Polom Etal SEGbook 2010

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Chapter 18

High-resolution SH-wave Seismic Reection for Characterizationof Onshore Ground Conditions in the Trondheim Harbor,Central Norway

Ulrich Polom 1 , Louise Hansen 2, 3 , Guillaume Sauvin 2 , Jean-Se bastien LHeureux 2, 3 ,Isabelle Lecomte 2, 4 , Charlotte M. Krawczyk 1 , Maarten Vanneste 2, 5 , and Oddvar Longva 2, 3

AbstractThe area around Trondheim Bay in central Norway is

affected by landslides, both onshore and within the fjord,with several events documented to have occurred in thelast century. As urban development, including land recla-mation, is taking place in the harbor, assessing in situ soilconditions is paramount for infrastructure and operationalsafety. To obtain better insight into the harbor setting interms of subsurface structures and potential coastal geo-hazards, a high-resolution multichannel SH-wave seismic-reection land survey was carried out during summer2008, which complements a dense network of high-resolu-tion, single-channel marine seismic proles over the deltaicsediments in the fjord. The SH-wave seismic reection waschosen because the resulting interval shear-wave velocityprovides a nearly direct proxy for in situ soil stiffness, akey geotechnical parameter. In total, 4.2 km of 2.5D SH-wave proles was acquired along roads and parking places.Highly resolved images of the sediments were obtained,overlying the bedrock at a depth of about 150 m. The highquality of the data is ascribed to the quieter ambient noise

conditions of the nighttime data collection and an efcientsuppression of Love waves arising from the presence of a high-velocity layer at the surface. Five main stratigraph-ic units were identied based on reection patterns andamplitudes. Distinct SH-wave reection events enabled

detailed S-wave velocity determination down to the bed-rock. Subsequently, interval velocities were remapped in-to soil stiffness. Low S-wave velocities of about 100 m / soccurring in the upper 50 m of the fjord-deltaic sedimentsuccession suggest low sediment stiffness (50 to 100 MPa)directly below the stiffer man-made ll that is 10 to 15 mthick. The results indicate that SH-wave seismic reectionis well suited for urban ground investigation.

IntroductionA good understanding of soil conditions is a prereq-

uisite for safe development of urban areas. This is parti-cularly true for areas prone to landslides or other criticalground movements. Traditionally, geotechnical investi-gations are performed through invasive techniques suchas drillings or in situ tests, after which soil properties areinterpolated between the individual drill holes. Geophys-ical techniques, such as P-wave seismic reection and elec-tromagnetic and electric methods, are useful to outline thespatial variability of sediments and their history. However,they do not provide soil parameters for geotechnical appli-

cations. In addition, their resolution in the upper soil unitsis often relatively poor, and their use in urban areas mightbe hindered by various noise sources.

In contrast, a range of geophysical techniques directlyprovides S-wave velocity, an elastic parameter related to

1 Leibniz Institute for Applied Geophysics (LIAG), Hannover, Germany.2 International Center for Geohazards (ICG), Oslo, Norway.3 Geological Survey of Norway (NGU), Trondheim, Norway.4 NORSAR, Kjeller, Norway.5 Norwegian Geotechnical Institute (NGI), Oslo, Norway.

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investigations were carried out in the fjord, essentiallythrough a combination of sediment core analysis and veryhigh-resolution marine seismic proling (LHeureux, 2009).

Onshore investigations in Trondheims harbor werelimited to sparse geotechnical drillings, laboratory testing,and some early geophysical measurements (gravimetry).However, none of the previously acquired geophysicaldata provided relevant and detailed information for geo-technical purposes. Therefore, we started to consider acqui-sition of SH-wave seismic-reection data to improve theonshore data coverage. During a preliminary site investi-gation in 2007, some 50-MHz ground-penetrating radar(GPR) proles were acquired in an attempt to map thetop part of the ground, i.e., the man-made ll and a fewmeters below. However, penetration was 5 to 10 m (allow-ing the bottom of the man-made ll to possibly be identiedat some locations), and the data were contaminated heavily

by air diffractions caused by buildings, trucks, and so on(Sauvin, 2009). Spectral analysis of Rayleigh waves froma small P-wave refraction seismic test (hammer blow and24 channels) revealed low to very low S-wave velocities.Hence, the site conditions for high-resolution imagingand S-wave velocity mapping using the planned SH-waveseismic-reectiondatawereconsideredappropriate. Acous-tic noise estimation in the harbor showed that tide noisecould be ltered out easily. However, seismic data couldbe acquired only without noise from the nearby trainstation, boats, and trucks / cars in the harbor area.

The SH-wave seismic-reection survey was carried out

in 2008, using an SH-wave vibrator and a landstreamersystem developed at the Leibnitz Institute for AppliedGeophysics (LIAG) in Germany. The goals of the SH-wave seismic investigation were to obtain high-resolutioninformation on the internal structure of the sediments, tomap the depth to bedrock, and to determine elastic prop-

erties (i.e., S-wave velocity) of the sediments as a keyproxy for their stiffness. Our objective is to present theacquisition technology and seismic poststack results of this SH-wave seismic-reection survey, including 2D depthsections and S-wave velocity proles down to the bedrock.We illustrate the ability and importance of this type of seismic surveying method in urban areas for the integrationof geologic and geotechnical site characterization.

Geologic SettingBedrock around Trondheim is dominated by westward-

dipping low-grade metamorphic, volcanic rocks of Precam-brian-Silurian age (Wolff, 1979). It is exposed locally onland and at the seabed. During repeated glaciations, bed-rock was eroded along weakness zones, shaping the land-

scape of valleys and fjords, including the present valleyat Trondheim.In the bay of Trondheim, core data combined with

high-resolution seismic proles show that a 125-m-thick succession of fjord-deltaic deposits on top of marine andglaciomarine clays covers the bedrock (Figure 2). The gla-ciomarine and marine sediments were deposited during andafter the end of the last glaciation (Reite, 1995; Rise et al.,2006; Lysa et al., 2007). After deglaciation, the area wassubject to glacioisostatic rebound, causing rapid fall of rela-tive sea level from a maximum of 175 m above sea level atTrondheim. As a consequence, glaciomarine deposits be-

came exposed locally above sea level, and the river outletin the fjord moved continuously northward in phase withHolocene delta progradation (Reite, 1995; LHeureuxet al., 2009). The shallow near-shore areas were reclaimedfrom the sea during urbanization in the last century(see Figure 1). Landslide activity in Trondheim Bay is

Figure 2. High-resolution offshoreP-wave seismic section (subbottomproling; see location in Figure 1).Modied from LHeureux et al., 2010.

Used by permission.

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witnessed by distinct escarpments of both prehistoric andknown historical age (Figure 1). Near-shore landslidingstarted with translational failure along specic weak layers(LHeureux, 2009; LHeureux et al., 2010).

MethodData acquisition was carried out using the vibroseis

method (Crawford et al., 1960), which is common prac-tice in onshore hydrocarbon exploration in which explosivecharges cannot be used. The advantages of this method are(1) control of the resulting source signal in terms of frequency bandwidth and energy, (2) control of the initialenergy applied to the subsurface to prevent damage of the

paved surface or buried structures, and (3) excellent repeat-ability of the seismic source signal. However, this methodrequires a sophisticated control of the vibratory sourceand the use of a recording system that can perform vibro-seis-correlation processing for eld quality control. Tolower the initial energy applied, the total amount of sourcesignal energy is stretched in time. Therefore, the recordingtime for each seismic record is several times longer thanrequired for impulsive seismic sources.

The S-wave source was combined with an S-wavereceiver-array system (landstreamer) designed for oper-ation on paved or compacted surfaces. Extension of theapplication toward SH-waves minimizes body-wave-typeconversions between P- and S-waves. Furthermore, thiswave type is decoupled widely from Rayleigh surfacewaves (ground roll). However, operating in SH-wavemode introduces another surface-wave type, i.e., Love

waves. In contrast to Rayleigh waves, Love-wave propa-gation requires a low-velocity layer at the surface above ahalf-space of higher velocity. In such cases, Love wavesintroduce strong distortions of the reected waves alongthe receiver spread. This complicates S-wave reectionanalysis because Love-wave velocities fall in a similarrange as body S-wave velocities in the shallow subsurface.In contrast, the propagation of Love waves is hindered bya high-velocity surface layer above a half-space of lowervelocity. Typically, this is found for, e.g., all-weatherroads constructed above soft soil, even if the surface is notpaved. Such a high-velocity layer is present in the area of

Trondheim Harbor, i.e., the man-made lls on fjord-deltaicsediments. Therefore, the area is well suited for SH-waveinvestigations using a modied landstreamer system.

The S-wave source

The S-wave source consists of a hydraulically drivenshaker unit installed below the modied frame of asmall truck originally designed for urban application(Figure 3). The seismic-source concept was developed byLIAG (Hannover), Kiefer GmbH (Dorfen), and Prakla-Bohrtechnik GmbH (Peine). The source system was

designed for shallow high-resolution applications and tomeet the technical requirements of the European Union(EU) concerning environmental safety and trafc require-ments. Beyond the integration of a S-wave vibrator unit,the buggy is designed conceptually for integration of avertical vibrator as well. The most important technicaldata on the source system are summarized in Table 1.

Mechanical vibrations of the shaker unit are decou-pled from the truck in three dimensions by an array of eight air-bag devices. The typical shaking orientation is

Figure 3. MHV4S hydraulic S-wave vibrator in operation.

(a) The shaker system is included in a light-weight baseplateof tensile-strength aluminum alloy mounted below a specialtruck frame. The hold-down force to support ground couplingby the truck weight was balanced to the best t of 3800 kg atthe center of the shaker unit by optimizing the truck framedesign. Photograph courtesy of L. Hansen. (b) S-wavelandstreamer during data acquisition on a detached mode inthe Trondheim Harbor area. Arrows indicate the orientation of the SH-wave particle motion of source (white arrow closestto the land streamer) and receivers (black arrow near bottomof image). Photograph courtesy of U. Polom.

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perpendicular to the frame of the truck, to generate SH-waves. A hydraulically driven rotation mechanism allowschanging to vertically polarized (SV) mode or other

desired horizontal-shaking polarization if required. Thisadjustment can be made by rotating the shaker masswithin the inner frame of the baseplate. After rotation, ahydraulic coupling mechanism leads to a force-lockedcontact of the shaking mass and the baseplate unit. Thisconstruction principle enables a low center of gravity of the whole unit to reduce tilt moving during shaking oper-ation. Ground coupling is achieved by an easily exchange-able plate below the shaker casing, attached with smallspikes, rubber pads, or similar friction-enhancement ma-terial, depending on specic surface conditions. For atasphalt surfaces in the area of Trondheim Harbor, a

rubber pad was preferred to prevent damage to the surface.A Pelton VibPro vibrator-control system was used

for radio-controlled operation, sweep-signal generation,hydraulic-shaker control, and phase locking. To obtainoptimal results and signal-to-noise ratio (S / N), numerousadjustments of the control system were carried out priorto the surveying. Ultimately, two 10-s sweep signals wereused at each source location, differing only by a 180 8

phase shift (i.e., opposite polarity) to enable the plus-minus operation and subtractive-stacking method for the

enhancement of S-waves (Polom, 2005a, 2005b). Frequen-cies of the upsweep ranged from 25 to 100 Hz, with 100-mstapers at either end, after several test sequences with the

intention to record a distinct response from the bedrock and to minimize airwave reections from surroundingbuildings. The pilot-sweep signal was recorded in geo-phone channel 121.

The receiver system

The landstreamer consists of 120 single geophone unitsat 1-m intervals (split into ve subsets with 24 channels).The ground coupling of the landstreamer geophones isachieved solely by gravitation force using a static, stable

three-point contact, attached by tough metal vats for roadapplication or plastic vats if damage to the road needs tobe prevented. The streamer unit was reeled on a specialtrailer attached with an electrically driven reeling unit fora fast roll-on, roll-off application. The horizontal geo-phones were oriented perpendicular to the proling di-rection (pure SH mode). During data acquisition, thelandstreamer was attached to a Geometrics Geode systemincluded in the tracking and recording car. The samplingrate was 1 kHz.

Table 1. Technical data of the MHV4S shear-wave vibrator.

Buggy unit Oscillation generator

Two-stage hydraulic four-wheel drive Full hydraulic drive

Four-cylinder diesel injection (IVECO) Hydraulic suspended reaction mass

2800 ccm, 92 kW at 3600 rpm, 290-Nmtorque at 1800 rpm

Hydraulic rotation and lock device

Meets Euro 3 emission requirements,additional cleaning by particle lter

Reaction mass weight 240 kg

Climbing performance 45% Oscillation house weight 160 kg, castfrom special aluminium alloy

Top speed 62 km / h Baseplate diameter 1200 mm, quick-change ground-coupling pads

Tandem pump 2 70 l/ min (drive),117 l/ min at Pmax 280 bar (shaking)

Theoretical peak force 30 kN

Full biodegradable hydraulic uid Frequency range 16 to 300 HzOffroad tires 315 / 55-16 3D vibrational decoupling

Rollover protection frame Servo valve: Mannesmann Rexroth

Length 4.5 m, width 1.5 m, height 2.12 m Drip-free quick-change clutch

Oscillator control: Pelton VibPro



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Seismic eld operation

To minimize ambient noise level and for crew safety,the seismic experiment was conducted only at night(11:30 P .M. to 5:00 A.M ) during 10 nights, 19 June to 30June 2008. Eleven 2D proles were acquired along roadsand car parks with a total length of 4.2 km (Figure 4).Local companies and the public authorities granted per-mission for eldwork and periodic road closures, eitherfor the whole harbor area or in the immediate vicinity of the survey. Detailed planning led to efcient acquisitionwith little downtime, e.g., for railway trafc or badweather (one night went without acquisition because of strong wind and heavy rain).

The general seismicacquisition parameters usedare listed in Table 2. Seis-

mic data were recorded andstoreduncorrelatedtoenableediting and ne-tuning priorto full-scale processing. Theoperator carried out quali-ty control during surveyingusing the display-correla-tion adjustment of theGeodesystem. In addition, seismicdata-quality control of eachproduction night was doneduring the day using theGEDCO-VISTA processingsystem to validate or mod-ify recording parameters foroptimal seismic-reectionprocessing.

Independent handlingof source and receiver units

allowsmuch more variability in thespreadlayout, includingthe undershooting of problematic zones (e.g., buildings,gateways). This is in contrast to the most often usedxedcon-guration, in which the landstreamer is attached to the source(e.g., Pugin, 2007). In general, the spread congurationshownin Figure 5 was used. The landstreamer move-up was doneonly after 40 m of source move-up by an ingoingand outgoingrampof80 m each. Thereby, the acquisitionschemeresultsin azigzag-formed CMP coverage after data processing.

The positions of every fortieth receiver and offset shotwere marked on the pavement during the survey. Unfortu-nately, GPS positioning did not work in the area, possiblybecause of electromagnetic noise. Therefore, geodetic survey-ing was carried out during the day, using a terrestrial-based

Figure 4. Prole location (white lines).(a) Side view showing the coastline of Trondheim and the fjord-reclaimedharbor. Photograph courtesy of Trondheim Port Authority. Used bypermission. (b) View above the site

with indication of the prole numbers(white text). Black triangles indicate thelocation of two CPTu piezocone tests,and the gray circle represents a referenceposition along prole 4 to be used later.Orthophoto source # NorwegianMapping Authority. Used by permission.

Table 2. Acquisition parameters of SH-wave proles after initial parameter tests.

Period 1930 June 2008, 10 nights of data acquisition

Instrument Geometrics GeodeChannels per record 120 1 auxiliary

Seismic source LIAG MHV4S shear-wave vibrator, adjusted to 60% peak forcefor friction contact by rubber pad

Sweep type 25- to 100-Hz linear, 10 s, 10-ms taper at front and end

Recording 14 s, 4 s after correlation

Sampling interval 1 ms

Recording lter Out

Spread type 2D variable-split spread, SH-SH conguration, roll-on operation

using a 40-m streamer shift intervalGeophone type SM6 H (10 Hz), single units attached to landstreamer unit

Receiver interval 1 m

Source interval 4 m

Vertical stack Twofold [ Y][Y] alternated vibrations, stored separately



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Trimble S6 DR300 system referenced to three topographicpoints (TP).

Processing

The general processing scheme of SH-SH reections issimilar to conventional processing of onshore P-P seismic-reection data (Kurahashi and Inazaki, 2006; Pugin et al.,2006). Examples of on-site preprocessed single shot seriesfrom prole 4 (location in Figure 4) are shown in Figure 6.The source-receiver conguration used for the SH-wave

survey in the harbor yields little conversion to P- andSV-waves and surface waves. The inverse velocity contrastfrom the surface pavement and man-made lls on top of softer soils in the Trondheim Harbor area also suppressesthe generation of Love waves and refracted SH-waves.However, Figure 6d shows some refractionlike waves(red line; velocity of about 1000 m / s) and surface waves(green line; velocity of about 100 m / s). The former couldcorrespond to S-to-P converted waves, especially if hetero-geneities are below the source; the latter are probably weak channeled Love waves traveling in the low-velocity zone

Figure 5. General source-to-receiverspread scheme used for operation withdecoupled source and receiver units.This spread enables high exibility indense building and trafc areas.

Figure 6. Shot-record examples of prole 4 at the location given inFigure 2b. The preprocessing applied onsite consisted of vibroseis correlation,stacking of the opposite-polarity records,300-ms AGC, and no lters.



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below the pavement. Source airwave attenuation, com-monly used in P-wave seismic-processing sequences to re-duce source noise, was found to be unnecessary becauseSH-SH source-and-receiver conguration is affected lessby direct airwaves than are common P-P source-and-recei-ver congurations. Some coherent noise remaining in thedata (e.g., black arrow in Figure 6), occurring before andafter the rst arrivals, is explained by ringing because of crosscorrelation with the sweep signal and is emphasized

further by amplitude scaling. This noise and the refraction-like arrivals are eliminated later by muting. The Lovechannel waves, although weak, are removed efciently by f-k lters.

During data acquisition, a preliminary processing se-quence using only raw geometry parameters was developedto perform on-site quality control and obtain an initial ideaof subsurface structure and S-wave velocity distribution.This sequence allows for better proling control in the area.Next, the preliminary processing sequence was extended ina rst run conducted after the survey, using crooked-linegeometryandanamplitude-preserving processing sequence,and was applied to all proles, followed by a rst combinedinterpretation (Polom et al., 2008b, 2009a; Polom et al.,2009b). The high-quality, well-resolved structure observedon the processed data led to further enhanced imaging of the shallow parts, i.e., the actual targets for geotechnical

applications (LHeureux, 2009). Then the processing se-quence also was reviewed and extended, and parametersettings were ne-tuned (Figure 7).

In addition, reviewing and testing of each individualprocessing parameter were carried out carefully for eachprole individually, to obtain optimal results in ampli-tude-preserving processing and to derive the best possiblesubsurface S-wave velocity information. The most impor-tant aspects of the processing are outlined below. Forfurther details and proles illustrating the results of all pro-cessing steps performed in GEDCO-VISTA, see Sauvin(2009). Prole 4 of Sauvin (2009) is used in the following

to illustrate the major results.

Vibroseis signal contraction

Because the data were stored as uncorrelated timeseries, precorrelation data-quality analyses were perform-ed. Time-frequency domain analysis (Gabor or Fouriertransform) shows generally good coherency of transmittedsignals and low noise levels with few spike and burst distor-tions. The amplitudes of harmonics, caused by nonidealvibrator-to-ground coupling, are also low relative to the

primary signal, indicating that no corrections were neces-sary prior to crosscorrelation. Next, signal contractionwas performed by common vibroseis correlation usingthe pilot sweep. For some shots from proles 1 and 2, thepilot-sweep signal was distorted because of a temporarymalfunctioning of the pilot signal generator in the record-ing car. These pilot sweeps then were reconstructed fromthe opposite polarity recorded at the same location. Ulti-mately, the trace length was 4 s after correlation (10-ssweep length, 4-s listening period).

Figure 7. Actual processing sequence. Solid-border boxesare attached to processing applied systematically to allproles. Dashed-border boxes represent optional processingapplied to some proles only.



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Poststack migration

For further improvement, essential for interpretation,the poststack data were migrated in time and depth. Differ-ent methods were tested for poststack time migration, in-cluding nite-difference (FD), Kirchhoff, and frequency-wavenumber ( f-k ) migration. Finite-difference migration

(ltered 458

to 658

algorithm) using an rms velocity eldyielded the best results. Prior to application, the velocityeld was smoothed slightly to prevent migration artifacts.Figures 9 and 10 show an example of the time-migratedsection of prole 4, with and without the interval velocityeld in the background, respectively. Similarly, severalalgorithms were tested for poststack depth migrationusing Seismic Unix instead of VISTA and including FD,Fourier-FD, Gazdag phase shift plus interpolation (PSPI;Gazdag and Sguazerro, 1984), and split-step Fourier mi-

gration (Stoffa et al., 1990). The latter two migrationschemes seem to give the best results (visual inspectiononly). As for time migration, slight velocity-eld smooth-ing was required. The PSPI depth-migrated section of prole 4 is given in Figure 11 with superimposed interpre-tation as explained below.

ResultsIn total, 4.2 km of SH-wave seismic reection proles

was gathered during 10 nights of time-restricted data acqui-sition, including experimental testing. The survey areaproved to be highly suited for the methods used, andhigh-quality recordings were obtained. All proles showa high-resolution image of the fjord-ll sediments andbedrock down to a depth of at least 200 m (vertical res-

Figure 9. Prole 4: Final poststack time-migrated section (45 8 to 658 FD).

Figure 8. Prole 4: Brute-stack section with the reference positiongiven in Figure 2b noted in blue, i.e.,corresponding to the shot proles of Figure 6.



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olution of 1 to 5 m). In addition, MASW acquisition wastested briey during the 2008 campaign, but along only apart of prole 2 (Figure 2), with 48 vertical geophonesevery 1 m (streamer) and a distance of 12 m to the source(5-kgsledgehammer). The resulting S-wave velocity prole(Sauvin, 2009) reached only 10 m in depth at that site. Thisprevents a proper comparison with the velocity proles

obtained with seismic reection (see below), which arenot sufciently constrained so close to the surface.

Geologic interpretation

Thehigh resolution of the SH-wave seismic data allowsfor a detailed geologic interpretation in depth (Figure 11).Five main stratigraphic units are identied by clear reec-tion patterns in the seismic-reection sections. The unitsmake up a typical fjord-ll succession above bedrock and

correspond to the stratigraphic succession documented inthe fjord (e.g., Figure 2) and general models of fjord-llsuccessions (Corner, 2006). Detailed features can be ob-served within some units. All proles have been inter-preted, correlating the picked events from one prole tothe other at the crossing points in the time-migrateddomain (Figure 12).

Unit A, interpreted as bedrock, forms the base of theseismic sections and contains irregular high-amplitude re-ections and an undulating upper boundary at depths of 90 to 160 m. Structures within bedrock are observed, butfurther investigation is needed for verication. Unit B,interpreted as mainly glaciomarine deposits, is 20 to 70 mthick and is characterized by continuous subparallel reec-tions draping and onlapping the bedrock (unit A). Theonlapping inll pattern reects an inuence from sedimentgravity ows during initial sedimentation after retreat of

Figure 10. Prole 4: Final poststack time-migrated section (45 8 to 658 FD)with superimposed S-wave intervalvelocity (V S ).

Figure 11. Prole 4: Final poststack depth-migrated section (Gazdag phaseshift plus interpolation) with geologicinterpretation.



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the glacier from the fjord basin. Distinct high-amplitudereections at the top might reect a change in the deposi-tional environment at the transition to the Holocene.However, more data are needed for conrmation.

Unit C, interpreted as consisting of fjord marine andprodeltaic deposits, conformably overlies unit B and isless than 50 m thick. It is characterized by partially continu-ous reections with local lenses displaying a more irregularreection pattern. Unit D, interpreted as deltaic deposits,displays wavy to gently fjordward-dipping continuous-to-discontinuous reections and is 10 to 50 m thick. Thefjordward-dipping reections are interpreted as delta fore-sets (Figure 11). Topsets also are present. Unit E displayscontinuous to irregular reections and is less than 15 mthick. It is interpreted as anthropogenic ll, which lies onthe previous tidal at / submarine part of the Nidelva

River delta (Figure 11).

Estimation of S-wave velocity andgeotechnical properties

Because of the distinct and continuous reectionevents, the S-wave interval velocity could be determinedwith good accuracy down to bedrock in the survey area.Interval velocities were calculated by the smoothed-gradi-ent method to prevent artifacts typically introduced by

the Dix algorithm. Estimated velocity errors by the rms ve-locity analysis were 10% to 20% on average but could in-crease to as much as 30% because of the strong 3D surfacetopography of the bedrock. This was reduced again by the

DMOand thesubsequent ttingof the2.5D prolenetwork.Velocity error after conversion to interval velocities mightbe somewhat more for some intervals. In general, S-wavevelocity ( V S ) increases from 100 m / s in top deltaic deposits(unit D, Figure 12) to 600 m / s at the bedrock interface (unitA, Figure 12). An average velocity of 300 m / s is recordedin the anthropogenic lls.

The measurement of dynamic or small-strain shearmodulus G max of a soil is important for a range of geo-technical applications (e.g., Hight and Leroueil, 2003).According to elastic theory, G max can be calculated fromthe S-wave velocity ( V S ) using

Gmax r V 2S , (1)

where G max is the shear modulus (in Pa), V S is the shear-wave velocity (in m / s), and r is the density of the soil(in kg/ m3 ). Numerous density logs exist in the area of interest, offshore and onshore, but they are very limitedin depth (a few meters), and no attempts have been madeso far to use them to generate spatially varying densitymodels. Soil density measured in several samples from

Figure 12. Three-dimensional view of interpreted poststack time-migrated SH-wave seismic sections. Five main strati-graphic units are indicated by lettersA through E.



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deconvolution. Prestack depth migration (PSDM) is thenext step for further quality control of the interval velocity

eld. Residual moveout of common-image-point gathers(CIG) can be used to improve the velocity model andthus possibly rene the seismic imaging. Seismic modelingalso should be used to better understand the complexwave propagation at very shallow depths (early arrivalsin Figure 6 and Love channel waves) and to control thevelocity model and seismic interpretation. Finally, a pre-served-amplitude approach of PSDM could allow for ex-traction of more information from seismic data via variousattributes, e.g., amplitudes.

The focus of future investigations will be to rene thevelocity model using additional CPTu or preferably othergeophysical methods, e.g., crosshole tests or seismic CPT(Ghose, 2008). In addition, at least one borehole down tobedrock is necessary for ground truth. Another focus isthe correlation of the onshore SH-wave data with the P-wave marine data, including P-to-S converted wavesacquired by Statoil and the Norwegian Geotechnical Insti-tute (NGI) a few years ago. The latter data set indeed canprovide a link between P- and S-wave velocity models off-shore. Finally, an ideal case would be to carry out SH-waveseismic reection offshore as well, but marine S-wavetechnology is not sufciently mature, although potentiallyinteresting results have been obtained in recent years (Wes-terdahl, 2004; Vanneste et al., 2007).

ConclusionFull SH-wave seismic acquisition using an S-wave

vibrator and a landstreamer system equipped with SH-geophones is highly suited for ground investigations inurban areas with a paved surface. The handling of thissystem in a exible manner enables advanced eld tech-niques and allows for sophisticated processing tools thatmake use of distinct waveelds. At this point, futurework also should encompass multicomponent acquisitionto provide the full waveeld. In addition, the 1-m verticalresolution of the seismic sections achieved here allowedus for the rst time to reasonably combine land seismicswith marine data sets. Furthermore, the high-resolutionSH-wave data yield detailed geologic information thatcan be integrated into a complete 3D ground characteri-zation. Nonetheless, a drill hole for calibration or additionalVSP data should constrain the subsurface model further.Determination of small-scale structure and the translationof seismic velocities to soil stiffness bear great potentialfor geotechnical applications. The dynamic shear modulesobtained in this study are comparable to those obtainedusing conventional geotechnical engineering techniques.Especially in urban areas, this noninvasive technique isone of the few that provides important types of informationdelineating the spatial extent of low-stiffness zones.

Acknowledgments

The authors acknowledge the Trondheim Port Author-ity and local municipalities and companies for their per-mission and support during the eld campaign. The high-resolution SH-wave seismic-reection survey was spon-sored kindly by Statoil. The authors are most thankful to

Figure 14. Comparison of G max estimated from S-waveseismic and derived from CPTu data. (see Figures 1 and 2b forthe locations of seismic prole 4, CPTu 2, and CPTu 5).



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the German acquisition crew (Stefan Cramm, SiegfriedGrueneberg, and Eckhardt Grossmann) and to the studentsinvolved in eld testing (Karl-Magnus Nielsen, Universityof Oslo, for preliminary site investigations in fall 2007 andto Eugene Morgan, TUFTS University, for MASW acqui-sition). Trakkvakta and Det FINNs Trakkhjelp AShelped with road closures. The Norwegian Public RoadAdministration and Rambll gave permission to use CPTudata. The authors thank Andre Pugin, Robert A. Williams,and one anonymous reviewer for their great help in improv-ing the manuscript. An academic license (ICG) of GEDCOVISTA software was used for most processing, and Seis-mic Unix (Colorado School of Mines) was used for depth-migration tests. This is ICG contribution no. 298.

References

Arsyad, I., U. Polom, and S. T. Wiyono, 2007, Shallowreection seismic shear-wave velocity analysis onpaved soils using a land streamer unit: 69th Conferenceand Technical Exhibition, EAGE, Extended Abstracts,P19.

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