Leibniz Institute for Applied Geosciences, Stilleweg 2, D - 30655 Hannover, Germany, http://www.gga-hannover.de

INTERREG III ARhin Supérieur Centre-Sud /

Oberrhein Mitte-Süd

SHEAR-WAVE VIBRATOR APPLICATIONS FOR SHALLOW SEISMIC INVESTIGATIONS – SOME EXPERIMENTAL RESULTS

U. Polom, S. Grüneberg, W. Rode

IntroductionShear-wave generation for shallow seismic investigations is often done byhorizontally directed alternating hammer blowing at a ground coupleddevice. We found that this technique works well for refraction studies. For VSP and reflection analysis, there is often a lack of resolution and continuous wavelet quality. This depends on the low frequency content of the initial seismic signal, the repeatability of the blow quality and thecontinuous damage of ground coupling conditions in the near sourceenvironment during a vertical stack sequence.

Application of a shear-wave vibrator source instead of hammer blowingneeds more technical effort and is not common in shallow seismicapplications. Hydraulic driven source systems available on the market (e.g. IVI MiniVib) are often too big and too expensive for the goals within theuppermost 100 m in depth. Some electrodynamic driven systems forshallow applications have been presented in the past, but they did not create the break-through at the market. Therefore, the development of electrodynamic driven shaker sources has been started from a small scaleprototype, also in order to gain more experiences in shear wave seismics.

The aim of the activities is the development of seismic sources for shallowshear-wave investigations in urban areas for e.g. earthquake microzoningand geotechnical applications. Due to the often complicated coupling and ambient noise conditions, repeatability and adjustable frequency bandwidthare important factors for shear-wave data production.

β-Version

Acknowledgements

The autors are greatfully acknowledged to Karl-Heinz Boegner and the management of Robert Bosch GmbH, Germany, for the support regarding electrodynamics.

This work is kindly supported by the European Community within the cross-border project INTERREG III A Rhin Supérieur Centre-Sud / Oberrhein Mitte-Süd.

Shear-wave spade source used foralternating hammer blow impacts. Integrated two sided sensitive triggersensor supports correct trigger timing. Plastic pads reduce airblast events.

Two orthogonally orientated shear-wave spadesources while VSP measurements showingdamages of the near source environment.

Left: Prototype of experimental shear-wave vibratorsource system for shallow applications consisting of a 12 V DC car battery , programmable sweep signalplayer (16 bit resolution, eprom based) and electrodynamic driven shaker unit. „Waffle iron“ patterned baseplate surface supports groundcoupling while different soil conditions. A switch isused for inverting shaking polarity to separate P-and S-waves.

Left: Improvedprototype versionof the shaker unit. This shaker typegenerates clearfirst break eventsover more than250 m distance (depending on ambient noiseconditions) withoutany kind of sourcegenerated airblastnoise.

Programable sweep signal player unit including a precise trigger signal generator (closed contact orvoltage driven). Storage capacity of eprom unitsare 512kb (256 ksamples) actually, using 5 sampling intervals from 0.25 ms (64 s, max. 1 khz) to 0.015625 ms (4 s, max. 32 kHz).α-Version

Result of of downgoing shear wave signal of thehammer source (left, stack of four alternatedblows) and the vibrator (right, stack of ten alternated sweeps, 10-100 Hz, 10 s) monitored bya buried 3-component geophone at 125 m depth.

Acquisition parameters

Location: Area Otterbach, Weil a.R.,March/April 2003

Instrument: GEOMETRICS StrataVisor NXChannels/rec: 67 (Hammer), 65 (Vibrator)Shot Locations: 64 (Hammer), 66 (Vibrator)Recording: 2 s (Hammer), 12 s (Vibrator)

(both 2 s after correlation)Sampling int.: 2 msRecording filter: outSpread type: 2D variable split spread,

SH-SH configuration,fixed receiver setup

Geophone type: SM 6H (10 Hz), single unitsReceiver int.: 3 mSource int.: 3 mVertical stack:

Hammer: 4-fold alternated blowsVibrator: 4-fold alternated sweeps

10-160 Hz lin., 10 s

Sweep frequency content could only be used up to 80 Hz due to mechanical problems of the shakerunit.

Example of depth converted stack section (SH-SH source-receiver configuration) generated by the shear-wave spade source (4 alternated blows at each sourcelocation). Test area was within a small deltaic region, where fluviatile gravel layers of Quarternary age are overlaying a Tertiary clay sequence below 18 m depth.

Same profile as left generated by the improved shaker unit (4 alternated sweepseach source location). Processing parameters are set identical to the hammer sourcesection.

Time[ms]

Example of shallow VSP SH shear-wave velocity-depth analysis from a glacial sand/clay sequence located in Northern Germany using the improved vibrator source (stack of two alternated sweeps, 10-100 Hz, 10 s). Boreholewas cased by a bentonit cementated plastic liner to obtain stability and good shear-wave coupling. Prior to picking, data has been rotated to source azimut. Density function (red) was derived by probe samples. Subsequently, shearmodulus vers. depth function (blue) was calculated for foundation analysis.

100 200 300 400 500Interval velocity [m/s]

50

45

40

35

30

25

20

15

10

5

0

Dep

th [m

] asl

+ 1

5 m

100 200 300 400 500Shear modulus [MN/m2]

-----------------------------------------

Shear-wave velocity VsDensity ρShear modulus G0max

1.4 1.6 1.8 2 2.2Density [t/m3]

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

50

45

40

35

30

25

20

15

10

5

0

Dep

th [m

] asl

+ 1

5 m

mS, fs, gsbraun

mS, fsgrau

fS, ms,Holzkohle,grau

fS, Holzkohlegrau

fS, Schlufflinsen

fS,msgrau

mS, fs, gsgraubraun

S, graumS, fs,

Holzkohlegrau

Schluff, fS, schwarzmS, fs, gs, fg

Holzkohlegrau

fS, ms, grauBraunkohle, schwarzfS, ms, Hk., schwarz

fS, u, hellgrau

fS, wenigGyttjalinsen

T, u, fS, ms

fS,u, grau

fS,ms, grau

T, halbfestdunkelgrau

Source offest 3 m, no graphic compensation

G = Vs2 ρ

Left: Comparison of 4-fold stacked shot recordsgenerated by the hammer source (left) and thevibrator source (right) from shot location 1 at areaOtterbach, Weil a.R. (AGC 500 ms applied). Acquisition details are noted below. Whole datarecording was affected by strong noise levelcaused by neighbouring highway, railroad and industry, therefore, a typical urban test area.The dominant seismic events are the SH-refraction(~600 m/s) from a gravel layer and surface wavesof Love type (phase velocity ~200 m/s). Betweenthe refraction and Love wave events is the windowof the potential seismic reflection events. The zonebelow the Love wave is dominated by scattered orreflected Love waves, wherein no reflection eventshave been observed until now.The entire vibrator record shows higher frequencycontent, better signat-to-noise ratio - also for therefraction event - and less energy within the Love wave. Zone of potential reflection events showsmore resolution and a clearer transition to the Love wave.

Love wave

SH-refraction

ConclusionsShear-wave generation by hammer blowing technique should be restricted to low resolution requirements, i.e. refractionsurveys. Shear-wave generation by vibrator technique results in better data quality, higher resolution and precise triggertiming. But in contrary to the hammer blow technique, vibrator technique needs more technical spends and modern recording equipment with a fast correlation routine implemented.

The e.g. five time longer recording cycles of the vibroseis method are almost compensated by the faster and easiersource positioning and coupling, which works also with different ground conditions. Once positioned, the source is remotecontrolled by the recording operator wacthing the noise screen for low noise conditions. This is advantageous forshortening the production cycles and reducing man power.

At the actual stage of development the presented shear-wave vibrator source system is a useful altenative to the hammerblow technique. But is not yet in a final, professional development stage. In contrast to the principal difference in seismicwave generation and regarding the general problems of near surface seismics, the similarity of the generatedseismograms is surprising. Due to the first restrictions regarding simple and low cost technical solutions of the shakerunits, the presented prototypes indicate a high potential of the technical possibilities.

Next important steps in source development are increasing the source power and the frequency bandwidth. Additionally, the source system has to be optimized for operation requirements. Further work will be spend to multiple sourceoperations and an extension to P-wave generation.

Time [ms]

Data channel Data channel

Time [ms]

Depth[m]

Depth[m]

Weil a.R., profile1, depth section hammer Weil a.R., profile1, depth section vibrator

Weil a.R., profile1, hammer source, record 1003 Weil a.R., profile1, vibrator source, record 1000

Leibniz Institute for Applied Geosciences, Stilleweg 2, D - 30655 Hannover, Germany, http://www.gga-hannover.de

INTERREG III ARhin Supérieur Centre-Sud /

Oberrhein Mitte-Süd

More current details from shear-wave vibrator applications:

Normal polarity plots Reversed polarity plots

Swee

pra

nge

10-1

00 H

zSw

eep

rang

e20

-200

Hz

Swee

pra

nge

30-3

00 H

z

Source parametersUsing the vibrator source, near surface reflection seismic resolution is strongly dependent fromthe given sweep parameters. The low frequency limit of the sweep range controls energy content, sharpness, and the amplitude contrast of the Love wave events regarding to the refraction and reflection events. A high amplitude contrast leads to distortion of the shallow surface reflectionsand refraction events due to the heavy sidelobe correlation artifacts of the Love wave.

Sufficient holddown force to support source coupling is important to lead the high frequencycontent from the source into the subsurface, which is neccessary for the resolution of nearsurface layers. Currently, we favor 2-3 times higher holddown forces than the source shakingforce, elastically decoupled by airbags. But with respect to the desires of more source power, thisis a heavy disadvantage for the operation logistic requirements of small and low weight sources.

Holddow

nforce ~ 500 N

Holddow

nforce ~ 1500 N

Holddow

nforce ~ 1000 N

Reflection and transmission behaviorIn contrast to P-waves, the reflection coefficient of S-waves is defined by the inverse polarity(remember the simple fixed/loose end rope experiment of school physics), whereas transmissioncoefficient yields the same sign. Therefore the SH-wave refraction events, which are onlycontrolled by the transmission formula, comming up from a subsurface boundary with thesame sign which was generated by the source. This is due to the particle motion isperpendicularly to the direction of wave propagation. In contrast to this, refracted P-wavescoming up with the inverse source sign due to the change in direction of particle motion.Furthermore, assuming an reflection impedance contrast I2>I1 for both SH- and P-waves, the SH-wave changes polarity of particle motion within reflection, wheras P-waves does not do so. Therefore, the refraction and reflection events from a same subsurface boundary haveopposite polarity for SH-waves, but same polarity for P-waves.

Love wave

SH-Refraction

?

?

70 mAux

SH-Refraction

SH-Reflections

SH-Reflection fromrefraction boundary

SH-Reflection fromrefraction boundaryshowing negative sign

SH-Refraction

Loerrach/Stetten, profile2, record 2015-R

Loerrach/Stetten, profile2, record 3115-RLoerrach/Stetten, profile2, record 3115-N

Loerrach/Stetten, profile2, record 2015-N

Loerrach/Stetten, profile2, record 1069-RLoerrach/Stetten, profile2, record 1069-N

Amplified sidelobecorrelation noise

by AGC

?

Sidelobecorrelation noise

All seismogramms above are produced by same source location, spread, and environment conditions within one hour of production.Stack of 2 alternated vibrations and AGC (100 ms) applied. Bandpass filter ranges were set to sweep ranges.

Contact: U.Polom@gga-hannover.de

Amplified sidelobecorrelation noise

by AGC

Amplified sidelobecorrelation noise

by AGC