Crosshole Seismic Tomography Research Articles

Physical characterization of mine goafs using high-resolution seismic cross-hole tomography via an adjoint method

Mine goafs often cause severe ground settlement and collapse, posing significant endangers to the safety of buildings and engineering constructions. Quantitatively detecting the spatial distribution and physical characteristics of goafs has always been challenging. To evaluate the potential threat of a fluorite mine goaf on a proposed expressway, we inverted cross-hole dataset to image the P-wave velocities between boreholes using a newly developed Eikonal equation based adjoint-state traveltime tomography method (ATTOMO). Independent comprehensive geophysical prospecting, including core analysis, borehole optical televiewer imaging and ultrasonic wave logging, indicates that our tomographic results at the central section could discern anomalies on a scale of 1 m or even smaller and detects velocity variations at a scale of 100–200 m/s. Our results show that the mine goaf is distributed in a northeastward trend with a width of 20–30 m, consistent with the exploiting features of the fluorite mine. The goaf is located approximately 20 m away from the bridge site, where only a small-scale goaf tunnel with dimensions of [Formula: see text] m was found at depths of 18–19.5 m. Our results suggest that the safety of the proposed expressway would not be significantly affected by the goaf, and thus no design changes are required.

Detect Multiple-Set Fractures by Crosshole Seismic Tomography for use in Environmental and Engineering Geophysics

Crosshole seismic tomography (CST) could be used to reconstruct the velocity section of media between two holes. One commonly used attribute of fractures reflected in seismic waves is the anisotropy of velocity. Therefore, in this study, the multiple symmetry axes of anisotropic media are utilized to simulate the multiple-set fractures in strata. The anisotropic algebraic reconstruction technique (AART) and anisotropic simultaneous iterative reconstruction technique (ASIRT), extensions of isotropic ART and SIRT, are developed to detect the medium with multiple symmetry axes and strong anisotropy. Full coverage of ray paths in azimuth is needed for these techniques. The performances of these techniques are tested by numerical and physical modeling. Testing results show that the proposed AART and ASIRT methods can successfully detect the multiple-set fractures when the observed data exceeds the estimated parameters. However, in complexly anisotropic and heterogeneous media, the estimation error will significantly increase when using these techniques if the number of estimated parameters is equal to or greater than the number of observed data.

Application of Cross-hole Seismic Tomography to Inferred Cavities Condition.

The construction of the Rajamandala hydroelectric power plant civil works has experienced critical problems that arise in the implementation process, with the occurrence of cracks cavities conditions in the headrace tunnel locations. We applied the cross-hole seismic tomography to detect important heterogeneities and mechanical proprieties of the formations between two boreholes in this case inferred cracks cavities condition. The seismic source is inside the borehole “a sparker” and the receivers “hydrophones” are in an adjacent one, the sparker is lowered into the borehole one step at the time. Seismic waves, propagating in the tunnel wall rock, spread widely, and are reflected and refracted when encountering interfaces between rocks with different acoustic impedances. Reflected waves returning to the receivers are recorded. After the processing of the first arrivals of the P waves we obtain a cross section of the seismic velocities in between the two wellbores. With the cross-hole seismic tomography, the basis could be provided to the tunnel construction and parameter adjustments to guarantee the construction safety. Our result shown that there is a weak zone above the tunnel with low Vp zone (~1.2 km/s) may be related to weak zone (may be fracture, unconsolidated rock, and fluid-filled rock or landslide caving). Checker-board resolution test is also conducted to determine the tomogram areas that are reliable to be interpreted. Our challenges are we have poor resolution due to not enough raypath. The Cross-hole seismic tomography can effectively and safely guide the excavation of the tunnel section working surface in combination with reconstructed images and excavation technology.

High-resoultion shallow anomaly characterization using cross-hole P- and S-wave tomography

Cross-hole seismic tomography is a high-precision method that can obtain the velocity structure between boreholes. At present, single-component geophones are mainly used for P-wave velocity estimation. However, S waves have a shorter wavelength than P waves, incurring a high-resolution tomography image. In this paper, we apply three-component cross-hole seismic tomography to shallow geological survey. First, three-component geophone is used to collect cross-hole seismic data. Secondly, the apparent velocity polarization method is used to separate the P and S wavefields from the recorded three-component waveform data. Then, a damped least-squares traveltime tomography is used to calculate the cross-hole P- and S-wave velocity structures. Finally, the distribution of Poisson's ratio between the boreholes is derived from the inverted P- and S-wave velocities. Numerical model experiments show that this method can solve some typical shallow subsurface problems, such as the detection of karst features and boulders. The proposed method is used to detect an underground air-raid shelter located in Hangzhou, Zhejiang Province, China, and the P- and S-wave velocities and the Poisson's ratio are obtained with high precision. The tomography image of the air-raid shelter agrees well with its known location. and the calculated Poisson's ratio distribution shows that the air-raid shelter may be partially filled with mucky clay. Three-component cross-hole seismic tomography achieves a high data acquisition efficiency and can simultaneously obtain shallow subsurface stratum velocities and Poisson's ratio with high precision. Thus, this technique can be used for shallow-subsurface surveys.

A case study on the application and workability of the crosshole seismic survey in a karstic area

Crosshole seismic tomography is currently one of the reliable methods used in identifying the seismic velocity patterns for the development in areas challenged by presence of karstic features due to dissolution of the underlying limestone bedrock formation. The crosshole seismic tomography which provides a persistent image of the borehole and its surroundings in the form of P-wave velocity, picks up in-depth information compared to other surface geophysical methods. A crosshole seismic survey which consists of 99 lines connecting a total of 48 boreholes were conducted at Jalan Cochrane for a proposed residential development to determine the subsurface geology of the area. The site was divided into four main towers, namely Tower A and B (East) and Tower A and B (West). Jalan Cochrane is underlain by the Kuala Lumpur Limestone formation. This survey was mainly conducted to identify the possible cavities and incompetent rocks which are fractured and infilled in the proposed area. The crosshole seismic survey tomography correlated well with the borehole information obtained. Several drop in velocity observed were concluded to be possible cavities in the bedrock. Data obtained in Tower A and B (West) are more uniform and have a uniform bedrock level compared to the data in Tower A and B (East) which has an undulating bedrock level as well as fractured rocks and infilled cavities. This paper presents the findings obtained from the crosshole seismic survey conducted at Jalan Cochrane which successfully mapped out the cavities and bedrock level.

Geophysical investigations for stability and safety mitigation of regional crude-oil pipeline near abandoned coal mines

Abstract This study aims for the protection of a crude-oil pipeline, buried at a shallow depth, against a probable environmental hazard and pilferage. Both surface and borehole geophysical techniques such as electrical resistivity tomography (ERT), ground penetrating radar (GPR), surface seismic refraction tomography (SRT), cross-hole seismic tomography (CST) and cross-hole seismic profiling (CSP) were used to map the vulnerable zones. Data were acquired using ERT, GPR and SRT along the pipeline for a length of 750 m, and across the pipeline for a length of 4096 m (over 16 profiles of ERT and SRT with a separation of 50 m) for high-resolution imaging of the near-surface features. Borehole techniques, based on six CSP and three CST, were carried out at potentially vulnerable locations up to a depth of 30 m to complement the surface mapping with high-resolution imaging of deeper features. The ERT results revealed the presence of voids or cavities below the pipeline. A major weak zone was identified at the central part of the study area extending significantly deep into the subsurface. CSP and CST results also confirmed the presence of weak zones below the pipeline. The integrated geophysical investigations helped to detect the old workings and a deformation zone in the overburden. These features near the pipeline produced instability leading to deformation in the overburden, and led to subsidence in close vicinity of the concerned area. The area for imminent subsidence, proposed based on the results of the present comprehensive geophysical investigations, was found critical for the pipeline.

Joint tomography of multi-cross-hole and borehole-to-surface seismic data for karst detection

Two-dimensional (2D) cross-hole seismic tomography is widely used in karst exploration. However, this method only obtains subsurface geological structures from the profiles of two adjacent wells and cannot acquire geological information from the lateral profile direction. The tomographic imaging area and accuracy are also limited owing to sparse ray density in the near-surface area. This approach is therefore not conducive to the spatial identification and evaluation of subsurface karst structures. To address this problem, we conducted multi-hole combined seismic tomography experiments and obtained high-density and high-quality seismic spatial data using an optimized observation system and acquisition parameters. We propose a new method of joint tomography of multi-cross-hole and borehole-to-surface seismic data. We performed 2D seismic tomography inversion by combining cross-hole data with borehole and surface data based on the theory of seismic tomography, and then used the data fusing from multiple wells based on the theory of kriging method. A pseudo-three-dimensional (3D) spatial velocity field of the experimental site is established. The results show that (1) the combined borehole and surface data provide richer ray-tracing information than the cross-hole dataset alone, (2) the pseudo-3D spatial velocity field and spatial distribution of seismic anomalies (karst) can be obtained using the proposed method, and (3) a comparison with actual borehole logging data verifies the effectiveness and accuracy of the proposed method in obtaining subsurface geological structures and detecting the spatial distribution of karst, which has significant practical and application potential.

Geophysical Methods and their Applications in Dam Safety Monitoring

The use of geophysical methods in dam sites investigations and safety monitory has proved their good value and versatility in many earthfill dam sites as early as the 1920s. In the following years great development has occurred in the methods, application procedures and tools used. They may be considered today as good ways for carrying out observation tasks on existing dams in non-intrusive and much faster and cheaper ways than the traditional geotechnical methods. It is possible using them to discover anomalies in the dam body or its foundation at an early stage and allowing quick intervention repair works. These methods seek to register and present variations in the basic geotechnical material properties in dams such as; bulk density, moisture content, elasticity, mechanical properties of rocks, electrical resistivity and mineralogy and magnetic properties and so forth. Such variations can indicate increasing seepage flow, progression in cracks’ sizes, formation of voids, caverns and other instability manifestations. Depending on how any investigation is carried out and the targeted anomaly, there is now selection of these methods such as: Electromagnetic Profiling (EM), Electrical Resistivity Tomography (ERT), SelfPotential (SP), Ground Penetration Radar (GPR), variety of Seismic Methods (SM) which can be applied using such equipment as in Seismic refraction, Seismic Reflection, Multi Analysis of Rayleigh surface waves (MASW) instruments, or using Refraction Micrometer (ReMi), macro-gravity method, and Cross-Hole Seismic Tomography. In addition, Temperature Measurements and other less used methods can be used like Microgravity measurement, Magnetic Profiling and Radio Magnetotelluric methods. An attempt is made here to cover the details of these methods, their advantages and limitations and to prove their usefulness in many dam sites all over the world. One observed issue is their adaptability to embankment dams more than to concrete dams and their popularity for checking seepage related problems and material changes within dam bodies and their foundations such as formation of voids and sinkholes.

Risk Assessment for Karst Hazards at a Facility in Saudi Arabia—A Case Study

The boom in industrial and infrastructure construction in Saudi Arabia requires in-depth knowledge of the underground conditions to build a sustainable project. This paper gave a practical example of the challenges facing an existing facility built on a karstic limestone formation in 1970’s. Multiple geophysical techniques such as Microgravity, Electrical Resistivity Imaging, Vertical Seismic Velocity Profiling, Cross Hole Seismic Tomography, Ground Penetration Radar, Multi-Channel Analysis of Surface Waves, as well as boreholes and down-hole video imagery, were deployed through the journey of assessing the karst associated hazards of potential sinkholes or general subsidence. This paper described the karst processes, site geology, geophysical and geotechnical exploration program, and characterizes the subsurface karst conditions at the eastern province of Kingdom of Saudi Arabia. Results and findings of preliminary, geotechnical and geophysical investigations have been utilized to identify the major factors influencing the karst formation. Analyses and integration of obtained results to estimate the type and magnitude of risk affected the major components of industrial facility at the project site using well-known statistical approach were presented in this paper. Consequently number of remedial measures implemented to combating the experienced and expected karst hazards at the studied industrial facility placed in karstified landform are given. A model for using integrated geophysical and geotechnical techniques to assess karst-related risks to a facility was further explored herein.

Modal analysis of Rayleigh waves using classical MASW-MAM approach: Site investigation in a reclaimed land

Surface wave tests such as the multi-channel analysis of surface waves (MASW) and microtremor array measurements (MAM) offer a fast and convenient way of measuring the shear wave velocity (Vs) of the ground for geotechnical site investigation due to their non-intrusive nature. However, it has been widely reported that the accuracy and resolution of the Vs profile depend on the construction of the dispersion image and its interpretation. While many advanced computational techniques have been proposed to process surface waves into a final Vs profile, we opted for the classical ‘picking and inverting’ approach in the current study. This is in view of the lower required computation power as well as the relatively uniform geology of the site. Surface wave tests (MASW, MAM and horizontal-to-vertical spectral ratio) were carried out at a remote reclaimed land in Singapore. A systematic methodology for the modal analysis of Rayleigh waves based on combined MASW and MAM tests is presented. The MAM dispersion image was constructed using the phase shift method, the extended spatial autocorrelation (ESAC), and the refraction microtremor (ReMi) method but a meaningful dispersion curve could only be obtained using the phase shift. Due to the modes of energy in the low frequency regime being unclear in the dispersion image, an idealized synthetic model of the site was set up to guide the mode identification process. The combined dispersion curve and HVSR profile were jointly inverted to construct the final Vs profile, which agreed quite well with the seismic cross-hole tomography experiments and bore-log information. It is also demonstrated that the inversion of the fundamental mode alone led to slight errors and low resolution in the final velocity profile, even for such a simple soil stratigraphy.

Stability analysis and design of stabilization measures for Chenab railway bridge rock slopes

This article presents the challenges encountered during the stability analysis and stabilization of two Himalayan rock slopes supporting the Chenab Railway Bridge in India, which will be the highest railway bridge in the world after completion. Owing to the importance of the project, extensive geological investigations, in situ and laboratory tests were carried out using conventional and advanced methods to evaluate various intact rock, rock joints, rock mass properties and rock mass quality present at the site. Once the conclusive results were made regarding these properties, stability of slopes were evaluated against both structurally controlled and stress-controlled failures using the developed probabilistic approach due to the presence of high variability in intact rock, rock mass and rock joints properties. It was observed that while the slopes were stable against stress-controlled failures, the possibility of structurally controlled failures exists as observed in the probabilistic approach in which the conventional deterministic approach was not able to detect. Different stabilization measures, i.e. rock bolts, slope flattening, grouting, drainage and shotcreting, were designed at the site to reduce the probability of failures for the slopes, deformability of rocks present due to cavities and high aperture joints, shallow block failures along the slope surface and pore-pressure build-up. To check the efficiency of the stabilization measures, cross-hole seismic tomography is carried out at the site and further in situ measured displacements are compared with the displacements estimated using the developed probabilistic approach. The case study used in this paper is a real-time ongoing bridge project of global interest and the data and analysis presented will be of great benefit to the researchers and practitioners working on similar problems.

Experience of using seismoacoustic methods for a detailed modeling of geological environment at design of high criticallity objects of nuclear construction

Engineering surveys for the design of facilities with high criticality rating require a switch from methods with discrete assessment of environmental parameters (drilling and static / dynamic sounding), to methods that allow to obtain these values continuously in а massif. The most efficient way to solve this problem is by performing crosshole seismic tomography (CST). The article presents the results of CST during geophysical studies at the site of projected nuclear power plant (NPP). Four cross sections were obtained as a result of survey at distances from 24 to35 metersbetween the boreholes. Requirements for the boreholes preparation are specified in article, along with description of equipment for seismic signals excitation and registration, the examples of seismic records (gathers), methods of analysis and data processing are also shown. The velocity sections for compressional and shear waves obtained as a result of tomographic inversion are presented. The values of dynamic characteristics of soil properties (Young modulus, shear modulus, Poisson ratio) and their distribution in the inter-well space were calculated with the use of data on the soil density of the massif. Also, the article shows the result of applying the technique of incident and reflected waves fields allocation, which were obtained as a result of seismic cone penetration test (SCPT) to depths of 20–30 meters aiming to get detailed geological cross-section.

The potential of seismic cross-hole tomography for geotechnical site investigation

Seismic cross-hole tomography has been commonly used in oil and gas exploration and the mining industry for the detection of precious resources. For near-surface geotechnical site investigation, this geophysical method is relatively new and can be used to supplement traditional methods such as the standard penetration test, coring and sampling, thus improving the effectiveness of site characterization. This paper presents a case study which was carried out on a reclaimed land in the Eastern region of Singapore. A seismic cross-hole test was performed by generating both compressional waves and shear waves into the ground. The signals were interpreted by using first-arrival travel time wave tomography and the arrival times were subsequently inverted using Simultaneous Iterative Reconstruction Technique (SIRT). A comparison with the borehole logging data indicated that P-wave velocity model cannot provide sufficient information about the soil layers, especially when the ground water table is near the surface. The S-wave velocity model seemed to agree quite well with the variation in the SPT-N value and could identify to a certain extent the interface between the different soil layers. Finally, P-wave and S-wave velocities are used to compute the Poisson's ratio distribution which gave a good indication of the degree of saturation of the soil.

Relationship Between Geological Structures and Velocity Anomalies in Two-Dimensional Tomography with First-Arrival Seismic Travel Time (FAST): A New Recognization

Two-dimensional (2D) tomography with first-arrival seismic travel time (FAST) is widely and effectively used to detect geological anomalies. For detecting small geostructures in a coalbed, 2D seismic tomography with FAST depends primarily on the seismic wave velocity. The seismic velocity tomography of a coalbed, sources and receivers are placed in the coal seam, differs from the cross-hole seismic tomography on the ground. The response of the seismic wave velocity anomaly to common geostructures, like the response of a high-velocity anomaly to a normal fault, has been comprehensively summarized. However, the interpretation of the response has not been fully discussed. Therefore, in this paper, a numerical simulation is used to corroborate the relationship. A special three-layer coalbed model was constructed and used to investigate the relationship between geostructures and velocity anomalies in coalbeds using seismic ray tracing technology. The results indicated that the thickness and inclinations of the coalbed, the size and position of abnormal geological bodies are the key factors for 2D tomography with FAST on the coal mining face. The morphology of the roof and floor strata determines the path that the seismic wave travels. Finally, the roof and floor strata are other important factors affecting the velocity anomaly.

Seismic Travel Time Tomography in Modeling Low Velocity Anomalies between the Boreholes

Travel time cross-hole seismic tomography is applied to describing the structure of the subsurface. The sources are placed at one borehole and some receivers are placed in the others. First arrival travel time data that received by each receiver is used as the input data in seismic tomography method. This research is devided into three steps. The first step is reconstructing the synthetic model based on field parameters. Field parameters are divided into 24 receivers and 45 receivers. The second step is applying inversion process for the field data that consists of five pairs bore holes. The last step is testing quality of tomogram with resolution test. Data processing using FAST software produces an explicit shape and resemble the initial model reconstruction of synthetic model with 45 receivers. The tomography processing in field data indicates cavities in several place between the bore holes. Cavities are identified on BH2A-BH1, BH4A-BH2A and BH4A-BH5 with elongated and rounded structure. In resolution tests using a checker-board, anomalies still can be identified up to 2 meter x 2 meter size. Travel time cross-hole seismic tomography analysis proves this mothod is very good to describing subsurface structure and boundary layer. Size and anomalies position can be recognized and interpreted easily.

Fracture detection and imaging through relative seismic velocity changes using distributed acoustic sensing and ambient seismic noise

Fracture systems are important pathways for fluid and solute transport and exert a critical influence on the hydraulic properties of aquifers and reservoirs. Therefore, detailed knowledge of fracture locations, connections, and evolution is crucial for both groundwater and energy applications (e.g., enhanced geothermal, oil and gas recovery, carbon sequestration, and wastewater injection). The innovative combination of distributed acoustic sensing (DAS) and ambient seismic noise techniques has the potential to detect and characterize fracture systems at high-spatial and temporal resolution without an active source. To test this, we conducted a multiphysics field experiment at Blue Canyon Dome, New Mexico. A novel energetic material developed by Sandia National Laboratories was used to generate fractures in two separate stimulations. Ambient noise was recorded before and after each stimulation using fiber-optic cables installed in the outer annulus of four boreholes surrounding the stimulation hole at a radius of 1.2 m. The Python package MSNoise was used to compute crosscorrelations and measure changes in velocity between each time period relative to the initial (prestimulation) time period. The majority of channel pairs showed a velocity reduction (average −3% relative velocity change) following both stimulations. We used a 3D Bayesian tomography approach to resolve spatial variations by utilizing differences between channel pairs. Results showed that the greatest velocity reduction was concentrated near the center of the test area and suggested the presence of a near-vertical fracture, oriented northeast to southwest for depths >19 m below ground surface and extending slightly to the southwest corner. These results were generally consistent with crosshole seismic tomography time-lapse images. DAS technology provides valuable sensing capability and — when used with a passive seismic approach — shows great promise for monitoring and characterization of fractured-rock systems.

Bridge pier foundation evaluation using cross-hole seismic tomographic imaging

An ambitious project connecting Jammu and Srinagar through a railway link in tectonically active and geologically complex Himalayan Mountain terrain is under progress. Under this project, the world's highest (359m) railway arch-bridge is under construction across the River Chenab in the northern territory of India. This mega engineering structure has a two-fold ribbed arch design, comprising of steel girders. During the excavation for one of the concrete pillars on the right abutment, wide open joints and weak/shear zones were noticed. The width of these joints varies from 30 to 50cm, trending along N170° with a dip of 65°. The foundation area of this pillar is 13m×24m and on the cut slopes of the right bank of Chenab River. These exposed joints and weak zones were treated with consolidation grouting to strengthen the foundation area. To delineate the extent of these joints and weak zones below the foundation level, seismic tomography was carried out in five boreholes drilled for this purpose to cover the 300sq-m area. The results of cross-hole seismic tomography reveals the presence of three low velocity (≤2600m/s) anomalous zones below the foundation area. This also ascertained the efficacy of grouting in consolidating the joints and weak zones. Later, rock-mass quality (Q) was determined based on the relationship between the P-wave velocity and the Q-value (Barton, 2002) to infer the support system for the slope stabilization below the foundation. 3-D visualization of the seismic velocity demarcates the extent of weak or untreated zones. This methodology facilitates to update the design parameters according to Q-values during the construction stage and estimate the required level of reinforcement and support system. Similar methodology can be applicable in other areas under same site conditions.

Characterizing CO2 plumes in deep saline formations: Comparison and joint evaluation of time-lapse pressure and seismic tomography

Monitoring the migration of sequestered [Formula: see text] in deep heterogeneous reservoirs is inherently difficult. Geophysical methods have been successfully used, but flow conditions are only indirectly linked to the measured properties. Besides geophysical methods, pressure tomography (PT) is proposed as an alternative method to depict the structure of deep saline formations for [Formula: see text] sequestration and to continuously delineate [Formula: see text] plumes. In contrast to more established geophysical measurements, pressure transients are directly related to flow properties, which allows for the estimation of permeability. We investigate the influence of aquifer heterogeneity on PT performance, and we compare the PT results to crosshole seismic tomography (ST). Multilevel fluid injections and high-frequency P-wave pulses are induced in a simulated deep borehole, and the recorded signals at another well are processed by a traveltime inversion scheme. The reservoir structure is inferred by clustering the inverted hydraulic diffusivity prior to [Formula: see text] injection, and the plume distribution is determined by clustering the tomograms of the inverted mixed-phase diffusivity difference and P-wave velocity difference. The clustered structures are then used for zonal calibration to acquire the saturation within the plumes. Modeling results indicate that PT provides clearer structural information on the [Formula: see text]-free aquifer due to its direct linkage to permeability. However, the plume depicted by PT can be ambiguous, whereas ST is less sensitive to the prevailing heterogeneity of permeability at postinjection and can thus image the plume more clearly. PT and ST can be complementary to each other through the joint clustering to improve plume shape identification and estimation of spatial [Formula: see text] saturation.

Multi-hole seismic modeling in 3-D space and cross-hole seismic tomography analysis for boulder detection

A boulder stone, a common geological feature in south China, is referred to the remnant of a granite body which has been unevenly weathered. Undetected boulders could adversely impact the schedule and safety of subway construction when using tunnel boring machine (TBM) method. Therefore, boulder detection has always been a key issue demanded to be solved before the construction. Nowadays, cross-hole seismic tomography is a high resolution technique capable of boulder detection, however, the method can only solve for velocity in a 2-D slice between two wells, and the size and central position of the boulder are generally difficult to be accurately obtained. In this paper, the authors conduct a multi-hole wave field simulation and characteristic analysis of a boulder model based on the 3-D elastic wave staggered-grid finite difference theory, and also a 2-D imaging analysis based on first arrival travel time. The results indicate that (1) full wave field records could be obtained from multi-hole seismic wave simulations. Simulation results describe that the seismic wave propagation pattern in cross-hole high-velocity spherical geological bodies is more detailed and can serve as a basis for the wave field analysis. (2) When a cross-hole seismic section cuts through the boulder, the proposed method provides satisfactory cross-hole tomography results; however, when the section is closely positioned to the boulder, such high-velocity object in the 3-D space would impact on the surrounding wave field. The received diffracted wave interferes with the primary wave and in consequence the picked first arrival travel time is not derived from the profile, which results in a false appearance of high-velocity geology features. Finally, the results of 2-D analysis in 3-D modeling space are comparatively analyzed with the physical model test vis-a-vis the effect of high velocity body on the seismic tomographic measurements.

Crosshole seismic tomography with cross-firing geometry

We have developed a case study of crosshole seismic tomography with a cross-firing geometry in which seismic sources were placed in two vertical boreholes alternatingly and receiver arrays were placed in another vertical borehole. There are two crosshole seismic data sets in a conventional sense. These two data sets are used jointly in seismic tomography. Because the local sediment is dominated by periodic, flat, thin layers, there is seismic anisotropy with different velocities in the vertical and horizontal directions. The vertical transverse isotropy anisotropic effect is taken into account in inversion processing, which consists of three stages in sequence. First, isotropic traveltime tomography is used for estimating the maximum horizontal velocity. Then, anisotropic traveltime tomography is used to invert for the anisotropic parameter, which is the normalized difference between the maximum horizontal velocity and the maximum vertical velocity. Finally, anisotropic waveform tomography is implemented to refine the maximum horizontal velocity. The cross-firing acquisition geometry significantly improves the ray coverage and results in a relatively even distribution of the ray density in the study area between two boreholes. Consequently, joint inversion of two crosshole seismic data sets improves the resolution and increases the reliability of the velocity model reconstructed by tomography.