960-968,Journal of Coastal Researchl- l- -'-- --'- '-__Fort Lauderdale, Florida r=Fall 1996

Digital Ground Penetrating Radar (GPR): A NewGeophysical Tool for Coastal Barrier Research(Examples from the Atlantic, Gulf and Pacific Coasts,U.S.A.)Harry M. 101, Derald G. Smith and Richard A. Meyers

Department of GeographyUniversity of CalgaryCalgary, Alberta T2N IN4, Canada

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ABSTRACT _

.JOL, H.M.; SMITH. D.G., and MEYERS, R.A., 1996. Digital ground penetrating radar (GPR): A new geophysical toolfor coastal barrier research (examples from the Atlantic, Gulf and Pacific coasts, U.S.A.I. Journal ofCoastal Research,12(4),960-968. Fort Lauderdale (Florida), ISSN 0749-0208.

Although digital ground penetrating radar (GPR) is still in its infancy, results indicate that it will soon become oneof the most significant geophysical instruments for coastal barrier studies. GPR can infer stratigraphic trends, andtherefore directions of progradation and/or aggradation, delineate sedimentary facies, and determine depth to thefresh-brackish water interface in shallow freshwater conditions (i.e., Galveston Island). Internal structure of barriersconsisting of sand, broken shell fragmr-nts and boulder-gravel in Georgia, Florida, Texas, Oregon and WashingtonStates are illustrated using GPR. Seaward dipping reflections (1-23°) from paleo-beach surfaces occur to depths of12m. Severe signal loss is noted on all barriers at approximately the level of the low tide. Gulf Coast barriers have thinfreshwater lenses « 4 m ), below which brackish watcr attcnuates electromagnetic energy. In contrast, the best resultsare from higb wave energy and high tidally-influenced (3.7 m) Pacific clastic sand and barriers with palco-beach dipsof 1-2" to 12 m deep. A boulder-gravel beach at Seaside, Oregon, has good results with inclined reflections of 23° to12 m deep. Shelly beaches at Anast.asia Island, Florida exhibit semi-continuous seaward inclined reflections (3-6'1 todepths of 6 m.

ADDITIONAL INDEX WORDS: Barrier island, geophysics, coastal stratigraphy, coastal sediments.

INTRODUCTION AND BACKGROUND

Acquisition of subsurface stratigraphie information ofmodern barriers has been restrained over recent decadesbecause of limited funds available for research and devel­opment. Drilling and geophysical systems for deep stratig­raphy acquisition has been motivated by high profitabilityin the exploration for hydrocarbons and minerals. Devel­opment of drilling and geophysical systems for shallow stra­tigraphy has been driven by the less profitable cnviroruncn­tal and geotechnical industries (ROMIG, 1993). A strati­graphic acquisition system is needed with high resolution(drn scale) that is affordable, portable, robust, and ti me andcost effective, Recent developments in ground penetratingradar (GPR) technology have made such a system available.CPR is a high resolution geophysical instrument useful forassessing stratigraphy and paleogeomorphology of barriers.Earlier analog GPR systems, which were revolutionary atthe time of their introduction (early 1980's) are now bulkyand lack basic acquisition parameters and post-processingcapabilities i.i.e., scaling changes, topography corrections).Digital GPR systems have dealt with these problems. Thedigital system has the advantage of high fidelity (better sig­nal-to-noise ratio) and, equally important, the capability to

.95010 recened 18 January 1.995; accepted in revision 17 May 1995.

record, process, and store data digitally. This latter featureallows data to be processed with sophisticated seismic pro­cessing software.

Barrier islands, spits and strandplains are elongate bod­ies of coarse grained sediments (usually sand), transportedand deposited by longshore drift and wave processes. NorthAmerican examples can be found along the Atlantic Coastand Gulf of Mexico and along isolated segments of the Pa­cific Coast, Gulf of St. Lawrence and the Arctic Coast (Alas­ka).

Over the last several decades, there has been an increas­ing interest in the stratigraphy and sedimentology of bar­riers, spits, and strandplains. Early interest was motivatedby the need for depositional models for oil and gas explo­ration (BERNARDet al., 1962; BVRNEet al., 1959; GOULDandMcFARLANE, 1959; HAYI<~S and KANA, 1976; HAVES, 1979;HOYT and HI'~NRV, 1965; KRAFT, 1971, 1978; MCCUBBIN,1982; REINSON, 1984), Recently, rapid urbanization of bar­riers has initiated additional research to enable coastal com­munities to better plan and mitigate sustainable freshwatersupplies, sewage and garbage disposal, and better under­stand long term erosion and depositional problems. There­fore, to better understand the dynamics and nature of bar­rier systems, new and different methodologies must be ap­plied to keep pace with this growing demand for informa­tion .

Digital Ground Penetrating Radar 961

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Although pioneering studies provided the basis for under­standing the origin and dynamics of barriers (FISK, 1959;BERNARD et al., 1962; HOYT, 1967; KRAFT, 1971; HAYES,1979), the internal structure and stratigraphic trends areinferred, not fully documented. Recently, the developmentand application of GPR to stratigraphic and geomorphicproblems has shown promise (ULRIKSEN, 1982; LEATHER­MAN, 1987; BERES and HAENI, 1991; JOL and SMITH, 1991;SMITH and JOL, 1992). Limited studies carried out in thelate 1980's established that analog GPR could detect sedi­mentary structures within barriers (LEATHERMAN, 1987;FITZGERALD et al., 1992; VAN HETERN et al., 1994); whilemore recently, digital GPR clearly demonstrated its full po­tential on the Willapa Bay Barrier of Washington State (JOLet al., 1994; MEYERS, 1994; MEYERS et al., 1994).

Our primary objective is to establish the effectiveness ofdigital GPR for data acquisition in the assessment of barrierstratigraphy from a variety of depositional settings (Atlan­tic, Gulf and Pacific Coasts) with different sediment com­positions (sand, shell fragments and boulder-grave\). A sec­ondary objective is to show how GPR can be used to deter­mine the depth and delineate the fresh/brackish ground­water contact in coasts that have a shallow lens of freshgroundwater.

METHODOLOGY

Digital GPR profiles are similar in appearance to seismicprofiles, except that GPR data are acquired by using tran­sient electromagnetic (EM) energy reflection. A short pulseof high frequency EM energy, usually in the 10 to 1,000megahertz (MHz) range, is transmitted into the ground.Some of the energy is reflected back to the surface from thecontacts between different subsurface lithologies, includingsuch changes as sediment grain size (facies change con­tacts), mineralogy, density, bedrock contact, and water con­tent (Figure 1; MOORMAN et al., 1991; DAVIS and ANNAN,1989). This effect enables the subsurface stratigraphy andground moisture conditions to be inferred from the charac­ter of the radar return signals. The resolution ofGPR at 100MHz (assuming a velocity of 0.1 mlns; ns = nanosecond =10-9 seconds) is approximately 25 to 50 em which is approx­imately 10 times greater than conventional high-resolutionshallow seismic which is between 3-5 m (JOL, 1988). GPRtheory and methodology are adequately explained elsewhere(ANNAN and DAVIS, 1976; ULRIKSEN, 1982; DANIELS et al.,1988; DAVIS and ANNAN, 1989).

We used a pulseEKKO@ IV radar system in reflectionsurvey mode with antennae frequencies of 50, 100 and 200MHz and either a 400 v or 1,000 v transmitter. At each site,prior to a decision being made for the complete survey, wetest all antennae frequencies. In this paper, we provide thedataset obtained with the antennae that we believe best il­lustrates the stratigraphy of each site. Traces at each sur­face location (0.5 or 1.0 m intervals) were digitized at a sam­pling time interval of 800 picoseconds and vertically stacked64 times. Profiles were processed and plotted usingpulseEKKO@ IV (version 4.0) software. The depth scales ofthe profiles are based on an average near-surface velocity

Figure 1. Ground penetrating radar profiling procedure. (a) The step­like procedure involves repetitive moves of both the transmitter and re­ceiver at a constant spacing. (b) Five schematic GPR traces, showing thearrival of air-wave and ground-wave pulses and a lower reflected wavefrom a subsurface reflecto.r (JOL and SMITH, 1991).

determined from common mid-point (CMP) surveys at eachsite (for additional information on CMP surveys see JOL andSMITH, 1991).

RESULTS AND INTERPRETATION

The studied sites include: Jekyll Island, Georgia; Anas­tasia Island, Florida; Galveston, Mustang, and Padre Is­lands, Texas; Netarts, Bay Ocean, Nehalem, Seaside, andClatsop barriers, Oregon; and Willapa and Ocean Shoresbarriers, Washington (Figure 2). From this extensive database, selected radar profiles are presented. For the sake ofclarity, the horizontal scale of all profiles is distance in me­ters, while the vertical scale is shown as both two-way trav­el time in (nsec) and depth in meters (m). It should be notedthat all profiles are vertically exaggerated. The two upper­most continuous reflections in all profiles represent airwave and ground wave arrivals respectively and are notpart of the stratigraphic data.

Jekyll Island (Georgia-Atlantic Coast)

Jekyll Island is located 13 km southeast of Brunswick,Georgia, and consists of fine grained sand. The 200 MHzprofile shows seaward dipping reflections, typical of mostbarriers studied to date. The profile is oriented northwest­to-southeast (perpendicular to shore) and is located on a lev­el playing field 400 m northeast of the University of Georgiafield station laboratories. Inclined reflections dipping south-

Journal of Coastal Research, Vol. 12, No.4, 1996

962 J ol, Smi th a nd Meyers

Figure 2, Loca t ion map of 8 GPR sites studied a long th e Atlantic, Gulfand Pacific coas ts of the United States.

ea st at 1_20 indi cate seawa rd paleo-pr ogradation of theshore face. The third nearly continuous hori zontal reflectionis from th e freshwater table. The inclined wavy-lik e deeperreflections ma y represent pr eser vation of offshore bars. Anerosiona l contact (inte rpre ted as sa nd to silt), at 20 m (hor­izontal scale) and 5 m deep, shows 2 m of stra ta with steeperinclined reflections that may represent a lith ofacies changea nd possibly a tidal inlet cha nnel-fill.

Figure 4 (100 MHz) perpendicularly in tersects Figure 3southwest from trace 70, and this profile indicates horizon­tal st ra t igra phy along th e depositional s t r ike . However, two

distinct radar facie s jo in at approximate ly 3.0-3.5 m deep.We interpret the continuously inclined upper facie s to re p­resen t th e beach face. The lower radar facies, between 3.0­3.5 to 7.0 m, is interpre ted as offsh ore bars and tidal inl et­fill.

Anastasia Islan d , located southeast of St. Augustine,nor theast Florida , cons ists of coars e gra ined sa nd from bro­ken shells, Ea stward dipping radar reflecti ons indi catela rge sca le gently inclined st ra ta (Figure 5). This 50 MHzline is from an a bandone d west-to-east road (perpendicula rto shore) , located 2 km northwest of St. Augustine Beach.The freshwater table was at-or-n ear the surface « 1 rn ),

Semi -continuous, inclined reflections dipp ing at 3 to 6° to adepth of at lea st 6 m indicate a seaward paleo-progradationof th e coas t. This pal eo-sh oreface is steeper than that atJekyll Island (Figure 4) which has incl ined reflections of 1­2° in fine sa nd. The abil ity of CPR to measure th e angle ofinclination of paleo-beachface surfaces ca n be us ed to infergrain size (steeper slopes are associated with coarsergrained sediments; P ETHICK, 1984 ). The importance of th eprofile is to show th at rad ar can perform reasonably well incoarse grain broken she lly materi al whi ch is typical of theea stern Florida coast. But, th e ab sence of vacant land suit­able for radar profil ing, free of bu ildings, power lines andothe r cultural structu res is a major problem in these con­tinuously urbanized coasts .

Anastasia Island (Florida-Atlantic Coast)

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Figu re 3. CP R profile (200 MHz) from J ekyll Island, Georgia, showing incli ned reflections dipping sea ward which re prese nt pale o-beach surfaces andoffshore bars. The st ra t igr aphica lly lower 2 m of reflect ions (20- 75 m) may represent a tida l inlet-fill . Solid line shows th e erosiona l contac t (sa nd to silt)or possibl y a fres h-brackish water contac t.

Journal of Coas tal Research, Vol. 12, No. 4, 1996

Digital Ground Pen etrating Radar 963

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300-Figur e 4. GPR profile (l Oa MHz) a long t he barrier axi s of Je kyll Island, Georg ia , show s horizontal reflect ions (extend ing southwest from Figure 3 at70 m). The abse nce of dips su ggests tha t the island was neither acc re t ing northeastward nor sout hwestwa rd at th e t ime of deposit ion .

Galveston Island (Texas-Gulf Coast)

The first profile (100 MHz) from Galv eston Island wasshot along 8-Mil e Road (nort hwest to southeast; Figure 6)in an attempt to verify the pioneering interpretation of sea­wa rd dipping shoreface structure propo sed by BERNARD etal. (1962). Galveston Island cons ists of fine grained sandand contains a shallow water table at mid -island (1.5 mdeep-3rd reflection). The semi-continuous radar reflectionsare dipping seaward at approximately 1_2 0 exte nding to adepth of about 5.5 m. This angle of inclination is th e sameas the angl e of radar refle ct ions at J ekyll Island. Below 5.5m, the radar signal is attenuated by either an increase ofsilt sized sediment, structureless burrowed sediment or pos­sibly brackish water.

The second profile from Galv eston Island (100 MHz, Fig­ur e 7) shows horizontal reflect ions, th en signal attenuationby brackish water below 2.75 m (depth to br ackish waterconfirmed by th e golf course head groundskeep er , PersonalCommunication, 1994 ). This site was s hot on a northwest­to-southeast golf course fairway at La fitt e's Cove GolfCourse, near the mid-b arrier. The upper horizontal reflec­tion (1.6 m) may represent ver t ical accretion strata from

wash over storm s or eolian pro cesses. Th e lower reflectionsare rin gin g (geophys ical nois e). Data from both Mustangand Padre Islands are simila r to Figure 8, with ver y sha llowradar pen etration, but th e availa ble data does su ggest ver­tical accr eti on bedding with in the fre shwater lens « 3 m intested locations ).

Figure 6 shows reflections exte nding down to 5.5 m; th eaddi t ional 3 m of data are possibl e to acqui re because thesurface elevation is approxim ately 3 m higher than th e sur­face in Figure 7 t i.e., a th icker sedimenta ry pile a bove thebrackish water table). The surficial sedimentary s tr ucturesin Figure 6 are most likely attributabl e to bea ch prograda­tion during storm and post-storm eve nts (MEYE RS, 1994 ).To expla in these structures, MEYERS (1994) sugges ts an ero­siona l phase during high magnitude storms whi ch concen­t ra te either hea vy min erals or slightly coarse r grai ned sed­iments as a lag. This eros ional phase is followed by lateralaccretion of sedime nt during summe r low wave intensity pe­riods. Figure 7 from Lafitte 's golf course shows an absenceof inclin ed strata and th e hori zontal st ra t a could be inter­preted as a washover simila r to th at shown by MCCUBBIN(1982 , p. 269).

Jo urnal of Coastal Research, Vol. 12, No.4, 1996

964 Jo l, Smith and Meyers

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200Figure 6. GPR profile (tOO MHz ) from mid-Galveston Island , Texa s, a long 8-Mi le Road show s inclin ed reflections dipping toward the Gu lf of Mexico,verifying th e deposit iona l in te rpreta t ions in th e "classic dri ll core profile" of BERN ARD et at. (1962). Dashed line shows th e wate r tab le.

J ournal of Coastal Research, Vol. 12, No. 4, 1996

Digital Ground Penet rating Radar 965

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Figure 7. GPR profile (100 MHz) from Ga lveston Island , Texas, showing hori zonta l stra t igraphy an d a loss of sign al below 2.75 m, the depth to brackishwater (head groundske eper , Lafitte's Cove Golf course, personal communication, 1994 ). The dashed line shows the wat er table. Profiles from Musta ngand Padre Islands were simila r.

Seaside Stranplain (Oregon-Pacific Coast)

The coas t at Se asid e, Oregon consi sts of a rai sed boulder ­gr avel stra ndpla in (RANKIN, 1983) with a rel a tively steepbeachface. This poorly sorted boulde r gr avel was derivedfrom the near by Tillamook headl and (RANKIN, 1983) . Th ewes t-to-east (perpendicular to shore) pr ofile (lOa MH z) endsat the boardwal k near t he nor th end of t he Tides Motel (F ig­ur e 8). The radar re flectio ns show inclined boulder-gravelstrata dipping up to 23° to a depth of 11 m and this is in­terpreted as paleo -beach surfaces , pr oba bly deposited dur­ing storms. The loss of signa l below the somewhat promi­ne nt reflect ion at 200 nsec may be du e to in trusion of saltwater a nd/ or a bedrock contact. Agai n, a sing le dri ll holewould verify th e ca use and GPR could then ma p the ent irearea . Over the 50 m of profile, the fres hwa ter table, rep re­sented by the prominent, nea rly conti nu ous horizontal ra ­dar re flection is 5.5 m below t he surface.

Willapa Barrier (SW Washington-Pacific Coast )

The Willa pa ba rrier is a mode rn, active barr ier spit con­sisti ng of fine gra ined sand derived from the mouth of theColumbia River via longsh ore t ra ns port (BALLARD, 1964). Itis 38 km long by 2-3.5 km wid e a nd is influ en ced by a 3.7m tidal ra nge and high-energy waves (MEYERS, 1994). Th e

6 m thick facies of shing le-like reflectio ns dip toward theocean at ap proximately 1_2° and t his pattern represents ah istory of progradat ion si nce 4500 BP (l00 MHz, Fig ure 9).The se prominen t an d con ti nuously incli ned reflections areinterpre ted as majo r storm depositional bedding sur faces inwh ich offsho re bars are not preserved, contrasti ng with th einferred offshore bars in the Jekyll Isla nd profi le (Figure 3).Profi les a long the depositional strike show hori zontal , ne ar ­ly con tin uou s re flect ion pat tern s sim ila r to th ose in Figure4. Th e radar facies below 6 m shows disconti nuous re flec­t ions an d loss of radar signal returns which may indicate alithofacies change, possib ly du e to biotur ba tio n destroyingpr imary sedimentary st ru ctu res. Abunda nt water well dataverify the abse nce of brackish water or silt in th e upper 30m of the barrier.

O cean Shores Barrier (Washington-Pacific Coast )

Located 26 km west of Aberdeen, Was hington, the ba rr ie rpro tects the northwest sec tor of Grays Ha rbor from the Pa ­cific Ocean. The ba rrier cons ists of fine grained sand trans­ported from the Columbia River by longsh ore curre nts (Pre­TERSON a nd PHIPPS, 1992). The GPR profil e , (100 MHz)from t he back barrier (F igure 10), shows a lower ra da r faciesof steeply inclin ed (2_8°) reflections steepening eastward

Journal of Coasta l Resea rch, Vol. 12, No. 4, 1996

966 Jol, Smith and Meyers

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in to the harbor; this may re prese nt a washover fan depositin the early evolu tion of th e ba rrie r spit Th e cha nge in slopea ngle from wes t (2°) to east (8°) is simila r to a conce pt ua ldiagram of washover fans (M CCUBBIN , 1982), Above th ewash over foresets (between traces 73 an d 150 m) and locat­ed between the surface and 5 m deep , the continu ous hori­zon tal reflections represent verti cal accretion from stormwas hover events du r ing sea level r ise through th e mid-Ho­locen e. Farther west (between traces 0 an d 73 m), re flec­tio ns dip seaward, incl ined at 1_2°. These re flecti on pat­tern s are simila r in an gle to th ose from th e Willapa , Ga l­veston and Jekyll Barrie rs ,

DISCUSSION

A GPR comparison of sa ndy barrier s shows a range of ra ­dar reflect ion pa tterns and depths of pen et ration , affected bystorm wave a nd tidal ene rgy, sed iment grai n size and min­era logy. Pac ific sa ndy barriers yie ld the highest qu ality da­ta sets (Figu res 9 and 10), which consist of cont inuous, prom­inent reflect ions to a depth of 12 m, as compared to less prom­inen t reflection s from J ekyll and Galvesto n Islands. Oth erbeach ma terial s suc h as boulder-gr avel (Figu re 8) and shellfra gments (Figure 5) provide discernible rad ar stra t igraphy ,but a re gene ra lly of a lower qua lity th an clastic sand domi­na nt systems. However , boulder-gravel deposi ts did allow oneof th e deepest penetrations for GPR in our study.

Pacific and Atlantic barriers receive 1.5-2.0 m of precipi­tation annua lly with moderate am oun ts of evaporation whichres ult in thi ck (30- 60 m) fresh wate r lenses (MEYERS, 1994).Th is compares to lower pre cipitation and higher evaporat ion

rates for Texas barrier s whe re fresh water lenses are 1-4 mthick. The brackish-sal ine water near the surface at Galves­ton, Pad re and Mustan g Islands severely limits the useful­ness of radar for stra t igra phic studies. However, if th e depthat which sign al loss du e to brackish water can be veri fied bydr illing a sin gle hole, then GPR can be used to map th e fresh­water lens in a barr ier . Th erefore , GPR provides a time ef­ficien t, cost effective and non-d estructive mean s for coastalgro undwate r stud ies (seasonal changes , drawdown cha ra c­ter isti cs due to water ext raction , and sa lt water intrusion).

All the rad ar data from th e barriers investigated indicatea dom inanc e of seawa rd progradation rather than downdriftaccret ion. Th e Willapa barrier was the most exten sively stud­ied and the 35 line kilom eters of GPR also show a predomi­nantly seaward progradation (MEYERS, 1994); oth er barriersneed rigorous investigation with GPR to indicate their longte rm progradation patterns.

CONCLUSIONS

We believe th at GPR is presen tly th e most promisin g de­vice availab le for subsurface investigations of coasta l sedi­mentary and groundwater environments. Shallow seismicdoes not provide the needed resolution for det ailed s tra tig­raph y. LEATHERMAN(1985) noted "convent iona l seismic sur­veying ha s been found to be useless on ba rrier island s anddetailed st ra tigraphic correlat ions are difficult based on bore­hole da ta". Vibracoring provid es the only inexpensive pointsource data , but has a very limited depth of penetrati on inbarriers. Six m was achieved by repeatedly rein ser tin g thecore tub e into th e sa me hole; an addit ional meter of core was

J ourn al of Coas ta l Research, Vol. 12, No.4 , 1996

Digital Gro und Pe netrati ng Radar 967

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Figure 9. GPR profile (lOa MHz) from the Will ap a ba rr ier spi t, Washingt on showing 1_2 0 inclined reflect ions dipping seaward . The a bse nce of offshoreba rs , cont ra ry to Jekyll Islan d, is attributed to reworking of th e beach surface by severe wint er stor m waves (MEYERS, 1994 ).

recovered with each attempt. Rotary au ger and trenching islimited due to sediment collapse becau se of sh allow ground­water tabl es. Geophysical wireline logging is limite d due tolittle change in sediment grain size and thi s method also re­qui res existing wells. Apart from GPR , no other sh allow sub­surface data-acquisition system with comparable resolutionand continuity of data is pr esently ava ilable.

GPR can be used effectively on coastal barriers, spits andstrandplainscomposed of clean coarse sediments (little to orno clay or silt) with reas onably thi ck freshwater aquifers.

The se a re important requirements since t he GPR signal isattenua ted in brackish/saline groundwater and silt/clay con­diti ons. GPR results can infer sedimenta ry facies and direc­tion s of paleo-deposition. Previous tests hav e shown t hat ra­dar facies compare well with structural facies (J OL andSM ITH , 1991; SMITH an d JOL , 1992; HUGG ENB ERGER, 1993).Dip an gle of inclined reflections may be asso cia ted with gr ainsize of sedime nts (1_2° in fine sa nd, 3_6° in coarse sa nd togranules of brok en shells , and up to 23° in bould er-gravel ),as well as wave energy. Paleo-directions and patterns of pro-

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10.0Figu re 10. GPR profile (l Oa MH z) from th e back barrier at Ocean Shores , Wash ington. Lower most ea stern stee ply dippin g st ra ta are prob able wash overforeset beds . Upp er facies of hori zontal refle ct ions east of 73 m are inte rpreted as vert ica l accretion bedding depos ited as part of the origi nal spit sandmas s. Farther wes t shingle-like seaward beach accretio n beds are indicated by prom inent cont in uous inclin ed reflecti ons.

Jo urn a l of Coastal Resea rch, Vol. 12, No.4, 1996

9liH .Iol, Smith and Meyers

gradation and aggradation in barriers can be useful in plan­ning coastal communities and navigational facilities. Finally,CPR can be used to accurately delineate and map the sub­surface fresh-brackish water contact beneath some sandycoasts for a fraction of the cost and time as compared to con­ventional drilling.

ACKNOWLEDGEMENTS

Financial support for field work and purchase of the CPRwas provided by grants from the Natural Sciences and En­gineering Research Council of Canada (NSERCl, the BarrierSedimentology Industrial Consortium (BSIC) and The Uni­versity of Calgary. Richard Davis, Jr., Mike Blum and JohnSuter are thanked for their suggested CPR sites. RichardBenson is thanked for reviewing the manuscript. BrandonBeierle and Murray Hay assisted in the field and carried outscaling and reductions of the CPR profiles.

liTERATURE CITED

ANNAN. AP. and DAVIS, J.1., 1976. Impulse radar sounding in per­mafrost. Radio Science, 11, 383-394.

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