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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 Gorman� tUS. Army Engineer tU.S. Army Engineer �U.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: Acoustipulse� 0.8-5.0 Bottom penetration to -30 m Uniboom� 0.4-14 15-30 cm resolution with 30-60 m penetration Bubble Pulser -0.4 Similar to Uniboom� 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&G�, Datasonics�, Reson�, 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 �0.305m +0.457m after World War I revolutionized river, lake, and offshore sur- ment one-sigma standard error (�0.5 ft) (�1.0 ft) (�1.5 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 � 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 �0.046 009 015 040 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 1000� (75 kHz), Simrad EM 950� (95 Khz), Krupp-Atlas Fan- depth for the highest quality surveys, transducers and re- sweep� (200 kHz), and the Reson SeaBat� (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 � 0.2 and ever-greater prolem.cessed multi-beam data from Figure 4, an example of processed multi-beam data from � 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 (� 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 � 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.~~/.?� ,-, ~~~~~~~~~~~.,, - ,. f,',. x i , ,.si;e2 , ? . 2 ,, ,' ~ ; __ _ ,___ (3 1� Vwhre V� 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 �heqight 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 �1/ 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-- ... . '-, . � .-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- Journal of Coastal Research, Vol. 13, No. 4, 1997 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 - --- .... ----�=--- 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- structure. The most successful results come from comprehen- graphic surveying. Engineer Manual EM 1110-2-1003. Washing- sive studies where the subsurface model is verified with cores ton, D.C. or other direct evidence. HUGHES, S.A.; PUCKETT, P.; TUBMAN, M.W., and CORSON, W.D., 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 lish this paper was granted by the Chief of Engineers, U.S. performance considerations. Underwater Systems Design, 17(4), Army Corps of Engineers. Citation of trade names does not 15-29. constitute an official endorsement or approval of the use of MAZEL, C., 1985. Side-scan sonar training manual. Klein Associates, Salem, New Hampshire. MCGEE, R.G.; BALLARD, R.F., JNR., and CAULFIELD, D.D., 1995. A technique to assess the characteristics of bottom and subbottom LITERATURE CITED marine sediments. Technical Report DRP-95-3, U.S. Army Engi- neer Waterways Experiment Station, Vicksburg, Mississippi. BATES, R.L. and JACKSON, J.A. (eds.), 1984. Dictionary of Geological MEISBERGER, E, 1990. Exploration and sampling methods for bor- Terms. Garden City, New York: Anchor Press/Doubleday, 571p. row areas. Technical Report CERC-90-18, U.S. Army Engineer BELDERSON, R.H.; KENYON, N.H.; STRIDE, A.H., and STUBBS, A-R., Waterways Experiment Station, Vicksburg, Mississippi. 1972. Sonographs on the Sea Floor. Amsterdam: Elsevier, 185p. MEISBERGER, E.P. and WILLIAMS, S.J., 1980. Sand resources on the BERNE, S.; AUFFRET, J-P, and WALKER, P., 1988. Internal structure inner continental shelf of the Cape May Region, New Jersey. of subtidal sandwaves revealed by high-resolution seismic reflec- Coastal Engineering Research Center Miscellaneous Report 80-4, tion. Sedimentology, 35, 5-20. U.S. Army Engineer Waterways Experiment Station, Vicksburg, CAULFIELD, D.D. and YIM, Y.C., 1983. Prediction of shallow subbot- Mississippi. tom sediment acoustic impedance while estimating absorption and MEYERS, R.A.; SMITH, D.G.; JOL, H.M., and HAY, M.B., 1994. Inter- other losses. Journal of the Canadian Society of Exploration Geo- nal structure of Pacific coast barrier spits using ground penetrat- physicists, 19(1), 44-50. ing radar. GPR '94-Proceedings of the Fifth International Confer- CAULFIELD, D.C.; CAULFIELD, D.D., and YIM, Y.C., 1985. Shallow ence on Ground Penetrating Radar, Canadian Geotechnical Society subbottom impedance structures using an iterative algorithm and and Waterloo Centre for Groundwater Research, Waterloo, Ontar- empirical constraints. Journal of the Canadian Society of Explo- io, Canada, pp. 843-854. ration Geophysicists, 21(1), 7-14. MORANG, A., 1987. Side-scan sonar investigation of breakwaters at CHRZASTOWSKI, M.J. and SCHLEE, J.S., 1988. Preliminary sidescan- Calumet and Burns Harbors on southern Lake Michigan. Miscel- sonar investigation of shore-defense structures along Chicago's laneous Paper CERC-87-20, U.S. Army Engineer Waterways Ex- northside lake front: Wilson Avenue groin to Ohio Street beach. periment Station, Vicksburg, Mississippi. Environmental Geology Notes 128, Department of Energy and Nat- OLHOEFT, G.R., 1988. Selected bibliography on ground penetrating ural Resources, Illinois State Geological Survey, Champaign, Illi- radar. Proceedings of the Symposium on the Applications of Geo- nois. physics to Engineering and Environmental Problems. Society of En- CLAUSNER, J.E. and POPE, J., 1988. Side-scan sonar applications for gineering & Mineral Exploration Geophysicists, Golden, Colorado, evaluating coastal structures. Technical Report CERC-88-16, U.S. pp. 462-520. Army Engineer Waterways Experiment Station, Vicksburg, Mis- SCHOCK, S.G.; LEBLANC, L.R., and MAYER, L.A., 1989. Chirp sub- sissippi. bottom profiler for quantitative sediment analysis. Geophysics, DANIELS, J., 1989. Ground penetrating radar. SAGEEP '89-Pro- 54(4), 445-450. ceedings of the Symposium on the Applications of Geophysics to SCHOCK, S.G. and LEBLANC, L.R., 1992. Sediment classification us- Engineering and Environmental Problems, Environmental and En- ing the chirp sonar. Paper OTC 6851, Offshore Technology Confer- gineering Geophysical Society, Englewood, Colorado, pp. 62-142. ence, Houston, Texas, pp. 363-368. DWuvAL, J., 1989. Radiometrics in geology. SAGEEP '89-Proceedings SCHWAB, W.C.; WEBB, R.M.T.; DANFORTH, W.W.; O'BRIEN, T.F., and of the Symposium on the Applications of Geophysics to Engineering IRWIN, B.J., 1991. High-resolution side-scan sonar imagery of the and Environmental Problems, Environmental and Engineering Manchas Interiores-Manchas Exteriores coral reef complex, Geophysical Society, Englewood, Colorado, pp. 1-61. Mayaguez, Puerto Rico. U.S. Geological Survey Open File Report FIELD, M.E., 1979. Sediments, shallow subbottom structure, and 91-436, Washington, D.C. sand resources of the inner continental shelf, central Delmarva SCHWAB, W.C. and RODRIGUEZ, R.W. (eds.), 1992. Progress of stud- Peninsula. Coastal Engineering Research Center Technical Paper ies on the impact of Hurricane Hugo on the coastal resources of 79-2, U.S. Army Engineer Waterways Experiment Station, Vicks- Puerto Rico. U.S. Geological Survey Open-File Report 92-717, burg, Mississippi. Washington, D.C. FITZGERALD, D.M.; BALDWIN, C.T.; IBRAHIM, N.A., and HUMPHRIES, SELLMANN, P.V.; DELANEY, A.J., and ARCONE, S.A., 1992. Subbot- S.M., 1992. Sedimentologic and morphologic evolution of a beach- tom surveying in lakes with ground-penetrating radar. CRREL ridge barrier along an indented coast: Buzzards Bay, Massachu- Report 92-8, U.S. Army Corps of Engineers Cold Regions Research setts. In: FLETCHER, C.H., III, and WEHMILLER, J.F., (eds.), Qua- & Engineering Laboratory, Hanover, New Hampshire. ternary Coasts of the United States: Marine and Lacustrine Sys- SHALOWITZ, A.L., 1964. Shore and Sea Boundaries, with Special Ref- tems. (SEPM) Special Publication No 48, Society for Sedimentary erence to the Interpretation and use of Coast and Geodetic Survey Geology, Tulsa, Oklahoma, pp. 65-75. Data. Volume 2, Publication 10-1, U.S. Department of Commerce, FLEMMING, B.W., 1976. Side-scan sonar: a practical guide. Interna- Coast and Geodetic Survey, U.S. Government Printing Office, tional Hydrographic Review, 53(1), 65-92. Washington, D.C., 749p. GRIFFITHS, D.H. and KING, R.F., 1981. Applied Geophysics for Ge- SHERIFF, R.E., 1977. Limitations on resolution of seismic reflections Journal of Coastal Research, Vol. 13, No. 4, 1997 Monitoring Coastal Environments: Geophysical Methods 1085 and geologic detail deriveable from them. In: PAYTON, C.E., (ed.), ULRIKSEN, A.P.F., 1982. Application of Impulse Radar to Civil En- Seismic Stratigraphy-Applications to Hydrocarbon Exploration. gineering. Geophysical Survey Systems, North Salem, New Hamp- Memoir 26, American Association of Petroleum Geologists, Tulsa, shire (reprint of doctoral dissertation completed at Lund Univer- Oklahoma, pp. 3-14. sity of Technology, Lund, Sweden, 1982). SHERIFF, R.E., 1980. Seismic Stratigraphy. Boston, Massachusetts: UMBACH, M.J. (Commander, NOAA), 1976. Hydrographic Manual. International Human Resources Development Corporation, 227p. 4th Edition. U.S. Department of Commerce, National Oceanic and SHERIFF, R.E. and GELDART, L.P., 1982. Exploration Seismology, Vol- Atmospheric Administration, Washington, D.C. (with updates ume 1: History, Theory, and Data Acquisition. Cambridge: Cam- 1979, 1980, and 1981). VAN HETEREN, S.; FITZGERALD, D.M., and McKINLAY, P.A., 1994. bridge University Press, 253p. Application of ground-penetrating radar in coastal stratigraphic SIECK, H.C. and SELF, G.W., 1977. Analysis of high resolution seis- studies. GPR '94-Proceedings of the Fifth International Confer- mic data. In: PEYTON, C.E. (ed.), Seismic Stratigraphy-Applica- ence on Ground Penetrating Radar, Canadian Geotechnical Society tions to Hydrocarbon Exploration. Memoir 26, American Associa- and Waterloo Centre for Groundwater Research, Waterloo, Ontar- tion of Petroleum Geologists, Tulsa, Oklahoma, pp. 353-385. io, Canada, pp. 869-881. Journal of Coastal Research, Vol. 13, No. 4, 1997