Crosshole Seismic Tomography Research Articles

Multiscale geophysical characterization of an unstable rock mass

Hazard mitigation from rockfalls and landslides is a priority in densely populated areas. A proper characterization of the inner structure of the rock mass is key to the comprehension of the mechanisms enhancing the slope instabilities. To this aim multi-scale geophysical methods can provide a novel and valuable tool for a high-resolution imaging of the internal structure of the rock mass and unique constraints on the physical state of the medium. We present here a cross-hole seismic tomography survey coupled with laboratory ultrasonic velocity measurements and physical properties determination on rock samples to characterize the damaged and potentially unstable granitic cliff of Madonna del Sasso (NW Italy).Results allowed to obtain: i) a lithological interpretation of the velocity field obtained at the site, ii) a systematic correlation of the measured velocities with physical properties (density and porosity) and macroscopic features of the granite (weathering and anisotropy) of the cliff. The multi-scale approach adopted within this study revealed to be crucial for the imaging at depth of the main fractures affecting the cliff (site-scale seismic tests) and for the understanding of the variations in the seismic velocity between altered and intact rock (laboratory-scale tests); similar approaches can be potentially used in further microseismic monitoring studies.

시추공 합성탄성파 기록을 통한 지하 불연속 경계면의 파악

지반 조사를 위해 흔히 수집하는 지표탐사 자료, 시추조사 자료, 지질공학 자료들을 서로 상관시켜 불연속 경계면 및 암반 파쇄대등의 분포를 파악하였다. 전기비저항 입체도와 공내 영상촬영을 통해 개략적인 지질 연약대의 주향 방향을 분석하고, 시추공 사이 탄성파 토모그래피 속도와 로즈 다이어그램을 통해 지층 및 암반 파쇄대의 공간적인 분포대를 파악하였다. 암반의 동적 물성을 파악하기 위해 S-PS 검층과 <TEX>${\gamma}-{\gamma}$</TEX> 검층으로 동적 탄성계수를 계산하여 푸아송 비 및 P파 속도와의 상관관계를 알아보았다. 지층의 불연속 경계면은 타격수 N값, 개별적인 밀도나 속도 정보를 이용하여 결정하는 것 보다 물리검층에서 수집한 속도와 밀도로부터 계산한 음향 임피던스의 대비, 즉 반사계수 자료와 시각적으로 잘 상관되었다. 암반에 발달한 주요 파쇄대 구간은 그 상부 경계면이 반사계수와 최적 리커 요소파의 곱말기로 계산된 합성탄성파 트레이스에서 극성이 음인 높은 진폭과 잘 상관되었다. 합성탄성파 기록으로 해석된 주된 파쇄대는 실제로 시추 코어 자료에서 관찰된 코어손실 구간 및 공내 영상촬영 자료에서 평가된 낮은 암질 구간과 잘 부합되었다. We integrated and correlated datasets from surface and subsurface geophysics, drilling cores, and engineering geology to identify geological interfaces and characterize the joints and fracture zones within the rock mass. The regional geometry of a geologically weak zone was investigated via a fence projection of electrical resistivity data and a borehole image-processing system. Subsurface discontinuities and intensive fracture zones within the rock mass are delineated by cross-hole seismic tomography and analyses of dip directions in rose diagrams. The dynamic elastic modulus is studied in terms of the P-wave velocity and Poisson's ratio. Subsurface discontinuities, which are conventionally identified using the N value and from core samples, can now be identified from anomalous reflection coefficients (i.e., acoustic impedance contrast) calculated using a pair of well logs, comprising seismic velocity from suspension-PS logging and density from logging. Intensive fracture zones identified in the synthetic seismogram are matched to core loss zones in the drilling core data and to a high concentration of joints in the borehole imaging system. The upper boundaries of fracture zones are correlated to strongly negative amplitude in the synthetic trace, which is constructed by convolution of the optimal Ricker wavelet with a reflection coefficient. The standard deviations of dynamic elastic moduli are higher for fracture zones than for acompact rock mass, due to the wide range of velocities resulting from the large numbers of joints and fractures within the zone.

Imaging of Sand Lens at a Barrage Site in the Indian Himalaya – A Case Study using Cross-hole Seismic Tomography

Abstract Drilling at the barrage site of a hydropower project usually gives a reasonable picture of the sub-surface rock mass condition. However, when the overburden (OB) comprising river borne materials is very thick and the basement rock is deep, the drilling results become unreliable. In the present case, the depth to the granitic gneiss basement is 45m from the river level and the height of the proposed diversion barrage is only 24.5 m. For this reason, stripping of the overburden up to the basement is not considered feasible. Cross-borehole seismic tomography (CST) was carried out at the barrage site to investigate the inclusions and structure of the OB material in order to facilitate a foundation design suitable for permeable formation. Three sets of CST surveys were carried out up to a depth of 30 m between the three vertical boreholes that were prepared at the vertices of an equilateral triangle with a separation of 17.8 m. The tomograms precisely imaged the nature and disposition of the different constituents of the overburden material. A lens shaped low velocity layer (LVL) with seismic velocity Vp=2000-2500 ms-1 was mapped at a depth of 14.5m from the surface. Hence, it was recommended to shift the barrage axis by 50 m towards the downstream side of the proposed axis. The outcome of this study proved valuable for the designer who finalised the foundation design. Successful application of CST in OB characterisation has reiterated the need of such studies for barrage site investigations particularly in the Himalayan terrain.

A critical appraisal of asymptotic 3D-to-2D data transformation in full-waveform seismic crosshole tomography

Seismic full-waveform inversion (FWI) is often based on forward modeling in the computationally attractive 2D domain. This implies the assumption of a line source extended in the out-of-plane medium invariant direction, with far-field amplitudes decaying inversely with the square root of distance. Realistic point sources, however, generate amplitudes that decay approximately with the inverse of distance. Conventionally, practitioners correct for this amplitude difference and the associated phase shift by transforming the recorded 3D field data to the approximate 2D equivalent by using simplistic asymptotic filter algorithms. Such filters assume straight raypaths, a constant velocity medium, and far-field recordings. We have assessed the validity of 3D-to-2D data transformation in the context of crosshole seismic full-waveform tomography by propagating 3D and 2D wavefields through 2D media and comparing 2D reference synthetics with their filtered 3D equivalent. The filter performs well in simple situations, which confirms the general applicability of the conventional asymptotic approach. However, we have observed substantial errors in more complex elastic models, associated with overlapping arrivals and strongly curved raypaths. To test if this error translates into deficient model reconstruction in FWI, we performed complementary inversion experiments using a frequency-domain algorithm. Purely acoustic waveform inversions of 3D-to-2D filtered data are only weakly affected, but in the case of elastic FWI, in which an S-wave influence is present, adverse effects increase substantially. Two-dimensional FWI in combination with filtering seems to be an acceptable strategy as long as the model is two-dimensional, the recording geometry is straight and perpendicular to strike, and only slight S-wave energy is contained in the data. The latter two conditions are generally met in exploration-type marine seismic surveys at short offsets and in some crosswell applications using explosive sources and nondirectional pressure receivers.

Imaging weak zones in the foundation using frequency domain attenuation tomography

Cross-hole imaging method using Time Domain (TD) and Frequency Domain (FD) parts of cross-hole radar tomography data acquired using Step Frequency Ground Penetrating Radar (SFGPR) was implemented. This method was adopted for imaging foundation of a dam to check if the foundation was free of geological weak zones. The dam site is characterised by massive and jointed-phyllites associated with major and minor shears. The cross-hole radar tomography data was acquired in the frequency bandwidth of 250 MHz, from the deepest level gallery up to a depth of 40 m in the foundation. In TD, first arrival time and amplitudes of radio waves were inverted using Simultaneous Iterative Reconstruction Technique (SIRT) resulting in velocity and attenuation tomograms. The tomograms showed nearly uniform velocity or attenuation structure in the respective tomographic plane. Subsequently, cross-hole radar tomography data was analysed in FD for a variation of spectrum-amplitude at different frequencies. Amplitudes picked at each single frequency were then inverted using SIRT for obtaining frequency domain attenuation tomogram (FDAT). The FDAT clearly showed presence of anomalous high attenuation zones in the depth range of 23–33 m of the tomographic plane. The anomalous zones in the attenuation tomogram are weak zones in the foundation. To validate the above observations, cross-hole seismic tomography was also done in the same boreholes. Cross-hole seismic tomography results showed low velocity (p-wave) zones around the same location corresponding to the high attenuation zone in FDAT, bringing the dormant weak zone to light. This enabled fine-tuning of the reinforcement design and strengthening the weak zone. This paper discusses the cross-hole radar tomography imaging method, the results of its application in imaging weak zones in the foundation and the comparison of cross-hole radar tomography results (in TD and FD) with the cross-hole seismic tomography results. • Cross-hole SFGPR tomography data analysis in TD and FD; FD attenuation (FDAT). • Inversion of TD and FD data by SIRT; TD - Velocity and Attenuation Tomograms. • FDAT (7.5-11 dB/m) at 200 MHz reveals weak zone in foundation. • Cross-hole P-wave low velocity (2200-3500 m/s) validates FDAT weak zone. • FDAT approach demonstrated in cross-hole imaging of dormant geological weak zone.

The method of interlineation of vector functions $ \vec{w} $ (x, y, z, t) on a system of vertical straight lines and its application in crosshole seismic tomography

The authors propose a method to construct interlineation operators for vector functions \( \vec{w} \) (x, y, z, t) on a system of arbitrarily located vertical straight lines. The method allows calculating the vector \( \vec{w} \) at each point (x, y, z) between straight lines Γk for any instant of time t ≥ 0. They are proposed to be used to construct a crosshole accelerometer to model Earth’s crust on the basis of seismic sounding data \( {{\vec{w}}_k}\left( {z,t} \right),\,k=\overline{1,M} \), about the vector of acceleration \( \vec{w} \) (x, y, z, t) received by accelerometers at each chink Γk.

Validity of the acoustic approximation in full-waveform seismic crosshole tomography

Acoustic waveform inversions are an increasingly popular tool for extracting subsurface information from seismic data. They are computationally much more efficient than elastic inversions. Naturally, an inherent disadvantage is that any elastic effects present in the recorded data are ignored in acoustic inversions. We investigate the extent to which elastic effects influence seismic crosshole data. Our numerical modeling studies reveal that in the presence of high contrast interfaces, at which P-to-S conversions occur, elastic effects can dominate the seismic sections, even for experiments involving pressure sources and pressure receivers. Comparisons of waveform inversion results using a purely acoustic algorithm on synthetic data that is either acoustic or elastic, show that subsurface models comprising small low-to-medium contrast ([Formula: see text]) structures can be successfully resolved in the acoustic approximation. However, in the presence of extended high-contrast anomalous bodies, P-to-S-conversions may substantially degrade the quality of the tomographic images. In particular, extended low-velocity zones are difficult to image. Likewise, relatively small low-velocity features are unresolved, even when advanced a priori information is included. One option for mitigating elastic effects is data windowing, which suppresses later arriving seismic arrivals, such as shear waves. Our tests of this approach found it to be inappropriate because elastic effects are also included in earlier arriving wavetrains. Furthermore, data windowing removes later arriving P-wave phases that may provide critical constraints on the tomograms. Finally, we investigated the extent to which acoustic inversions of elastic data are useful for time-lapse analyses of high contrast engineered structures, for which accurate reconstruction of the subsurface structure is not as critical as imaging differential changes between sequential experiments. Based on a realistic scenario for monitoring a radioactive waste repository, we demonstrated that acoustic inversions of elastic data yield substantial distortions of the tomograms and also unreliable information on trends in the velocity changes.

Using stochastic crosshole seismic velocity tomography and Bayesian simulation to estimate Ni grades: Case study from Voisey's Bay, Canada

Modeling of grades is a key step and the major source of error in appraisal stage of mining projects. We used a geostatistical approach to explicitly integrate seismic travel time data, as well as acoustic and core logging data into the estimation of nickel grades in the Voisey's Bay deposit. Firstly, the crosshole seismic travel times are inverted using a stochastic tomographic algorithm. This algorithm allows for the inclusion of acoustic log data and seismic covariance into the inverse problem, leading to high-resolution velocity tomographic images of the orebody. Secondly, grade realizations are generated using a Bayesian sequential Gaussian simulation algorithm, which integrates the ore grades measured on the core logs and the previously inverted tomographic data. The application of the presented method to the Voisey's Bay deposit yields an improved knowledge of the geology setting and generates grade models with realistic spatial variability compared to conventional methods.

Crosshole seismic tomography including the anisotropy effect

Velocity anisotropy caused by fine layering has a strong effect on the travel paths at large vertical offsets in crosshole seismic data. This effect must be considered in seismic waveform tomography, so that the inversion procedure can match the waveforms in real data. In this paper, we propose to properly take into account this anisotropic effect in two stages. First, we invert for the anisotropy parameter simultaneously with the (horizontal) velocity in travel time tomography. Then we incorporate this estimated anisotropy parameter model in the waveform simulation during waveform tomography, to refine the velocity image. When considering this anisotropic effect, far-offset data are properly used in the inversion, then both travel time and waveform tomography may have a better ray coverage, producing a velocity image with a higher resolution.

시추 및 물리검층자료의 상관해석을 통한 청주화강암체의 물성 정보 및 절리 분포

To clarify the distribution of joints and fracture zones in the Cheongju granitic mass, we analyzed drill-core and geophysical well-logging data obtained at two boreholes located 30 m from each other. Lithological properties were investigated from the drill-core data and the samples were classified based on the rock mass rating (RMR) and on rock quality designation (RQD). Subsurface discontinuities within soft and hard rocks were examined by geophysical well-logging and cross-hole seismic tomography. The velocity structures constructed from seismic tomography are well correlated with the profile of bedrock depth, previously mapped from a seismic refraction survey. Dynamic elastic moduli, obtained from full waveform sonic and logging, were interrelated with P-wave velocities to investigate the dynamic properties of the rock mass. Compared with the correlation graph between elastic moduli and velocities for hard rock at borehole BH-1, the correlation points for BH-2 data showed a wide scatter. These scattered points reflect the greater abundance of joints and fractures near borehole BH-2. This interpretation is supported by observations by acoustic televiewer (ATV) and optical televiewer (OTV) image loggings.

Photogrammetry and Tomography Technique methods, its comparison with the conventional methods of geotechnical laboratory testing

This paper presents the investigation and testing of the foundation of the Cliff, whom has deteriorated due to the hydraulic and massive ice pressure and its freeze thaw cycles, a visual inspection and core samples were taken for the core reconnaissance of rock formation and elastic properties of the rock strata.Exploring these cavities by using the Crosshole Seismic Measurements and Tomography Technique methods [6] and comparing it with the conventional methods of geotechnical laboratory testing and its results, These Methods of the Crosshole Seismic and Tomography Techniques are used to determine the dynamic elastic properties of subsurface materials, to detect the fracture zones, and to delineate geological boundaries. Also to give other information which assists in the analysis of the Cliff stability or any foundation in order to have the correct and suitable methods and ways that helps in the reinforcement, renovation or reconstruction of any geotechnical foundation.

Rock-Socketed Large Diameter Bored Pile and Geophysical Survey in Cavernous Karst Area: Tin Shui Wai Public Library Cum Indoor Recreation Centre

This paper will present the geological and engineering considerations in the planning, design and construction of a public library cum an indoor recreation centre in cavernous karst area at Tin Shui Wai, Hong Kong. An appraisal was carried out to determine the effect of geological consttraints on the foundation design and to assess the suitability of difffferent types of foundation systems. Sinkhole hazard is a major challenge to engineers when founding a structure and/or carrying out excavation in cavernous karst area. Formation of a sinkhole usually leads to a sudden depression on the ground surface caused by the collapse of cavities. This paper will describe how the most appropriate foundation system was chosen and how the sinkhole hazard was investigated and/or dealt with at the planning, design and construction stages, especially the experience in using cross-hole seismic tomography to delineate the spatial extent of cavities within the karst area. Moreover, in Hong Kong, although rock-socketted steel H-piiles and large diameter bored piles have widely been employed as foundation, limited local experience and design parameters have been published for the piles installed in cavernous karst area. This paper will discuss the allowable shaft friction between marble and concrete pile at the socket section being adopted in this project.

Nonlinear traveltime inversion scheme for crosshole seismic tomography in tilted transversely isotropic media

Crosshole seismic tomography often is applied to image the velocity structure of an interwell medium. If the rocks are anisotropic, the tomographic technique must be adapted to the complex situation; otherwise, it leads to a false interpretation. We propose a nonlinear kinematic inversion method for crosshole seismic tomography in composite transversely isotropic media with known dipping symmetry axes. This method is based on a new version of the first-order traveltime perturbation equation. It directly uses the derivative of the phase velocity rather than the eigenvectors of the body-wave modes to overcome the singularity problem for application to the two quasi-shear waves. We applied an iterative nonlinear solver incorporating our kinematic ray-tracing scheme and directly compute the Jacobian matrix in an arbitrary reference medium. This reconstructs the five elastic moduli or Thomsen parameters from the first-arrival traveltimes of the three seismic body waves (qP, qSV, qSH) in strongly and weakly anisotropic media. We conducted three synthetic experiments that involve determining anisotropic parameters for a homogeneous rock, reconstructing a fault embedded in a strongly anisotropic background, and imaging a complicated four-layer model containing a small channel and a buried dipping interface. We compared results of our nonlinear inversion method with isotropic tomography and the traditional linear anisotropic inversion scheme, which showed the capability and superiority of the new scheme for crosshole tomographic imaging.

Seismic tomography of the Excavation Damaged Zone of the Gallery 04 in the Mont Terri Rock Laboratory

SUMMARY An endoscopic antenna is used to perform a seismic cross-hole tomography in the Excavation Damaged Zone (EDZ) of the new G04 gallery of the Mont Terri Underground Rock Laboratory (Switzerland) excavated in Opalinus clay. More than 800 seismic traces were recorded between two vertical boreholes by combining 22 source and 48 receiver locations. A vertical area of 1.2 × 3.4 m under the floor of the gallery is investigated with a high-resolution tomography. Data with a very good quality allow to determine the traveltimes and the amplitudes of a 40 kHz source wavelet propagating between the two boreholes. The analysis of the traveltimes shows that the wave velocity is homogeneous but anisotropic with a minimum value of 2490 ± 45 m s−1 in the direction normal to the bedding and a maximum of 3330 ± 90 m s−1 parallel to the bedding. The amplitude of the first arrivals strongly varies depending on the source–receiver locations, and suggesting an heterogeneous distribution of the attenuation coefficient of the seismic waves. A Bayesian inversion provides likely models of attenuation that are compared with geological observations. The areas where fractures or cracks exist in the Opalinus clay appear as highly absorbing the seismic waves.

High-resolution aquifer characterization using seismic cross-hole tomography: An evaluation experiment in a gravel delta

We performed field experiments at a well-studied aquifer test site to evaluate the geophysical method of seismic cross-hole tomography for high-resolution aquifer characterization. We address the challenge of finding meaningful relationships between seismic velocity and hydraulic conductivity at the field scale for hydraulic conductivity estimation. The resolution of seismic cross-hole data is limited by inversion ambiguities and diffraction effects, which are not accounted for using conventional ray-based inversion algorithms for velocity reconstruction. We investigate the impact of diffraction effects on seismic velocity inversion and implications for hydraulic modelling. Our ray-based seismic tomograms correlate well with sedimentary cores and hydraulic conductivity logs allowing us to interpret a zone model with distinct geophysical/hydraulic zones. We use this zone model to constrain seismic inversions using two approaches for forward modelling: ray tracing (infinite-frequency approximation) and wavefield modelling at a realistic frequency, which accounts for diffraction. We show that using the common ray tracing approach, seismic velocities in low-velocity zones may be significantly overestimated. Based on the available data, we observe a negative linear relationship between logarithmic hydraulic conductivity and seismic velocity. This relationship is used to parameterize hydraulic models based on seismic velocity models. Computer simulations of advective contaminant transport demonstrate that their results are extremely sensitive to an accurate determination of seismic low-velocity zones. We conclude that seismic tomography is indeed very useful for high-resolution aquifer characterization. However, an inversion approach that accounts for diffraction effects, a priori knowledge of the subsurface structure and additional well data should be applied for quantitative interpretations.

In-Mine Geophysics – Cutting costs and finding ore

After a number of years of development and implementation Inco has a vigorous mines geophysics program in place at our sulphide nickel operations in Canada. This involves geophysical support in exploration, delineation and production. The work in these areas is mutually reinforcing with a common set of physical properties being exploited at a wide variety of scales. The exploration work continues to generate new ore zones in an established but prolific camp and the production related work has resulted in ongoing savings and more efficient mining operations.We review a number of the main applications including blasthole surveying, televiewers, crosshole seismic tomography, pulsed neutron borehole probes, and BHEM. These programs and a number of other initiatives have been technically successful and are invarious stages of implementation. Overall our mines geophysics work has been a very successful with significant new in-mine discoveries and savings in production. We believe that this is still an underdeveloped area with many more opportunities for geophysical imaging of ore at multiple scales

An Adaptive Wavenumber Sampling Strategy for 2.5D Seismic-Wave Modeling in the Frequency Domain

Simulation of seismic waves from a 3D point-source in a 2D medium may be performed in the frequency-wavenumber domain (called 2.5D modelling). It involves computing the Fourier-transformed Green's function for a number of frequency (ω) and strike direction wavenumber (k y ) values and doubly inverse transforming to convert to the traveltime and distance space. Such modeling produces a wavefield with 3D features but the computation becomes pseudo 2D (i.e., in the xz-plane) rather than 3D (in the xyz-frame). The common sampling strategy for the wavenumber is inefficient for 2.5D wave modeling because it employs a large number of wavenumbers (k y ). This leads to a high cost of computer time in the linear-equation-solving processing, which detracts from the advantages of 2.5D modeling. In this paper, we use two analytic frequency-wavenumber-domain solutions for seismic waves in a homogeneous medium and an inhomogeneous media (two semi-infinite media in contact) to investigate the properties of the solutions and an efficient sampling strategy for choosing the wavenumbers. We have carried out analytic and numerical experiments with these solutions, and present adaptive Gauss–Legendre abscissae for the wavenumber sampling in terms of a modeling situation. We show that the effective range and the number of sampling points of the wavenumber define the adaptive sampling strategy, and they can be estimated in terms of the wavelength and the maximum source-receiver offset. We apply this sampling strategy to the finite-element method and demonstrate that the range and number of sampling points may be adapted for obtaining significant computational efficiency and satisfactory accuracy for every frequency component. Such 2.5D wave modeling can be readily applied for frequency-domain full-waveform inversion for seismic surface measurements and crosshole seismic waveform tomography.

Nondestructive imaging of small size voids at Akrotiri archaeological site, Thera Island, Greece, by seismic inversion techniques

High-resolution travel time tomography was used to explore the volcanic basement rock at the Akrotiri archaeological site, Thera (Santorini) Island. The survey was carried out in the context of a large scale project, in which the protective roof cover of old monuments is being replaced by a new environmentally friendly structure, which will be supported by 95 pillars drilled into the volcanic basement rock. Man-made or natural cavities (empty or half-filled with stones), ceramics, and other materials of archaeological interest were unveiled during the excavation of foundation shafts. The objective of this geophysical investigation was the detection of such voids in the vicinity of the excavated shafts, so that the overhead structure can be better supported and protected in the case of an earthquake event. The cross-hole seismic tomography technique was adopted for this purpose. A number of synthetic examples and a calibration experiment at a shaft with a known natural cavity clearly indicated that the tomographic inversion is capable of providing high-resolution 2-D velocity models. High S/N ratios ensured field seismic records of high quality. A set of stability tests was run to check the consistency of the method. Travel time residuals verified the validity of the final velocity depth sections, while model complexity trends showed a consistency between models after a certain number of iterations. The reconstructed velocity fields were quite consistent with the expected velocity structures based on the geologic descriptions of formations encountered during the drilling of the shafts. Impressive low-velocity structures attributed to natural or man-made cavities were reported to the constructing group of engineers, and a remedial action plan was being undertaken to support and improve the ground behavior.

Crosshole seismic tomography: working solutions to issues in real data travel time inversion

In travel time inversion of crosshole seismic tomography, when dealing with real data, the following three issues need to be addressed: how to suppress the effect of data errors, how to balance the data contribution and reference model, and how to properly set the geological constraints (well logs) in an inversion. Signal-to-noise ratios of travel time data picked from real crosshole seismic surveys likely depend on the vertical offset between a source and a receiver, since the overall attenuation will typically be greater for a longer ray path. Therefore, to suppress the effect of data errors in a least-squares solution, a data confidence matrix is used in practice where the weighting is set logically based on the vertical offset. When a model constraint is adopted in the inversion to seek a solution with least deviation from some prescribed background that is most likely to have no spurious structure, a trade-off parameter is used to explicitly balance the dependence of the inversion solution to the data contribution and the reference model. The latter is built by combining sonic logging information, which is hard geological evidence at the vicinity of wells, and an initial estimate from the travel time back-projection, a preliminary solution of the travel time tomography. Geological constraints are set so that the inversion solution should have the least deviation from the hard geological evidence, and more deviation for the remaining part of the survey region, since the true velocity model might differ from such an initial estimate and the true ray paths might deviate from the initial ray paths.

Crosshole ultrasonic tomography and its application to karst investigation

Crosshole ultrasonic tomography is referred to ray tracing by griding method and image reconstruction by combining the successively linearized iteration with damping least-squares QR decomposition, with which the result has better precision than that with crosshole seismic tomography by using the same inversion method in engineering geological investigation. It has been proved in practice that the investigation with crosshole ultrasonic tomography can clearly show the location and shape of the abnormal geological unit with the volume more than 60 cm when the crosshole distance exceeds 30 m. The result of investigation has been confirmed by the drilling. The crosshole ultrasonic tomography method will be extensively used in the engineering geological investigation.