Combining Seismic Velocity Tomography and Single-hole Reflector Tracing to Improve Volumetric Ground Imaging

8/6/2019 Combining Seismic Velocity Tomography and Single-hole Reflector Tracing to Improve Volumetric Ground Imaging

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COMBINING SEISMIC VELOCITY TOMOGRAPHY AND SINGLE-HOLEREFLECTOR TRACING TO IMPROVE VOLUMETRIC GROUND IMAGING.

CASE STUDIES

Descour Jozef M.1, Roger Surdahl

2, Yamamoto Takuji

3, and Shirasagi Suguru

3

1C-Thru Ground Inc., CO, USA: [email protected]

2Federal Highway Administration, CO, USA; [email protected]

3Kajima Technical Research Institute, Tokyo, Japan; [email protected] and [email protected]

ABSTRACT

Applications of seismic tomography typically use travel times of the fastest waves between sourcesand receivers in the ground. Reconstructed velocity models are limited to panels between pairs ofboreholes. However the resolution deteriorates rapidly with distance from neighboring panels.

The authors combined cross-hole tomography with 3D reflector tracing around individual boreholes toenhance ground images between neighboring panels. The technique uses an array of sources and

receivers placed at the same borehole (Single-Hole TRT) to generate and detect seismic wavesreflected from anomalies in the surrounding ground due to changes in acoustic impedance.

The Single-Hole TRT uses volumetric velocity model derived from the velocity tomography tomeasure distances from the survey borehole to surrounding anomalies. However it cannot define angularlocations of these anomalies.

A single-hole approach may be adequate for anomalies which direction from the borehole is known orimplied by cross-hole tomography. Also, for shallow targets, adding an array of sources on the surfacearound the survey borehole can improve the image. For other targets, a truly three-dimensional imagingof anomalies in the ground requires triangulation from single-hole surveys in at least three different holes.Presented Case Studies demonstrate different applications for this technique, from mapping naturalgeological features, to studies of unknown infrastructures (bridge piles, sewers).

INTRODUCTION

It continues to be a long-standing goal of being able to convert geophysical data into three-dimensional digital images of underground structural features that could be viewed and measured at anydirection, angle and depth. Using a combination of transmitted and reflected seismic waves the authorswere able to produce visual images allowing to identify piles and their depth, boreholes, lamination, oldsewer walls, and even general shape of solution mining caverns.

STATE-OF-ART REVIEW

Seismic velocity tomographySeismic body waves traveling away from their source interact with varying elastic ground features. In

general, seismic waves travel faster and are least attenuated in more competent grounds, and are slowerand more attenuated in weaker grounds. Seismic velocity tomography uses the inversion of travel times

measured between known source and receiver locations (trans-illumination) to reconstruct the velocitymodel of the ground as an image of the ground features. The reconstruction is digital in nature, and isaccomplished by converting the surveyed volume into a grid of uniformly spaced nodal points forming anorthorhombic lattice (Kittel, 1971). The spacing between the closest nodal points defines resolution of thereconstruction in each direction.

The velocity and attenuation changes result in bending trajectories/raypaths of energy flow (Aki et al,1980; Shea-Albin et al, 1991). This bending additionally affects the travel times measured for seismicbody waves passing along the quickest path between each pair of source and receiver locations. Thus,


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the proper tomographic inversion nowadays must include bending of the ray paths to reliably reconstructthe velocity model of surveyed ground.

Furthermore, the reliability and resolution of reconstructed seismic velocity models are determined bythe geometry and the number of seismic sources and receivers which in turn define the density ofcoverage, and the length of raypaths.

For most of seismic tomography surveys, the configuration and length of boreholes control the accessand geometry for sources and receivers. As shown in figure 1 for a panel formed by a pair of parallelboreholes, the tomographic reconstruction of the velocity model returns velocity values alongreconstructed raypaths. However, the velocity in islands between raypaths along the panel is imprecise atbest, and can only be smoothed between the raypaths by reducing the resolution (figure 1b).Furthermore, for a number of panels connecting a reasonable array of parallel boreholes (figure 2a) thereare significant volumetric pockets of no coverage between the panels (figure 2b). This considerablyreduces the resolution of velocity tomography thus increasing the size of anomalies that can remainundetected between the panels (figure 3a).

Since velocity tomography uses only the arrival times of a particular phase for seismic waveforms, awealth of additional information carried by the remainder of each waveform is discarded.

Fig. 1. Seismic velocity tomogram for (a) the grid five-fold finer (0.1 m) than the spacing betweensources, and between receivers, and (b) for matching grid (0.5 m).

Fig. 2. Reliability of seismic velocity tomogram along (a.), and between survey panels (b.) for gridsize matching spacing between sources and between receivers.

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In search for more effective and still practical three-dimensional imaging of the ground features, theauthors introduced and continue to improve a new technique described below that uses reflected seismicwaves.

Three-dimensional reflector tracing (TRT)

Any change in mechanical properties of the ground can be characterized by the size of the anomalyand the contrast of associated acoustic impedance (the product of density and seismic velocity).Anomalies of a size comparable with wavelengths of incident seismic waves produce detectablescatter/reflection of these waves (figures 3b and 4).

Fig. 3. Enhancing detection of structural features by combining direct wave seismic velocitytomography (a), with 3D Reflector Tracing (b).

These anomalies act as reflectors returning part of incident energy of seismic waves to a detector.These reflected waves are analyzed to determine the location and nature of each reflecting boundary.The reflection coefficient for normal incidence of seismic energy onto a boundary between media 1 and 2is defined (Waters, 1978; Aki et al, 1980) by:

1122

1122

VV

VVR

+

=

where: R is the reflection coefficient, is the rock mass density, and V equals the seismic wavepropagation velocity in the ground.

A transition from a material with lower acoustic impedance to one with a higher value results in apositive reflection coefficient, and vice versa. Thus, seismic waves in soil, reflected from steel or concrete

foundations would have the same polarity as the incident waves. Features such as fractured zones withina more solid rock mass will give rise to reflections of reversed polarity. The larger the size of a reflectorand the acoustic impedance contrast, the larger the reflection coefficient, and the stronger and easier todetect are the reflected waves.

The anomalies reflecting seismic waves can be imaged by transposing time records of these wavesback to the location of their origin, using the proper velocity model to convert time to distance, and thenstacking these waveforms at that location (Ashida, 1993, 2001, Neil et al, 1999; Hanson et al, 2000).Figure 5 shows the principal concept behind this operation. For each seismic source and receiver ofknown location the locus of all possible reflector positions for the same two-way (source-reflector-

Borehole w. source

Borehole w. hydrophones

KEY:

Rock

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receiver) travel time defines ellipsoidal surface in three-dimensional space. It is also called the equi-traveltime surface by Ashida et al (1993). For the proper velocity model the transposed reflected signalsmatch in phase, and are stack in space forming a well defined wave-like anomaly. That anomaly starts atreflector surface and may stretch behind that reflector, away from the source-receiver location.

Fig. 4. Relation between rock quality (acoustic impedance) and reflection coefficient.

Fig. 5. Principal concept for transposing reflected waves onto the reflector.

The polarity of its first peak is controlled by the contrast in acoustic impedance across the boundaryof the reflector. The technique typically employs the average velocity measured for the direct wavestraveling between sources and receivers. In some cases the velocity model can be produced by seismicvelocity tomography generated for all sources and receivers.

A discrete image of reflecting anomalies in the ground is calculated using specialized software, for agrid of regularly spaced nodal points contained in a pre-defined rectangular survey volume. This volumehas to include all sources and receivers which seismic records are used to make an image.Presented technique for imaging reflecting structures is referred to as Three-dimensional Reflector tracing

(TRT). This technique is currently used for imaging anomalous ground conditions ahead of excavatedtunnels with an array of sources and receivers coupled to the tunnel walls.

It requires a strict phase and frequency matching for reflected waveforms at the reflector location.This is particularly true for smaller targets comparing to their distance from sources and receivers. Alsothe frequency and phase matching condition can be easily compromised if the spread of sources andreceivers is too large compared to the distance to a reflector. The main compromising factors are: (1)

Receiver

Receiver

Source

Reflector

Tunnel

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Reflecting

surface

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reflectioncoefficient

Medium 1

Competent

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differences between the velocity model and the real velocity distribution in the ground, and (2) phasechanges due to changing directional characteristics of sources, receivers and reflectors, and also due todifferent ground response mainly at the source. Therefore in this method, the receiver points should belocated relatively close to the source points.

Single-hole TRT - a new imaging tool

Modification of the TRT technique designed for using in boreholes has been introduced recently toimprove three-dimensional detection of anomalies, particularly the ones that were missed by the velocitytomography. This technique is referred to as the Single-Hole TRT (Descour et al, 2005a, 2005b).

Survey principlesThe technique uses a string of 6 to 8 hydrophones or other sensors that are coupled through water or

mechanically to the borehole walls. The source is placed either in the middle of the sensor string, or at itsbottom, or at its top, dependent on the target location with respect to the depth of a borehole.

The source sends seismic waves into the ground. Measuring travel times for waves traveling alongthe hole provides basic data for an average velocity assessment in the surrounding ground.

Seismic waves traveling away from the hole become reflected from anomalies in acoustic impedance

and the reflected waves are recorded by the string of sensors.Using the same concept of equi-travel time converted to distance through the velocity model, the

specialized software generates an axially-symmetric reflectogram for all reflected waves.

EquipmentA slim swept frequency source constitutes the core element for using the technique in shallow

surveys targeting man-made or natural features in the ground. However, the same concept was usedsuccessfully in deep boreholes using small explosive charges for impact sources. In such case, thesource and the receivers have to be separated by the proper distance along the same hole, or two closespaced wells should be used.

A string of hydrophones or mechanically coupled sensors is required to be equipped with a broadband, high sensitivity, and low noise sensors responding either to pressure waves in water, or toacceleration for seismic waves in the ground.

The diameter is important for the source and the sensors, as they both are placed in the same hole,and the diameter of this hole should be possibly small particularly for shallow investigation, and to keepthe cost of drilling low.

Typical applicationsThe Single-Hole TRT is very useful and economical for imaging targets which direction with respect

to the borehole alignment is known, or for targets that are nearly axially-symmetric with respect to theborehole.

For other targets a triangulation from at least two and preferably three boreholes for better three-dimensional definition of anomalies in the surrounding ground should be used.

This technique is particularly well suited for imaging slim one-dimensional targets like pilessupporting foundations of any type.

This technique offers an excellent addition to a traditional cross-hole tomography as it allows imagingtargets between and away from tomography panels connecting pairs of boreholes. On the other hand italso benefits from the cross-hole velocity tomography through better definition of the velocity model in itssurvey range.

Four case studies demonstrate benefits of using the Single-Hole TRT technique for imagingunderground structural features.


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CASE STUDY 1 - STEEL PILES OF BRIDGE FOUNDATION

This study was funded by the Federal Lands Highway as part of seeking emerging technologies foridentifying unknown bridge foundations. The investigation included cross-hole tomography, and the newThree-dimensional Reflector Tracing (TRT). The study was conducted for foundations of a few yearsold bridge. It was only known to the investigators that one of the abutments had a spread footing, and theother was supported by H-beam steel piles.

Procedure

Velocity tomographyBoreholes 9 to 11m deep were drilled on each side of the two bridge abutments. The cross-hole

tomography survey was conducted along each abutment for source in one borehole on one side of theabutment, and the hydrophones in the second borehole on the other side of the same abutment (Figure6). The source was activated at 0.5m centers along its borehole. The same spacing was used forhydrophones in the second borehole. The travel times were measured for direct P-waves for eachsource-receiver combination and were processed by the RockVision3D proprietary software to generatevelocity tomograms under each foundation.

Single-hole TRT

Two boreholes, one per each abutment were drilled with larger diameter (ID 76mm) to accommodatefor placing hydrophones and the source in the same hole together for the Single-Hole TRT survey. Themeasurements were conducted with the source-receivers array repeating measurements from the bottomof the hole up at 2 meter intervals.

The results were processed individually by the same software for each array depth to generate animage (reflectogram) of features in the ground along a narrow corridor below each bridge footing. Then allthe images for the same abutment were merged together.

Survey results

Figure 6 shows the velocity tomograms for each of the abutments. The top tomogram shows ratheruniform ground conditions below the east abutment.

There is only a single higher velocity structure in the middle of the image rising from bedrockdelineated at Elev. 2448.5m. The bottom velocity tomogram for the second abutment shows some highervelocity elements in rather chaotic arrangement across most of the ground above bedrock (Elev. 2447m).However, it is impossible to identify the nature of these elements.

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Fig. 6. Seismic velocity tomograms generated under east and west bridge abutments.

Each tomogram was used as a base to build an average velocity model for processing TRT datacollected under the respective abutment.

The top reflectogram in figure 7 appears to confirm the existence of a single object, more competentthan the surrounding ground, which was identified in the velocity tomogram under the footing of the eastabutment (Figure 6). On the other hand, the bottom reflectogram for the west abutment shows relativelyregular pattern of structural features appearing linear and descending from the footing down towardbedrock. Moreover, the anomalies at each end of this abutment appear inclined.

2455

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400 2300P-wave velocity (m/s)0 1 2 3 4 5 6 7 8 9 10

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Fig. 7. Single-Hole TRT reflectograms generated under east and west bridge abutments. Contoursof piles as designed are superimposed on West Abutment reflectogram.

The detected features were interpreted as somewhat distorted images of individual piles. This findingis not surprising considering that the integrity of the images produced by TRT technique stronglydepends on reflected signals matching in phase when transposed to the reflector location. And that matchis controlled by inaccuracies in the velocity model, and by complex or small profile of reflectors (in thiscase small profile steel piles).

After this investigation was completed, the findings about the piles were confirmed including a matchbetween the inclined anomalies, and the battered piles at each end of the east abutment.This case represents a situation when the Single-Hole TRT survey conducted in one, properly locatedborehole is capable of detecting a row of reflectors (piles) due to their radial alignment from the boreholelocation.

CASE STUDY 2 - DEEP SOLUTION CAVERN

This study was conducted to characterize the shape of cavities developed by solution miningnahcolite (Ramey et al, 2005). These cavities are difficult to characterize because solution miningremoved less than 25% of the cavity volume leaving the cavern filled with insoluble material turned torubble. The depth of the target was between 490m and 670m. The Single-Hole TRT survey wasconducted in two 18mm ID production casings grouted side by side in a 0.5m diameter borehole. The

production was suspended for the time of survey.

Procedure

A high temperature hydrophone set of 10 hydrophones at 6m centers was used for this investigation.A multiple round borehole perforation gun was used to fire shots at 6m centers as well.The survey was conducted for three sets of sources and receivers (Figure 8). For the first set thehydrophones were lowered to the depth range from 469 to 523m. The shots were fired at the range from657 to 603m.

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For the second set the hydrophones were at the same range, and the shots were fired at the depthrange from 566 to 512m. For the third set, the hydrophones were at the range from 390 to 444m, and theshots were fired at the range from 504 to 450m.

The velocity for data processing could not be measured directly from seismic records as they weredominated by waves traveling either through steel casings, or through fluid columns in these casings.Therefore the velocity was calculated using elastic constants for the type of a deposit. The calculationyielded P-wave velocity of 3,200m/s, and S-wave velocity of 1,250m/s.

Fig. 8. Section through axially symmetric reflectogram, and profiles of solution caverns generatedfor three sets of sources and receivers placed in two casings grouted in one access borehole.

Survey results

The contour image of the cavities was obtained from both P- and S-wave reflectograms mergedtogether. Figure 8 shows profiles for all imaged cavities. The profile was found to correlate with geologicaldata. Also the configuration and horizontal extent of imaged cavern profiles were consistent with the mostlikely development and horizontal extent of the caverns. The volumetric assessment of the caverns basedon seismic data was verified by another direct method (partial fluid displacement). Also some statistical

analysis of the production output was conducted for the same reason. This investigation has shownseismic evaluation of the cavern sizes matching other methods within 10% margin of error.

CASE STUDY 3 - MAPPING GEOLOGICAL PROFILE

This study was conducted in a test facility in Japan. Its purpose was to evaluate the efficiency ofcross-hole tomography and Single-Hole TRT for mapping inclined geological strata near the surface.Both surveys were conducted using four vertical holes drilled in one row that was oriented perpendicularto the strike of strata as shown in figure 9. The average depth of the boreholes was 40m.

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et3

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Procedure

Velocity tomographyThe velocity tomography was conducted for three panels between consecutive pairs of boreholes. For

each panel the seismic waves were generated by a swept frequency source at 1-m centers in one of the

holes. The other hole was instrumented with hydrophones also at 1-m centers. In addition, a number ofsource points were spread along and up to 5m away from the line of boreholes. The waves from thesesource points were recorded by hydrophones placed at each borehole.

Fig. 9. Geological profile and configuration of survey boreholes for the test site in Japan.

Single-hole TRTThe TRT survey was conducted in boreholes 3 and 4. The survey was conducted using an array of

6 hydrophones at 2m centers with the source placed within 1.8-m long section in the middle of thehydrophone array. The entire set was lowered to a number of depths in each of survey boreholes with thesource activated multiple times at each depth.

Survey results

Figure 10 presents the velocity tomograms combined for all surveyed panels. The image appears toaccurately address the boundary between mudstone and sandstone layers. It also shows a weatheredzone below the ground surface. However, there is no indication of a thin conglomerate zone in sandstone.Figure 11 shows the reflectogram superimposed on velocity tomograms. It correctly delineates themudstone-sandstone boundary. It also appears to show some lamination in sandstone, and a chain of

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anomalies matching known alignment of the conglomerate. Moreover, it appears to show verticallyaligned anomalies matching location of the boreholes. An additional chain of anomalies showingreversed inclination may be a mirror image of another geological boundary to the right of Hole 4 outsideof the velocity tomogram range.

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Fig. 10. Seismic velocity tomograms generated for cross-hole tomography survey at the geologicaltest site in Japan.

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CASE STUDY 4 - OLD SEWER PERIMETER

A century old, approximately 13-m deep and 3.3-m outer diameter sewer line runs near the proposedcaisson location. The sewer shell is made of concrete and protected from outside with brick. The task was

to locate the lateral profile of the sewer structure within 0.15 m margin of error.

Procedure

Four boreholes, each approximately 16-m deep, were drilled two on each side of the anticipatedsewer line as shown in Figure 12. The holes were cased with 76-cm inner diameter PVC pipes. The pipeswere grouted to the ground with a rigid grout, and were filled with water.

Velocity tomographyIn the first step the seismic velocity cross-hole tomography survey was conducted along four side

panels connecting pairs of boreholes. The source was activated at 0.5-m centers along one hole of eachpair. Seismic waves were recorded by a string of hydrophones in the second hole, also with 0.5m spacing.The travel times for detected direct P-waves were processed using the RockVision3D software to

generate velocity tomograms along all four panels.

Then the same software was used to generate an average velocity model using 3D interpolationthroughout the space between the panels. The velocity model was required for a subsequent tracing ofreflectors.

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Hole related anomalies

Conglomerate?

Mirror image

0 5 10 15Distance (m)

Fig. 11. Single-Hole reflectogram from holes 3 and 4 superimposed on the velocity tomogram.

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Fig. 12. Horizontal profile of the old sewer and location of boreholes used for seismic imagingsurvey.

Single-hole TRTIn the second step, the source and hydrophone array were placed together in the same borehole and

the Single-Hole TRT survey was conducted over the bottom 10 meters of the borehole depth. Thesurvey was repeated in each hole.

The acquired data and the velocity model generated by the cross-hole tomography were used by adifferent module of the same RockVision3D software to generate three-dimensional image (reflectogram)of the ground features reflecting seismic energy.

Survey resultsFigure 13 shows P-wave velocity tomograms along two opposite side panels crossing the sewer path.

The tomograms show a weak image of the sewer shell with its top and bottom relatively well defined.

Figure 14 shows two sections through an image generated by the Single-Hole TRT. The image isoriented looking along the sewer alignment. In this image the side walls of the sewer are quite welldefined. The inside of the sewer appears practically free of anomalies. The image also appears showinga vertical structure of a manhole well. The space outside both the sewer and the manhole appears full ofnoise.

The lateral extent of the sewer shell was assessed by animation, moving d turning a model of thesewer (cylindrical contour in both images) across both the velocity tomograms, and the reflectogram until

an optimum match - well within required margin of error - was received (Figure 12).

Approximate sewercenterline

Approximatehorizontal extent of

sewer brick wall

Approximatelateral error

Panel TB-3/TB-4

Panel TB-1/TB-2

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Fig. 13. Seismic velocity tomograms along two panels crossing the sewer alignment as marked infigure 12.

Fig. 14. Two sections through 3D TRT reflectogram looking along the sewer alignment.

SUMMARY

A new, Single-Hole TRT technique offers a significant step toward being able to see throughan opaque ground by means of using seismic waves.

The technique successfully complements the traditional velocity tomography by tracing anomalieshiding in pockets of insufficient coverage between tomographic panels formed by cross-holesurveys.

Its significant benefit stems from using the entire wave record, and not only the first arrivals ofdirect seismic waves typical for cross-hole tomography.

The technique benefits from using swept frequency rather than impact sources in boreholes. Thisapproach provides for an exceptional repeatability of the source. It also helps to expand and

0 2 4 6

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control the frequency band, and subsequently the resolution of seismic imaging. However it alsorequires highly sensitive sensors of the frequency range matching that of the source.

For a single borehole data, the technique offers an axially-symmetric reflectogram. This typeimage should be sufficient for structures which direction from the survey hole is known, or if thehole is drilled in the middle of axially symmetric structure.

For truly three-dimensional ground imaging this technique requires at least two, and preferablythree boreholes to assure a unique ground imaging solution.

This technique is well suited for imaging shallow man-made targets such as piles, sewers, andother mainly linear infrastructure elements. However it may also be used for deeper targetsassociated with old, abandoned mine workings or shafts.

The technique still requires an improvement so the images can be made sharper, and less pronefor disruption by multiple reflections e.g. between a cluster of piles typical for pier foundations.

REFERENCES

Kittel, Ch., 1971. Introduction to solid state physics. 4th

edition. John Willey & Sons, New York.Shea-Albin, V. R., D. R. Hanson, and R. E. Gerlick, 1991, Elastic Wave Velocity and Attenuation as Used

to Define Phases of Loading and Failure in Coal. USBM Report of Investigation 9355, U.S.Bureau of Mines, Department of the Interior, 43 pp.

Aki, K, and P. G. Richards, 1980. Quantitative seismology. Theory and methods. Vol. 1. W. H. Freemanand Co., N. York.

Watters, K. 1978. Reflection seismology. A tool for energy resource exploration. John Willey & Sons, NewYork.

Hanson, D. R., K. Y. Haramy, and D. M. Neil, 2000. Seismic Tomography Applied to SiteCharacterization. Paper in Proceedings of Geo-Denver 2000, Denver, CO.

Ashida, Y., 2001. Seismic imaging ahead of a tunnel face with three-component geophones. InternationalJournal of Rock Mechanics & Mining Sciences 38, 823-831.

Neil, D. M., K. Y. Haramy, J. Descour, and D. Hanson, 1999. Imaging ground conditions ahead of theface. World Tunneling, V. 12, No. 9, November, pp. 425-429.

Ashida, Y., and K. Sassa, 1993. Depth transform of seismic data by use of equi-travel time planes.

Exploration Geophysics 23: 341-346.Descour, J. and T. Ross, 2005. Geophysical investigation of an unknown bridge foundation. Publication

No. FHWA-CFL/TD-05-02.Descour J., T. Yamamoto, and K. Murakami, 2005. Improving 3D imaging of unknown underground

structures. Proceedings of FHWA Unknown Foundation Summit, Lakewood, CO. November 15-16, 2005.

Ramey, M., J. Descour, and M. Hardy. 2005. Cavity shape characterization of a rubble-filled solution-mined cavity. Proceedings of Solution Mining Research Institute; Spring 2005 Technical Meeting,Syracuse, New York.

Documents

Combining Seismic Velocity Tomography and Single-hole Reflector Tracing to Improve Volumetric Ground Imaging