Seismicsic na 3rocessinc in

enc ineerinc si:e inures:ica:ion-acase ~is:oryby ANGELA M. DAVIS*, BSc, MSc, FGS 8t PETER J. SCHULTHEISS~, BTech

IntroductionTHE NEED FOR in situ dynamic testing ofsedimentary materials, in addition to thestandard static tests carried out duringsite investigations, has been recognised in

the last few years. Knowledge of the dy-namic elastic properties of a foundationmaterial allows predictions to be madeboth of its response to dynamic loading(such as that imposed on the sea-floor bylarge oil platforms under storm conditions)and of its total strength by empirical cor-relations (Taylor Smith, 1975).

While seismic tests have always beenavailable as part of any civil engineeringsite investigation (usually to define thethickness of the overburden and its vari-ation), the velocity data derived whenused in any diagnostic manner (to assessrock quality for instance) nearly always

*Demonstrator, and liResearch Assistant, MarineScience Laboratories, University Co'lege of NorthWales, Menai Bridge, Gwynedd

10

=01

0010001 001 0 1

010 100

Fig, 1. Gas saturation % for a fine sand ofporosity 40%

refer to compressional wave propagation.For a complete assessment of the dynamicproperties there is a need to measureshear wave phenomena. The relevant in-terrelationships between compressionalwave velocity (V„), shear wave velocity(Vs) and the various elastic moduli are

4Vn

— (B + —G) //,

V,. = (G//i)!

where />

BEG

densitybulk modulusYoung's modulusshear modulus or rigiditymodulusPoisson's ratio

E = 2V,.c/i (1 + /t)

or E = /iV,c (1 + ft) (1 —2/t)/(1 —tt)

B = E/3 (1 —2p)

~ i (if/i

l" 3'~3'M .'lfr;~.

;a~~ IW

~ ~ ( ( lal

Fig. 2. Signal Averaging System being tested in shear wave survey during drilling operations

44 Ground Engineering

As can be seen from the various equa-tions, the shear wave velocity is a vitalparameter in deriving the magnitudes ofthe moduli; it is also extremely useful in

that it is not dependent on the propertiesof the pore fluid and used with the com-pressional wave velocity is a good de-tector of gas (Fig. 1).

Because of the importance of seismictesting to give information about the engi-neering properties of the sea-floor, a shearwave research programme has been estab-lished for the last five years at the MarineScience Laboratories with the ultimateaim of producing seismic site investiga-tion techniques for use at sea. In the pro-cess of perfecting the system a numberof tests have been performed on land(Davis, 1977). This case history describesa field survey at Fleet, Dorset, carried outin conjunction with Atlas Copco CraeliusLtd., which was conducted to test newand improved systems.

Test requirementsOne of the first requirements for rapid

efficient in situ geophysical surveying isthat the equipment to be used in the fieldis lightweight and portable. For seismicsurveying on land, when utilising a mech-anical energy source, severe restrictionsare imposed by the source in that theamount of energy transmitted to theground is directly related to its size. Toavoid increasing the source bulk, it isnow possible to overcome these problemsby employing a signal averaging systemwhich acts to improve the signal-to-noiseratio and hence enhance the receivedseismic pulses. This proves advantageouswhen working in 'noisy'nvironmentssuch as urban areas, nuclear power sta-tion sites, littoral zones, etc., and wherethere are restrictions on the use of ex-plosives.

Various signal processing systems areavailable on the market for use withstandard mechanical sources. The majorityof these are enhancement instrumentswhich operate by stacking incoming sig-nals produced by a repetitive energysource and by this means increasing theamplitude of the seismic event. The fund-amental principle behind the operation ofthe ABEM Signal Averaging System, theinstrument used in the Dorset survey, isthat a series of stacked signals can beaveraged and by such a process, noisewhich is random cancels out, whereas theseismic signal of interest remains unalt-ered and is enhanced relative to back-ground noise. The improvement in signal-to-noise ratio by this process means thatthe capability of small mechanical energysources can be improved because theinstrument gain can be increased to allowweak signals to be detected (Fig. 2).

Both compressional wave and shearwave measurements on sedimentary mat-erials can be made utilising standard re-fraction shooting techniques. The refractionmethod is based on the measurement andanalysis of waves critically refracted fromboundaries across which there is a veloc-ity increase. From the plotting of time-distance graphs, derived from the refractedwave arrivals, it is possible to indicatevelocity magnitudes and depth of the sub-surface layers. The technique has provedto be extremely successful for engineeringstudies where an idea of soil propertiesis often required prior to drilling.

Methods for the generation and detec-

tion of P-waves are reasonably standard.A falling weight source provides predo-minantly compressional energy togetherwith some vertical shear, The compres-sional wave being the fastest can easilybe detected on the seismic record as afirst arrival. Assessment of shear wavepropagation however is not so straight-forward; the shear wave being a late ar-rival at the detecting geophone is oftenmasked by earlier arrivals and hence needssome method by which it can be madeto stand out for identification purposes.The easiest way of achieving this is touse a separate impulsive source whichpolarises the vibration perpendicular to thedirection of propagation. One such sourceis the ABEM Multipulse which consists ofa ground pole, an impact hammer and ham-mer support. The ground pole is driven50cm vertically into the surface materialand the hammer pivoted above it. Thehammer on striking the side of the groundpole generates mainly horizontal shear(diagrammatically shown in Fig. 3). Iden-tification of the shear wave on the seis-mogram can be aided by a polarity rever-sal technique, i.e. reversal of the impactdirection through 180'ives a 180'hasereversal of the S-wave.

Surface seismic testing in the field pro-vides a quick, inexpensive method of ob-taining the dynamic elastic properties of asedimentary material. However, some

borehole work is essential to fully definesub-surface variations of moduli. Down-hole shooting requires one borehole andutilises a surface source and down-holereceivers. With such a technique intervalvelocities can be calculated over a chosendepth range. Cross-hole shooting necessi-tates the use of three boreholes for effi-cient working and employs a down-holesource and down-hole receivers. Collecteddata relate to a specific horizon withinwell-defined limits (Davis 8t Taylor Smith,1979),

Shear wave /

/I I(1 /rI/ Horizontal / r

//r'eismometers

/ /r/

/r

Shear

orizontaenergysource

Fig. 3. Source/receiver configuration forsurface shear-wave shooting with a hori-zontally polarising source

II-;;„::;:;:-:fi

86 c

97 3

Vertical impulse - 1 shot

I@I 5

152 5

163 6

17i 5

185 7

196 r

I 'ff IIff "i '

metres)

r86 c

108 3

119 6

130 6

Vertical impulse - 34 shots

I lsz s

163 6ill

174 5

f,!I185 7

196 C

IiItIIjIIIQ~'

IsJ

I

,li'

I

I: ll

i:rli

I:::(b)i

'ay,

1980 45

Fig. 4 (a & bj. Illustration of seismic recordimprovement using signal processing—P-wave shooting

Shot distance(metres)

86 4

97 3

108 3 unuur~u~v~, ~

130 6

141 5

152 5

I,

r

185 7

196 4

Fig. 5 (a 1').Record improvement by signal processing —S-wave shooting

Shot distance 1

(meires)Hortzontoi impulse - 65 shots

86 4

97 3

108 3

119 6

130 6

I r. I 5

152 5

163 6

174 5

185 7

st 196 4

Test procedure1. Preliminary survey

A preliminary survey was conducted onthe Oxford Clay at Fleet using the surfacerefraction technique. A series of reverse-line profiles were shot for both P and S-waves, up to one hundred metres spread,to examine the general subsurface geologyof the site. The maximum spread lengthwas determined by the energy distributionof the seismic wave generators and theprevailing 'noise'onditions.

A second intensive survey was planned

on the basis of data collected from thepreliminary work. This was to include fur-ther refraction work to demonstrate thecapabilities of the ABEM Signal AveragingSystem, down-hole shooting and cross-hole shooting. (Note: ABEM SAS was apre-production model being tested duringthe final stages of its development).

2. Main survey(a) Refraction shooting

Using the same energy sources and seismometers as in the initial surveys, it be-

came possible to extend the seismic lineto 200 metres by averaging a number ofstacked shots with the SAS, Previouslythe prevailing noise conditions had re-stricted the gain setting on the seismo-graph amplifiers such that beyond 100metres the seismic pulse was lost. How-ever, by recording and averaging multipleblows, up to 256, the gain levels could beincreased and seismic signals enhancedrelative to background noise.

For compressional wave measurementsthe string of 12 vertically sensitive seis-

—120

—100

Ei—60—

ShotDistance, mShot

Fig. 6. Travel-time graphs for surface refraction compressionalwave shooting

46 Ground Engineering

North South

~W120— ~e/e100—

~+ ~W

E 80— ~~, el —80Ql eMe

—60~~40- e< —40

We20— -20

p I I I I I I I I I I I I I I I I I I +II p

t20 40 60 80 100 120 140 160 180 200t

North500—

—400

Ei- 200—

Fig. 7. Travel-time graphs for surface refraction shearwave shooting

South—500~e

400— e~ ~~~PC'00—

~~ —300

100— +e —100

p I I I I I I I I I I I I I I I I I I I Xl

20 40 60 80 100 120 140 160 180 200t0

Impulse Distance, m Impulse

Horizontal shearsource

Tl toseismicrecorder

Receiver borehole I m pul se

Shothole

Trioseismicrecorder

Receiverborehole I

I

Receiverborehole 2

ole 3.componenteter positions

Fig, 8. Well-shooting with the Multipulse shear-waveenergy source

Down-hole 3-componentreceivers

Fig. 9. Cross-hole shear wave technique using an SPThammer source

mometers were spaced at approximately10 metre intervals and the line extendedby re-locating a number of the receiversfor further testing. With a similar spreadlength and geophone positions, shear wavetests were carried out using the MultipulseS-wave generator and 12 horizontallymounted seismometers,

For both P and S-wave tests signalswere averaged and played back on theseismic recorder at chosen intervals ofstacked shots to monitor the rate of im-provement in signal/noise ratio. A seriesof records is displayed in Figs. 4 and 5.Time-distance plots have been drawn forone seismic line and velocity variationswith depth computed (Figs. 6 and 7).(b j Down-hole shooting

A borehole was put down in the OxfordClay and a three-component geophonelowered down the hole and fixed againstthe clay wall, The geophone was fittedwith a clamping device (Scarascia et al.,1976) so that a good coupling could bemade between the instrument and themedium, A lowering system using inclino-meter tubes allowed the receiver to beprecisely orientated at depth, this beingcritical to the experimental accuracy,

With the geophone orientation known,the shear wave energy source was set upat the surface some distance from thehole and testing carried out at 3m inter-vals as the receiver was lowered downthe hole (Fig. 8). Observed travel timeswere corrected for actual slant distancesto produce vertical travel times from whichinterval velocities were calculated (Tel-ford et al., 1976), Tests were repeatedusing a compressional wave source at thesurface, the P-wave signal being recordedon the vertical component of the down-hole receiver. The results are summarisedin Table I.(cj Cross-hole shooting

A technique was devised to generatevertical shear at depth down a borehole.For this purpose a standard penetrationtest (SPT) hammer was used (Stokoe 8tAbdel-razzak, 1975) and energy transferredto the base of the borehole by means ofa series of drill rods (Fig. 9). Vertical shearwaves propagating in a horizontal layerwere detected by two receivers placed in

adjacent boreholes at the same depth asthe energy source (Fig, 10). Velocitieswere computed from delay-times betweenreceivers, By increasing the source and

receiver depths in incremental steps, ashear wave velocity profile was drawn. Asmall amount of compressional waveenergy was also generated along the seis-mic line from which compressional wavevelocities were obtained. The results arepresented in Table II.

DiscussionData from the preliminary survey and

main investigation illustrate well the ap-plication of signal processing in surfacerefraction shooting using small mechanicalenergy sources. The use of averaged mul-tiple blows enabled the spread length ofthe seismic line to be doubled, hence in-creasing wave penetration depth, Time-distance profiles indicate a large increasein velocity with depth for shear waves anda comparatively minor increase for P-waves; S-wave velocities range from220m/sec at the surface to 770m/sec atdepths in excess of 25m, whereas the vari-ation in P-wave velocity is only 60%. Thiscontrast is due to the sensitivity of shearwave propagation to sediment fabric vari-ations; increasing overburden pressureleads to a variation in rigidity of the totalsediment fabric. P-wave propagation how-

*i .l ! ti't!i t !tii»i.t; .!.t'it;, i i i ! o i 1 i l,!. l r " t i; 'i! . t,l t i r

tel�

!i, t t * I !to* it. t t tt, t i~w...xt c!i e v Ni » !.

:.„.!j;]jji!fiji!I

filf! fll

'ross - hole shooting.Spt down- hole source,

R'l *";:' tteceiver depth smetres.Receiver separation tt 7S metros.

I

l'ill iljll'i

'III

p

(,IIIii

!i;I;:I

ll i l

::;I

j

Fig. 10, Cross-hole seismogram recorded by two down-hole geophones

May, 1980 47

TABLE I. IN-SITU DOWN-HOLE MEASUREMENTS

Depth range,metres

V„m/sec

V„m/sec

EMN/mz

GMN/mz

0- 2.66

2.66 — 5.51

5.51 — 8.43

8.43—11.65

11.65—14.59

14.59—17.64

17.64—20.65

255

274

291

298

1273

1308

1437

1750

1750

1881

267

413

477

538

718

565

1315

0.4997

.4992

.4993

.4993

.4996

.4982

89

138

159

179

239

188

TABLE II. SUMMARY OF CROSS-HOLE DATA

Depth,metres

V,m/sec

Vm/sec

EMN/mz

GMN/mz

13

312

336

389

1750

1750

1750

619

718

0.4995

.4993

.4987

239

321

Depth,metres

S»m/sec

GMN/ms

Sim/sec

GMN/mz

4.9 228 110 125 33

7.8 333 235 182 70

11.2 375 298 192 78

14.3 311 205 171 62

S< ——horizontal shear wave velocityS, = vertical shear wave velocity

ever is primarily dependent on the porefluid and although useful in defining thedepth to the water-table has little appli-cation on its own in the evaluation ofdiscrete changes in elastic propertieswithin a given sediment body.

Down-hole and cross-hole shooting pro-vides a means for continuous logging ofdynamic elastic moduli with depth. Thecollected data relate to well-defined hori-zons within the medium and hence suchtechniques are advantageous for accuratelogging of subsurface layers.

Accurate dynamic and static testing onrecovered core samples can provide sup-plementary data relating to the field con-dition, Small-scale seismic shear wavestudies are proving to be of great interestin the analysis nf clay fabrics. The OxfordClay samples appear to be anisotropic in

terms of horizontal and vertical shear (seeTable III) .

ConclusionsShear wave propagation in shallow sedi-

mentary layers can be studied using thetechniques described. While the OxfordClay at Fleet provided a good site for anappraisal of new and improved systems,further measurements have successfullybeen made on a range of materials includ-ing loose sand and consolidated rocks. Theapplications of such work in both the ter-restial and marine field of foundation en-gineering are widespread. Development ofa sea-bed shear wave refraction system isunderway at the Marine Science I abora-tories, UCNW, and deployment of such

48 Ground Engineering

a system w;'ll contribute to our knowledgeof sea-floor sediment conditions.

AcknowledgementsThe authors would like to thank Atlas

Copco Craelius Ltd., for their help in thepreparation and execution of the drillingoperations and Atlas Copco ABEM for theloan of the Signal Averaging System. Thetest site was kindly made available for useby the Ministry of Defence. The shearwave research programme at UCNW ispartially supported by a contract from theEngineering Geology Unit, Institute of Geo-logical Sciences; the authors gratefullyacknowledge this assistance. The authorsare also grateful to Messrs. F. C. Dewesand D. Taylor Smith for their assistanceduring the field survey and with the pre-paration of this case history,

ReferencesDavis. A, (1977): "A technique for the in situmeasurement of shear wave velocity". ABEM CaseHistory No. 90180,Davis, A., si Taylor Smith, D. (1979): "Dynamicelastic moduli logging of foundation materials".Proc. Int. Conf. on Offshore Site Investigation,l.ondon, March.

Scarascia, S., Colombi, B., si Cassinis, R, (19'76)iSome experiments on transverse waves", Geo-

physical Prospecting 14, pp. 549-568.

Stokoe, K. H., 8i Abdef-razzak, K. G. (1975): Shearmodulus of two compacted fills". Proc. Conf lnSitu Mess. Soil Props. Raleigh, N. C ASCEpp. 422-449.

Taylor Smith, D. (1975): "Geophysical assessmentof sea-floor sediment properties". Oceanology In-ternational

Telford, W, M., Geldart, E. P., Sheriff, R, E,Keys, D. A. (1976I: Applied Geophys cs, Cam-bridge University Press.

TABLE III. LABORATORY SHEAR WAVE DATA FROM RECOVERED CORE SAMPLES

&.Trade Literature

Slurry Walls, by Petros P. Xanthakos. Pub-lished by Mcgraw-Hill Book Company,1979, in McGraw-Hill's series on ModernStructures. 622 pp; price US $34.50. Avail-able from McGraw-Hill Book Co. (UK)Ltd., Shoppenhanger's Road, Maidenhead,Berks., price f19.30.

This book, which has a Forward byProfessor R. B. Peck, assembles the theoryand practice of slurry walls related toconstruction into a form that can bereadily used by the practising engineer orconstructor. In so doing the author hasdrawn widely on a large number of refer-ences and reports which are availablethroughout the world. The author has as-sembled, evaluated and organised pre-sent knowledge about properties andfunctions of clay slurries, about the tech-nology of construction and the placementof concrete under slurry and about thestructural requirements and design ofwalls as well as the limitations of presentslurry wall construction practice.

Thus the user is provided with the tech-nical background against which to makejudgements about proposed methods ofuse. To achieve this the author has dividedthe text into 13 well illustrated and ref-erenced chapters. These take the readerthrough the following general subjects;

1. The state-of-the-art of slurry trenchuses throughout the world.

2. The stability of trenches and cutssupported by bentonite.

3. The interaction between the soil andthe slurry in the trench.

4. The technology, preparation and uses,and controls of slurried construction.

5. Methods of excavation and equip-ment available.

6. Slurry trench cut-off walls.7. Semi-rigid and rigid cut-off walls.8. The concrete technology and con-

struction9. Construction fundamentals of dia-

phragm walls.10. A review of various wall systems in

use.11. Load-bearing elements and founda-

tions.12. Uses and applications of slurry

trench walls in various civil engin-eering problems.

13. Topics relevant to analysis and de-sign.

This is a major work which has beenassembled over a period of years andthere is no doubt that it is the best textavailable on this general subject. It is writ-ten by a practising civil and structuralengineer who has paid equal attention todesign aspects, theory and constructionconsiderations and consequently it can berecommended without reservation to allengineers involved with underground en-gineering whether they be architects, con-sultants, contractors or owners of theutilities concerned.