Applications of Ground-penetrating Radar (Gpr) to Sedimentological, Geomorphological and...

7/30/2019 Applications of Ground-penetrating Radar (Gpr) to Sedimentological, Geomorphological and Geoarchaeological Stu

1/34

Geological Society, London, Special Publications

doi: 10.1144/GSL.SP.2000.175.01.12p139-171.

2000, v.175;Geological Society, London, Special PublicationsAdrian Neal and Clive L. Robertsgeoarchaeological studies in coastal environmentssedimentological, geomorphological andApplications of ground-penetrating radar (GPR) to

serviceEmail alerting

new articles cite this articleto receive free e-mail alerts whenhereclick

requestPermission

part of this articleto seek permission to re-use all orhereclick

Subscribe

CollectionLondon, Special Publications or the Lyell

to subscribe to Geological Society,hereclick

Notes

2013 The Geological Society of London

at CAPES on May 6, 2013http://sp.lyellcollection.org/Downloaded from


2/34

Appl icat ions of ground-penetrat ing radar (GPR) tosed imento log ica l , geomorpho log ica l and geoarchaeo log ica l s tud ies

in coasta l environmentsA D R I A N N E A L & C L I V E L . R O B E R T S

School o f Appl ied Sc iences , Univer s it y o f W olverhampton , Wul f runa S t ree t , WolverhamptonW V 1 1 S B , U K ( e - m a i l : c s l 8 1 5 @ w l v . a c . u k )

Abs t rac t : Acquis i t ion of high resolut ion da ta regarding the s t ra t igraphy and inte rna ls t ruc ture of coas ta l sedimentary sequences i s becoming increasingly impo r tant in man ysedimentological , geom orpho logical and geoarchaeological studies. Such informa tion isusually obta ined from the logging of f ield exposures, shallow trenches and cores. How ever ,ground-pene t ra t ing rad ar (GPR) , a re la tive ly new and rapidly deve loping non- invasivegeophysical technique, m ay also aid such inv estigations in cer tain coastal settings, provid ingaddi t iona l , compl imentary and of ten unique da ta se ts . The technique i s based on thetransm ission, reflection an d reception of high frequency electromagnetic (radar) waves, withreflections occurr ing in the subsurface due to the v arying electrical properties o f thesediments. Such ref lections can occur due the presence of pr imary sedimentary structures,l i thological/m ater ial changes or water conten t changes. Examples presented from the U Kindicate that the technique can accurately delineate the strat igraphy and internal sedi-me ntary structure o f coastal barriers, spits and strandplain s, both ab ove an d below a freshgroundw ater table. Sand an d/or grave l -domina ted beach-dune systems on mod era te to highwave energy, macrot ida l coas ts appear to provide opt imum se tt ings for G PR deployment .In addit ion, the technique also has the potential to locate, map and provide strat igraphiccontext for a wide range of a rchaeologica l fea tures tha t a re comm only found in such coasta lenvi ronments .

U n t i l r e c e n t l y it h a s g e n e r a l l y b e e n n e c e s s a r y t or e l y o n t h e l o g g i n g o f f i el d e x p o s u r e s , s h a l l o wc o r e s a n d s u r f a c e e x c a v a t i o n s t o d e t e r m i n e t h ed e t a i le d s t r a t i g r a p h y , i n t e r n a l s e d i m e n t a r y s t r u c-t u re a n d a r c h a e o l o g y o f u n c o n s o l i d a t e d c o a s ta ls e d i m e n t a r y s e q u e n c e s . H o w e v e r , s u c h a n a p -p r o a c h h a s a n u m b e r o f a ss o c i at e d p r o b le m s :( 1) g o o d f ie l d e x p o s u r e s a r e o f t e n s c a rc e a n dl i m i t e d t o t h e z o n e a b o v e t h e g r o u n d w a t e rt ab l e. F o r e x a m p le , m a n y U K c o a s ta l d u n es y s t e m s a r e h e a v i l y s t a b i l i z e d b y v e g e t a -t i o n ; s u i t a b l e a n d e x t e n s i v e e x p o s u r e s a r et y p i c a l l y o n l y r e v e a l ed i n f r o n t a l d u n e s t h a th a v e b e e n e r o d e d d u r i n g s i g n i fi c a n t s t o r m s .( 2) e x t e n si v e s h a l lo w c o r i n g o r e x c a v a t i o nw o r k i s o f t e n t i m e c o n s u m i n g a n d e x p e n -s i v e . C o n s e q u e n t l y , c o r e s a n d t r e n c h e s a r eu s u a l ly l im i t ed i n n u m b e r a n d / o r h a v ec o n s i d e r a b l e s p a c i n g , m a k i n g c o r r e l a t i o nb e t w e e n t h e m d i ff ic u l t. T h e r e s t r i c t e d n a t u r eo f b o r e h o l e s a m p l i n g a l s o e n s u re s t h a t o n l yl i m i t e d i n f o r m a t i o n r e g a r d i n g t h e i n t e r n a ls e d i m e n t a r y s t r u c t u r e o f t h e d e p o s i ts c a nb e o b t a i n e d . I n a d d i t i o n , t h e a p p l i c a t i o n o f

s u c h i n v a s i v e t e c h n i q u e s m a y b e u n d e s i r -a b l e o r u n f e a s i b l e d u e t o i m p o r t a n t c o n -s e r v a t i o n c o n s i d e r a t i o n s a t t h e s i t e .D u r i n g t h e l a s t d e c a d e , h o w e v e r , r a p i d d e v e l -o p m e n t s i n t h e f ie l d o f e n v i r o n m e n t a l g e o p h y s i c sh a v e b e g u n t o a l l o w t h e n o n - i n v a s i v e a c q u i s i -t i o n o f a w i d e r a n g e o f s h a l lo w s u b s u r f a c e d a t a( R e y n o l d s 1 99 7). I n p a r t i c u l a r, g r o u n d - p e n e t r a t -i n g r a d a r ( G P R , a l so r e f er r ed t o a s i m p u l s e r a d a r ,g r o u n d - p r o b i n g r a d a r o r g e o r a d ar ) h a s fo u n d aw i d e r a n g e o f a p p l i c a t io n s i n t h i s f ie ld . G P R h a sp r o v e n v a l u a b l e i n m a n y g l a c io l o g i c al , h y d r o g e o -l o g i c a l , e n v i r o n m e n t a l , e n g i n e e r i n g a n d f o r e n s i ci n v e s t i g a t i o n s ( se e e x a m p l e s i n R e y n o l d s ( 19 9 7) ).I t a l s o h a s t h e p o t e n t i a l t o a i d f u t u r e p l a n e t a r ye x p l o r a t i o n p r o g r a m m e s ( O r i & O g l i a n i 1 9 9 6 ;H e r i q u e & K o f m a n 1 9 97 ). H o w e v e r , w i t h re s p e c tt o c o a s t a l s t u d i e s i t i s r e c e n t d e v e l o p m e n t s i n g e o -l o g i c a l a n d a r c h a e o l o g i c a l a p p l i c a t i o n s t h a t a r eo f p a r t i c u l a r i n t e re s t . G P R h a s s u c c e ss f u ll y b e e nu s e d t o d e t e r m i n e t h e e x t e n t , t h i c k n e s s , s t r a t i -g r a p h y a n d i n t e r n a l s e d i m e n t a r y s t r u c t u r e o f aw i d e ra n g e o f u n c o n s o l i d a t e d s e d i m e n t s . T h e s eh a v e i n c l u d e d a e o l i a n , f l u v i a l , g l a c i a l, l a c u s t r i n e ,

From: PYE,K . & ALLEN,J. R. L. (eds) . Coastal and Estuarine Environments: sedimentology, geomorphologyand geoarchaeology.Geo logical Society, Lond on, Special Publications , 175, 139-171. 0305-8719/00/$15.00 TheGeologica l Society of Lo ndo n 2000.



3/34

140 A. NE A L & C . L . RO BER TSdel t a ic and f r eshw ater pea t depos i t s ( fo r examp le ,Jo l & Sm i th 1991; Smi th & Jo l 1992 , 1997; Ga w-t h o r p e e t a l . 1 9 9 3 ; H u g g e n b e r g e r 1 9 9 3 ; S c h e n ke t a l . 1 9 9 3 ; T h e i m e r e t a l . 1 9 9 4 ; B r id g e e t a l .1 9 9 5 , 1 9 9 8 ; M e l l e t 1 9 9 5 ; O l s e n & A n d r e a s e n1995; Bristow e t a l . 1 9 9 6 ; H a r a r i 1 99 6; L o n n e &Lau r i t s en 1996; Busby 1997; Lec le rc & Hick in1997; Rober t s e t a l . 1 9 9 7 ; V a n d e n b e r g h e & v a nO v e r m e e r e n 1 9 9 9 ) . I n t a n d e m w i t h t h e g r o w t hin geo log ica l app l i ca t ions there has been an in -c reas ing use o f G PR in a rchaeo log ica l inves t iga-t io n s . G P R c a n q u i c k l y a n d a c c u r a t e l y lo c a t e a n dd e f i n e m a n y b u r i e d a r c h a e o l o g i c a l f e a t u r e s a n d ,in many cases , de te rmine the i r s t r a t ig raph icc o n t e x t ( V a u g h a n 1 9 8 6 ; S t e r n b e r g & M c G i l l1 9 9 5 ; A r n o l d e t a l . 1 9 9 7 ; C o n y e r s & G o o d -m a n 1 9 9 7; R e y n o l d s 1 99 7) . C o n s e q u e n t l y G P R isa geophys ica l t echn ique tha t has r ecen t ly ga inedw i d e a c c e p t a n c e w i t h i n t h e a r c h a e o l o g i c a l c o m -m u n i t y (C o n y e r s & G o o d m a n 1 99 7). H o w e v e r ,f ew of the a rchaeo log ica l inves t iga t ions r ep or tedin the l i t e r a tu re have u t i l i zed GPR in t ru lycoas tal set t ings .A pre l iminary inves t iga t ion in to the l ike lyc o a s t a l g e o m o r p h o l o g i c a l a n d s e d i m e n t o l o g i c a la p p l i c a ti o n s o f G P R w a s fi rs t u n d e r t a k e n b yL e a t h e r m a n ( 1 9 8 7 ) . S u b s e q u e n t s t u d i e s h a v econf i rmed i t s po ten t i a l , par t i cu la r ly in Nor thA m e r i c a ( F i t z g e r a l d e t a l . 1 9 9 2 ; D o t t & M i c k -e l son 1995; Jo l e t a l . 1 9 9 6 ; M e y e r s e t a l . 1996;v a n H e t e r e n e t a l . 1 9 9 6 ; v a n H e t e r e n & v a nde P las sche 1997; van Heteren e t a l . 1998; Smithe t a l . 1999) , bu t a l so in Europe (S t r and Pe te r sen& Andre asen 1989; van O verm eeren 1994 , 1998;C l e m m e n s e n e t a l . 1 9 9 6 ) a n d e l se w h e r e ( B a k e r1991; Roy e t a l . 1 9 9 4 ; H a r a r i 1 9 9 6 ) . H o w e v e r ,desp i t e th is r esearch G PR s tud ies in coas ta l env i-ronm ents a re s ti ll very much in the i r in fancy . Th i si s due , a t l eas t in par t , t o the f ac t tha t manycoas ta l s c i en t i s t s who may wish to u t i l i ze GPRin thei r s tudies do not have a suff icient ly detai ledk n o w l e d g e o f g e o p h y s ic a l s u r v e y i n g a n d i n te r -p re ta t ion to use the t echn ique wi th conf idence .T h e l i m i te d n u m b e r o f st u d ie s u n d e r t a k e n s o fa r ,c o m b i n e d w i t h t h e n e e d t o e x a m i n e b o t h t h ere levan t geo log ica l / a r chaeo log ica l and geophys i -cal l i terature, means that i t i s of ten di f f icul t forthe non- spec ia l i s t t o eas i ly ascer t a in whether thetechn ique wi l l p rov ide the r equ i r ed in format iona t a par t i cu la r s tudy s i t e . Wi th these p rob lems inmind , the a ims of th i s paper a re as fo llows : ( a ) toi n t r o d u c e t h e G P R t e c h n i q u e , e x p l a i n i n g h o w ,why and where i t i s mos t l ike ly to work ; (b ) toe x a m i n e s o m e o f th e i m p o r t a n t i ss u es s u r r o u n d -ing survey des ign , da ta p roces s ing and in te r -p re ta t ion ; ( c) to i l lus t ra t e a r ang e o f coas ta lg e o m o r p h o l o g i c a l a n d s e d i m e n t o l og i c a l a p p l i c a -t ions by presen t ing examples f rom a s e r i es o f

s tud ies r ecen t ly under t aken in the UK; (d ) tod i scus s the po ten t i a l geoarchaeo log ica l app l i ca-t i o n s o f G P R i n c o a s t a l e n v i r o n m e n t s t h r o u g h aqua l i t a t ive eva lua t ion o f a r chaeo log ica l t a rge t s inthe coas ta l dunef i e lds o f South Wales .

T h e G P R t e c h n i q u eI n m a n y r e s p e c t s G P R i s a n a l o g o u s t o t h eperhaps more f ami l i a r r e f l ec t ion s e i smic ands o n a r t e c h n i q u e s ( M c C a n n e t a l . 1 9 8 8 ) . A t a ni n d i v i d u a l s u r v e y p o i n t t h e G P R s y s t e m e m i t s ashor t pu l se o f h igh f r equency e lec t romagnet i ce n e r g y i n t h e M H z r a n g e ( D a v i s & A n n a n 1 9 8 9 ;Reynolds 1997) . Th i s i s t r ansmi t t ed in to thegrou nd , where i t encou nter s m ater i a l s o f d if f e r-ing e l ec t r i ca l p roper t i es . Var ia t ions in theseproper t i es l ead to changes in the ve loc i ty o f thep r o p a g a t i n g e l e c t r o m a g n e t i c w a v e . W h e r e v e l o -c i ty changes a re abrup t , wi th r espec t to thed o m i n a n t w a v e l e n g t h o f t h e r a d a r w a v e , apro por t io n o f the energy wi ll be r e fl ec ted . Th i swi l l t hen be t r ansmi t t ed back to the sur face ,w h e r e i t c a n b e r e c e i v e d b y t h e G P R s y s t e m(Fig . l ) . The t ime t aken fo r the s igna l to bet r ansmi t t ed , r e f l ec ted and then r ece ived i s afunc t ion o f the dep th o f the r e f l ec ting hor i zonand the average ve loc i ty o f the e l ec t romagnet i cwave . Th i s i s r e f e r r ed to as the two-way t r ave lt im e a n d is m e a s u r e d i n n a n o s e c o n d s ( 10 - 9s e c o n d s ) . I n m o d e r n G P R s y s t e m s t h e r e c e i v e ds igna l i s d ig i t i zed and s to red on a l ap- top com-puter , which a l so a l lows r ea l - t ime da ta d i sp laya n d s i m p l e p r o c e s s i n g . W h e r e s t a n d a r d r a d a rre f l ec t ion p rof i l ing i s be ing under t aken , the da taco l l ec tion p roces s i s r epea ted a t s equen t i a l su rveypoin t s . The r esu l t ing t r aces a re then p laced nex tto each o ther to bu i ld up the r a da r r e f lec t ion p ro-f i le of the subsurface (Fig. 1) .G P R s y s t e m s o p e r a t e e i t h e r i n m o n o s t a t i cm o d e , w i t h a s i n g l e a n t e n n a o r b i s t a t i c m o d ew i t h s e p a r a t e t r a n s m i t t i n g a n d r e c e i v i n g a n t e n -n a e . M o n o s t a t i c s y s t e m s t e n d t o b e t o w e d a l o n gthe g round and the hor i zon ta l ax i s i s r ecordedon a t ime-base . Conver s ion to a d i s t ance-base i sthen r equ i r ed . B i s t a t i c sys tems a re usua l ly oper -a t e d i n a s t e p - w i s e m a n n e r a n d t h e h o r i z o n t a lax i s i s r ecorded on a d i s t ance-base .

T h e o r e t i c a l b a c k g r o u n dI n t h e M H z f r e q u e n c y r a n g e a n d a t l o w c o n d u c -t i v i t i e s ( < 1 0 0 m S / m ) a n d l o w r e l a t i v e m a g n e t i cpermeabi l i t i es (=1 fo r non-magnet i c mater i a l s ) ,i t is the d ie lec t ri c p roper t i es o f mater i a l s tha tde te rmines the i r r esponse to p ropaga t ing e l ec-t r o m a g n e t i c e n e r g y ( D a v i s & A n n a n 1 9 8 9 ;



4/34

a) DISPLAY~- ANDRECORDCONTROLUNIT(TIMING)

TRANSMITTERJ L RECEIVER

~ REFLECTEDSIGNAL ~ ~ - -

Distance (m)b) M') O

AIR WAVE ) ,G R O U N D W A V E )

PRIMARYREFLECTIONS

0.050.0100.0 "~"c

d~E150.0

200.0250.0

Fig . 1. Gr oun d p ene t ra t ing radar d a ta acquis i t ion and the resul ting radar re f lect ion profi le . (a ) GP R da taacquis i tion a t an indiv idua l survey point showing the GP R sys tem com pone nts and the subsurface re f lec torconf igura t ion (as de f ined by the presence of pr im ary sedimenta ry s t ruc tures , wa te r con tent va r ia t ions andli thology changes). (b) Radar reflect ion profi le result ing from the sequentia l plot t ing of individual t races fromadjacent su rvey points . The air wave is the first arrival and results fro m part of the e lectromagn etic pulsegenera ted a t the t ransmi t t ing anten na t rave l ing di rect ly throug h the a i r to the rece iv ing antenna . The g roun dwave i s the second a rr iva l an d result s f rom par t of the e lec t romagne t ic wave in i t i ated a t the t ransm i t t ing antennabe ing s t rongly re frac ted wi th in the upp ermo st pa r t of the subsurface. The a i r and g roun d wave a re fo l lowed by aseries of primary reflect ions result ing from the changing dielectric propert ies of the subsurface.



5/34

142 A . N E A L & C . L . R O B E R T STab le 1 . T y p i c a l e le c t ri c a l p r o p e r t ie s o f s o m e c o m m o n g e o l o g i c a l m a t e r i a l s a t 8 0 - 1 2 0 M H z ( b a s e d p r i m a r i l y o n v a nH e t e r e n et al . 1 9 9 8 , b u t w i t h a d d i ti o n a l d a t a f r o r n D a v i s & A n n a n 1 9 8 9 ; T h e i m e r et al . 1 9 9 4 ; v a n O v e r m e e r e n 1 9 9 4 )M e d i u m R e l a ti v e E l e c t ro m a g n e t i c C o n d u c t i v it y A t t e n u a t i o nDi e l e c t r i c wa v e v e l o c i ty (m S / m ) (d B / m )P e rm i t t i v i t y (m / n s )A ir 1 0 .3 0 0Fre sh w ate r 80 0 .03 0 .5 0 .1Sea w ate r 80 0 .01 30 000 1000Un s a t u ra t e d s a n d 2 .5 5 -7.5 0 .1 -0 .2 0 .0 1 0 .0 1 -0 .1 4Satu ra ted sand 20-31 .6 0 .05 -0 .08 0 .1 1 0 .03 -0 .5Un sa tu ra te d sand and g rave l 3 .5 -6 .5 0 .09 0 .13 0 .007-0 .06 0 .01 -0 .1Satu ra ted sand and g rave l 15 .5 -17 .5 0 .06 0 .7 -9 0 .03 0 .5Un sa tu ra te d s il t 2 .5 -5 0 .09 -0 .1 2 1 -100 1 300*Sa tura ted s i l t 22 30 0 .05 -0.07 _


6/34

G P R A P P L I C A T I O N S I N C O A S T A L S T U D I E S 143v e l o c i t y a n d t h e l o w e r t h e a t t e n u a t i o n ( T a b l e 1 ).A s f r e s h w a t e r h a s a h ig h R D P c o m p a r e d t o a i ra n d t h e c o m m o n r o c k - f o r m i n g m i n e r a l s ( O le -h o f t 1 98 1), t h e d i e l e c t r ic p r o p e r t i e s o f c o m m o ng e o l o g i c m a t e r i a l s a r e p r i m a r i l y c o n t r o l l e d b yw a t e r c o n t e n t ( T o p p e t a l . 1 9 8 0 ; D a v i s & A n n a n1 98 9) . H e n c e i n u n c o n s o l i d a t e d s e d i m e n t s r a d a ri s s e n s i ti v e to v a r i a t i o n s i n t h e s e d i m e n t : a i r :f r e s h w a t e r r a t i o . C o n s e q u e n t l y , c h a n g e s i n t h ef lu id occupy ing the po re s pace ( e .g . a i r t o f r e s h -w a t e r ) o r v a r i a t i o n s i n t h e p o r o s i t y i t s e l f l e a d t os i g ni f ic a n t c h a n g e s i n th e R D P a n d c o r r e s p o n d -i n g r a d a r w a v e v e l o c i t y , t h u s c a u s i n g r e f l e c ti o n s .I n a d d i t i o n , r a d a r i s a l s o s e n s it i v e t o c h a n g e s i ns e d i m e n t c o m p o s i t i o n a n d g r a i n s h a p e , o r i e n t a -t i o n a n d p a c k i n g ( D a v i s & A n n a n 1 9 8 9 ; B a k e r1991) . As a r e s u l t o f thes e r e l a t ion s h ip s , f ea tu r e ss u c h a s t h e w a t e r t a b l e , s e d i m e n t a r y s t r u c t u r e sa n d l i t h o l o g i c a l b o u n d a r i e s a l l g i v e s i g n i f i c a n tr e f l e c t i o n s ( T a b l e 2 ) . H o w e v e r , t h e s e s e e m i n g l ys i m p l e r e l a t i o n s h i p s c a n b e c o m p l i c a t e d b yt h e i n t r o d u c t i o n o f s ig n i f ic a n t a m o u n t s o f h i g hc o n d u c t i v i t y p o r e w a t e r ( e . g . s e a w a t e r ) o r s e d i -m e n t ( e . g . s a t u r a t e d c l a y ) . T h e s e w i l l o f t e n p r o -v i d e s t r o n g r e f l e c t i o n s w h e n e n c o u n t e r e d , b u ta l so r a p i d l y a t t e n u a t e t h e s i g na l , p r e v e n t i n g a n yf u r t h e r s i g n i f i c a n t p e n e t r a t i o n .F o r a n y g iv e n f r eq u e n c y a n d R D P , t h e p r o p a -g a t i n g e l e c t r o m a g n e t i c w a v e h a s a s p e c i fi c w a v e -l e n g t h a n d t h i s l i m i t s t h e h o r i z o n t a l a n d v e r t i c a lr e s o l u t i o n t h a t c a n b e a c h i e v e d ( J o l 1 9 95 ; S m i t h &J o l 1995 ; R eyno lds 1997 ) . Wi th r e s pec t to ve r t i ca lr e s o l u t i o n , w a v e t h e o r y i n d i c a t e s t h a t t h e b e s tt h a t c a n b e a c h i e v e d is o n e - q u a r t e r o f th e d o m i -n a n t w a v e l e n g t h ( S h e r i f f 1 9 7 7 ) . A l l r e f l e c t i o n st h a t o c c u r w i t h i n t h a t v e r t i c a l d i s ta n c e w i l l i n t e r -f e r e c o n s t r u c t i v e l y t o a g r e a t e r o r l e s se r e x t e n t a n dfo rm the f ina l obs e rved s ing le r e f l ec t ion (F ig . 2 ).H o r i z o n t a l r e s o l u t i o n is d e t e r m i n e d b y t h e s iz e o ft h e f i r st F r e s n e l z o n e , w h i c h i t s e l f i s a f u n c t i o no f w a v e l e n g t h a n d d e p t h o f t h e re f l e c to r ( S h e r i ff1977 ; R eyno lds 1997 ) . Dep th to the r e f l ec to r i ss i g n i f i c a n t b e c a u s e t h e d o w n g o i n g s i g n a l t ra v e l si n a n e v e r - e x p a n d i n g e l l i p t ic a l c o n e . C o n s e q u e n t -l y f o r a g i v e n w a v e l e n g t h , a s t h e d e p t h i n c r e a s e sthe s i ze o f the f i r s t F r es ne l zone a l s o inc r eas es ,l e a d i n g t o l o w e r h o r i z o n t a l r e s o l u t i o n .I n a l l G P R s u r v e y s th e r e i s a t r a d e o f f b e t w e e nd e p t h o f p e n e t r a t i o n a n d r e s o l u t i o n ( D a v i s &A nn an 1989; J o l 1995; S m i th & J o l 1995 ). F o ra n y g i v e n e a r t h m a t e r i a l , a s t h e a n t e n n a e f r e -q u e n c y i s i n c r e a s e d t h e r e i s a n i n c r e a s e i n r e s o -l u t i o n b u t a d e c r ea s e i n t h e d e p t h o f p e n e t r a t i o n( F i g . 3 ) . H o w e v e r , t h i s r e l a t i o n s h i p c a n b e c o m -p l i c a t e d b y s i g n i f i c a n t c h a n g e s i n t h e d i e l e c t ri cp r o p e r t i e s o f t h e s u b s u r f a c e w i t h d e p t h .A s a c o n s e q u e n c e o f t h e t e c h n i c a l li m i t a t i o n sp l a c e d o n t h e v e r t i c a l a n d h o r i z o n t a l r e s o l u t i o n

a )

Reflector

Source/detec tor

~J4

Fresne] zone

c)X/4 for tow X/4 for highf l ~ q u e n c y f r e q u e n t ,

Low frequency zone

Fig. 2. T he horizontal resolution of unm igrated rada rdata is given by the width of the Fresnel zone.(a) Electromagnetic waves propogate through theground in an ever-expanding elliptical cone, with theapex of the cone at the centre of the transmittingantenna . A ll reflections within on e-quarter of thedom inant wavelength will interfere constructively toform a single reflection (as observed by the receivingantenna). (b) Waves closest to the Fermat pathcontribute most to the amplitude of the reflection.(e) The width of the Fresnel zone at any particulardepth is a function of the depth to the reflector andthe wavelength of the electomagnetic wave, which isin turn frequency dependent. The higher the frequency(and the shorter the wavelength) the higher thehorizontal resolution. Modified from Emery &Myers (1996).

o f a r a d a r r e f l e c ti o n p r o fi l e , i t m u s t b e v i e w e d a ss o m e f o r m o f a v e r a g e o r a p p r o x i m a t i o n o f th et r u e s u b s u r f a c e s t r u c t u r e . T h e d e g r e e o f a p p r o x -i m a t i o n d e p e n d s o n t h e r a d a r w a v e l e n g t h ( w h i c hi n i t s e l f i s a f u n c t i o n o f t h e c e n t r e f r e q u e n c y o f t h e



7/34

UUUV

UUU~

E~5~50UUUI

0

0

I+

,.

~.

+;I;'::I::,:

+

+

'

tCJUu++u

"++

o

o

u

d

o

i

I

1,

p,~-

0

O.

Q

0

0

q

o

Q

0

O.

0

O.

(q

0

o

g

g

o

o

g

g

o

o

o

o

o

~o

o

_

(sew+

(saW!_

tElC)~aJ~Lutuu!+uAul

0

0

r

~

d

~yo

++

,+

t+O

I

:

;

I

."++++o,+--+.

l~

I

I

~

i

I

I

l

o

Q

o

o

o

o

Q

o

o~

o

$

o

~0

g

"G"

(sew!


8/34

G P R A P P L I C A T I O N S I N C O A S T A L S T U D I E S 145r e t u r n i n g r a d a r w a v e s a n d t h e d i e le c t ri c p r o p e r -t i es o f t h e s u b s u r f a c e ) a n d d e p t h i n t h e p r o f i le .T h i s i s a n i m p o r t a n t p o i n t t o c o n s i d er w h e na t t e m p t i n g t o i n t e r p r e t r a d a r r e f l e c t i o n p r o f il e s .

S y s t e m s e t- u p a n d s u r v ey d e si gnP r i o r t o u n d e r t a k i n g a f u ll G P R s u r v e y an u m b e r o f c r i t i c a l d e c i s i o n s r e g a r d i n g s y s t e ms e t- u p a n d s u r v e y d e si g n m u s t b e m a d e . T h e s ew i l l b e g u i d e d p r i m a r i l y b y t h e n a t u r e a n d s c a l eo f t h e fe a t u r e s o f p o t e n t i a l i n t e r e s t a n d t h ee l e c t ri c a l p r o p e r t i e s o f t h e g e o l o g i c a l a n d / o ra r c h a e o l o g i c a l m a t e r i a l . I t s h o u l d b e n o t e d t h a tw i t h r e s p e c t t o s y s t e m s e t - u p t h e c o n s i d e r a t i o n so u t l i n e d b e l o w a p p l y t o t h e m o r e r e c e n t d ig i t a lG P R s y s t e m s .(1 ) Antennae frequency. T h i s w i l l b e g u i d e d b yt h e r e so l u t i o n a n d d e p t h o f p e n e t r a t i o nr e q u i r e d o r p r e d i c t e d ( se e p r e c e d i n g d i s -cus s ion ) .(2 ) Time window. T h i s i s t h e a m o u n t o f t i m e

o v e r w h i c h t h e s y s t e m is t o r e c o r d d a t ab e i n g r e c e iv e d . T h e s iz e o f w i n d o w r e q u i r e dc a n b e p r e d i c t e d b a s e d o n t h e e s t i m a t e dr a d a r w a v e v e l o c i ty a n d d e p t h o f p e n e t r a -t i o n . A w i n d o w l a r g e r t h a n t h a t a c t u a l l yr e q u i r e d i s u s u a l l y d e s i r a b l e , i n c a s e t h e e l ec -t r i c a l p r o p e r t i e s o f t h e s u b s u r f a c e c h a n g ea l o n g t h e s u r v e y l i n e , a l l o w i n g g r e a t e rd e p t h o f p e n e t r a t i o n a n d h e n c e t h e d et e c-t i o n o f r e f l e c t o r s w i t h l o n g e r t r a v e l t i m e s .(3 ) Trace stacking. T h e s t a c k i n g o f i n d i v i d u a lt r a c e s i s a m e a n s o f i n c r e a s i n g t h e s i g n a l t on o i se r a t i o b y a r i t h m e t i c a l l y a v e r a g i n g o u tt h e r a n d o m a n d v a r i a b l e p o r ti o n s o f t h er e f l e c t e d w a v e ( F i s h e r et al. 1992 ; C onyer s& G o o d m a n 1 99 7). D i g i t a l G P R s y s te m sa l l o w t h i s t o b e p e r f o r m e d i n t h e f i e l d , b u ti t s h o u l d b e b o r n e i n m i n d t h a t t h e g r e a t e rt h e n u m b e r o f s ta c k s t h e l o n g e r i t ta k e s t oa c q u i r e d a t a a t a n i n d i v i d u a l s u r v e y p o i n t .(4 ) Ga i n s . A s r a d a r w a v e s p a s s d o w n t h r o u g ht h e g r o u n d , e n e r g y i s l o s t d u e t o c o n i c a ls p r e a d i n g a n d a t t e n u a t i o n . C o n s e q u e n t l y ,l a t e r a r r i v a l s o n a r e f l e c t i o n t r a c e w i l l g e n -e r a ll y h a v e l o w e r a m p l i t u d e s t h a n e a r li e r

a r r i v a l s . I n o r d e r t o e n h a n c e t h e s e l a t e r a r r i-v a l s g a i n s a r e a p p l i e d t h a t a m p l i f y th ew e a k e r s ig n a l. U n l i k e t h e s y s t e m p a r a -m e t e r s o u t l i n e d s o f a r , t h e a p p l i c a t i o n o fg a i n s t o t h e d a t a i s n o t a l w a y s f i x e d o n c et h e s u r v e y h a s b e g u n . I n d i g i t a l G P Rs y s t e m s g a i n s c a n b e a p p l i e d t o t h e r e a l -t i m e d a t a d i s p l a y w i t h o u t a f fe c t in g t h e r a wd a t a . A s e n s i b le g a i n m u s t b e a p p l i e d a t t h i sp o i n t i n o r d e r t o e n s u r e t h a t a l l s i g n i f i c a n tr e f l e c ti o n s a r e b e i n g o b s e r v e d .

(5 ) Survey lin e orientation and length. W i t hr e s p e c t t o g e o m o r p h o l o g i c a l a n d s e d i m e n -t o l o g i c a l f e a t u re s , e . g. b e d f o r m s , s u r v e yl i n e s s h o u l d i d e a l l y b e a l i g n e d e i t h e r p a r a l -l el o r n o r m a l t o t h e a s s u m e d d e p o s i t i o n a ls t r i k e . T h e y s h o u l d a l s o b e l o n g e r t h a n o n eb e d f o r m w a v e l e n g t h . R e c t a n g u l a r g r i d s o rr e g u l a r l y s p a c e d p a r a l l e l s u r v e y l i n e s a l l o wt h e a c q u i si t i o n o f p s e u d o - 3 D d a t a . W h e r el i n e a r a r c h a e o l o g i c a l f e a t u r e s a r e t h e t a r g e t ,f o r e x a m p l e w a l l s a n d d i t c h e s , s u r v e y l i n e ss h o u l d i d e a l l y b e n o r m a l t o t h e i r s tr i k e a n de x t e n d w e l l b e y o n d t h e i r h o r i z o n t a l l i m i t s .H o w e v e r , f o r m o s t a r c h a e o l o g i c a l a p p l i c a -t i o n s a r e c t a n g u l a r s u r v e y g r i d i s r e q u i r e d( C o n y e r s & G o o d m a n 1 99 7). F o r r o u t i n er e c o n n a i s s a n c e w o r k t h e i n d i v i d u a l g r i ds q u a r e s m u s t b e s m a l l e r t h a n t h e s m a l l e s tr e a l i st i c t a r g e t . F o r d e t a i l e d s u b s u r f a c em a p p i n g c a r e m u s t b e t a k e n t o e n s u r e t h a ts u rve y l ines a re s u f f i c i en t ly c lo s e to a vo i ds p a t i a l a l i a s i n g . I n r e a l i t y s u r v e y l i n e o r i e n -t a t i o n a n d l e n g t h a r e u s u a l ly a c o m p r o m i s eb e t w e e n w h a t i s i d e a l a n d w h a t i s f e a s i b l eg i v e n t h e g r o u n d c o n d i t i o n s a t t h e s i t e .(6 ) Horizontal sampling interval. I n o r d e r t oc o n s t r u c t a n i m a g e o f t h e s u b s u r fa c es e q u e n t i a l t r a c e s f r o m i n d i v i d u a l s u r v e yp o i n t s a r e p l a c e d n e x t t o e a c h o t h e r t op r o v i d e a c o n t i n u o u s p r o f i l e ( F i g . 1 ) . T h eh o r i z o n t a l s a m p l i n g i n t e r v a l m u s t b e s m a l -l e r t h a n t h e f e a t u r e o f i n t e re s t , i d e a l l ye n s u r i n g t h a t i t s f u l l c h a r a c t e r i s t i c s i n t h ep l a n e o f t h e s u r v e y a r e re p r e s e n t e d o n t h er a d a r i m a g e . W h e r e d a t a a r e b e i n g r e c o r d e db y a c o n t i n u o u s l y m o v i n g a n t e n n a , t h e h o ri -z o n t a l s a m p l i n g i n t e r v a l i s a f u n c t i o n o f th e

Fig. 3. The relationship between resolution and de pth o f penetration , as demo nstrated b y radar reflection profilesobtained at three different an tennae frequencies alo ng survey line B (see Fig.7) a t R aven M eols blowo ut on theSefton coast. (a) 50 M H z profile. V ertical resolution = 0.89 m, horizon tal resolution at 5m de pt h= 3.41m(b) 100 M H z profile. V ertical resolution = 0.38 m, horizo ntal resolution at 5 m depth = 2.9 m (e) 200 M Hz profile.Vertical resolution = 0.3 m, horizon tal resolution at 5 m depth = 2.8 m. The survey line was o rientated roughlyparallel to the axis of the depositional lobe of the blowout. The s tron g horizon tal reflection at 5.5 m o o is thewater table (WT ). The intersection with Raven Me ols survey line A (RM SL-A ) is indicated (see Fig. 8). Furthe rdetails regarding the radar s tratigraphy of the profiles can be found by reference to Figs 8 & 9 and Table 3.



9/34

1 46 A . N E A L & C . L . R O B E R T S

(7 )

( 8 )

r a te o f m o v e m e n t a n d t h e t r an s m i s s i o na n d r e c o r d i n g ra t e s. W h e r e d a t a a r e b e in gr e c o r d e d i n a s t e p - w i s e m a n n e r , t h e h o r -i z o n t a l s a m p l i n g i n t e r v a l i s m e r e l y a f u n c -t i o n o f t h e d i s t a n c e b e t w e e n i n d i v i d u a ls u r v e y p o i n t s .Topographic survey. A s a G P R s y s te m m e a -s u r e s o n l y t w o - w a y t r a v e l t i m e t o r e f l e c t o r si t d o e s n o t t a k e i n t o a c c o u n t t h e e f f e c t o fc h a n g i n g t o p o g r a p h y o n t h e r e l a t i v e p o s i -t i o n o f a d j a c e n t t ra c es . C o n s e q u e n t l y , w h e r es i g n i f i c a n t t o p o g r a p h y i s e n c o u n t e r e d as t a t i c c o r r e c t i o n m u s t b e a p p l i e d i . e. i n d i v i -d u a l t r a c e s m u s t b e m o v e d u p o r d o w n t ot a k e i n t o a c c o u n t c h a n g e s i n e l e v a t i o n a l o n gt h e s u r v e y l i n e . I n o r d e r t o d o t h i s e f f e c -t iv e ly , a n d t o a v o i d d i s t o r t i o n s o f t h e r a d a ri m a g e , a t o p o g r a p h i c s u rv e y m u s t b e u n d e r -t a k e n . T h e s a m p l i n g i n t e r v a l w i ll b e af u n c t i o n o f th e t e r r a i n e n c o u n t e r e d . A l lb r e a k s o f sl o p e s h o u l d b e a c c o u n t e d f o r .Velocity surveys. W h e r e t r a n s m i t t e r a n dr e c e i v e r a r e i n d e p e n d e n t , i t i s p o s s i b l e t oc a rr y o u t C M P ( C o m m o n M i d - P o i n t) s ur -v e y s t o e s t i m a t e s u b s u r f a c e r a d a r w a v ev e l o c i ti e s ( A n n a n & D a v i s 1 97 6; R e y n o l d s1 9 9 7 ) . I n o r d e r t o o b t a i n t h e m o s t r e l i a b l er e s u lt s , re f l e c ti o n s t h a t a p p e a r h o r i z o n t a lo n t h e s t a n d a r d r a d a r r e f le c t io n p ro f i le ss h o u l d b e u s e d f o r t h e v e l o c i t y a n a l y s is .T h e v e l o c i ti e s a r e d e r i v e d f r o m t h e C M Pp r o f i l e s i n e x a c t l y t h e s a m e w a y a s i n r e fl e c-t i o n s e i s m i c s ( R o b i n s o n & ~ o r u h 1 9 8 8 ,p . 8 8 - 9 0 ) . O n c e o b t a i n e d , t h e v e l o c i t ie s c a nb e u s e d t o c o n v e r t t w o - w a y t r a v e l t i m e t oe s t i m a t e d d e p t h o n t h e r a d a r r e f le c t io n p r o -f il es . W h e r e t h e s u b s u r f a c e r a d a r r e s p o n s e i ss e e n t o v a r y s i g n i f i c a n t l y a l o n g a s u r v e y l i n e ,a s er ie s o f C M P s u r v ey s s h o u l d b e u n d e r -t a k e n t o d e t e r m i n e t h e n a t u r e a n d m a g n i -t u d e o f a n y v e l o c i t y v a r i a t i o n s . A n y s ig n if i-c a n t c h a n g e s w i ll h a v e c o n s i d e r a b l e b e a r i n go n s u b s e q u e n t i n t e r p r e t a t i o n . I f t r a n s m i t t e ra n d r e c ei v e r a r e n o t i n d e p e n d e n t ( e.g . m o n o -s t a t i c s y s te m s ) , s u b s u r f a c e v e l o c i ti e s m u s tb e e s t i m a t e d i n a n o t h e r m a n n e r . T h i s i su s u a l l y b e st a c h i e v e d b y c a rr y i n g o u t r a d a rr e f l e c t i o n p r o f i l i n g o v e r o b j e c t s o r s t r a t i -g r a p h i c h o r iz o n s o f k n o w n d e p t h , a l l o w i n gd i r e c t m e a s u r e m e n t o f r a d a r w a v e t r a v e lt im e s . T h i s c a n a l s o p r o v i d e v e r i f ic a t i o n o fv e l oc i ti e s o b t a i n e d f r o m C M P s u rv e y s.

D a t a e d i ti n g a n d p r o c e s si n gO n c e G P R s u r v e y d a t a h a v e b e e n a c q u i r e d i nt h e f i e l d a c e r t a i n a m o u n t o f d a t a e d i t i n g a n d

p r o c e s s i n g i s r e q u i r e d . T h i s s h o u l d b e d o n e i n as y s t e m a t ic m a n n e r , i n o r d e r t o e n s u r e c o m p a r -a b i l i t y b e t w e e n i n d i v i d u a l s u r v e y l i n e s . A l l d a t ap r e s e n t e d i n t h i s p a p e r w e r e c o l l e c t e d u s i n g aS e n so r s a n d S o f t w a r e P u l s e E K K O TM 1 0 0 G P Rs y s t e m a n d e d i t e d a n d p r o c e s s e d u s i n g v e r s i o n4 . 2 o f t h e s y s t e m s o f t w a r e . A f l o w d i a g r a m o ft h e e d i t i n g a n d p r o c e s s in g p r o c e d u r e a p p l i e d t ot h e d a t a i s s h o w n i n F i g . 4 . T h e n e e d f o r c e r t ai n ,p a r t i c u l a r l y i m p o r t a n t , o p e r a t i o n s i s m o r e f u l l ye x p l a i n e d b e l o w .I t i s e ss e n t i a l t h a t t i m e z e r o , w h i c h e f f ec t iv e l ym a r k s t h e p o s i t i o n o f t h e g r o u n d s u r f a c e o n t h er a d a r r e f l e c t io n p r o f il e , is l o c a t e d i n t h e c o r r e c tp o s i t i o n i . e . a t t h e z e r o o f f s e t t i m e . T h e s p a c i n gb e t w e e n t h e t r a n s m i t t e r a n d t h e r e c e i v e r m e a n st h a t t h e f i r s t s i g n a l ( t h e d i r e c t a i r w a v e ) i sr e c e i v e d a t a s m a l l , b u t f i n it e , t i m e a f t e r t h e p u l s ewa s in i t i a te d . As a r e su l t , the z e ro o f f se t t ime l i e sj u s t a b o v e t h e a i r w a v e , b y a n a m o u n t o f t i m ed e t e r m i n e d b y t h e v e l o c i t y o f t h e a i r w a v e a n dt h e a n t e n n a e s e p a r a t i o n ( C o n y e r s & G o o d m a n1 9 9 7 ) . I n t o w e d s y s t e m s a n t e n n a s e p a r a t i o n w i l lb e d e t er m i n e d b y t h e r a t e o f m o v e m e n t a n d t h et r a n s m i s s i o n a n d r e c o r d i n g r a te s .I t i s u s u a l l y d e s i r a b l e t o e s t i m a t e t h e r e t u r nc e n t r e f r e q u e n c y f o r t h e p r i m a r y r e fl e c ti o n s in as t a n d a r d r a d a r r e f l e c t i o n p r o f i l e . T h i s i s b e c a u s ea s a n e l ec t r o m a g n e t i c w a v e p r o p a g a t e s t h r o u g ht h e g r o u n d t h e h i g h e r fr e q u e n c ie s a r e a t t e n u a t e dp r e f e r en t i a ll y . C o n s e q u e n t l y , t h e r e t u r n i n g e le c-t r o m a g n e t i c w a v e h a s a l o w e r c e n t re f r e q u e n c yt h a n t h e i n i t i a l p u l s e g e n e r a t e d b y t h e t r a n s m i t -t i n g a n t e n n a . T h e u s e o f th e r e t u r n c e n t re f re -q u e n c y , i n c o n j u n c t i o n w i t h t h e r e l e v a n t v e lo c i t ye s t i m a t e s , l e a d s t o a m o r e a c c u r a t e e s t i m a t e o fr a d a r w a v e l e n g t h a n d h e n c e v e r t i c a l a n d h o r i -z o n t a l r e s o l u t io n .I f t h e r e a r e s i g n i f i c a n t c h a n g e s i n t h e r a d a rw a v e v e l o c i t y w i t h d e p t h , i t is n o t a p p r o p r i a t e t op l o t t h e r a d a r r e f l e c t i o n p r o f i l e s w i t h e l e v a t i o n /d e p t h s ca le s d e r iv e d f r o m a n a v e r a g e v e l o c i t y f o rt h e w h o l e s e q u e n c e . T h i s w o u l d l e a d t o i n a c c u -r a t e d e p t h e s t im a t e s f o r m o s t o f th e p r o f i le a n dd i s t o r t i o n o f t h e i m a g e d u e t o a n i n a p p r o p r i a t ed e g r e e o f t o p o g r a p h i c c o r r e c t i o n . I n s t e a d v e l o -c i t i e s f o r i n d i v i d u a l l a y e r s w i t h i n t h e p r o f i l em u s t b e d e r i v e d . F o r e x a m p l e , w h e r e a w a t e rt a b l e i s e n c o u n t e r e d i n a n o t h e r w i s e u n i f o r ms e q u e n c e o f s a n d y s e d i m e n t s a t w o - l a y e r v e l o c i t ym o d e l i s r e q u i r e d , a s t h e r a d a r w a v e v e l o c i t y i nu n s a t u r a t e d s a n d i s ty p i c a l l y o v e r t w i ce t h a t i ns a t u r a t e d s a n d . C o n s e q u e n t l y , s p l i t e l e v a t i o n /de p th s c a le s a re r e qu i re d (e . g . F ig . 3 ) .V a r i o u s f i lt e rs c a n b e a p p l i e d t o r a d a r d a t a ,b u t t h i s m u s t b e d o n e w i t h c a u t i o n . W i t h r e s p ec tt o P u l s e E K K O TM G P R s y s te m s t h e o n l y f i lt e rt h a t t e n d s t o b e u n i v e r s a l l y a p p l i e d i s ' D e w o w '



10/34

G P R A P P L I C A T I O N S I N C O A S T A L S T U D I E S 147

I T O P O G R A P H I CP R O , F I L E ( S I I

I G E N E R A T E ITO PO G RAPHI CCO RREC TIO N F I L ES

I C O N V E R TTO O . D IFNECESSARY I->I

J R A D A R R E F L E C T I O NP R O F I L E I S I II C E C K ' & C O RR E C ' i -...H E A D E R F IL ES .I D I S C A R D N W A NT F T DTRA CES (e. g. uplicates)4 ,

(correct ion for t im e zero'jitter')

M ERG E REL EVANT F IL ES

I A O J O S T T ' M E Z E R O T OT H E Z E R O O F F S E T TIM E4 ,CAL CUL ATE RETURNC E N T R E F R E Q U E N C YFO R PRI M ARYREFLECTIONSCAL CUL ATE VAL UES

FO R HO RI ZO NTAL ANDVERTI CAL RESO L UTIO N4 ,APPL Y TO PO G RAPHI CCORRECTION+

AP F;'EY RE LEVA NT IVEL O CI TY , G A I NS &'DEW OW' F ILTER

P L O T R A D A R IREFLEC TION PROFILE

AM END EL EVAT IO NSCAL E I F TW O L AYERVEL O CI TY M O DE L ISREQUIRED

C O M M O N M I D - P O I N TP R O F I L E ( S ) I

I C H E C K& C O R R E C T IHEADER F I LE

I DISCARDUNWANTEDTRA CE S (e.g. dupl icates)

4 ,A D J U S T T I M E Z E R O T OT H E Z E R O O F F S E T T IM E4 ,I APPL Y RE L EVANT G AI NS& 'DEW O W ' F IL TER I4 ,PL O T CM P PRO FI LE( S )+

I - I P E R F O R M V E L O CA N A L s S

Fig. 4. Flow diagram of the editing and processing procedu re applied to the data in this pap er using Sensors andSoftware Pulse EKKO T M 100 system software.

o r ' S i g n a l S a t u r a t i o n C o r r e c t i o n ' . T h i s i s a t im ef i l t e r t h a t r e m o v e s t h e s l o w l y d e c a y i n g l o w f r e -q u e n c y n o i s e w h i c h m a y b e g e n e r a t e d b y t h e l a r g et r a n s m i t p u l s e o f t h e G P R s y s te m . B e y o n d t h is ,p e r h a p s t h e s i m p l e s t a n d m o s t w i d e l y a p p l i e d

f il te r s t o G P R d a t a a r e t ra c e - t o - tr a c e a n d d o w n -t h e - t r ac e a v e r ag i n g . B o t h h e l p t o r e d u c e r a n d o mn o i s e b y a v e r a g i n g i t o u t . T r a c e - t o t r a c e a v e r -a g i n g , w h i c h i s o n e o f a s u i t e o f s p a t i a l f i l te r s , a l s oe m p h a s i z e s h o r i z o n t a l o r g e n t l y d i p p i n g f e a tu r e s



11/34

148 A. N EA L & C . L . RO BER TSa n d s u p p re s s e s ra p i d l y c h a n g i n g o n e s . H o w e v e r ,a n y f o r m o f a v e r a g i n g , e s p e c i a l l y w h e n h e a v i l yapp l i ed , e f f ec tive ly r educes the r eso lu t ion tha tcan be ach ieved and sub t l e f ea tu res a re o f t en los tf r o m t h e d a t a s e t . O f t e n i t i s m o r e a p p r o p r i a t eto use one o f a s e r ies o f mo re com plex t imef i l ters . These select ively remove cer tain f requen-c ies f rom the da ta s e t , t r ace by t r ace . Theyinc lude bandpas s f i l t e r ing , which r emoves h igha n d l o w f r e q u e n c y n o i s e o u t s i d e t h e p o r t i o n o fspec t rum of in te res t , l owpass f i lt e r ing , wh ichremoves h igh f r equency no i se above a spec i f i edcu t -o f f po in t , an d h ighpas s f i l t er ing , which r e -mov es the low f r eque ncy s igna l be low a spec i fi edcu t -o f f po in t .O f the var iou s f il te r s ou t l ined above , on ly'Dewow' f i l t e r ing has been rou t ine ly app l i ed tothe da ta p resen ted in th i s paper . An ex tens iveseri es o f exper imen t s ind ica ted tha t fu r the r f i lt e r-ing d id no t s ign i f i can t ly enhance the f ea tu res o fin te res t and tha t w hen he avy f i lt e r ing was app l i edfea tu res were los t o r d i s to r ted . W here subse quen tf i l ter ing is deemed to be des i rable i t i s bes tp e r f o r m e d w i t h r e f e re n c e t o h a r d - c o p i e s o f th eessent ial ly unf i l tered ref lect ion prof i les , thusensur ing tha t f ea tu res observed a re ' r ea l ' andnot a r t i f ac t s o f the f i l te r ing p roces s .M o r e c o m p l e x p r o c e s s i n g p r o c e d u r e s , s u c h a sm i g r a t i o n a n d d e c o n v o l u t i o n , a r e n o t a s y e trou t ine ly app l i ed to r adar da ta , un l ike in r e f l ec -t ion seismics . This ref lects the relat ive infancy ofG P R s tu d i es , t h e r a t h e r c r u d e n a t u r e o f s u c hproces s ing in much of the ava i l ab le r adar sof t -ware and the d i f f e ren t su rvey t echn iques andd e m a n d s o f t h e e n d u s er s. H o w e v e r , e v e n r e la -t ive ly s imple migra t ion rou t ines can enhanceradar da ta s ign i f i can t ly in some ins tances . As inseismic ref lect ion s tudies , the ef fects of mig rat io nare th ree- fo ld : ( a ) d ipp ing r e f lec t ions a re r e tu rnedto , o r neare r to , t he i r t rue hor i zon ta l and ver t i ca lpos i t ion ; (b ) s t ruc tu ra l d i s to r t ions on s t eep lyundula t ing r e f l ec t ions a re par t i a l ly o r whol ly r e -moved; ( c ) d i f f r ac t ion pa t t e rns genera ted f rompoin t r e f lec to r s and s t rong ly curved r e f lec to r s a r epar t i a l ly o r whol ly r emoved . In add i t ion to us ingp r o c e s s i n g s o f t w a r e s u p p l i e d b y G P R m a n u f a c -t u r e r s , r a d a r d a t a c a n a l s o b e c o n v e r t e d t of o r m a t s t h a t a r e c o m p a t i b l e w i t h m o r e c o m -plex re f l ec tion s e ismic packa ges . Resea rch to da tes u g g e st s t h a t t h i s a p p r o a c h m a y p r o v i d e a ne f fe c ti ve m e a n s o f p e r f o r m i n g m o r e a d v a n c e dproces s ing (F i sher e t a l . 1 9 9 2 ; T o d o e s c h u c k e t a l .1992; Fisher e t a l . 1996) . However , i t shou ld no tb e f o r g o t t e n t h a t a l t h o u g h t h e r e a r e m a n yobvious s imi la r i t i es be tween the r adar r e f l ec t ionand se i smic r e f l ec t ion t echn iques , the phys ica lbas i s fo r the genera t ion , p ropoga t ion and r e f l ec -t ion o f energy is fun dam enta l ly d i f f e ren t .

Interpretat ion of radar ref l ect ion prof i l es:radar s trat igraphyA l t h o u g h a r e a s o n a b l y s i g n i f i c a n t n u m b e r o fG P R s tu d i es h a v e n o w b e e n c a r r ie d o u t , a m e a n sfor the sys tema t i c in t e rpre ta t ion o f r ad ar r e fl ec-t ion prof i les for geological appl icat ions is onlyjus t be ing deve loped . Researcher s have begun toapp ly s e i smic s t r a t ig raph ic t echn iques (Mi tchume t a l . 1977) to the in te rp re ta t ion o f r ada r r e fl ec-t ion p rof i les (Beres & H aeni 1991; Jo l & S mi th1 99 1) . T h e d e v e l o p m e n t o f ' r a d a r s t r a t i g r a p h y 'h a s a l l o w e d t h e d e l i n e a t i o n a n d m a p p i n g o fgene t i ca l ly r e l a t ed s t r a t ig rap h ic un i t s w i th in s ed i-m e n t a r y d e p o s i t s ( G a w t h o r p e e t a l . 1 9 9 3 ; H u g -genbe rger 199 3; van O verm eeren 1994 , 1998;Br idge e t a l . 1 9 9 5 ; B r i st o w 1 9 9 5 ; L e c le r c &H i c k i n 1 9 9 7 ; R o b e r t s e t a l . 1997; Smith & Jol1997; van Hete ren e t a l . 1998).As in s e i smic s t r a t ig raphy (Mi tchum e t a l .1977; Emery & Meyer s 1996) , r adar s t r a t ig raphyrel ies on the ident i f icat ion of sys temat ic ref lec-t ion t e rmina t ions (F ig .5 ) . These t e rmina t ionsdef ine r a d a r s e q u e n c e b o u n d a r i e s . Geolog ica l lythese a re be l i eved to r epresen t s ign i f i can t non-d e p o s i t i o n a l o r e r o s i o n a l h i a t u s e s ( G a w t h o r p ee t a l . 1993) . Radar s equence boundar i es def inegene t i ca l ly r e l a t ed pa ckag es o f re f l ec tions t e rmedr a d a r s e q u e n c e s . G a w t h o r p e e t a l . (1993) definet h e s e a s t h e ' f u n d a m e n t a l s t r a t i g r a p h i c u n i t s 't ha t can be iden t i f i ed on r adar r e f l ec t ion p ro-file s. W o r k i n g w i t h in t h e f r a m e w o r k o f r a d a rsequences i t i s then poss ible to def ine var iousr a d a r f a c i e s . T h e s e a r e m a p p a b l e t h r e e - d i m e n -s iona l packages o f r e f l ec t ions wi th d i s t inc t iveconf igura t ions , con t inu i ty , f r equency , ampl i tude ,ve loc i ty charac te r i s t i cs and ex te rna l fo rm ( Jo l &S m i t h 1 9 9 1 ; G a w t h o r p e e t a l . 1 9 9 3 ; H u g g e n b e r -ger 1993).Desp i t e the obv ious ap pea l o f such a sys tema-t ic a p p r o a c h t o t h e i n t e rp r e t a t i o n o f r a d a r r e fl ec -t ion p rof il es, a numb er o f p rob lem s r em ain .F i r s t ly , var ious type o f e l ec t roma gnet i c ' no i s e 'a r e c o m m o n l y s e e n o n m a n y r a d a r r e f l e c t i o nprof i l es and care mus t be t aken to avo id mis -iden t i fy ing apparen t r e f l ec t ion t e rmina t ions as so-c ia t ed wi th the r esu l t ing in te r f e rence pa t t e rns .S o u r c e s o f n o i se i n c lu d e t h e G P R e q u i p m e n ti t s e l f , t he ambien t env i ronment ( e .g . r ad io andte lev i s ion s igna l s ) and , where unsh ie lded an ten-nae a re dep loyed , s ca t t e r ing due to sur f ace ob-j ec t s in c lose p rox imi ty to the survey l ine (Sun &Young 1995) . In add i t ion , s econdary r e f l ec t ions ,ana logous to those s een on s e i smic p rof i l es , cana l so occur . These inc lude r e f l ec t ion hyperbo laec a u s e d b y p o i n t s o u r c e s o r m a j o r l a t e r a l d i sc o n -t inu i ti es in the subsur face , a l th oug h in bo th casest h e se c a n b e r e m o v e d b y m i g r a t io n . I t s h o u ld b e



12/34

G P R A P P L I C A T I O N S I N C O A S T A L S T U D I E S 149

T o p l a p

" , , ' " " " ' - - . , . T r u n c a t i o n" < , , ~ - -- - " - . . k - - - - ] R a d a r S e q u e n c e~ " ~ 1 " ~ - - - ~ - - Z ~ . . I B o u n d a r ie s

- - - . . . _ _ _ _ 2Downlap

Fig. 5. The basic descriptive termino logy associated with th e definition of radar sequences and their boundaries.Modified f rom G awthorpe e t a l . (1993).

n o t e d t h a t i n m a n y a r c h a eo l o g i c al a p p li c a ti o n st h e i d e n t i f i c a t i o n o f s u c h p o i n t s o u r ce s i s f u n d a -m e n t a l t o t h e l o c a t i o n o f p o t e n t i a l t a r g et s .S e c o n d l y , n o t a l l o f t h e p r i m a r y r e fl e c ti o n s a ret h e r e s u l t o f t h e p r e s e n c e o f g e o l o g i c a l l y s i g n if i -c a n t u n i t s o r s e d i m e n t a r y s t ru c t u r es . A c o m m o ne x a m p l e i s t h e p r e s e n ce o f a f r e sh g r o u n d w a t e rt a b l e . T h e t o p o f t h i s f e a t u r e g i v e s a p a r t i c u l a r l ys t r o n g r e f l e c t io n w h i c h o f t e n c r o s s - c u t s r a d a r s e-q u e n c e b o u n d a r i e s a n d r a d a r f a c i e s ( e . g . F i g . 3 ) .T h i r d l y , t h e p r i m a r y g e o l o g i c al r e fl e c ti o n p a t -t e r n s o b s e r v e d a r e t o s o m e d e g r e e d e p e n d e n tu p o n t h e r e t u r n c e n t r e f re q u e n c y o f t h e s ig n a la n d t h e d i e l ec t ri c p r o p e r t i e s o f t h e e a r t h m a t e -r i a l s ( J o l & S m i t h 1 9 9 1 ; H u g g e n b e r g e r 1 9 9 3 ) .I t h a s a l r e a d y b e e n n o t e d t h a t r e s o l u t i o n i s af u n c t i o n o f t h e se t w o p a r a m e t e r s a n d t h i s w i l lc l e a r l y a f fe c t t h e s c al e o f g e o l o g i c a l f e a t u r e t h a tc a n b e d e t e c t e d a n d t h e r e s u l t i n g r e f l e c ti o n c o n -f i g u r a t i o n s . F o r e x a m p l e , l a t e r a l v a r i a t i o n s i nb e d t h i c k n e s s c a n l e a d t o a p p a r e n t r e f l e c t i o nt e r m i n a t i o n s w h e r e v a l u e s f a ll b e l o w t h e l ev e l o f

r e s o l u t io n . A s a r e s u l t, t e r m i n a t i o n s m a y b ep r e s e n t w h e r e t h e r e t u r n c e n t re f r e q u e n c y i s l o w( e.g . 5 0 M H z a n t e n n a e ) a n d / o r t h e R D P o f t h em a t e r i a l i s l o w ( e . g . u n s a t u r a t e d s a n d a b o v e aw a t e r t a b l e ), b u t a b s e n t o n a n e q u i v a l e n t p r o fi l ew h e r e t h e f r e q u e n c i e s a r e h i g h e r ( e. g. 2 0 0 M H za n t e n n a e ) o r o n t h e s a m e p r o f il e w h e r e t h e R D Pi s h i g h e r ( e . g . s a t u r a t e d s a n d s b e l o w a w a t e rt a b l e ) . C o n s e q u e n t l y , i n a d d i t i o n t o t h e r a d a rs e q u e n c e a n d r a d a r f a c i e s c h a r a c t e r i s t i c s , t h er e s o l u t i o n a n d t h e s c a l e o f r e f l e c t i o n s a r e a l s oi m p o r t a n t c o n s i d e r a t i o n s w h e n i n t e r p r e t i n gr a d a r r e f l e c t io n p r o f i le s g e o l o g i c a l l y ( B r i s t o w1 9 9 5 ; v a n H e t e r e n e t a l . 1998) . I t is in this wayt h a t r a d a r s t r a t i g r a p h y d i ff e rs s i g n i f ic a n t ly f r o ms e i s m i c s t r a t i g r a p h y . R e f l e c t i o n s e i s m i c s c a n a tb e s t r e s o l v e s e q u e n c e s a n d f a c i e s t h a t a r e ' e q u a l

i n s iz e t o o r l a r g er t h a n c o m p l e t e m a c r o f o r m s i nm a j o r e l e m e n t s o f d e p o s i t i o n a l s y s t e m s ' ( v a nH e t e r e n e t a l . 1 99 8). B y c o n t r a s t G P R h a s t h er e s o l u t i o n t o i m a g e t h e i n t e r n a l s t r u c t u r e o fa r a n g e o f s e d i m e n t a r y u n i t s f r o m i n d i v i d u a lb e d f o r m s ( e .g . a e o l i a n d u n es ) u p t o c o m p l e t em a c r o f o r m s ( e . g . a b a r r i e r i s l a n d ) .A m o r e q u a l i t a t i v e a p p r o a c h o f r a d a r f a ci esd e s c r i p t i o n a n d i n t e r p r e t a t i o n h a s b e en a d o p t e di n s o m e i n s t a n c e s . T h i s i s b e c a u s e i n m a n yi n s t a n c e s : (a ) n o n - g e o l o g i c a l p r i m a r y r e f l e c t o r ss u c h a s t h e w a t e r t a b l e c a n b e o f c o n s i d e r a b l ei m p o r t a n c e ; ( b ) s e c o n d a r y i n t e r f e re n c e p a t t e r n ss u c h a s r e f l e c ti o n h y p e r b o l a e c a n a i d g e o l o g i c a l( o r a r c h a e o l o g i c a l ) i n t e r p r e t a t i o n a n d (c) m o d -e r n h i g h r e s o l u t i o n s u r v e y s g e n e r a t e v e r y c o m -p l e x p r o f i l e s w i t h h i g h n u m b e r s o f r e f l e c t i o nt e r m i n a t i o n s ( B r i s t o w 1 9 9 5 ; v a n O v e r m e e r e n1 9 9 8; v a n H e t e r e n e t a l . 1998). As a c onse q ue n c e ,a n u m b e r o f a tt e m p t s h a v e b e en m a d e t o a t t a c hd e f i n it i v e i n t e r p r e t a t i o n s t o v a r i o u s r a d a r f a ci e si n s p e c i fi c d e p o s i t i o n a l s e t t in g s ( B e r e s & H a e n i1 99 1; H u g g e n b e r g e r 1 99 3; v a n O v e r m e e r e n 1 99 4,1 9 9 8 ; B u s b y 1 9 9 7 ; v a n H e t e r e n e t a l . 1998).H o w e v e r , a l t h o u g h t h i s i s p r o b a b l y u s e f u l o n al o c a l o r r e g i o n a l s c a l e , t h e a p p l i c a t i o n o f t h e s et o d a t a f i 'o m n e w s t u d y s i te s i s s o m e w h a t d i f fi -c u l t . T h i s s i t u a t i o n m a y i m p r o v e a s t h e t o t a ld a t a s e t i n c r e a s e s .

Geom orphological and sedimentologicalapplicat ions in coastal environments.T h e o r y a n d p r a c t i c a l e x p e r i e n c e s u g g e s t t h a tG P R s u r v e y i n g i s l i k e ly t o b e m o s t s u cc e ss f ulw h e r e g r o u n d c o n d u c t i v i t i e s a n d r e s u l ti n ga t t e n u a t i o n a r e l o w ( c f . T a b l e 1 ) , t h u s a l l o w i n gs i g n i fi c a n t p e n e t r a t i o n o f t h e r a d a r s i gn a l .



13/34

150 A. NE AL & C. L. RO BE RT SConsequen t ly , coas ta l depos i t s domina ted bysands , g rave l s and pea t s , tha t a re e i the r unsa tu -ra ted o r be low a f re sh g roundwa te r t ab le , a rel ike ly to be the mos t am enab le to rada rref lec t ion prof i l ing (L eath erm an 1987; Baker1991; Clemmensen et al . 1996; Meyers et al .1996; van Heteren e t a l . 1996; van Heteren & vande Plassche 1997; van Heteren et al. 1998; vanOverm eeren 1998) . Dep osi ts dom inat ed by sig-n i f ican t amoun ts o f f ine g ra ined sed imen t ( e .g .sa l tmarshes) or areas with a brackish to sa l inegroundwater are usual ly far less su i tab le , due tocons ide rab le a t t enua t ion o f the e lec t romagne t ics ignal (Le ath erm an 1987; Jo l et al. 1996; vanH e t e r e n et al . 1996; van Heteren et al . 1998).In o rde r to demon s t ra te the types o f geomor-pho log ica l and sed imen to log ica l da ta tha t can beob ta ined f rom GPR su rveys in su i tab le coas ta lse t t ings , examples f rom a ser ies of recent ly com-ple ted U K based s tudies wil l be presented . Thea im i s no t to in t roduce comple te da ta se t s andfu rn i sh fu l l in te rp re ta t ions , bu t to ind ica te then a t u r e a n d r a n g e o f i n f o r m a t i o n t h a t c a n b eob ta ined and i t s po ten t ia l con t r ibu t ion to fu tu recoasta l research .

S e f t o n co a st , N W E n g l a n dThe Se f ton coas ta l dune ba r r ie r sy s tem in NWEn gla nd (Fig . 6) consis ts of a ser ies of unco nso-l ida ted Ho locene sed imen ts , o f ten ove r 30mth ick . The sho re face , fo re sho re and dune depos -i t s o f the ba r r ie r cons i s t p redominan t ly o f f ine -gra ined and well-sor ted sand (Pye 1991; Neal1993; Pye & Ne al 1993). Th e se dimen ts of thefo rmer back ba r r ie r a rea cons i s t o f f r e shwa te rand es tuar ine peats , c lays and s i l ts and arepar t ia l ly over la in by a t leas t 12 m of dune sandon the ir western (seaward) margin (Neal 1993;Pye & Neal 1993) . Severa l per iods of duneac t iv ity , s epa ra ted by phases o f dune s tab i l i za-t ion and so i l fo rma t ion , have taken p lace up tothe present day (Pye 1990; Neal 1993; Pye &Neal 1993, 1994; Pye et al. 1995). As a con-sequence the barr ier sys tem consis ts of a th ickand ex tensive sui te o f coas ta l s ed imen ts tha t a resui tab le for radar prof i l ing . In par t icu lar , thereexis ts a wide range of deposi ts associa ted withact ive and recent ly ac t ive coasta l dunes , inadd i t ion to the ex tens ive mid - to l a te -Ho loceneaeol ian sequences .

( 1 ) R a v e n M e o l s b lo w o u tAt R aven Meo ls , in the sou the rn pa r t o f thecoasta l d une system (Fig . 6), a la rge ac t ive t rou gh

.... ii~ii,,4iiiiiiiiiiiiiiiii

i!!~ ,

: ==

Formby /Point /\

Raven bBiowc

Blown sandpresent-da~~ Blown sand~ Peatm Triassic san

:~ Alluvium (fnestuarine)Shirdley HillGtacial till

Fig. 6. Loca tion and surficial geology of the Seftoncoast, immediately to the north of Liverpool in NWEngland. Locations of the two study sites at RavenMeots and Woodvale are also indicated. Modifiedfrom Pye et al. (1995).b lowo u t i s deve loped . The b low ou t has fo rmedduring approximate ly the las t 100 years . I t l ieslandw ards o f a b road be l t o f pa ra l le l fo redunesthat have been de velopi ng s ince a t leas t 1880 (Pye1990) . The b low out is recorded on the f i rs t aer ia lpho to g raph s taken o f the a rea in 1945, when i twas app rox im a te ly 60 m long and 30 m wide . I t i sn o w a r o u n d 3 0 0 m l o n g a n d 1 0 0 m w i d e , w i t h ama jo r depos i t iona l lobe a t i t s downwind (no r th -eas tern) end (Fig . 7) .A ser ies of in tersec t ing radar ref lec t ion pro-f iles were col lec ted a t the s i te a t var ious ante nna ef requenc ie s . CM P su rveys ind ica ted tha t r ada rwave ve loc i t i e s above and be low the wa te r t ab lewere s ign i f ican tly d i f feren t and tha t a two- laye rve loc i ty mode l shou ld be employed . As a con -sequence , e levat ion sca les on the prof i les pre-sen ted a re d i f fe ren t above and be low the g round



14/34

5m /

\

~

,

yJ

/

-~

//

"

,

O)

/ /,

\1

D

STON

LOB

/

/r,

///

\1/

/-

-

/"

/-

/

\1

~

"~

\1

(

~DWA

\1

\1

\1

SB

\1/

\1

pn

.

d

d

~

rmd

-~bes

v

aehmmo

d

~~daopan

.

crest

.

hmichzs

\1v

aed

b

bowo

o 7 t :Z :z - r~

Fg7Tmangmopoc

euoRnMesbowahooohr

reopeshwnFg38&9


15/34

v t=

SW

o

I

I

~

~

~

-

!

00 5010102020303040

RSC

I

Y

1

Dsa

m)

8 1

'

RSB

t

1T

NE20

~0dw? LM

0

Fg81MHa

eeopoeomR

MesbowsuvnAT

suvnwoeaepaeohasohbow

hoFg7

runaohbow

amp11mahd

oo12mR

s

b

eRM-ABaaeRM-123ahooo

thweaeWTaenceaaehne

owhsuvnBRMSBaCRMSCAudpoohasagaohpoea

isgocnepeaocbo

nTe3


16/34

G P R A P P L I C A T I O N S I N C O A S T A L S T U D I E S 153wa te r t ab le (e .g . F ig . 3 ) . Ground t ru th ing f romhand auge r ho le s ind ica ted tha t the e leva t ion /dep th e s t ima te s were remarkab ly accu ra te , ly ingwi th in the l ike ly e r ro r s a s soc ia ted wi th auge r ing(e .g . 4-0.1 m a t 8 m depth) . This p rob abl y ref lec tsthe h igh ly un i fo rm na tu re o f the dune sands a tthe s tudy s i te . Resul ts f rom three in tersec t ingsu rvey lines tha t t r ave rse a numbe r o f the ma ingeomo rpho log ica l f ea tu res o f the b lowou t(Fig . 7) are presente d h ere (Lines A, B and C;Figs 8, 3 & 9).

Dis t inc t r ada r sequence bounda r ie s and fac ie scan be ident if ied on a l l the ref lec t ion prof i lesob ta ined , a l lowing the cons t ruc t ion o f a r ada rs t ra t ig raphy fo r the b lowou t . Th is s t r a t ig raphycan be in te rp re ted geo log ica l ly wi th the a id o ff ield obse rva t ions , auge r ho le s and ae r ia l pho to -graph analysis (Table 3) . The th in dune soi lf o r m i n g r a d a r s e q u e n c e b o u n d a r y R M - B p r o -vides a d is t inc t and la tera l ly cont inuous ref lec-t ion tha t can easi ly be t raced in the subsurface .The radar fac ies ident i f ied have ref lec t ion char-ac ter is t ics tha t c lear ly indica te they representpacke ts o f p r im ary sed im en ta ry s t ruc tu re s . Th isi s con f i rmed by f ie ld obse rva tions . Fu r th e rmo re ,the na tu re o f the va r ious rad a r f acie s can bere la ted to the fo rma t ion o f sed imen ta ry s t ruc -tu re s unde r d i f fe r ing depos i t iona l cond i t ions .In pa r t i cu la r , the dune in te rna l s t ruc tu re rep re -

sented by radar fac ies RM-3 can be c lear lyre la ted to the geomorpho log ica l deve lopmen t o fthe deposi t ional lobe , as ident i f ied on recentae r ia l pho tog raphs .The s t rong re f lec t ion f rom the wa te r t ab le i sthe mos t impor tan t non -geo log ica l f ea tu re tha tcan be ident if ied on the radar ref lec t ion prof i les .I t fo rms a nea r ho r izon ta l and la te ra l ly con t in -uous ref lec t ion tha t can be t raced across thestudy s i te . Associa ted with th is ref lec t ion are acomplex se r ie s o f in te r fe r ing hype rbo lae , whoseapex appea r to l ie a t the wa te r t ab le and obscu rem a n y o f t h e p r i m a r y s e d i m e n t a r y s t r u c t u r e sbenea th . The cause o f these hype rbo lae i s un -c lea r , bu t may be re la ted to d i scon t inu i t i e s onthe water tab le surface a nd the large s ize of theFresne l zone a t dep ths o f up to 10 m. H owever ,wha teve r the i r o r ig in , they p reven t de ta i l ed geo -log ica l in te rp re ta t ion benea th the wa te r t ab le .(2) Woodvale, Ainsd ale HillsThe Ainsda le Hi l l s l i e in the cen t ra l pa r t o fthe Sef ton coasta l dune barr ier complex (Fig . 6) .At th i s po in t the dune be l t is app rox ima te ly 3 kmwide , a l though the W oodv a le a rea has been level -led for cul t iva t ion or housing. Boreholes indi-ca te tha t the ma in body o f the dune complex ,bo th a t Ainsda le and e l sewhere , cons i s t s o f a

SEDistance (m)

o RMSL-A oc~ O 0 00 0

NW

c

EmF-

0.050.0

100,0

150.0200.0250.0

300.0

L15.0

i0.0

WT

C3d

Ec~>

t~

Fig. 9. 100 MH z rad ar reflection profile from R aven M eols blowout su rvey line C. The survey line was orientate dnorm al to the axis of the blowout (Fig. 7), cross-cutting the ramp (10-25 m) and the depositional lobe (0 10mand 25-43 m). Radar sequence boundaries (RM-B), radar facies (RM-2, 3) and the location of the water table(WT) are indicated, as is the intersection with survey line A (RMSL-A). A full description of the radarstratigraphy of the profile and its geological interpretation can be found in Table 3.



17/34

Te3Dpoanepaoohrasraga?mRMesbow

ashwnFg38&9Tgocnepaowadbhays

ohaheoedeuaaapoa

R

Wa

Vc

D

po

Inepeao

Voy

Rouo

R

FeRM-1

00m/n

02m

S

B

yRM-A

R

FeRM-2

01m/n

04m

S

B

yRM-B

R

FeRM-3

01m/n

04m

Seosuhzaaeay

cn

supaeeeo

Payo

ebaseocmpe

ineenhbadobow

thoohweae

O

ebcmpeseoneen

hbadobowhoo

thweae

Seococneeowh

lmieaeacnyReo

h

owomoaeaedpa

dwaooRM-Aaoaoo

RM-B

Leaycn

uaneeo

Leaycn

supae

gaymoaeohgae

reeoO

oydspaow

aecocneaohpA

reeodwaooRM-B

Up

oehesmesd

eapoa

poaob

dcmpeNowyn

bowaehgo

weae

Cabw

hu

ynb

d

so

RM-1aoyndd

soRM-2

Cmpeseoaaco

aasaeb

cocnb

nsua

ncvo

ddomenhpe

oaea

pavaoc

Tnhmichzeenad

sasosuawchbbebh

reyd

eaasmesoRM-3

P

socoaad

eahvao

fed

ooohbowoh

ee

poevynhewdLw

aehdodb

nsuao

oy

seenminevosua

Z > t~ t- -q


18/34

Dsa

m

00 50

100.0

10

20

20

30

WVBo

y

.

8

wy

50 002 d E g

Fg11MHaeeopoeomWo

esuvnATpoesoeaewoenmaohpeumegod

oskohd

syemR

s

b

eWVABa

aeWV12ahooohweaeWTaenceashne

owhsuvnB

(WVBAudpoohpoesasagaasgocnepeaocbo

nTe4

C > > .q :Z :Z > r > r~


19/34

156 A . N E A L & C . L . R O B E R T Ss e r i e s o f a e o l i a n s e d i m e n t s a t l e a s t 1 2 m t h i c ka n d e x t e n d i n g w e l l b e l o w t h e c u r r e n t g r o u n dwa te r l e ve l (N e a l 1993 ; Pye & N e a l 1993). Se v-e r a l b o r e h o l e s a l s o i n d i c a t e t h a t p a r t s o f t h ea e o l i a n s e q u e n c e c o n t a i n a s e ri e s o f p e a t s a n dh u m i c h o r i z o n s , t h e o l d e s t o f w h i c h c a n b e d a t e dt o a p p r o x i m a t e l y 5 1 0 0 14C y e a r s B P ( N e a l 1 99 3;P y e & N e a l 1 99 3) . H o w e v e r , c o r r e l a t i o n o f t h e s ep e a t s a n d h u m i c h o r i z o n s i s e x t r e m e l y d i f f i c u l td u e t o t h e l i m i te d n u m b e r a n d w i d e s p ac i n g o ft h e b o r e h o l e s a n d t h e s m a l l n u m b e r o f s am p l e ss u i t a b l e f o r r a d i o c a r b o n d a t i n g . I n o r d e r t o t r ya n d o v e r c o m e t h e s e d i f fi c u lt i es , a s e r ie s o f r a d a rr e f l e c t io n a n d C M P p r o f i l e s w e r e c o l l e c t e d a l o n ga n u m b e r o f s u r v ey l in e s ac r o ss t h e A i n s d a l eH i l ls , i n c lu d i n g W o o d v a l e . T h e s u r v e y l in e s w e r ed e s i g n ed s o a s t o r u n a p p r o x i m a t e l y p a r a ll e l o rn o r m a l t o t h e p r e s u m e d g r o s s d e p o s i t i o n a l s t r ik eo f t h e d u n e s y s t e m , b a s e d o n t h e o r i e n t a t i o n o fs u r f ic i a l b e d f o r m s .

A t W o o d v a l e t h e f r e s h g r o u n d w a t e r t a b l ew a s g e n e r a l l y w i t h i n l e ss t h a n 1 m o f t h e s u r f a c ea l o n g a l l o f t h e s u r v e y l i n e s . A t a n a n t e n n a ef r e q u e n c y o f 1 00 M H z t h e r a d a r s i gn a l w a s a b let o p e n e t r a t e t h e f u l l d u n e s e q u e n c e t o d e p t h s o fa t l e a s t 8 m ( F i g s 1 0 & 1 1 ) . C M P s u r v e y s a g a i ni n d i c a t e d t h a t r a d a r w a v e v e l o c i t i e s a b o v e a n db e l o w t h e w a t e r t a b l e w e r e s i g n i f i c a n t l y d i f f e r e n ta n d t h a t a t w o - l a y e r v e l o c it y m o d e l s h o u l d b ee m p l o y e d . T h e c o m p l e x i n t e r f e r e n c e h y p e r b o l a ea s s o c i a t e d w i t h t h e w a t e r t a b l e a t R a v e n M e o l sb l o w o u t w e r e n o t o b s e r v e d o n t h e p r o f i le s f r o mW o o d v a l e . T h i s a l l o w e d a d e t a i l e d r a d a r s t r a t i -g r a p h i c a n d g e o l o g i c a l i n t e r p r e t a t i o n f o r t h ea e o l i a n s e d i m e n t a r y s e q u e n ce b e n e a t h t h e w a t e rt a b l e ( T a b l e 4 ) .T h e l a c k o f r a d a r w a v e p e n e t r a t i o n b e n e a t ht h e a e o l i a n s e q u e n c e ( i . e . b e l o w s e q u e n c e b o u n -d a r y W V - A ) a n d e v i d en c e f r o m a n u m b e r o fn e a r b y b o r e h o l e s i n d i c a t e s t h a t t h e d u n e s e d i -m e n t s o v e r l i e a p e a t , w h i c h i s i n t u r n u n d e r l a i n b yt h i c k e r s e q u e n c e s o f e i t h e r si lt s o r m u d d y s a n d s .W i t h i n t h e a e o l i a n s e q u e n c e a d i s t i n c t s e q u en c eb o u n d a r y ( W V - B ) c a n b e i d e n ti f ie d a n d i s in t e r -p r e t e d a s a b o u n d i n g s u r f a c e m a r k i n g a s i g -n i f i c an t h i a t u s i n d u n e s a n d d e p o s i t i o n . S h a l l o wb o r e h o l e s a t t h e s t u d y s i t e w e r e u n a b l e t o l o c a t ea n y m a j o r l i t h o l o g i c a l c h a n g e a t t h i s b o u n d a r y( e . g . a h u m i c h o r i z o n o r p e a t ) . C o n s e q u e n t l y ,i t c a n b e a s s u m e d t h a t t h e b r e a k i n d e p o s i t i o nw a s n o t s u f f ic i e n t ly l o n g t o a l l o w s i g n i f i c a n t s o i lf o r m a t i o n .T h e b o u n d i n g s u r f a c e d e f i n e d b y W V - Bs e p a r at e s t w o m a j o r p h a s e s o f a e o l i a n a ct i v i ty ,r e p r e s e n t e d b y r a d a r f a c i e s W V - 1 a n d W V - 2 .R e f l e c t i o n c o n f i g u r a t i o n s w i t h i n b o t h r a d a rf a c i e s a r e i n t e r p r e t e d a s b e i n g t h e p r o d u c t o fp r i m a r y s e d i m e n t a r y s t r u c t u r e s w i t h i n t h e d u n e

s e d i m e n t s . I n t e r p r e t a t i o n o f th e s e s t ru c t u r e s i n d i -c a te s t h a t i n b o t h p h a s e s o f d u n e a c t iv i t y d e p o -s i t i o n w a s r a p i d , w i t h l i t t l e o r n o v e g e t a t i o nc o v e r . D u r i n g t h e f i r s t p h a s e o f a e o l i a n a c t i v i t y( W V - 1 ), a s e ri e s o f b r o a d , l o w , u n d u l a t i n g d u n e sd e v e l o p e d i m m e d i a t e l y o n t o p o f t h e u n d e r l y i n gp e a t . T h e l a c k o f e v i d en c e fo r b o u n d i n g s u r fa c esw i t h i n t h i s s e q u e n c e a n d t h e a p p a r e n t p r e s e r v a -t i o n o f t o p s e t s , f o r e s e t s a n d b o t t o m s e t s s u g g e s t st h a t s e d i m e n t a t i o n r a t e s w e r e v e r y h i g h . T h ed u n e t o p o g r a p h y a s s o c i a te d w i t h t h e e n d o f t h i sp h a s e o f d u n e a c t i v i t y a p p e a r s t o h a v e b e e n p r e-s e r v e d d u r i n g t h e s e c o n d p h a s e o f a c t i v i t y ,r e p r e s e n t e d b y W V - 2 . T h e v e r y l o w a n g l e , s u b -p a r a l l e l n a t u r e o f t h e r e f l e c t i o n s i n r a d a r f a c i e sW V - 2 s u g g e st s t h a t t h e s e c o n d p h a s e o f d u n ef o r m a t i o n w a s p r o b a b l y a s s o c i a t e d w i t h e x t e n -s iv e s a n d s h e e t d e v e l o p m e n t . R a p i d b u r i a l o f t h ep a l a e o - d u n e t o p o g r a p h y i s i n d i c a t e d b y t h ed i v e r g e n t fi ll p r e s e n t i n t h e t o p o g r a p h i c d e p r e s -s i o n s a s s o c i a t e d w i t h W V - B . L e v e l l i n g o f t h ea r e a f o r a g r i c u l t u r a l p u r p o s e s , d u r i n g t h e l a t t e rp a r t o f th e n i n e t e e n t h c e n t u r y a n d e a r l y p a r t o ft h e t w e n t i e t h c e n t u r y , i s l i k e l y t o h a v e r e m o v e da n y s m a l l d u n e s t h a t w e r e p r e s e n t d u r i n g t h ef i n a l p h a s e o f s a n d s h e e t s t a b i l i z a t i o n .

T h e A y r e s f o r e l a n d , I s le o f M a nT h e A y r e s c o a s t a l f o r e l a n d l i e s a t t h e n o r t h e r nt i p o f t h e I s le o f M a n ( F i g . 1 2 ) . I t i s o v e r 7 k ml o n g a n d w i d e n s p r o g r e s s i v e l y f r o m w e s t t o e a s t.A t i t s e a s t e r n m a r g i n i t is o v e r 2 .5 k m w i d e . T h el a n d w a r d m a r g i n o f t h e A y r e s is m a r k e d b y ac l if f c u t i n t o t il l. T h i s c l i ff i s n o t i c e a b l y d e g r a d e da l o n g i t s c e n t r a l a n d e a s t e r n p o r t i o n s . S e a w a r do f t h i s c l i f f a r e a s e ri e s o f H o l o c e n e d e p o s i t s ,p r i n c i p a l l y o f c o a s t a l o r c o a s t a l - m a r g i n o r i g i n .F i e l d s e c t i o n s , e x p o s e d a l o n g t h e e r o d i n g e a s t e r ne d g e o f t h e f o r e l a n d , s u g g e st t h a t t h e i n n e r p a r to f t h e A y r e s c o n s i s t s o f a s e r ie s o f s h a l l o w b a s i n s( F i g . 1 2) c o n t a i n i n g a c o m p l e x s e q u e n c e o f f in e -g r a i n e d c l a s t i c a n d o r g a n i c d e p o s i t s ( P h i l l i p s1 9 6 7 , 1 9 6 9 ; W a r d 1 9 7 0 ; D a c k o m b e & T h o m a s1985; Gonz a le s et al. 2000) . T he se ba s ins a res e p a r a t e d e i t h e r b y t i l l o r b e a c h f a c e s a n d s a n dg r a v e l s . T h e m o s t s o u t h e r l y o f t h e s e b a s i n s , a tP h u r t , i s a l s o a n i m p o r t a n t g e o a r c h a e o l o g i c a ls it e ( D a c k o m b e & T h o m a s 1 98 5; G o n z a l e s et al.2 0 0 0 ). B y c o n t r a s t , t h e o u t e r p a r t o f t h e fo r e -l a n d c o n s i s t s e n t i r e l y o f s a n d s a n d g r a v e l s w i t hd i s t in c t s et s o f b e a ch r i d g e s ( D a c k o m b e &T h o m a s 1 98 5). D i s c o n t i n u o u s a n d t h i n ( < 3 m ,L a m p l u g h 1 9 0 3 ) c o a s t a l d u n e s a n d d e p o s i t sf o r m t h e t o p o f t h e s e d i m e n t a r y s e q u e n ce , b o t ho n t h e i n n e r a n d o u t e r A y r e s .



20/34

G P R A P P L I C A T I O N S I N C O A S T A L S T U D I E S 1 5 7

r..,O

Z

00 '

v

O0 .~ ..,

( 0 ' 0 set~eLu) uo!;eAeB

I ~ I ~ I : ~ I :~ ; ; ;o o o o q o oo o 8 g 8 g ~0 ~ 0,1 0,t CO

(su) ew!

; . .~

~=._o ~4--'o o

~ ' ~

o~_g

F: .~ e

[ -. , ~ ~

~ ~.~

~ m

e ~ o=.,-.~

~N s

~ m.~>



21/34

Te4Dpoaneao[hrsg

JoWoeAneHaswnF1&1Tgocneaowadbh

ayop

au

b

eo[ha

R

Wa

Vc

D

po

Gocnepeao

Voy

Rouo

S

B

yWVA

Leaycn

uaneeo

Aoaesmaneeehba

R

FeWV1

00m/n

02m

S

B

yWVB

R

FeWV2

00m/n

02m

Leaycn

wpae

reeoSmemedvgMu

deoaedpo0o2

C

dwhodwanoo

WVATaeaohpwhWVB

Mamumhc

45m,bgay

~m

Leaycn

uaneeo

who

aouo25m

Leaysucn

w

pae

reeoDvg

whn

deoaoaewhuan

WVBApedpo0o2~Dpo

reeocoebc

d

reaohpwhWVBb

h

Mamumhc

5m,bgay

~m

Bohds

U

anbpa

thehmuosysmespmayo

inedogn

Peminypaessoaaco

aa

whwdyvynoeaoOedspa

acmpeeseoosoesa

bomssB

nsuaaenpe

S

sadd

oamudeo

dgowhnasndminebaseo

bo

owuand

wheon

vaoc

B

nsuamaknadsnhaund

sd

oNoed

osoomao

Paessoaaco

aadanh

u

yndsasosuaS

s

radd

oapoaagsshwh

leonvaoc


22/34

G P R A P P L IC A T IO N S IN C O A S T A L S T U D IE S 159

plFISTO( eNF BkACH FACE LAGOO NAL ALOI IAN

Pointof. ~ -~ A y rejJ ~. / "~.~ ~ Outer Ayres

N ~o~ . ~r , ~ g~>


23/34

~

~d~

~

i g%

g

Q

Tm:

I

I6

EeomOD

7

0 ,rC

2

.~

Z

~O

atCAPESonMay6,2013 http://sp.lyellcollection.org/ Downloadedfrom


24/34

G P R A P P L IC A T IO N S IN C O A S T A L S T U D IE S 161

ENEDistance (m)

o OASL-A f

0 ,0

~" 100.0

200.0

300.0

,, 1- - } ~ 1

WSW8o~

I

dOrrglLli

Fig. 14. 200 MH z r ada r reflection profile from Out er Ayres survey line B. The profile is orie ntate d paral lel to oneof a complex series of beach ridges th at characterize this part of the Ayres (Fig. 12b). The lo cation of the watertable is indicated (WT), as is the intersection with survey line A (OASL-A).

wave ve loc i ty var ia t i on was mi n im al (e .g . F ig . 13) ,appears deeper whe n AY-3 i s p resen t . Moreov er ,the degree o f d is to r t io n i s g rea te r the th ic ker theAY-3 un i t . Th is i s because i t i s two-way t rave lt im e o f th e r a d a r w a v e, r a th e r t h a n d e p th , t h a t i sac tua l ly be ing measured dur ing da ta co l lec t ion .The two-way t rave l t ime to the wate r tab le i s afu n c t io n o f t h e a v e ra g e r a d a r w a v e v e lo c i ty o fthe mate r ia l above i t . As AY-3 th ickens a long asec t ion of p rof i le , the average radar wave ve lo-c i ty becomes lower as the lower ve loc i ty mate r ia lo f AY-3 assumes grea te r impor ta nce . Conse-quent ly , the two-way t rave l t ime to the wate rtab le leng thens and the wate r tab le appearsdeeper in the re f lec t ion prof ile . Mor e impor-t a n t ly , h o w e v e r , i n a d d i t i o n to t h e d i s to r t i o n o fthe wate r tab le , the re a re a lso d is to r t ions in thed ip and conf igura t ion of the re f lec t ions f romrada r sequence bounda r ies and ra dar fac ies un i t s .As these cann ot eas i ly be cor rec ted , they must bec o m p e n sa t e d fo r in a q u a l i t a ti v e m a n n e r d u r in gth e i n t e rp r e t a t i o n o f t h e r a d a r s t r a t ig r a p h y .In te rpre ta t ion of the radar re f lec t ion prof i lesob ta ined (Table 5 ) , supplemented by a se r ies o fcommerc ia l boreholes and t r ia l ho les , sugges tsthe p resence of a complex sequence of beachr idge depos i t s o f d if fe ren t in te rn a l charac te r andsed imenta ry fac ies assoc ia t ions to those o f theoute r A yres . The base o f the sequence , wherereached , i s charac te r ized by sequence boundary

A Y -A , w h ic h d ip s g e n t ly t o t h e n o r th a n dposs ib ly wes t. Bo reholes ind ica te th a t th is is theti l l surface. Above this t i l l base a dist inct crosssec t iona l beach r idge morphology i s observed ,d e l in e a t e d b y r a d a r s e q u e n c e b o u n d a ry A Y -B .Boreholes ind ica te tha t ra dar fac ies AY-2 , whichl ies be tween these two radar sequence bound-ar ies , is charac te r ize d by medi um to coarse sandsand sub-ro unde d to round ed grave l . I t s re f lec t ionconf i gura t i on i s in te rpre te d as represen t ing thepresence of a comp lex se ries o f p r ima ry sed imen-ta ry s t ruc tures wi th in coarse beach r idge depos-i ts. Such structu res suggest s ignif ica nt spatia l an dte m p o ra l v a r i a t i o n in b e d fo rm ty p e a n d m ig ra -t i o n d i r e c t io n d u r in g b e a c h r idg e e m p la c e m e n t .L y in g a b o v e r a d a r s e q u e n c e b o u n d a ry A Y -Bis a complex t rough f i l l de f ined by radar fac iesAY-3 . Boreholes and t r ia l ho les ind ica te tha tthis f i ll is a com plex mix of san dy a nd si l ty clays,sands and sa

Applications of Ground-penetrating Radar (Gpr) to Sedimentological, Geomorphological and...

Documents

Transcript of Applications of Ground-penetrating Radar (Gpr) to Sedimentological, Geomorphological and...