsed time an acoustic pulse
takes to travel from a generating transducer to the seafloor     curacy and adequacy of the final data. Calibrations are time-
and back. If the velocity of sound in water is known, the trav-  consuming and reduce actual data collection time. Neverthe-
el time of the reflected wave can be measured and converted      less, this must be countered with the economic impact re-
into distance:                                                   suiting from low quality data that may be useless or may
                         v -t                                   even lead to erroneous conclusions (leading, in turn, to incor-
                     d      =  2  + k + dr               (1)     rectly designed projects and possible litigation). With the in-
                                                                 creasing use of Geographic Information Systems (GIS) for
where:                                                           analysis and manipulation of data, high standards of accu-
 d = depth from reference water surface                          racy are imperative. Planning and successfully implementing
 v = average velocity of sound in the water column               offshore surveys are sophisticated activities and should be
 t = measured elapsed time from transducer to bottom and        carried out by personnel or contractors with experience and
      back to transducer                                         a record of successfully achieving the accuracies specified for
 k = system index constant                                       the particular surveys.
dr = distance from reference water surface to transducer
      (draft)                                                   Positioning System Criteria
  Values of v, t, and dr cannot be exactly determined during       Table 3 depicts positioning systems which are considered
the echo sounding process, and k must be derived from pe-        suitable for each class of survey. The table presumes that the
riodic calibrations of the equipment. The calibration proce-     typical project is located within 40 km (25 mi) of a coastline
dure is also not precise. The measured time t depends upon       or shoreline reference point. Surveys further offshore should
the reflectivity of the bottom as well as the signal processing  conform to the standards in the NOAA Hydrographic Manual
methods used to detect a valid return. The shape or sharp-       (UMBACH, 1976).
ness of the return pulse plays a major role in the accuracy
and detection capabilities of depth measurement.
                                                                 Causes of Survey Errors
Survey Classes and Accuracy Criteria                               Errors in acoustic water depth determination are caused
  Hydrographic surveying requires the application of two         by the following physical and mechanical factors:
technical disciplines: horizontal positioning and water depth
measurement. The quality, and cost, of the final results is        Velocity of Sound in Water
directly related to the accuracy and precision of both ele-
ments. For shallow coastal and inland water work, the Corps        The velocity, V, in near-surface water ranges from 1,400 to
of Engineers has standards for three classes of hydrographic     1,525 m/sec (4,600 to 5,000 ft/sec), but varies with water den-
surveys:                                                         sity, which is a function of temperature, salinity, and sus-
                                                                 pended solids (HEADQUARTERS, USACE, 1994; p. 8-14). An
* Class 1-Contract opayment surveys-dhighu accuracy              average of 1,500 m/sec is assumed for many surveys in salt
* Class 2-Project condition surveys-medium accuracy              water. In estuaries or river mouths, water density can vary
ï¿½ Class 3--Reconnaissance surveys--low accuracy
                                                                 greatly within the water column, and in areas subject to
Table 2 lists the maximum allowable errors for each class.       freshwater runoff, it is not valid to assume that an average
Although the requirements of geologic site surveys may not       V can be used over the entire area and for all water depths.
be the same as those of Corps of Engineers hydrographic sur-     For example, a 10%o (parts per thousand) salinity change can
veys, the accuracy standards are useful criteria when speci-     change the velocity by 12 m/sec (40 ft/sec), or 0.12 m in 15 m
fying quality control requirements in contractual documents.     (0.4 ft in 50 ft). Therefore, for highest precision surveys, the
The frequency of calibration is the major distinguishing fac-    acoustic velocity must be calibrated onsite frequently using a
tor between the classes of survey and directly affects the ac-   bar check.



                                          Journal of Coastal Research, Vol. 13, No. 4, 1997






                                          Monitoring Coastal Environments: Geophysical Methods                                    1067


Table 3. Allowable horizontal positioning system criteria.               Boat-Specific Corrections

                                     Estimated                         As the survey progresses, the vessel's draft changes as fuel
                                      Positional   Allowable for      and water are used or as loads (equipment and personnel)
                                      Accuracy    Survey Class        are exchanged. Depth checks should be performed several
                                      (meters,
          Positioning System           eMS')      1    2    3         times per day to calibrate the echo sounders.
Visual Range Intersection              3 to 20   No   No  Yes
Sextant Angle Resection                2 to 10   No   Yes  Yes           Survey Vessel Location with Respect to Known
Transit/Theodolite Angle Intersection  1 to 5    Yes  Yes  Yes           Datums
Range Azimuth Intersection           0.5 to 3    Yes  Yes  Yes
                                                                        An echo sounder on a boat simply measures the depth of
Tag Line (Static Measurements from Bank)                              the water as the boat moves over the water column. However,
 <457 m (1,500 ft) from baseline     0.3 to 1    Yes  Yes  Yes
 >457 m (1,500 ft) but <914 m (3,000                                 the boat is a platform that moves vertically depending on
   ft)                                1 to 5    No   Yes  Yes        oceanographic conditions such as tides and surges. To obtain
 >914 m (3,000 ft) from baseline      5 to 50+  No   No   Yes        water depths that are referenced to a known datum, echo
Tag Line (Dynamic)                                                    sounder data must be adjusted in one of two ways. First, tides
  <305 m (1,000 ft) from baseline      1 to 3    Yes  Yes  Yes        can be measured at a nearby station and the echo sounder
  >305 m (1,000 ft) but <610 m (2,000                                 data adjusted accordingly. Second, the vertical position of the
   ft)                                3 to 6    No   Yes  Yes        boat can be constantly surveyed with respect to a known land
  >610 m (2,000 ft) from baseline      6 to 50+  No   No  Yes
                                                                      datum and these values added to or subtracted from the re-
Tag Line (Baseline Boat)               5 to 50+  No   No   Yes        corded water depths. For a Class 1 survey, either method of
High-Frequency EPS2 (Microwave or                                     data correction requires meticulous attention to quality con-
   UHF)                               1 to 4    Yes  Yes  Yes        trol.
Medium-Frequency EPS                   3 to 10   No   Yes  Yes
Low-Frequency EPS (Loran)             50 to 2,000 No   No   Yes         With the first procedure, the water surface is normally ref-
                                                                      erenced to an on-shore reference benchmark or gauge. The
                                                                      most common source of error is the assumed stability of the
 Doppler                            100 to 300  No   No   No
 STARFIX                                 5      No   Yes  Yes         water surface between the on-shore gauge and the survey
                                                                      vessel. In coastal projects subject to tides, ebb/flood flow, or
NAVSTAR GPS':
                                                                      riverine discharge, surface gradients between the gauge and
  Absolute Point Positioning (No SA)   15to100  No   No   Yes          the vessel can amount to more than 0.6 m, and depth data
 Absolute Point Positioning (w/SA)   50 to 100  No   No   Yes
 Differential Pseudo Ranging          2 to 5    Yes  Yes  Yes         must be corrected. For this reason, tide staffs are normally
  Differential Kinematic (future)     0.1 to 1.0  Yes  Yes  Yes        established in the immediate project area (i.e., it is not valid
'Root Mean Square                                                     to observe the tide on an intracoastal waterway for an off-
'Electronic Positioning System                                        shore survey). To reduce the effect of wind setups, surveys
3Global Positioning System                                            should be conducted under low wind conditions (less than 15
4Selective Availability                                               knots). Table 4 lists requirements for water level measure-
From HEADQUARTERS, USACE (1994)                                       ments based on class of survey.


                                                                        Waves

                                                                        As the survey boat pitches up and down, the seafloor is
                                                                      recorded as a wavey surface. To obtain the true seafloor
Table 4. Tide and water level measurement criteria.

                                                                                     Minimum Standard per Survey Class
                      Criteria                                                1                    2                      3
Gauge/Tide Staff Location'                                                 On-site              On-site               Near-site
Tidal Zoning Requirements                                                 Determine on case-by-case basis           Not required
Gauge Reading Frequency                                            ---------------------- As needed for 0.1 ft (0.03 m) surface change -------------------------
Leveling frequency-Gauge to Benchmarks per Project2                ------------- Start and finish of project -----------------   Project start only
Start/Finish Difference in Gauge Reference Elevation                   0.05 ft (0.015 m)     0.1 ft (0.03 m)    ------ -------------------
Staff Marking Intervals                                            -------- ------------------------- - 0.1 ft (0.03 m)   -------------------------------------
Least Count of Readings                                            -----------------------------------------  0.1 ft (0.03 m) ------------------------------------------------
Stilling Wells Required if Sea States Exceed                            0.5 ft (0.15 m)      1.0 ft (0.31 m)        2.0 ft (0.61 m)
'An on site gauge is defined as a being in a location relative to the project area such that not more than the following surface gradient exists between
gauge and vessel:
  Class 1: 0.1 ft (0.03 m)
  Class 2: 0.3 ft (0.09 m)
  Class 3: 0.8 ft (0.24 m)
Tidal or surface gradient zoning is required if these criteria cannot be met
2FGCC 3rd order levels-2 benchmarks required
From HEADQUARTERS, USACE (1994)



                                              Journal of Coastal Research, Vol. 13, No. 4, 1997






1068                                               Morang, Larson and Gorman

Table 5. Estimated depth measurement accuracy                     of tidal modeling are much greater than inaccuracies caused
                                                                  by wave noise.
                        Estimated Standard Error per Condition      Figure 2 is contoured digital bathymetric data from the
                                                                  Yaquina entrance, Oregon, collected with a single-beam sur-
                                Average   Average                 vey system. This is a complicated terrain with exposed rock
                                <20 ft    >20 ft
                                Error Source  Ideal  (6 m)  (6 m)  Coastal  reefs. Line spacing was close and the survey was conducted
                                                                  under tight specifications, equivalent to Class 1. Running
Measurement System     0.015     0.015     0.03      0.06
System Calibration     0.011     0.03      0.06      0.09         high quality surveys is difficult in the North Pacific because
Resolution             0.03      0.03      0.03      0.06         of frequent stormy weather and high seas.
Draft/Index                      0.015     0.03      0.06
Reference datum:                                                             MULTI-BEAM ECHO SOUNDERS
 Vertical              0.015     0.015     0.015     0.015
 Tide/Stage           0.006      0.06      0.06      0.15           Recently, multi-beam echo sounders, capable of generating
Platform Stability     0.015     0.06      0.09      0.3          remarkably detailed images of the seafloor or of submerged
Velocity                         0.03      0.03      0.06         structures, have been marketed. The shallow water multi-
Density/Sensitivity    0.015     0.015     0.3       0.15
Density/Sensitivity    0.015     0.015     0.3       0.15         beam systems are compact, high-frequency, high-resolution
                                                    Resultant MSE  ï¿½0.046  009  015  040  units that produce multiple beams from a single transducer
From HEADQUARTERS, USACE (1994) (converted to metric units)       head using arrays of miniature transducers and electronic
                                                                  signal control (Figure 3). Examples include the Sea Beam
                                                                  1000ï¿½ (75 kHz), Simrad EM 950ï¿½ (95 Khz), Krupp-Atlas Fan-
depth for the highest quality surveys, transducers and re-        sweepï¿½ (200 kHz), and the Reson SeaBatï¿½ (455 kHz). These
ceivers are  sometimes installed on heave-compensating            systems are now practical for small boat operations as a re-
mounts. These allow the boat to move vertically while the         suit of the simultaneous development of several critical tech-
instruments remain fixed. Electronic heave compensation in-       nologies: rapid-response heave-roll-pitch sensors, precise po-
struments are available that filter the wave signal as the        sitioning (in particular, differential global positioning sys-
survey progresses. Both methods are effective.                    tems (DGPS)), computerized integration of navigation with
                                                                  the sensor systems, and computerized data management and
Estimated Depth Measurement Accuracy                              display. An important note regarding multi-beam systems:
                                                                  the amount of data collected at any site is dramatically great-
  Even with the best efforts at equipment calibration and         er than the amount collected with conventional survey meth-
data processing, the maximum practicable achievable accu-         ods. Much more computer software and hardware is needed
racy for coastal surveys using echo sounders is about + 0.5       to process these data, and many  agencies are not yet
ft (0.15 m) (HEADQUARTERS, USACE, 1994, pp. 9-29). Under
 ft (0.15 * ) (HEADQ UARTERS, USA.E, 1994, pp. 9-29). Under       equipped to manage it. Archiving these files will become an
average river and harbor project conditions, the estimated        ev er-greater problem.
accuracy of an individual sounding falls between ï¿½ 0.2 and        ever-greater prolem.cessed multi-beam data from
                                                                    Figure 4, an example of processed multi-beam data from
ï¿½ 0.5 ft (0.61-0.15 m). Table 5 lists quantitative estimates of   the SeaBat 9001, shows the north side of the Yaquina north
depth measurements under different survey conditions. The         jetty, off Newport, Oregon (see Figure 2 for location). The
resultant depth accuracy of an overall survey is highly vari-     bumpy texture of the jetty is armor stone. The portion of the
able, regardless of the class specified for the project. For ex-  jetty imaged by the SeaBat extended from -1.5 m mlaw down
ample, a survey intended to be Class 1 conducted 10 km off-       to about -5 or -6 m  This image  is composed of 145,000 data
shore with poor tidal modeling may actually be accurate only      points To  monitor  th e  condition  of the jetty above the wat
                                                                  points. To monitor the condition of the jetty above the water
to Class 3 criteria (ï¿½ 1.5 ft). Thus, estimated resultant error   line, Portland District used low level aerial photography,
                                                                  line, Portland District used low level aerial photography,
must be evaluated on a project-by-project basis.
                                                                  plotting and comparing the location of individual armor
                                                                  stones with Geographic Information System (GIS) software.
Examples                                                            Thanks to rapidly-improving processing and data-recording
  Figure 1 is an example of analog echo sounder data from         capabilities, the state of science is constantly improving in
offshore Palm Beach County, Florida. This data was recorded       this field. Users need to contact equipment manufacturers
on the paper charts at a range of 0 to 110 ft, so to read the     and review Journal of Coastal Research, Journal of Geophys-
depths, a user should use the 0 to 55-ft printed scale and        ical Research, Marine Technical Society Journal, International
multiply by 2. For example, the top of the prominent lump         Underwater Systems Design, and trade journals for more in-
near Fix 299 has a depth of about 52 ft. This data was col-       formation.
lected a kilometer offshore without any tidal modeling nor          Many ships and most submersibles are now equipped with
with establishment of offshore tide gauges. Therefore, accu-      ahead-look sonars (ALS). This, too, is a new technology great-
racy is Class 3 or worse, and we can assume, at best, an          ly dependent on sophisticated signal processing and trans-
accuracy of ï¿½ 2 ft. This means that we can only state that        ducer design. Ahead-look sonar mounted on small remote-
the lump is between 50 and 54 ft deep. Because sea condi-         operated vehicles has been used to inspect underwater por-
tions were mild, wave noise is a minor problem in these re-       tions of coastal structures in environments where a manned
cords. In any case, here the inaccuracies caused by the lack      survey boat cannot approach the structure safely. LOGGINS



                                           Journal of Coastal Research, Vol. 13, No. 4, 1997






                                          Monitoring Coastal Environments: Geophysical Methods                                  1069









                                  _ I--- ~ ~ ~ ~ ~              7i ma                                             - -t--i ---i- -i: :_ ;




           .i.____    5         .         I7                                                       LL   iL: r_:;, I. ..._  _...

                                                   -~~iiBW '-;'I ---                                    '
-~ ~,j.ji '                                                                                                                :5::0di  '  i-ii



      Fiure 1. Analog echo sonder record rom offshore Palm Beach County, Florida (Coastal E-nineeri;g Reserch Center (CERC) data). Shore is to the
    II                                   ::                                                          E        i9  ': phi- I :: --  I'i.   : i,. 















  left. The lump near Fix 299 is a coral  _ outcrop.





                                                                       ourishment).
  High-resolution geophysics refers to the use of acoustic
sources, sound receivers, signal processing equipment, and                                         i

graphic displays to define water depth and provide cross-sec-










tional views of the sediments and strata below the seafloor Tsm ission  of t wavs   through edi-e nt a nd r-;
                                                                          Transmissio of ai ws C sdimt a   ro   0











Figure 1 Analog echo sounder record from offshore Palm Beach County Florida (Coastal pEngineering Research Center (CEsC) data) Shore is to thecom-


(HRanGD19895) lists frequenciesi resolusition, and other characteristics  ta and gastructures con thent   (50 or 60 m of the sedi-
is noise. The pvariounciples ALSo systems.                           ment columnis. Typical appoications in clude reconnaissance
                  damentagly the same as those of acoustological surveys, foundati on studies for offshore platforms,
 HIGH-RESOLU           ON SUBBOTOM PRO LING                          hazards surveys to locate buried debris and gareceivers empockets, andlower
Definitions                                                          surveys to identify mineral resources (eg., sand for beach ren-








tional views of the sediments and strata below the seafloor            Transmissioatter of confusion, seismic suwaves through sediment and rocky,
ble 1). "High-resolution" general en s that the surveys very high power sources for deep  penetrationes such as oi exploration, com-




are intended for engineering purposes or for identifying stra-       are not called "low resolution."



                                               Journal of Coastal Research, Vol. 13, No. 4, 1997





1070                                                 Morang, Larson and Gorman














                                              ~~~~~~~~~~~~~~~~~~~~~~~oil!



























Figure 2. Contoured bathymetry off Yaquina Entrance, Oregon, collected with a single-beam acoustic system with close line spacing and tight specifi-
cations (from HuGHEs et at. (1995)).


dia (similar to the treatment used in optics or ocean waves)         is reflected and refracted is described by Snell's Law (SHFR-
such that:                                                           IFF and GELDART, 1982):

                         T =-                                                            sin 0. sin 2 = 2
                              V                                                            V,        V2
                               1   V                                 where:
                          v = - = _
                              T    X                                  V, = velocity of sound in the upper media
                         V = Vx                             (2)       V2 = velocity ofsoundinthelowermedia
                            where:.,. 7,,,                                =  angle. of incidence


















                 where:                                               6~~~~~~~~~~~~2 =  angle of refraction
 T = period of the acoustic wave                                     The quantity p is called the raypath parameter. The above
 v = frequency                                                       relationship assumes a planar surface and, therefore, specu-
 X = wavelength                                                      lar reflections. If the surface is irregular and has bumps of
                  V  =   s peed      of    the   ave "disturbance"height d, reflected waves from the bumps reach the receiver
When a wave encounters an abrupt change in elastic prop-             before the waves from the rest of the surface by a distance
erties, part of the energy is reflected while the balance is re-    2d. These can be neglected where 2d /X < 1/4 (the "Rayleigh
fracted into the other medium. The proportion of energy that         criterion"), i.e., when d < X/8 (SHERIFF and GELDART, 1982).



                                            Journal of Coastal Research, Vol. 13, No. 4, 1997
                                    '1/ "C.








                                    ~-'".~~,S',"... Qpp                                                                    -   :~'it''"   

                                        . .t.~~/.?ï¿½ ,-,
                     ~~~~~~~~~~~.,, -   ,. f,',.





                              x                                                           i   , ,.si;e2
                                                ,  ?   .   2    ,,  ,'  ~        ;        __   _ ,___                            (3
                              1ï¿½   Vwhre
                           Vï¿½                                          1=vlct  fsudi  h   pe   ei

                              V~~~~~~ ~~~ ="X()                       V        eoiyo sudi  h   oe   ei
            whre                                                      0       agl  o  ncdec
                                       /~~~~~~~~~~~~~~~ nl frfato 
                T~ ~ ~ ~ ~~~~~ = eido h cutcwv                       Teqatt  scle h ryahprmtr h bv









Fiur =. speoned of athewae"iturbace                   ï¿½heqight 1cra, Oregof olected wihavesig-ba   frousi y m wthe bumse riesachn n tihe speciver


eries (smiart tof the temenergye is repetedwile the balancwaes is re-fl.Teted cand befraglcted wh esrie2d /by-  (thel' "Rayleigh-
fractdittheohrmdu.Tepootnofnrg  that:   crtro") p.  he   i<)8(HRF  and GELDART, 1982).


                                  k    si~~ouna of Cosa Rseach Vo.1,No  ,19






                                               Monitoring Coastal Environments: Geophysical Methods                                              1071



                                                        Seabat 9001 Multibeam Sonar


                                 60 Simultaneous Beams,
                                 Beomwidth 1.5 Degrees
                                 Covers 90 Degrees Arc







                                                                                                    Survey Vessel





                              Coostal
                             Structure



Figure 3. Beam pattern of the SeaBat 9001, showing its ability to image submerged portions of breakwaters.










                                                                                                           YAQUINA NORTH JETTY
                                                                                                                Newport, Oregon
                                                                                                                       6/26/94















                      Sand Seafloor




                                                                                                                               Jetty Tip









    Vertical Exag.                                                                         / 2:1

     Highest Elev. = -1.5 m mllw                                         Toe of Jetty

Figure 4. Processed SeaBat 9001 image of Yaquina North jetty. Image shows about 90 m of the seaward (north) side of the rubblemound structure from
a depth of -1.5 m mllw down to the seafloor.




                                                    Journal of Coastal Research, Vol. 13, No. 4, 1997





1072                                                Morang, Larson and Gorman


                                                                   ance between the two materials increases, R increases, thus
                                                                   resulting in more reflected energy. For example, a hard sea-
                                 Source  Hydrophne                 floor produces a stronger return than a soft seafloor. For most
                                                     Water         interfaces within the earth, impedance contrasts are small
                                                     Surface       and typically less than 1 percent of the energy is reflected.
                                                                   This is why sophisticated data processing and noise-reduction
                                                                   procedures are needed to reveal strata deep within the earth.
                                                     Bottom        Because the seafloor, the sea surface, and the base of the
                                                                   weathering layer are relatively strong reflectors, they are re-
                                                     -Horizon 1    sponsible for most of the multiple reflectors that often ob-
                                                                   scure portions of subbottom returns.
                                                     Horizon 2       Lack of signal penetration is caused by many conditions.
                                                                   Coarse sand and gravel, glacial till, and highly organic sedi-
                                                                   ments are often difficult to penetrate with conventional sub-
                                                     Horizon 3     bottom profilers, resulting in records with data gaps. The lack
Figure 5. Subbottom seismic surveying from a small boat.           of penetration itself is a diagnostic tool. For example, gassy
                                                                   sediments cause serious signal degradation and gaps in re-
                                                                   cords (Figure 6). Often, little useful subbottom data can be
                                                                   collected in estuaries and river mouths because they contain
This tells us that there is a practical limit to the size of fea-
                                                                   so much organic material. For example, much of Chesapeake
tures that can be detected on a surface which depends on the                      oa     t. r
frequency (and hence the wavelength), of the acoustic signal
source. For example, if a Bubble Pulser source is used with        these conditions, cores may be necessary to fill in the missing
                                                                   geological information. Digital signal processing of multi-
a dominant frequency of 400 Hz (Table 1), the wavelength in        geological  information,               Digital signal  processing of multi-
sandstone, assuming a velocity of 2,000 m/s, is equal to 5 m.
Therefore, an irregularity d would not be detected if it were      signal penetration or noise. However, signal processing is not
less than about ï¿½1/ X 5 or 0.6 m high.                             magic and there are limits to what it can achieve in difficult
  The strength of a reflected signal, and hence the ability to     environments.
detect an interface, depends upon the partitioning of energy         Several kinds of spurious signals (i.e., noise) cause difficul-
as the signal is partly reflected and partly refracted at the      ties in interpreting analog seismic records4:
material interface. Mathematical relationships known as
Zoeppritz' equations (detailed in SHERIFF and GELDART              * Direct arrivals-signal received directly from the sound
(1982)) describe this partitioning. The fractions of energy re-       source
flected and transmitted are given by ER and ET, where E, +         * Multiple reflections-repeated echoes from a strong reflec-
ET = 1.0. ER is calculated from:                                      tor, usually the seafloor
                                                                   0 Water surface reflection
                   ER = (Z     I -Z    = R                (4)        Side echoes-reflections from irregular bottom or hard ob-
                         a Z, + Z,]                                  jects such as man-made structures
where:                                                             0 Single point reflections-reflected energy radiated from
                                                                     small point objects such as rock pinnacles or pipelines
Zi = V X p (velocity times density) (i.e, acoustic impedance)
R = reflection coefficient (also known as the reflectivity)

  Table 6 lists densities of common materials encountered in         4 From booklet prepared by EG&G Corporation, Waltham, Mas-
seismic prospecting. It shows that as the difference in imped-     sachusetts, 1977.


Table 6. Energy reflected at interface between two media.

                                     First Medium             Second Medium
        Interface                Velocity'    Density2   Velocity      Density         Z~/Z2             R              E,
"Soft" ocean bottom                1.5         1.0         1.5          2.0               0.50         0.33           0.11
"Hard" ocean bottom                1.5         1.0         3.0          2.5               0.20         0.67           0.44
Ocean bottom with gas sand         1.5         1.0         2.2          1.8               0.38         0.45           0.20
Surface of ocean                   1.5         1.0         0.36         0.0012        3,800           -0.9994         0.9988
Base of weathering                 0.5         1.5         2.0          2.0               0.19         0.68           0.47
Shallow interface                  2.1         2.4         2.3          2.4               0.93         0.045          0.0021
Sandstone on limestone             2.0         2.4         3.0           2.4              0.67         0.2            0.04
'km/s
2g/cm3
Condensed from SHERIFF and GELDART (1982)



                                           Journal of Coastal Research, Vol. 13, No. 4, 1997






                                       Monitoring Coastal Environments: Geophysical Methods                                1073













































Figure 6. Subbottom profiler record from Charleston Harbor showing gassy sediment. Data collected with a digitally-recording boomer system.




Resolution                                                         smaller, about 0.6 m, and layers about 0.15 m thick can be

  The two most important parameters of a subbottom seismicdect.
                  The to mot imprtan paraeter  of asubbttomDuring the 1980's, improvements in data acquisition and
reflection system are its vertical resolution and penetration.Duigte18',mpomnsindaaciiinad
                                                                  signal processing made it possible for scientists to detect thin-
                           yof the output signal increases, the   ner and thinner layers using high-frequency systems while
resolution, or the ability to differentiate closely spaced re-     still achieving reasonable penetration, even in sands or other
flectors, becomes more refined. Unfortunately, raising the         difficult materials. For example, BERNE, AuFFRET,  and
frequency of the acoustic pulses increases attenuation of the      WALKER  (1988) were able to image the internal structure of
signal and consequently decreases the effective sediment pen-      sandwaves off Normandy, France, using a 2.5 kfz subbottom
etration. Thus, it is a common practice to use two seismic
reflection systems simultaneously during a survey; one of
high-resolution capabilities and the other capable of greater      Iterretion Pitfalls
penetration.
  The thinnest bed or layer that can be detected is about k/4        Acoustic characteristics are related to lithology so that seis-
(SHERIFF, 1977). Using the example of a 400 Hz signal in           mic reflection profiles can be considered analogous to a geo-
sandstone with k = 5 m, layers as thin as 1.25 m should be         logical cross section of the subbottom material. However, be-
detectable (providing, of course, that there are sufficient        cause of subtle changes in acoustic impedance, reflections can
acoustic impedances to produce measurable reflections). If a       appear on the record where there are only minor differences
3.5 kHz profiler is used, the wavelength in sandstone is much      in the lithologies penetrated. Also, significant lithologic dif-



                                            Journal of Coastal Research, Vol. 13, No. 4, 1997






1074                                               Morang, Larson and Gorman



              281 282    284    286    288    290    292    294    296    298    300    302    304    306    308    310
      o-     o    I  I  I  IIIII                  IIIIII                 II     II      III        IIII
     20--
     30--   12 2
     40  -                                                  SEAFLOOR
           24 -~---          ---
     70-- a     - _ .   .
            G__~ ~ ~             -~---------------
     110--    L   i     NE

    120-- 2 48 -
     19o0--    36-
    14O0 - -   60                  -                                                                                    _.
                110  ~~~~~~~~~~~~~~~~-- --------
    I20-    48-
    140--
    ,so --   60-
    160 --
             72-



               ;; -. .--,t,,  :._ ...

                                                       2..~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~n
                   -                _                A                                  N   -_e
                               ...... ;""~:~-~"~::',,';~~~~~~~~~~~ ~--L-O ....   . :  ~                        ....


             I .   1.          _. *.4. , ,AC, ~.. ,_ ..... A... 
         ;., . . _.'- .:_........                                             ....:.... ...._..... ; ' ' : . .
      '.......:  ......! /   _  _- ,_: ___                              _       _      _ _ . _. _,
   149901  -4-lo' ""t! '   j''   , ~..~~,' "'- ' _;Z-''" ~ 7. 1...__,,*c"~ -'_' "A-- ... . '-, .  ï¿½  .-Aj ',.".L"'',~ ' . .  ' .l ..1 .'~   ,".'~ -   :',.







   Fiur 7. Analo Bubble' Pe reor fromt off Palm Bec Couty Flrdsoigcrloucosadsn"ais
                             maskin byyr4 gasC4~~'io~ k (SERFF 1980) As, SH F (  hs wr i                                ''
             ten'" ', "a






      blance~~~~~~~i*'                               >~47'2  toti stattapi cross~i setin tha they invite direct   2t''C/ i ~ ~     4 ~ -
                                      'V       It ~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~''~~~~~~~~~~~' ~i' fFrf tJArhn4%m v


Figure 7. Analog Bubble Pulser re cord fro m o ff Palm Beach County, Florida, showing coral outcrops and  sand basins.



ferences may go unrecorded due to similarity of acoustic im-      corded in units of  milliseconds of two-way travel time. The
pedance across interfaces, minimal thickness of  the units, or    following equation  converts  from milliseconds to depth:
masking by gas (SHERIFF, 1980). As SHERIFF (1977) has writ-  T
ten, "Modern seismic sections often bear such striking  resem-                             D    - x V                       (5)
blance to stratigr    aphic cros    s s ecti ons that they invite direct                        2D
interpretation      by p  eople w ho do not appreciate geophysical  where:
limitations... Because most reflections are interference com-
posites, there is               no one-to-one    c orrespondence between seis-  D  =         depth
mic events and interference     s in the ea rth."      The user of geo-  T = Two-way travel time
physical data should be careful not to assume that, in noise-       V = average speed of acoustic signal
f ree areas        of good  signal penetration, every waveshape va-  To compute the depth scale shown in Figure 7, V was as-





                 by ~~~~~~~~~~~~~Thepobig                          compute theow  depth locatisown with Figue dept Vecordeds-
ation has a geologic meaning or represents a buried feature.
  Because of                                          the dangers of incorrectly interpreting acoustic
artifacts, seismic stratigraphy should always be considered                          T (msec)
tentativ       e  until                       s       upported by direct lithologic evidence from                   se2
core samples. In shallow coastal areas, it is common practice




                 by  ~~~~~~~~~~~~~~~uthpe Coccrps oftwoginees th ue water jepth prbngdt displays
by the Corps of Engineers to use water jet probing to accom-      Note  that accurate sound velocity data    is seldom available.
pany subbottom seismic surveys. This is especially important      Therefore, layer thickness can only be approximated.
when there is a thin veneer of sand over more resistant sub-        Th      e seafloor in Figure  7 is recorded as a h eavy double line.




                 1). ~~~~~~~~~~~~~~~The sertclsaloflaaog  esi   eord  isotn Fgre 7 mis recorded musta hecavyuntt donuble muliples.it
strate and the actual thickness of the veneer can be measured     The best way to determine the actual seafloor position is to
by the probing.                                                   compare a known depth location with the depth recorded on
                                                                    the echo sounder record; in this case, the correct seafloor
Interpretation Example                                            depth is just below the upper thick line. The first seafloor
                                                                    multiple occurs at two times the water depth and displays
  Figure 7 is an example of Bubble Pulser data from offshore      twice the slope of the original. Around Fix 294, third and
Palm Beach County, Florida (the same line shown in Figure         fourth multiples have been recorded. An interpreter of seis-
1). The vertical scale of analog seismic records is often re-     mic records must be careful not to confuse multiples with



                                            Journal of Coastal Research, Vol. 13, No. 4, 1997






                                       Monitoring Coastal Environments: Geophysical Methods                               1075


genuine reflectors caused by buried structures or sediments!      and WILLIAMS (1980) ran lines in a rectangular grid and col-
Unfortunately, it has become a geophysicist's truism that a       lected cores at selected intersection points.
multiple invariably ruins the part of the record where the          For offshore areas where little is known about the surficial
most interesting data should be found.                            geology, an alternative procedure is to run survey lines in a
  The low bump near Fix 300 is one of the coral reefs that        zigzag pattern approximately perpendicular to the coast (Fig-
parallel the southeast Florida coast. The reef at Fix 304 is      ure 9).
almost flush with the surface. Coral reefs are distinguished
by their steep sides and, typically, by the lack of coherent      Quantitative Analysis of Subbottom Sediments
reflectors within the masses.                                       For many years, experienced geophysicists could identify
  From Fix 282 offshore to the first exposed reef, a shallow      and predict some properties of subbottom sediments based on
basin appears to be filled with relatively transparent, paral-    the appearance of their analog records, especially if they sur-
lel-bedded sediments, probably sand. The greatest thickness,      veyed in an area in which they had experience. This was an
about 5 m (15 ft), occurs near Fix 296. A small pocket of sand    imprecise art at best, and numerous attempts have been
has collected between the two reefs (Fixes 301 to 303). Recall    made to develop quantitative methods to analyze signal re-
that earlier we estimated the resolution of a Bubble Pulser       turns to predict sediment properties.
signal would be about 1.25 m (4 ft) in sandstone. Therefore,        The Chirp Sonar, developed in the 1980's, was originally
thin layers of possibly cemented sands might not be revealed      sold as a high-resolution, quantitative profiling system. The
by this tool. This underscores why cores are necessary to pro-    Chirp is a system in which a minicomputer generates a fre-
vide additional lithologic information, especially if the pur-    quency-modulated pulse that is phase- and amplitude-com-
pose of the survey program is to identify sediment suitable       pensated to correct for the sonar system response. This pre-
for beachfill.                                                    cise waveform control helps to suppress correlation noise and
                                                                 source ringing. When the reflected signals are received,
Survey Patterns                                                   mathematical algorithms estimate the attenuation of subbot-
                                                                 tom reflections by waveform matching with a theoretically
  As with most other types of offshore investigations, there      attenuated waveform. Details of the theory and mathematics
is no "best" way to lay out a survey grid. The survey pattern     behind Chirp sonar are presented in SCHOCK, LEBLANC, and
must be based on the total area to be covered, types of targets   MAYER (1989), LEBLANC, PANDA, and SCHOCK (1992) and
being investigated, equipment and work boat to be used, time      SCHOCK and LEBLANC (1992). Chirp appears to work well in
available, weather, regulations, regional hazards (such as        unconsolidated fine sediments but less successfully in sands.
shipping channels), and, maybe most important, funding            Its main weakness is that the final product degrades when
available for the field studies. Often, experience with the use   the mathematics cannot accommodate frequency or phase
of certain tools and their efficiency in a particular geological  changes of the returning pulses.
terrain is the best guide to laying out the tracklines. The         Another acoustic impedance system designed to assess bot-
program should be flexible and amenable to changes based          tom and subbottom sediments was developed at the Water-
on interpretation of the data as it is collected (MEISBERGER,     ways Experiment Station and has recently been tested suc-
1990). This underscores how important it is that data be re-      cessfully at a number of coastal sites (MCGEE, BALLARD, and
viewed immediately, and not just collected and saved for fu-      CAULFIELD, 1995). This is an empirical technique which com-
ture interpretation. By then it will be too late to adjust field  pensates for absorption in each layer as a function of the cen-
parameters, and the records may be of little value.               ter frequency of a band-limited seismic trace, corrects for
  It is generally most appropriate to run seismic surveys in      spherical spreading, and uses classical multi-layer reflective
a pattern that is perpendicular to the suspected prevailing       mathematics to compute reflection coefficients at sediment
geologic structures or surficial topography (MEISBERGER,          horizons. This method uses discrete frequencies and is an ex-
1990). Existing scientific literature and bathymetric maps
should be consulted to help plan the surveys. Along most          (1983) and CAULFIELD, CAULFIELD, and YIM (1985).
coasts, seismic lines are run perpendicular to the shore. For                  SIDE-SCAN SONAR SURVEYS
example, along southeast Florida, two or three reefs run par-
allel to the shore and outcrop from the seafloor (Figure 7).      Side-Scan Sonar Theory
Between the reefs are accumulations of sand of varying thick-       Side-scan sonar is a system of imaging underwater objects
ness. Surveys run perpendicular to the shore can identify the     using high-frequency acoustic signals. Originally developed
extent of the sand accumulations and the areas of hard bot-       during World War II to detect enemy submarines, commer-
tom.                                                              cial systems designed for scientific use became available in
  If the prevailing offshore geology is not parallel to the       the 1960's and since then have been extensively used by
shore, the survey lines should be adjusted to best image the      oceanographic institutions, universities, pipeline and marine
terrain. For example, off Ocean City, Maryland, ridges extend     construction companies, archaeologists, and treasure-hunt-
from the shore in a northeast direction. In this area, FIELD      ers. Side-scan sonar has become an invaluable tool to evalu-
(1979) ran seismic lines in a grid at an angle to the shore       ate the condition of breakwaters, bridge piers, and other un-
allowing him to run both parallel and perpendicular to the        derwater structures (CHRZASTOWSKI  and SCHLEE (1988),
ridges (Figure 8). Off Cape May, New Jersey, MEISBERGER           CLAUSNER and POPE (1988), MORANG (1987)).



                                           Journal of Coastal Research, Vol. 13, No. 4, 1997







1076                                                               Morang, Larson and Gorman





                  7,5020                                   75-l                                     75000'                                   74'50



                                        DELAWARE   -                      ~       -         .*







                                                                       ISLE~~~~~~~o                0OF2         Coo... 






                                -5520~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~2



















                                                                                            C~~~~~~~~~~3~-













                                                                                                    1'    Vibratory Core Location

                                                                                            C              ~~~~~Geophysics Trackline with

                            ~~~~~~C-Nvg ato F i x 07Ao~a

                                ~~~~~~~~5 atclMiRles






                                                                   ,.~~~~f,,~~~  4.6
                   -3ev-00,~~~~~~~ 38.'
                                             C.





            CINCOTEAGUE                      ,.-e







                                                      75,l0                                   75100                                     74-50'


Figure&8 Rectangle grid survey pattern used by FIELD (1979) off the Delmnarva Peninsula.









                                                Journal of Coastal Research, Vol. 13, No. 4, 1997






                                        Monitoring Coastal Environments: Geophysical Methods                                 1077



                                 Or8s"    ar"oo           55    '      50     ' '    45'        1 '40'




                                      -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~2930 \, +++ -0





                                        25'                                                      +               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~25-
                              \~~~~~~~~~                    %





                                                                               EXPLANATION

                                           \   a      l       a             9~~~~~~~~~~'_S.RVEY UK A-0 WAVIWMII Ml


             - 2'             +    .            . ..                 '       --- +                +o'-



                                       \ I0*  tal ,,,s         ,Se

                   ,,.  ~~+             \   "                /a       u     


                                DAYTONA\
                                  BEACH    \ "*' ,; "            ,




                                                                             /,.~~~~~~~~~~~~~~~~~~~~j




                                                                                                    + ,~~~~~" o.-
              _olo            +            +\                    .,X'?             +             +                lo-

                                                                                     144t   He    <<.






                                                                         \   >           '&r 
                   05' ~ ~    110          I0             5             04                       C4
                                               PONCEINDE                            21~4 
                                               LEONINE                    q *ese











                 mone  i    yroyaiclysremoesiga a lae tim  or enacn  the displyofatiua
                                            c       5   ' .' ' ' [ 0m  \   .2t.,,,0,, _.
                                          SCALE IN NAUTICAL MILES  fau4

             lined body(towfst  below t 2  3 * b  6  7a c                              i dve             t    se
                              +            +            +                                                       IS-55+

                              Bl..os'      $[.,oo'        .,s'         .,0'          ,4,'       eo.,4o"'  '

Figure 9.  Zigzag reconnaissance survey  pattern fro m the Florida  eas  ost coast (from MEISBURGER (1990)).



  The basic side-scan system consists of three parts:               to record the incoming signals, allowing additional signal pro-
                                                                    cessing at a later time or enhancing the display of particular
(1) The transducers, mounted in a hydrodynamically stream-          feat
   lined body (towfish), towed at a depth below the turbu-  Deployed a certain distance'above the seafloor, the towfish
   lence of the survey vessel's propeller wash
                    lence o the srvey vssel's ropellr washemits a pulse of acoustic energy. This narrow pulse is trans-
(2) A graphic chart recorder combined with a signal trans-          mitted at right angles to the tow direction and reflects from
                    mitter and processor ~~~~~mitted at right angles to the tow direction and reflects from
   3)Amitter and processor                        1)objects on the seafloor. Transducers in the towfish detect the
                 (3) A tow cable connecting the two units (Figre 10)  reflections, convert them to electrical energy, and send them
Many modern systems also include a magnetic tape recorder           to the signal processing unit onboard the survey boat. Even



                                            Journal of Coastal Research, Vol. 13, No. 4, 1997





1078                                                Morang, Larson and Gorman









                                                                                    LAKE ~~~~~~~HARBOR


             Pan beam         Stlarboard beam




                                                                                            SECTION A


Figure 10. Side-sonar in operation from a small boat.


when the signals are recorded on magnetic tape, they are                       '01
typically also recorded in analog form on paper strip charts
as the survey progresses. Each returning signal is plotted on              H       A RBOR
the paper a distance from the center line corresponding to                                               "'   _
the time it was received. The center line on the paper rep-
resents the towfish's trackline. Seafloor objects which are
close to the trackline are displayed near the center line, while
objects located near the limit of the selected horizontal range
are printed at the edges of the record. Objects directly un-
derneath the towfish are normally not imaged because of the
geometry of the sonar's beam pattern.
  The recorded image is called a sonograph and is analogous
to a continuous aerial photograph. It can give indications of
the nature of the reflecting surface because the stronger the
returning signal, the darker the corresponding mark on the                                    SECTION B
paper. The intensity of the reflected signal is a function of      Figure 11. Example of wood crib breakwater with stone riprap toe pro-
material properties as well as of relief. Hard objects such as     tection and concrete cap. Breakwaters of this type are common through-
boulders and steel produce an intense reflection, whereas a        out the Great Lakes (from MoRANG 1987).
flat, soft clay seafloor reflects very little signal. On most side-
scan systems, the reflection of an object is recorded as black
while the acoustic shadow behind the object is white (the op-      between 25 and 500 m. The choice of horizontal range is
posite of what we see in a photograph). The printing on some
side-scan recorders can be reversed, which makes the image               o               , yp            p   r   o
resemble a photograph, but we do not recommend this change         of target  and sesired re        n a the image  is
because it confuses experienced interpreters who are familiar
with the traditional black return/white shadow display. The        desired, while a general reconnaissance survey will be run at
width of the shadow zone and the position of the object rel-       a range of 100 I  or more.
ative to the towfish can be used to calculate an  object's           For marine surveys close to shore or in shallow inland wa-
height. BELDERSON et al. (1972), FLEMMING (1976), LEEN-            ters, lack of water depth severely restricts the horizontal
HARDT (1974), and MAZEL (1985) provide additional details          range that can be achieved. A traditional "rule of thumb" is
on the use and theory behind side-scan sonar. In an effort to      that the fish should be towed at a distance above the seafloor
characterize bottom type, some researchers have been devel-        of about 10 percent of the selected horizontal range (FLEM-
oping relationships between the intensity of backscattered         MING, 1976). As an example, in 10 m water, the minimum
side-scan acoustical energy and the grain size of the bottom       tow depth will be about 2 m (to keep the fish below waves,
materials (e.g., ScHwAB et al. (1991) and ScHwAB and ROD-          surface turbulence, and propeller wash), leaving the fish
RiGuEZ (1992)).                                                    about 8 m above the seafloor. Therefore, maximum horizontal
                                                                  imaging range will be about 80 m and the recorder scale
Side-Scan Sonar Practical Considerations                           should be set at 100 m.
  The horizontal distance that is imaged can be selected by          Many factors affect image resolution. Vessel speed is one
the operator and, for most commercial side-scan sonars, is         of these: a slow speed enhances resolution because it allows



                                           Journal of Coastal Research, Vol. 13, No. 4, 1997






                                      Monitoring Coastal Environments: Geophysical Methods                                1079




















                         i55+00_111   SH  OWS
















Figure 12. Lakeside, Calumet Harbor breakwater (near Indiana-Illinois border) (from MORANG 1987).


more signals to be transmitted for a given linear distance of      by changing vessel speed or cable layback. Analog side-scan
seafloor. Typically, survey speed must be kept below 3-4           and subbottom signals are often seriously degraded when us-
knots for satisfactory record quality. Wave action also de-        ing tow cables longer than 500 m. The quality and electrical
grades quality. In rough seas, as the boat rocks and rolls, the    integrity of connectors and cable splices is especially critical
tow cable is constantly jerked, in turn causing the tow fish       when using long tow cables. Newer digital systems may not
to twitch and jerk. Various shock-absorbing mounts using           suffer from signal degradation problems to as great an extent
elastic bungee cords can be rigged up to support the cable,        as analog systems.
but this author has not found these measures to be particu-
larly helpful. In shallow water, when using survey boats up        Planning SideScan Sonar Surveys
to about 20 m length, 1.0-to-1.25-m waves are about maxi-
mum for satisfactory records. In deeper water, the longer tow        One of the worst mistakes a researcher can make is to sim-
cable absorbs considerable shock. However, even when using         ply contract with a survey company to go to sea and collect
50-m oil-field workboats on the continental shelf, side-scan       side-scan records based on vague criteria of looking for coral
record quality severely degrades once waves exceed 2 m.            reefs, sand ridges, or other geology. Several critical factors,
Sometimes, in long-period swell conditions, survey lines can       that greatly affect the cost of the project, must be considered
be run with the seas, allowing the engines to be throttled         before offshore reconnaissance surveys begin:
back. However, opposing the seas requires more power, and            (1) What is the resolution, or the size of the objects that
the extra turbulence and vibration often ruins the records.        must be identified? If, for example, a researcher needs to
  Other problems affect deep-water operations. Ringing or          identify 10-m2 coral outcrops up to several km offshore, the
strumming can occur when the frequency of the cable match-         surveys should be rather simple to conduct. If he insists on
es the motions of the vessel. Bungee cord shock absorbers or       identifying individual coral heads that are 10 x 10 cm, it can
plastic streamers (resembling fuzz or hair) can reduce the         be accomplished but only at extraordinary cost.
strumming effect. Sometimes strumming can be eliminated              (2) What is the precision of the surveying; i.e., the repeat-



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1080                                                Morang, Larson and Gorman





      .- -SOUTHEAST                    C-- :                                                           NOR               -



                                                          '5.~~~~~~~~~~~~-







     SURVEY BOAT DIRECTION                                              : 
                                                                                       J-.--- BREACH            











                    -:DIFFRACTION
                     HYPORBOLA                         CT    AND DEBRIS
                                                                                             W .  f_, ~      BREACH

Figure 13. Lakeside, Calumet Harbor breakwater (near Indiana-Illinois border) (from MORANG 1987).


ability of reoccupying a specific site? For a broad area recon-     cataloging data, and converting datums. All too often, side-
naissance (for example, Corps of Engineers Class 3 specifying       scan (and other seismic) records are given to the inhouse "ex-
two-dimensional one-sigma RMS positional error not to exceed        pert" to interpret, but this proves to be false economy consid-
100 m), surveys can be run at modest cost. If the user needs        ering the time required to sort through the survey logs, pre-
Class 1 (RMS positional error not to exceed 3 m), costs will be     pare base maps, plot features, and prepare a summary or re-
dramatically higher. A potential data user must also consider       port. As stated earlier, field data should be interpreted im-
the issue of how was the survey precision validated? Specifying     mediately, preferably as the survey is in progress so that, if
a standard for a survey is the first step; the contractor has to    necessary, the program can be adjusted to enhance the records
deliver this quality and document that it was achieved.             or cover in greater detail unexpected or especially interesting
  (3) When are the surveys to be conducted? The calmer the          features. We recommend that the principal investigator of a
weather, the better the quality. How much weather downtime          geophysics project be involved in all aspects of the program:
can the client afford to pay while the crew waits for optimum       planning, field data collection, and interpretation.
seas? Off the Oregon coast, where seas often exceed 2 m, it           (6) Is there a need for a high precision bathymetric survey
may be wise to charter as big a boat as can be afforded. In the     at the same time? Normally, side-scan sonar and bathymetry
Gulf of Mexico, smaller and less costly vessels may suffice.        surveys should be run simultaneously because one tool com-
  (4) In what form is survey data needed? Most surveys are          plements the other during interpretation. However, conduct-
now recorded digitally so that the tapes can be replayed and        ing bathymetric surveys is an expensive specialty. What pre-
reprocessed to make mosaic maps or enhance particular fea-          cision is needed? As discussed above, a Class 1 survey costs
tures. But for a broad-area reconnaissance, analog paper re-        dramatically more than a Class 3 survey.
cords may be sufficient.
  (5) WHO WILL INTERPRET AND MAP THE SURVEY                            EXAMPLES OF SIDE-SCAN INTERPRETATION
DATA! This is far from a trivial matter. In many cases, it is
advantageous to have the survey company do the interpreta-            Many turn-of-the-century breakwaters in the Great Lakes
tion so that they are faced with correcting navigation errors,      consist of wood frames, known as cribs, that were built on



                                           Journal of Coastal Research, Vol. 13, No. 4, 1997






                                       Monitoring Coastal Environments: Geophysical Methods                                1081






































Figure 14. Lakeside, Burns Harbor, Indiana (southeast corner of Lake Michigan).



land by skilled carpenters, floated into place, and filled with    pile cells at Calumet Harbor, protected with stone riprap. In
stone rubble (Figure 11). In recent years, many of these           this case, the returns from the vertical steel walls were so
breakwaters have begun to deteriorate. Figure 12 is a sono-        intense, the pattern of the cells is better seen on the opposite
graph from Calumet Harbor in southern Lake Michigan. The           (upper) side of the sonogram in the form of ghost images. The
wooden crib breakwater, protected with stone riprap, is seen       strong reflections from curved sides of each cell have pro-
in the bottom of the figure. The edge of the breakwater is         duced diffraction hyperbolas, similar to the hyperbolas that
marked with multiple lines, images of the wood beams. Near         are created by sharp subsurface reflectors in subbottom geo-
the right side of Figure 12, a discontinuity in the lines may      physical records (for example, from the edges of reefs or rock
represent a displaced crib that has begun to settle or tip. The    outcrops). A damaged section of the breakwater shows up as
coarse stone riprap that protects the toe is also evident. Fur-    a wide area of rock debris with a depression in the center.
ther offshore, several oval deposits of coarse material lie on     The harbor floor in the area is almost featureless and consists
the lakebed. These may be piles of material dropped in the         of clay with little or no sand. The gains and thresholds on
wrong location during construction or rehabilitation. The          the side-scan system (at least on a manually-adjusted sys-
mound near the center figure appears to have considerable          tem) must be set so that even a featureless seafloor will pro-
relief because the side closest to the towfish path (the center    duce a slight signal return-this is necessary in order to dis-
line) is dark, representing a strong reflection, whereas the       tinguish places where there is no return, such as from behind
opposite side is in shadow (which recorded as white). Ripples      objects or in holes.
and sand waves can be seen on the lake bottom, indicating            Rubblemound breakwaters produce dark, irregular, blocky
the presence of sand.                                              reflections. Figure 14, an example from Burns Harbor, Indi-
  Another method used to build breakwaters in the Great            ana, shows that the breakwater is built on a clayey lake bed.
Lakes was to drive steel sheet pile in the form of large cir-      Thin veneers of sand cover portions of the bed. As at Calumet
cular cells. These units were filled with rubble and capped        Harbor, there are piles of unidentified coarse material on the
with concrete. The lower half of Figure 13 shows the sheet         lake bed some distance from the jetty.



                                            Journal of Coastal Research, Vol. 13, No. 4, 1997






1082                                              Morang, Larson and Gorman


                                                                d = depth of a reflector (cm)
                          R 1 4                                  t = echo time delay (ns)
                                                                 c = speed of electromagnetic waves in a vacuum (30 cm/ns)
600                500                400                30      E = n2 (relative dielectric constant); for water, e = 81.

                                                                This formula applies to reflections from flat, horizontal in-
                                                                terfaces at least several wavelengths long or to scattering
                               ~ -   ~....._ ..... . .... from point sources. It can be applied to successive layers if E
                              - ---  ....         ----ï¿½=---      is known for each layer and the time delays to each layer can
                                                                be picked off the record.
                                                                  Signal attenuation is caused by several factors (SELLMANN,
                   *,  A     , 1T              , I              DELANEY, and ARCONE, 1992):

                                                                * Conductive and dielectric absorption
                                                                * Interface transmissions
                                                                * Spherical beam spreading

                                                                The last factor is compensated by automatic Time Range
                                                                Gain, which applies an amplitude gain that increases with
                                                                time of return. ULRIKSEN (1982), DANIELS (1989), and Du-
                                                                VAL (1989) cover in detail fundamentals of GPR and its use
300                200                100                 0      in civil engineering and geology. OLIOEFT (1988) provides a
                                                                bibliography of earlier GPR papers.

                                     __; ______ _ X- ,Applications of GPR

                                                                  Using both acoustic profiling equipment and ground-pen-
            .... ....----- --- --/ ~                            etrating radar in freshwater surveys permits researchers to
                                                                obtain more complete subbottom data because the two ap-
                                                                proaches respond to different physical properties and have
                    i'  7     '"                '         '      different spatial sensitivities (SELLMANN, DELANEY, and AR-
                                                                CONE, 1992). The resolution of GPR is typically less than that
                                                                of high-resolution acoustic profilers. For example, the pulse
                                                                from a 50 mHz (center frequency) radar has a duration of
                                                                about 50 ns, which in water is about 1.7 m long. A 100-mHz
                                                             '  commercial radar with pulse duration of 28 ns has a pulse
                                                                length of about 0.8 m. In comparison, a 7 kHz profiler has a
                                                                wavelength of about 0.2 m. However, despite the lower res-
Figure 15. Ground-penetrating radar record from St. Joseph, Michigan  olution, GPR is valuable because it can sometimes image ar-
(data collected and processed by Western Michigan University).
                                                                eas that are opaque to acoustic energy (e.g, gas-charged sed-
                                                                iments) or do not possess impedance contrasts adequate to
      GROUND-PENElTRIATING RADAR (GPR)                          produce acoustic signal returns.
                                                                  For the most part, GPR has not been used in oceanic coast-
Background                                                       al areas because of subsurface units that cause severe signal
                                                                attenuation. These typically include fine-grained estuarine
  Commercially-available short-pulse radar equipment used
                                                                and lagoonal clays and coarse-grained units that contain salt
for subbottom imaging consists of a control unit, magnetic
tape recorder, and power supply, and a combination transmit       t al (1994) have successfully t  al.P(92) and vAN HETEREN
and receiving antenna unit. Electromagnetic energy is re-
flected from earth materials because of variations in dielec-    ture and  tratgraphy of beach rdge s m       N ew E ngland, and
tric contrast and electrical resistivity. These contrasts differ  MEYER  et al. (1994) have reported similar success  on the
                                                                Pacific Coast. In general, GPR is successful when imaging
and may exceed the acoustic anomalies produced by the same
                                                                wide and high barriers where there is a thick lens of fresh-
materials; therefore GPR can sometimes reveal strata and
material changes that might not be revealed by acoustic
methods. Radar data interpretation is usually based on the
echo delay formula (SELLMANN, DELANEY, and ARCONE,               Lake Michigan GPR Examples
1992):                                                             The following GPR examples are from a Coastal Engineer-
                              ct                                ing Research Center monitoring project conducted along the
                         d   2(6)                                southeast shore of Lake Michigan near the town of St. Jo-
                                                                seph, Michigan. One of the purposes of the study was to de-
where:                                                           termine the thickness of the sand layer overlying glacial till.



                                          Journal of Coastal Research, Vol. 13, No. 4, 1997






                                       Monitoring Coastal Environments: Geophysical Methods                             1083






























Figure 16. Lakefloor structure off St. Joseph, Michigan, based on parallel GPR lines.



The GPR system in this study, developed by Western Mich-         * What precision and accuracy is needed (or at least de-
igan University, used a 145 mHz monostatic dipole antenna          sired)?
mounted on a plastic sled. An acoustic transducer pointed        * What is the budget for the project? This directly affects
upward to measure water depth as the sled was towed along          whether the requested precision and accuracy can be
the lake bottom. By keeping the sled on the bottom, the an-        achieved considering factors such as project location, mo-
tenna achieved better coupling with the sediment and re-           bilization costs, distance from harbors and other support
duced the signal attenuation that occurs when an antenna is        facilities, and data processing.
towed through the water. When the data were processed, the       * Who is the customer and how does the customer intend to
records were corrected to show the correct water depth. In         use the data? This directly affects what type of output is
Figure 15, an example of the processed records is compared         needed (raw digital data, finished maps, etc.).
with an interpreted section, similar to the type of interpre-    0 How will the project be jeopardized by weather delays?
tation usually used with acoustic subbottom profiler records.
Figure 16 is a diagram showing nine GPR lines from the St.         Subbottom geophysical surveys need to be planned with
Joseph study.                                                    the above considerations in mind along with additional fac-
                                                                 tors:
                       SUMMARY                                  * What is the scale (size) of objects that need to be analyzed?
  Geophysical survey methods are powerful tools for deline-        What is the minimum resolution that will image the tar-
ating subsurface structure and stratigraphy in coastal areas.      get?
But these tools must be used carefully by experienced geo-       * Who will analyze and interpret the records?
physicists and contractors. Projects must be planned thor-       * How will the output be displayed or plotted?
oughly to take advantage of the particular instruments being     * Is there a significant chance that the survey area is acous-
used and the scale and nature of features that are to be iden-     tically opaque because of gassy sediments? If so, acoustic
tified.                                                            geophysical tools may not be suitable. Possibly an alter-
  For bathymetric surveys, project planners should ask, as a       native method like ground-penetrating radar can be used.
minimum, the following questions and plan their surveys ac-        We reiterate that geophysical data must be interpreted im-
                ~~~~~cordinm~~gly:            ~mediately, preferably while the survey is underway. This
* What are the boundaries of the survey area and how far         way, mistakes can be corrected and survey parameters can
  offshore will the area extend? This affects navigation and    be adjusted if the data is not revealing the structures that
  tidal modeling.                                                are considered important. We must also emphasize that geo-



                                           Journal of Coastal Research, Vol. 13, No. 4, 1997






1084                                                   Morang, Larson and Gorman


physical tools are indirect windows into the world beneath                ologists and Engineers-the Elements of Geophysical Prospecting.
the sea-they provide a model of sediments and strata and                  Oxford: Pergamon, 223p.
                                                                       HEADQUARTERS, U.S. ARMY CORPS OF ENGINEERS, 1994. Hydro-
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                                                                         1995. Monitoring of the Yaquina Bay entrance North Jetty at
                                                                         Newport, Oregon, monitoring summary and results. Technical Re-
         ACKNOWLEDGEMENTS AND NOTES                                      port CERC-95-9, U.S. Army Engineer Waterways Experiment Sta-
                                                                         tion, Vicksburg, Mississippi.
  This paper was supported by various work units at the U.S.           LEBLANC, L.R.; PANDA, S., and SCHOCK, S.G., 1992. Sonar attenu-
Army Engineer Waterways Experiment Station, including                     ation modeling for classification of marine sediments. Journal of
the Civil Works Guidance Update Program and the Coastal                   the Acoustical Society of America, 91(1), 116-126.
Structures Evaluation and Design Program. We thank Mr.                 LEENHARDT, O, 1974. Side scanning sonar-a theoretical study. In-
                                                                         ternational Hydrographic Review, 51(1), 61-80.
Richard McGee for his thorough review. Permission to pub-               LOGGINS, C.D., 1995. Ahead-look sonars: design comparisons and
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                                                                         Salem, New Hampshire.
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                                              Journal of Coastal Research, Vol. 13, No. 4, 1997






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                                                 Journal of Coastal Research, Vol. 13, No. 4, 1997