,tllllllll:. - flvc
TRANSCRIPT
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
,tllllllll:.~~.~~+ +-
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
Direction of Survey ___
, Arrival ofreflected wave
(a) ---+--f----f----f-------lL-----+--" Ground
suriace
/Reflector3-----'--'"
Source/instant
(b) - - --t- - +-- +-- +-- -+-- --=--_ Arrival ofair wave
~ Arrival ofdirect wave
(ground wave)
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)
~.:.
- -~\,
if
. ' -. ( \..;h . -,/~,.~/",-__ . j ,J( ~ 1" /" -) 0::' / '. .. (, ij ( I /~ \ " ' ,
\
" /VI"·_L)~ . "..,.>. j I ' ~ oC') r : «> .J
~ j~l'\~
_:: 1), : " I
. I J ~ __ .l I /~_J- I " G \.~'.Yl\Tc'» ', i: \ _. '" . Isbna
J~J~ fif- <. '.~f~---' ;""- An",,·l'
\
lo.......~ - '"\ bland
Ga\v ('",to n Is\ a.nd :.. f \
r~~~~'i~[;~~I.na ' .
L-"
'r:
(.:~ -ii'.l \ /I.
: O Cl.'. l n Shorr)"• ~Jl rT~ 8 ,-,1('1/I ·<'; '· ,J:> idc ,-
I( U'"('!: l li l
)
NWo
".-...,
~ 100'-"
Q)
8 I~~.-E-
200
10 20Distance (m)
30 40 50SE
60 70I I I
.. . . . . . . ');,1;;......; ............,1......,1...; ;;; ; ; ; ;
0123??
"'048='5a
'--'6
7-8
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
NE99 lQO
(
.-.
Distance (m)4
10
59 6p 7p 80(c?«mIT/r"'7.:h77777;rrr;~77777(;((7
30I
((
20II
10I
swo
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
wa
Distance (m)10 20 30 40 50 60
E
70 80 90 100
o~~<{~~c;~et(crll(((((~_2~
~rl-
~~~14'::8
6'-"~~\\~~ twi{(~~""~f\W!~U( \~~
200Figure 5. GPR profile (50 MHz) from Ana stas ia Isla nd, Florida, (broken she ll ma terial) showing seawa rd dip ping incli ned reflections. The profile is froman aba ndoned road 2 km northwest of St. August ine Beach .
~
~-z»
. ~
w-v-e-z»:,-;
, ''lC.Cc..'-l;(I;. II
S<\1Q~ \lf)~•IT
~ ,-~ 100-<;»
Cl)
8.~
~
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
Distance (m)NW
oI
o
200
20I
, , \ ,
30I
40I
SE
50I
o
6
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
wo 10
Distance (m)20 30 40
I
rrtrrrru
~-s
~
~
~~
;)
~\~d
'-:..-l ~~
~~"io. ljj
f.!.:" rc:
s:
~~"-~"-~'-.l:C<l. LCrl''''''''. '''U
'" "t/»'
~
~Vi
M«
ii'
))~
\15
Fig ure 8. CPR profile (100 MHz) from Se aside , Oregon, of a raised boulde r-gravel beach showing inclined reflection s d ippin g seaward. Note th e waterta ble at 05 .5 m and paleo-beach surfaces inclined to 23" angle.
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
E
100Distance (m)60 804020
~?,
wo
300
~ 5~S ~~ 200
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-
wo 20 40
Distance (m)60 80 100 120 140
E
300
II. E • • ~.=••;-o.o
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.
BALLARD, RL., 1964. Distribution of beach sediment near the Columbia River. Department of Oceanography, University of Washington, Technical Report No. 98, 82p.
BERES, M., JR. and HM;NI, F.P., 1991. Application of ground-penetrating radar methods in hydrogeologic studies. Ground Water, 29,19-22.
BEHNARD, H.A.; LEBLANC, RJ., and MAJOH, C.F., 1962. Recent andPleistocene geology of southeast Texas. In: RAINWATER, E.H. andZIN(;ULA, RP. (eds.), Geology ofthe Gulf' Coast and Central Texas,Guidebook of' Excursions. Houston, Texas: Houston Geology Society, pp. 175-224.
BYRNE,.r.v., LERoy. D.O., and RILBY, C.M., 1959. The chenier plainand its stratigraphy, southwestern Louisiana. Gulf' Coast Association Geological Society, Ninth Annual Meeting, Houston, Transaction No.9, pp. 237-260.
DANIELS, D.J.; GUNTON, D.J., and SCOTT, H.F., 1988. Introductionto subsurface radar. lEE Proceedings, 135, F4, 277-320.
DAVIS, ,J.1. and ANNAN, AP., 1989. Ground penetrating radar forhigh resolution mapping of soil and rock stratigraphy. GeophysicalProspecting, 37, 531-551.
FISK, H.N., 1959. Padre Island and Laguna Madre mud flats, southcoastal Texas. Second Coastal Geography Conference, CoastalStudies Institute, Louisiana State University, pp. 103-151.
FITZ(iERALn, D.M.; BALDWIN, C.T.; IBRAHIM, N.A, and HUMPHH.IBS,S.M., 1992. Sedimentologic and morphologic evolution of a beachridge barrier along an indented coast: Buzzards Bay, Massachusetts. In: FLETCHER, C.H., III and WEHMILLER, J.F. (cds.I, Quaternary Coasts of the United States: Marine and Lacustrine Systems. SEPM-IGCP Special Publication no. 48, pp. 65-75.
GOULI), H.R and McFAH.LANE, E., JR., 1959. Geologic history of thechenier plain, southwestern Louisiana. Gulf' Coast AssociationGeological Society, Ninth Annual Meeting, Houston. TransactionNo.9, pp. 261-270.
HAYES, M.O., 1979. Barrier island morphology as a function of tidaland wave regime. In: LEATHERMAN, S.P. (cd.). Barrier Islands[rom the Gulf' of' St. Lawrence to the Gulf' of' Mexico. New York:Academic, pp. 1-28.
HAYES,M.O. and KANA, T.W., 1976. Terrigenous clastic depositionalenvironments: Some modern examples. Technical Report II-CRD,Coastal Research Division, University of South Carolina, part I,pp. 1-131.
HOYT, ,J.H., 1967. Barrier island formation. Geological Society of'America Bulletin, 78, 1125-1136.
HOYT, ,J.H. and HENHY, V.eJ., JIL, 1965. Significance of inlet sedimentation in the recognition of ancient barrier islands. Wyoming
Geological Association, Nineteenth Field Conference Guidebook, pp.190-194.
HU(;(;ENHEI(;EH, P., 1993. Radar facies: Recognition of characteristic braided river structures of the Pleistocene Rhine gravel (NEpart of Switzerland). In: BEST, J. and BRISTOW, C.S. (eds.), Braided Rivers. Geological Society Special Publication 75, pp. 163-175.
JOL, H.M., 1988. Seismic Stratigraphic Analysis of the SoutheasternFraser River Delta, British Columbia. M.S. Thesis, Burnaby, British Columbia: Simon Fraser University, 176p.
JOL, H.M. and SMITH, D.G., 1991. Ground penetrating radar ofnorthern lacustrine deltas. Canadian Journal of' Earth Sciences,28, 1939-1947.
JOL, H.M.; MF:YEH.S, RA; LAWTON, D.C., and SMITH, D.G., 1994. Adetailed ground penetrating radar investigation of a coastal barrier spit, Long Beach. Washington, U.S.A. In: BELL, RS. and LEPPER, C.M. (eds.I, Symposium on the Application of' Geophysics toEngineering and Enoironmental Problems (SAGGEP), Proceedings,1, 107-127.
KRAFT, J.C., 1971. Sedimentary facies patterns and geologic historyof a Holocene marine transgression. Geological Society of'AmericaBulletin. 82, 2131-2158.
KHAFT, J.C., 1978. Coastal stratigraphic sequences. In: DAVIS, RA.JI\. (ed.), Coastal Sedimentary Enoironments. New York: SpringerVerlag, pp. 361-383.
LEATHERMAN, S.P., 1985. Geomorphic and sedimentary analysis ofFire Island. Marine Geology, 63, 173-195.
LJ';,\TH~;J{MAN, S.P., 1987. Coastal geomorphological applications ofground penetrating radar. -lournal of' Coastal Research. 3, no. 3,397-399.
McCI !IlHIN, D.G., 1982. Barrier-island and strand-plain facies. In:SCHOLLE, P.A and SI'~;ARIN(;, D.R (eds.), Sandstone DepositionalEnvironments, American Association of Petroleum Geologists,Memoir 31, pp. 247-279.
MEYERS. RA., 1994. Willapa Barrier Spit ofS.W. Washington State:Depositional Processes Inferred from Ground Penetrating Radar.M.S. thesis, Calgary, Alberta: University of Calgary, iOlp.
MEYERS, RA; SMITH, D.G.; JOL, H.M., and HAY, M.B., 1994. Internal structure of a Pacific coast barrier spit using ground penetrating radar. Fifth International Conference on Ground Penetrating Radar, Proceedings, 2, 843-854.
MOORMAN, B.J.; Juixnc, A.S., and SMITH, D.G., 1991. Examiningfluvial sediments using ground penetrating radar in British Columbia. In: Current Research, Part A, Geological Survey ofCanada,Paper 9I-IA, pp. 31-36.
P~;TERSON, C.D. and PHIPPS, ,J.B., 1992. Holocene sedimentaryframework of Grays Harbor Basin, Washington, U.S.A In:FL~;TCIlER, C.H., III and WEHMILL~;R, J.F. (eds l, QuaternaryCoasts of' the United States: Marine and Lacustrine Systems.SEPM-IGCP Special Publication no. 48, pp. 273--285.
PETHICK, J., 1984. An Introduction to Coastal Geomorphology. London, England: Edward Arnold, pp. 96-99.
RANKIN, D.K., 1983. Holocene Geologic History of the Clatsop PlainsForedune Ridge Complex. M.S. thesis, Portland, Oregon: PortlandState University, 189p.
REINSON, G.E., 1984. Barrier-islands and associated strand-plainsystems. In: WALKEB, RG. (cd.), Facies Models. Geological Association of Canada, Geoscience Canada, Reprint Series 1, pp. 119140.
ROMII;, P., 1993. Overview of technological tools that could be developed for environmental applications. Society of' Exploration Geophysicists, 63rd Annual Meeting and International Exhibition,Expanded Abstracts with Biographies, pp. 1391.
SMITH, D.G. and JOL, H.M., 1992. Ground penetrating radar investigation of a Lake Bonneville delta, Provo level, Brigham City,Utah. Geology, 29, 1083-1086.
ULRIKSEN, C.P.F.. 1982. Application of Impulse Radar to Civil Engineering. Ph.D. thesis, Lund, Sweden: Lund University of Technology, (republished by Geophysical Survey Systems Inc., Hudson,New Hampshire) 175p.
VAN HETERN, S.; FITZ(;~;HALll, D.M., and McKINLAY, P. A., 1994.Application of ground penetrating radar in coastal stratigraphicstudies. Fifth. International Conference on Ground Penetrating Radar, Proceedings, 2, 869-881.
Journal of Coastal Research, Vol. 12, No. ·1. 1996