Engineering geology in practice in Britain: 6- ieuseo ceooiysica--;=:docs inencineerinc ceo ocyPart 1:Seismic techniques

by A. N. BURTON", BSc,ARCS, FGS

THE SEISMIC METHOD of subsurfaceinvestigation consists essentially of intro-ducing seismic energy into the ground tobe investigated and measuring the timetaken for the energy to travel along adefined path through the ground to a seis-mic detector located a known distancefrom the energy source. The energy traveltime between source and detector is meas-ured to an accuracy of 1 milli-second orbetter, by a seismic recorder.

The principal seismic techniques in com-mon use at the present time are:—continuous seismic reflection profiling

for marine investigations, and—seismic refraction profiling for landinvestigations.

Continuous seismic refraction profiling isalso used in offshore investigations andseismic reflection profiling onshore, butthese applications tend to be more expen-sive although there may be occasions whenthe value of the information obtained mayjustify the greater expense.

In reflection profiling the time (t) re-quired for seismic energy to travel fromsource S to detector D along the path

soirector, Hunting Surveys Ltd., Sorehamwood,Herts.

shown in Fig. 1 is given by:

+ d'........(1)where V, = seismic velocity of the upper

layerx = the distance between source

and receiver andd = the thickness of the upper

layerSimilar expressions can be obtained for

cases with more than one refiecting boun-dary.

In refraction profiling the expression fort for the path shown in Fig. 2. is:

VoVr ) VrVo

where V, = the seismic velocity of thelower layer while the othersymbols have the same mean-ing as in equation (1) above.

The path shown in Fig. 2. is known asthe critical refraction path since the angleof incidence (ic) of the energy is such thatthe angle of refraction is a right angle.

vo

vi

Fig. 1. Diagram illustrating reflection ofseismic energy at subsurface interface

Continuous seismic reflectionprofiling

Numerous articles have been written oncontinuous seismic reflection profiling. Aselection of these publications is listedin the bibliography.

Seismic energy incident on an interfacebetween layers of two different materialsmay be either totally reflected, or partiallyreflected and partially transmitted. For nearvertical incidence of approximately planewavefronts the coefficient of reflection Rand the amplitude transmission coefficientT are given by:

Vo,

Vi8

Fig. 2. Diagram illustrating refraction of seismic energy at subsurface interface

32 Ground Engineering

Vrpr —Vrpr

V„p„+ V,p,

T = 2Vipr

V.ps+ V p

".(3)

".(4)

where V,, V„and p, p, are the seismicvelocities and densities of the two layet3.

The term Vp is known as the acousticimpedance of the material. R is negativewhen Vr» ) V,». Thus when energy isreflected from an interface between a denselayer overlying less dense material a 180phase change occurs.

In general, the depth of penetration of aseismic pulse into the seabed is inverselyproportional to the frequency of the pulse,while the sub-seabed layer resolution isdirectly proportional to frequency. Thus, asingle energy source may not be capableof both the depth of penetration and thelayer resolution required for a particularinvestigation. Other factors affecting theperformance of continuous seismic profil-ing (CSP) systems are power output andthe firing rate of the energy source.

A wide variety of CSP systems are com-mercially available for off-shore investi-gations. These systems have been designedto meet differing requirements, rangingfrom structural investigation of bedrock todepths of several hundred metres for atunnel project, to the detailed investigationof the upper few metres of the seabed fora submarine pipeline. A typical CSP record .is shown in Fig. 3.

The CSP systems currently available aresparkers, boomers, pingers and air guns.Sparkers produce a seismic shock wave inthe sea by the explosive formation ofsteam bubbles resulting from the dischargeof stored electrical energy between twoor more electrodes immersed in the sea.Boomers operate by the explosive repul-sion of a metal plate, spring-loaded againstan insulated coil, when stored electricalenergy is discharged through the coil.Pingers operate with an acoustic pulsegenerated by the oscillation of piezoelec-tric or maqneto-strictive transducersfollowing the discharge of electrical energyinto the transducer. Air guns produce ashock wave by the explosive release ofhigh pressure air from a pressure chamberimmersed in the sea.

Sparker and boom er energy sourcesdevelop a double pulse of energy unlesssteps are taken to suppress the second, so-called, "bubble" pulse. The latter is causedby the contraction and explosive re-expan-sion of bubbles of steam formed after theinitial energy discharge (see Sargent,

TABLE I. PERFORMANCE CHARACTERISTICS OF CSP SYSTEMS

(a) Sparkers

Descri ption Power output Firing rate(joules) (per second)

Frequency Pulse length Penetrationband (Hz) (milliseconds) (milliseconds) Remarks

Typesavailable

Heavy duty sparker 1 000-8 OXy'.25-4 50-1 000 8-20 )150 Penetration good in

nearly all geologicalenvironments and usefultherefore for investi-gating structure in

bedrock. Unsuitable fordetailed studies ofshallow sub-seabedgeology

—EGG Sparkarry—AquatronicsSuper Sparker

Sparker 200-1 000

Light duty sparker 20-200

2-10

2-20

200-2 000

50-10 000

5-10

3-5 Has been used success-fully in a wide varietyof surveys includingharbour and pipelineroute surveys. Equip-ment is compact andrequires relativelysmall power supplies

—Teledyne

—Huntec Sparker

Multi-electrode 200-1 000 2-4 200-1 000sparker

*Higher power outputs are not used, generally, for site investigations

2-4

(b) Boomers

50-150 Has comparable depthresolution to thelight duty sparker butgreater penetration

Boomer 0,25-3 200-3 000 3-5 )100 Depth resolution similar —EGGto that obtainable withthe light duty sparker,but penetration superior.Boomers have been used,therefore for similartypes of investigationsto sparkers but arepreferred for water depthsin excess of 80-100m.They have practical dis-advantages of being heavyand cumbersome andrequiring larger powersupplies.

Precision boomer 100-300 2-6 400-14 000 0.5-2 30-50 Combine good depthresolution and goodpenetration in mostgeological situations.

—Huntec—EGG Uniboom

Pinger

(c) Pingers

up to 20 1 000-12000 0.2-1 2-50 Depth resolutionexcellent but penetrationvaries in inverse ratio tothe grain size of the seabed sediments.

—EGG—ORE—Edo Micro-profiler—Sonia

Air gun 0.1-4 7-2 500

(d) Air gun

70-1 500 Capable of deep pene-tration in shallow tomoderate water depths,moderate penetration in

deep water.

—Bolt—PettyUnipulse—Birdwell

1969). Suppression of the "bubble" pulsereduces the power output and, hence, thedepth of penetration of the system, butreduces the pulse length and thus enhan-ces the layer resolution.

Pinger sources are pure tone sourcesand do not give rise to a "bubble" pulse.However, their use is limited by theirrelatively high frequencies, the lowest fre-quency attainable with these sources beingabout 1Kc/s.

The principal characteristics of some of

the better known CSP systems are listedin Table I. This gives a general guide to thecapabilities of the various systems, but itis advisable to consult an experienced firm

or consultant before selecting a system orcombination of systems for a particularinvestigation.

In considering the depth of penetrationof the various systems listed in Table I itshould be borne in mind that in practicethe depth to which CSP records can beinterpreted may be limited by the depth of

water. This is because multiple reflectionsof energy between the seabed and watersurface may mask useful reflections re-ceived from sub-seabed depths greaterthan the water depth.

Other limitations of the CSP methodare:(i) Penetration may be reduced or not

achieved at all in seabed soils with highacoustic impedance (e.g. very stiff or hardboulder clay) in deep water, and highlyorganic soils (for example, peat) in

January, 1976 33

=««J«r

5 III}}00

aIz

Fig. 3. Typical CSP profile

% . ~» "--0g, »S 5 I }y

~~~}at 55

00

5 i+I 5 II

150 1)555}

8z

-.===-'IIII

-'=:-=-~iINK),.is} 5 f}}sIQg ~J~i%%i( AM }li}5.~

f,}}Il:,II!}%-'I(xi

fj"..':::

any water depths.(ii) Rough sea conditions with high ambi-ent noise levels may make operation ofCSP systems impractical due to masking ofthe seismic signal by the noise. This prob-lem may be overcome to some extent bymounting the energy source/detector sys-tem on the hull of the vessel, beneath thezone of turbulence, as with the ORE pinger,or by deep towing of the system.

Refraction profilinPublished literature on the seismic refrac-

tion method is even more abundant thanthat on the CSP method. However, anexcellent review article published recently(Green, 1S74) provides a very useful guideto the literature and the present stage ofdevelopment of the method.

Rocks and soils normally transmit twomain types of seismic shock wave, namelyshear (S) waves and longitudinal (P)waves. The relationships between thevelocities of shear and longitudinal waves(denoted V, and V, respectively) and theelastic moduli of the material (assumed tobe isotropic) are:

V„= —;V, =,.„(5g

fp3 gm ~ 'ts- ~"A"-'k}}w'll

@, '!r,00''.00,.„".0I„.: ' } 3ri@9$

Fig. 4. Typical 12-channel seismic refraction record

, 5,, @AC.~%w,'f'I:XL':,'L5%,.'*)B5}h0 '.<

aECC~%a, =,:-'4 =:--;8::}}-0}." .~f-',=';":z..-J-

..;-'I},

)~J I /:" ('JJI '.-, '

I

where n = the shear modulus of thematerial

K = the bulk modulus of thematerial

p——the bulk density of the material

Clearly V, is greater than V, and, there-fore. P-waves are generally the first toarrive at the seismic detector(s), and, withmost recording equipments currently usedin site investigations, they mask subse-quent arrivals. A typical seismic refractionrecord, consisting of P-waves, obtainedusing a 12-channel equipment, is shown inFig. 4.

Shear wave detectors are available, andare used where measurements of shearmodulus and Poissons Ratio are required.Since the shear modulus of fluids is zero,

TABLE II. PERFORMANCE CHARACTERISTICS OF REFRACTION PROFILING SYSTEMS

Instrument Manufacturer Recordingchannels

Recordingspeed Gains

RS-4 Dresser Sie 12 Standard 11.6in/sec Adjustable fromMax 23.2in/sec 100 to 6 000 in

ten 6 db steps1-10db vernier

Filters AGC Blaster Timing lines

Integral 10 millisecs

RS-44 Dresser Sie

TRIO ABEM

TRIO ABEM

SIGNAL BisonENHANCE- InstrumentsMENT 1570

24 Standard 11.6in/secMax 23.2in/sec

12 Controlled:—50, 100, 200cm/secUncontrolled3-4m/sec

24 ditto

SINGLE Sweep time inmilliseconds;25, 50, 100, 250500 switched

ditto

5 to 1 500adjustable in8 steps

ditto

Geophone GainControlAdjustable fromOto10

4 positionsOff—No filter16 Hz35 Hz75 Hz

2 AGCpositionsHigh and lowsensitivity

ditto

Separate 10 millisecs

Separate 2 millisecs

Separate 2 millisecs

Separate Digital timemeasure-ments todesiredpoints onseismicwave form

ER 75 Electrotech 12 Recording time 2 0000.2-0.4secsadjustable

Integral

34 Ground Engineering

S-waves are not transmitted by water.Energy sources used in seismic refrac-

tion profiling must be able to producesufficient energy at a frequency whichmatches the band-pass frequency of thedetecting and recording equipment. Inpractice the two energy sources commonlyused are gelignite explosions and the im-pact of a dropping weight, or sledge ham-mer blow on a steel plate embedded in

the ground. The latter source is used wheredepth of investigation is not greater thanabout 10m, the former for deeper investi-gations. A dropping weight device is illus-trated in Fig. 5.

At any single firing of the energy sourcethe proportion of the released energytransmitted downwards into the groundunder investigation determines the depthof investigation achieved. Burying an ex-plosive charge in the ground and tampingis advantageous, but if the surface layer iscomposed of loose unconsolidated materialor peat it may be necessary to insert thegelignite charges below the surface layerto obtain the required penetration.

The characteristics of some of therefraction seismic systems commerciallyavailable at the present time are listed inTable II. A problem affecting the perform-ance of all systems is that cultural andatmospheric noise occupies the same fre-quency band as the seismic signal and maymask the latter. This problem has beenovercome by the integrating seismographdeveloped by Bison Instruments Inc.,which adds the signals received fromrepeated firings of the energy source ateach location. This reinforces the seismicsignal because it is in-phase, whereas therandom noise tends to cancel out.

The seismic refraction method will givereliable results providing the followingconditions are satisfied:(i) The seismic velocity of successivelydeeper subsurface layers increases withdepth below ground level.(ii) The velocity to thickness ratio of eachsubsurface layer is less than a certain criti-cal value in relation to the overlying andunderlying layers.(iii) The seismic velocities of the subsur-face layers are effectively constant overthe length of the geophone spread.

If condition (i) is not fulfilled, and a lowvelocity layer is present, critical refractiondoes not occur. The seismic energy is

refracted towards the normal to the inter-face and passes downwards into a highervelocity layer before being returned to thesurface by critical refraction. Thus, the lowvelocity layer, called a "blind-zone", is notdetected, and consequently the calculateddepths of the deeper refractors will begreater than the true depths.

If condition (ii) is not satisfied a thinlayer will not be detected because theenergy refracted by it will be masked byenergy refracted by the underlying highervelocity layer. This is known as the "hid-den-layer" effect, and results in the cal-culated depths of the deeper refractorsbeing less than the true depths.

Condition (iii) is probably fulfilled lessfrequently than the other two particularlyin the case of superficial deposits and isprobably the most frequent cause of depthdetermination errors in the interpretationof seismic refraction records. Velocity vari-ations in the bedrock are less serious inthat they affect depth calculations to asmaller degree than the velocities of over-lying layers and are usually detectable.

These limitations indicate the importanceof boreholes to control the interpretationof the geophysical data, not only for corre-lation of seismic velocities with geologicalstrata, but to check whether the conditionsmentioned above are satisfied.

Tidal zone investigationsSeismic investigations in the tidal zone,

between high and low water, are carriedout by a combination of CSP and seismicrefraction methods. The CSP method isused, at times of high water, to obtaininformation as far inshore as the vesseland conditions will allow; the refractionprofiles are carried as far offshore as possi-ble during the low water periods. The zoneof overlap between the two methodsshould be of the order of 500m, if possible,to ensure a good correlation between thetwo methods.

Interpretation of seismic recordsInterpretation of seismic records is very

much a question of the skill and experienceof the interpreter. The latter should be aqualified geologist as well as a geophysicistsince the end-product of the interpretationis essentially a geological profile. A know-ledge of the engineering classification ofsoil and rock materials is also useful in

applying the results of correlation bore-holes to the interpretation of seismic sur-vey work.

Seismic velocities and hence the thick-nesses of sub-seabed layers cannot bedetermined directly from CSP recordsobtained using an energy source and onedetector with fixed separation (see equa-tion (1) above). Seismic velocities aredetermined from correlation boreholes orby the seismic refraction method.

The sub-surface layer resolution whichan experienced interpreter can derive fromsparker and boomer CSP records is oftenconsiderably better than the figures forpulse length given in Table I would indi-cate. This is due to the fact that signifi-cant reflections frequently can be followedthrough the "bubble" pulse trace on theCSP record (see Sargent, 1969). Thus,layer resolutions of 3 milliseconds for theEGG Sparkarray and 1 millisecond for theHuntec sparker are obtained in practice.

Precision boomers, which are designedto remove the "bubble" pulse by operatingnear the sea surface, may give a layerresolution better than 0.4 milliseconds infavourable conditions.

The identification of phase reversedsignals received from interfaces betweena dense layer and underlying less densematerial is possible by an experiencedinterpreter when the energy pulse isasymmetric and half wave rectified (seeGauss, 1970). This is of considerableimportance in site investigations for thefoundations of offshore structures, and fordredging feasibility investigations.

In refraction work the seismic velocitiesof the subsurface layers and layer thick-nesses can be determined directly from theseismic data (see equation 2 above). V,and V, in equation (2) can be determinedby plotting a graph of t against x (seeFig. 6). The diagram in Fig. 6 shows theresults of four firings or shots of the energysource. Shots made close to the end geo-phones of the spread (end-shots) producethe two profiles composed of two linearsegments representing different seismicvelocities. The V, segment represents thedirect wave travelling in the surface layer,while the V, segment represents theenergy refracted from the lower layer. Thethickness d of the surface layer can becalculated either in terms of the time inter-cept t, and the velocities V, and V,, or in

I IIJ, 4I

Fig. 5. Drop-weight devicein operating position behind Land Rover

January, 1976 35

Xc DISTANCE

terms of the critical distance xo and thesevelocities. The respective formulae aregiven below:

V,V,

2 (Vr' Vo'j~""(6)

«,(v, —v.)< ""(7)

The purpose of firing from each end ofthe spread is to take account of any dipof the lower layer.

The time/distance lines for the shotsbeyond the ends of the spread (out-shots)show a single velocity V„ indicating thatall the first arrivals from these shots trav-elled in the lower layer. This enables thevelocity of the latter to be determinedmore accurately and confirms that it is atrue refracting layer. If the change in slopebetween V, and V, for the end-shots hadbeen caused by a 'lateral change in velocity,then a similar time-distance graph wouldhave resulted from both the end andout-shots.

Equation (6) above enables a depthdetermination to be made at the end ofeach geophone spread in terms of thehalf-time intercept t,. It is sometimes possi-

2ble to calculate the effective value of thehalf-time intercept under each geophoneso enabling depth calculations to be madealong the entire length of the spread(Hawkins 1961).

The theory outlined above can be exten-ded to three or more layers for both hori-zontal and dipping strata.

Measurements of the compressionalwave velocity (V„) can be used to derivevalues for the dynamic modulus of elasti-city of subsurface layers. Brown andRobertshaw (1953) obtained the followingempirical expression for Youngs Modulus:

E = V,' 10-'b/in'-. (6)

This relationship appears to be valid forvalues of V, greater than 3km/sec and formaterials in the density range 2.2 to 2.6.

36 Ground Engineering

V, is also related to rock quality. Deereet al (1967) have shown that:

(s

x 100 R.Q.D. (9)V, Laboratory

This relationship is used in practice toestimate the rippability of sub-surface rockformations.

Shear wave velocity (V,) measurementscan be used to derive the dynamic shearmodulus of sub-surface materials from therelationship:

n = Vstp ............(10)(see equation (5) above)

Equations (8), (9) and (10) should onlybe used with caution and due regard totheir inherent limitations, but providedproper control is exercised, useful infor-mation can be obtained.

Seismic techniques in siteinvestigation

Seismic techniques are used to a muchgreater extent in site investigations thanany other geophysical methods. Offshorea sub-bottom CSP survey, combined witha side scan sonar and depth soundingsurvey all carried out at the same time, isalmost standard procedure at the presenttime. Onshore, seismic refraction is notemployed so extensively. This differencein utilisation is due mainly to economicfactors. Offshore, the costs of obtainingsub-seabed information by drilling andcoring are so high that 8 seismic surveypays for itself many times over by provid-ing the information required for planningan economical drilling and coring pro-gramme. Onshore, although the seismicmethod is capable of performing the samerole, the cost advantages are far less obvi-ous, since onshore drilling is less expensivethan offshore drilling, and land seismicsurvey costs more than marine seismicsurvey per unit length of line surveyed.

The role of seismic methods in siteinvestigations generally can be summarisedbrieRy as follows:—(i) At the preliminary investigation stage—to determine as accurately as possible,in conjunction with correlation boreholes

Fig. 6. Diagram illustrating time-distance plots for seismic energy refracted from asubsurface layer

or seabed cores, the geological stratapresent on the site and their depths.(ii) At the detailed investigation stage-to check the continuity of strata betweenboreholes; determine the dynamic moduliof sub-seabed or sub-surface materials;estimate the rippability of hard strata;locate the position of geological faults andother bedrock structures.

A seismic survey carried out as part ofa preliminary site investigation providesbasic information on the geology of thesite, such as depth to bedrock. This infor-mation can be used for planning thedetailed site investigation, by drilling andother direct methods, to give the bestresults for the least expenditure.

During the detailed investigation, theinterpretation of the seismic work carriedout during the preliminary stage should becontinuously up-dated to take account ofthe new borehole information available.

In view of this it is advisable to retainthe services of the geophysicist employedon the preliminary stage survey during thedetailed investigation stage.

Future developmentsIn the marine field the major develop-

ments in progress at present are towardsoperations in deeper water. The attenua-tion of the seismic signals by a deep watercolumn limits the use of present surfaceor near-surface towed systems to waterdepths of about 150m. To overcome thisproblem deep towed sparker and boomersystems have been developed and trialswere carried out in the North Sealast year. However, there is a practicallimit to the depth of water in which sys-tems towed behind a surface vessel can beemployed to survey the seabed. When thislimit is reached seabed survey work willhave to be carried out from submersibles.

In shallower water, improvements inexisting instrumentation can be expectedto continue both as regards data collectionand data processing. These improvementsshould lead to improved results from seis-mic surveys for harbour and other shallowwater investigations.

Onshore also, instrumentation is beingsteadily improved, but probably thegreatest scope for advance in this area isin the way seismic techniques are em-ployed in site investigations. A recent arti-cle (West and Dumbleton, 1975) conclu-ded that the best way of using geophysicsin site investigation was to integrate itfully with other methods. If this advice isfollowed by engineers it is considered thatthe resulting improvement in the value ofthe data obtained from the geophysicalwork would be increased considerably.

ReferencesBrown. P. D. and Roberrshaw, J. (1953): "Thein-situ measurement of Youngs Modulus for rockby a dynamic method" Geotechnique 3.7.283.Deere et al (1967): "Design of surface and nearsurface construction in rock" Proc. 8th SymposiumRock Mechanics. Minnesota 237-302.Gauss, G. A. (1970): "Acoustic techniques forgeorogicar studies with particular reference todredgmg problems" Norspec 70.Green, R. (1974/i "The seismic refraction method—a review" Geoexploration 12, 259-284.Hawklns. L. V. (1961): "The reciprocal method ofroutine shallow seismic refraction investigations"Geophysics 26, 6. 806-819.Kelland, N. C. (1970): "Improved interpretationusing a dual channel continuous seismic profilingsystem" European Association of ExplorationGeophysicists.Safgenr, G. E. G. (1969): "Further notes on theapplication of sonic techniques to submarine geo-logical investigations" Ninth CommonwealthMining and Metallurgical Congress.West and Dumbleton (1975}: "An assessment ofgeophysics in site investigation for roads lnBritain" Transport and Road Research LaboratoryReport 680.