Cross-hole Seismic Surveys Research Articles

Cross-Hole Full-Waveform Inversion—A New Approach for Imaging Quartz Vein-Hosted Gold Deposits

To enhance the efficiency of mine planning, mining companies wish to understand the structure and extent of ore-bearing rocks as well as possible. Conventional seismic reflection surveys are not well suited for this purpose as they provide an image containing only the location of reflectors, and do not provide physical property information to discriminate between ore and gangue material. Full-waveform inversion (FWI) is a powerful inversion technique, which is able to recover the physical properties of the subsurface at a far greater spatial resolution and accuracy than conventional seismic methods. In this study, we synthetically examined the feasibility of using FWI to image quartz vein-hosted gold deposits. We utilised the Curraghinalt gold deposit in Northern Ireland to parameterise our models, where mineralisation is bound entirely to thin (1–3 m) and steeply dipping (>45°) quartz sulphide veins. Firstly, we demonstrated that a conventional surface seismic reflection survey geometry alongside FWI is infeasible for imaging quartz vein-hosted gold deposits. Secondly, we explored a cross-hole seismic survey geometry consisting of sources and receivers placed down vertical boreholes. This cross-hole survey geometry is capable of generating synthetic datasets such that FWI can recover the position of the veins in space accurate to within 0.5 m relative to their true positions, and recover their physical properties with an accuracy greater than 90%, beginning from an entirely homogeneous starting model. We conclude it is essential the source and receiver boreholes be positioned such that both transmitted and reflected arrivals are present in the datasets, otherwise FWI will fail to accurately recover the position and physical properties of the veins. This opens a new avenue for FWI to play a major role in the planning stages and development of gold mines around the world.

Incorporating geotechnical and geophysical investigations for underground obstruction detection: A case study

Determining the location and boundary of underground obstructions and/or anomalies is a common problem and often a great challenge for tunneling and underground construction. In this study, geotechnical investigations (penetration tests and borehole drilling/sampling) and geophysical investigations (surface wave method and cross-hole seismic method) were conducted with the aim of identifying the location and boundary of rock obstructions in Changi East, Singapore. The surface wave method is frequently used in the sites with lateral homogeneity in previous studies, but its application in the sites with rock obstructions is rare. The experimental results of this study indicate that the surface wave method is also able to determine the upper surface of rock obstructions, but difficult to identify the lateral and bottom boundaries of rock obstructions. To improve the precision of detection, the full waveform inversion (FWI) method was used to process the data from the cross-hole seismic survey. The results indicate that the inversion precision of P-wave is higher than that of S-wave. The horizontal and vertical ranges of rock obstructions in the P-wave inversion results are 14–26 m and 7.5–11.0 m respectively, roughly consistent with the results of penetration tests (about 15–25 m) and borehole log (8.85–10.80 m). This result proves that the sequential application of first-arrival time analysis and FWI can effectively delineate the boundary of rock obstructions. Finally, the results of various detection methods were analyzed and compared in this study. Considering the advantages of various methods, we propose a cost-effective and high-precision workflow containing both geotechnical and geophysical investigations.

Determination of Bearing Capacity of the Soil Using Cross-Hole Seismic Survey for a Water Treatment Plant, Thi Qar, Southern Iraq

The geophysical method (Cross-Hole Survey) was used to assess the bearing capacity of the soil in the study area to build a water treatment plant. It was noted, that the velocities of the primary and secondary waves decrease at the depths of 4 and 5 meters compared to the depths above and down, so, the lithology or physical properties of this specific soil have changed. It was also observed that seismic wave velocities are low in general due to the medium density and hardness of the soils in the study area. Furthermore, the elasticity modulus values were shown to decrease at depths of 4 and 5 meters. It is further evidence of the soil's weakness at these depths. When the type of layer changed, the elastic modulus changed as well. The geotechnical properties between the wells were evaluated based on the velocities of seismic waves (S-waves and P-waves) for classifying the load capacity values with the stiffness. The bearing capability of the soil was found to be medium-stiff at depths of 2 and 3 meters, but soft at depths of 4 and 5 meters. At depths of 6, 7, 8, 9, and 10 meters, the bearing capacity is stiff to very stiff.

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.

Evolution of excavation damaged zones in Opalinus Clay shale inferred from seismic investigations

We present the results of new and repeat gallery- and borehole-scale seismic experiments carried out in two excavation damaged zones (EDZs) of 10 (gallery 08) and 20 (gallery 98) years of age at the Mont Terri Underground Rock Laboratory in St. Ursanne, Switzerland. The initial geometry and rock mass properties of the EDZs were studied during and shortly after excavation—allowing for a direct comparison of measurements collected in 2018 to those completed in the past.In gallery 08 parts of the EDZ remain unchanged over ten years, whilst other zones show improvement (i.e., self-sealing) or considerable degradation. In addition, velocity tomograms from 2018 measure extensive areas of p-wave velocity reduction in comparison to the surrounding rock mass at depths shallower than 3.5 m. Isolated extensile fractures are still found in boreholes up to a maximum radial extent of 4.2 m. Data from borehole interval velocity measurements and cross-hole seismic surveys from this gallery suggest the damaged zone still extends up to 1.9 m and 3.1 m radial depth.Refraction seismic results in gallery 98, in contrast, suggest a less pervasive, shallower (<1.0 m) EDZ with a lower frequency of extensile fractures. The smaller circumference of this gallery can in part explain this; however, when compared to various measurements collected during and after excavation, some reduction in EDZ extent is inferable. Both borehole interval velocity measurements and seismic refraction data record p-wave velocity values higher on average than those in gallery 08 and without obvious areas of velocity reduction. Most interval velocity measurements likewise do not measure zones of decreased velocity (e.g., corresponding to rock mass damage) at this location. This suggests more self-sealing has occurred over the twenty-year post-excavation period in gallery 98 in comparison to the ten-year period in gallery 08. As repository safety depends upon the integrity of the rock mass surrounding an excavation, these results are relevant for mid-term assessments of safety (i.e., especially prior to repository closure).

Environmental hazard assessment for ground failure in Jeddah city, western Saudi Arabia, through cross-hole seismic testing

The environmental threats of soil subsistence in Jeddah city have been assessed using cross-hole seismic testing. These threats affect the state of health of roads, buildings, near-surface foundations ensuring a reliable and economic infrastructure design, as unanticipated site conditions can cause significant traffic problems, soil subsidence, and structural collapse. Cross-hole seismic testing is of utmost importance in determining velocity profiles of P- and S-waves for environmental hazard assessment. These velocity profiles are required to calculate the geotechnical parameters of soil nearby important structures. A cross hole seismic survey was conducted at boreholes close to Jeddah desalination plant. A sledgehammer horizontally striking a wooden plate was used to generate S-waves polarized in the crossline and inline directions. A vertical hammer hitting a metal plate to generate the P-wave then the first break arrival times were inverted for the P- and S-wave velocity structure estimation. The equipment and procedures comply with the American Society for Testing and Materials (ASTM) D 4428/D 4428M-91 standard test method for cross-hole seismic testing. The estimated velocities were utilized to calculate the elastic moduli, geotechnical parameters and density variations within the mapped area. These parameters agree with the lithological variations of near- surface sediments from borehole logs. Finally, the study area illustrated soft, fractured, and poorly graded sediments, which, in turn, require special treatment from engineering point of view before establishing the important facilities in the area. These results are highly recommended and have to be considered to mitigate the environmental hazards in Jeddah city.

Sediment Thickness Estimation in Serpong Experimental Power Reactor Site Using HVSR Method

A study about sediment thickness mapping was conducted in Serpong Experimental Power Reactor Site. This reactor is planned to be built in a high seismic activity area. Sediment thickness is one of many factors that could affect the site response. A high sediment deposit could amplify and prolong the earthquake waves. This condition could make the reactor building more vulnerable to seismic hazard. The sediment thickness was estimated using the microtremor HVSR method. The parameter used for sediment thickness calculation was obtained from the borehole and cross-hole seismic survey and was tested to ensure that the chosen parameter and the method were accurate. The deviation found in the borehole data was also calculated to determine the accuracy level of the calculation. The current planned location for the reactor is in a relatively low sediment deposit area with 8.8 m of thickness, however there is an area, west of the current location that might be a better location for the reactor based on the result of this study.

Delineation of Cavity in Downstream Surge Chamber at Punatsangchhu-II Hydroelectric Project, Bhutan

On 3rd March, 2016, there occurred a massive roof fall incidence in one of the major caverns of Punatsangchhu-II Hydroelectric Project (PHEP-II), Bhutan which halted the operations in the major caverns and required additional strengthening measures. Prior to concluding anything on the treatment measures for the muck flown into the cavern and the cavity formed over the crown of Downstream Surge Chamber, there was a strong need to determine the extents of cavity. Even to understand the influence of cavity formation on adjacent caverns, the cavity needs to be delineated. Therefore, the management of PHEP-II adopted several techniques to decipher the shape and dimensions of the cavity like, surveying, geophysical, cross-hole seismic surveys, borehole scanning, and exploratory drilling. The tentative shape of the cavity could finally be established. The findings from each method are explained in this paper.

Visco-acoustic data modeling using optimum layer approximation technique: greenwood oil field example, USA

It is desirable to use Finite Element (FE) or Finite Difference (FD) method to simulate elastic data for cross-hole seismic surveys. But FE and FD are computationally intensive and time-expensive for full wave-field modeling of large cross-hole survey. The computation cost – space and time- of elastic data modeling inhibits the use of synthetic data for providing solution to cross-hole geophysics problems .On the other hand, acoustic synthetic data require less computation time but lacks some of seismic arrivals seen in the field data. In this study a recursive reflectivity method incorporating P and S wave properties and attenuation effects is formulated to model cross-hole seismic data. The model is tested using a set of field data acquired in Green Wood field, USA. The visco-acoustic data modeled by the recursive reflectivity method show improvement over the acoustic data and gave a perfect match to the elastic and field data. The visco-acoustic data featured the different seismic arrivals seen in the field and elastic synthetic data. The data simulation method is stable, efficient, and fast. The inclusion of a scheme for determining the optimum layer model further reduced the computation time without compromising the quality of the results: only about one-eight of the time require for elastic data is needed to produce the equivalent visco-acoustic data. Because of its low computation cost, the study highlights the strong potentials for solving the problem of inefficient algorithm for processing and imaging cross-hole data, with a consequence of using cross-hole technology as a routine in oil and gas industry.Keywords: Cross-hole seismic survey; synthetic data; acoustic wave simulation; visco-acoustic

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.

Tomographic inversion of near-surface Q factor by combining surface and cross-hole seismic surveys

The estimation of the quality factor Q plays a fundamental role in enhancing seismic resolution via absorption compensation in the near-surface layer. We present a new geometry that can be used to acquire field data by combining surface and cross-hole surveys to decrease the effect of geophone coupling on Q estimation. In this study, we drilled number of receiver holes around the source hole, each hole has different depth and each geophone is placed geophones into the bottom of each receiver hole to avoid the effect of geophone coupling with the borehole wall on Q estimation in conventional cross-hole seismic surveys. We also propose a novel tomographic inversion of the Q factor without the effect of the source signature, and examine its stability and reliability using synthetic data. We estimate the Q factors of the near-surface layer in two different frequency bands using field data acquired in the Dagang Oilfield. The results show that seismic absorption in the near-surface layer is much greater than that in the subsurface strata. Thus, it is of critical practical importance to enhance the seismic solution by compensating for near-surface absorption. In addition, we derive different Q factors from two frequency bands, which can be treated, to some extent, as evidence of a frequency-dependent Q.

Micro gravity and cross-hole seismic to monitor water storage changes in aquitards

SUMMARY Aquitards surround many aquifers. Due to their comparatively high storage capacity, they will control the hydraulics of aquifers in the long term. Long term management of aquifers therefore requires a good understanding of aquitard characteristics. To quantify water storage changes in an alluvial sequence a micro gravity station has been set up adjacent to an agricultural field in the Liverpool Plains in NSW, Australia. Gravity changes over time will provide an integrated signal of the total water storage changes at the site. Piezometers at the site will allow to determine at which depth the storage change is occurring. To derive water storage changes from the water level changes in the aquitards and confined aquifers at the site, the specific storage (S s) of the formation needs to be known. Two insitu methods to derive S s are applied. Firstly, S s is derived based on the bulk modulus, which is obtained in a cross hole seismic survey and secondly S s is derived based on pressure analysis in the lysimeters. Ss values derived from the pressure analysis in the lysimeters are one magnitude higher than those based on the cross hole seismic. This difference needs to be further investigated and might be due to the thick unsaturated zone (20 m) at the site, which appears to interfere with the pressure analysis in the lysimeters.

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.

Cross-hole seismic measurements for detection of disturbed ground beneath existing structures

Cross-hole seismic surveys are well established for non-invasive investigations of geological structure and are well suited for detecting hazards beneath buildings. Inversion of the seismic data confirms the value of tomographic imaging as a means of expressing the results in a form that is of practical value to the engineer. Physical property research has identified petrophysical factors, which control the attenuation and velocity of compressional and shear waves in reservoir-type rocks. This indicates that there is value in assessing changes in amplitude of seismic waves in addition to their velocity. The practical value of such surveys is demonstrated using case histories. While minimising the losses between the borehole casing and the formation (e.g. using bentonite grout) is required for amplitude studies, it is noted that diffraction causes a bias towards higher values in velocity studies. Two surveys are described where cross-hole compressional waves have been used to investigate the foundation conditions under domestic dwellings in response to subsidence. The results show that a wide range of velocities and signal loss is possible above the water table and that consideration of amplitudes enables areas of disturbed ground to be identified. Automatic data acquisition and processing has enabled a 3-D dataset to be generated using 16 pairs of boreholes and over 5000 individual seismic rays.

Crosshole seismic imaging for sulfide orebody delineation near Sudbury, Ontario, Canada

Crosshole seismic instrumentation based on a piezoelectric source and hydrophone detectors were used to gather seismograms between boreholes at the McConnell orebody near Sudbury, Ontario. High‐frequency seismograms were recorded across rock sections 50 to 100 m wide containing a continuous zone of massive sulfide ore. First‐arrival traveltimes obtained from a detailed scan were used to create a P-wave velocity tomogram that clearly delineated the ore zone. Refraction ray tracing on a discrete layer model confirmed the main features of the tomogram. The survey demonstrated that it is possible to conduct cost‐effective, high‐resolution crosshole seismic surveys to delineate ore bodies on a scale useful for planning mining operations.

A 15 kHz cross-hole seismic survey across a fracture at the AECL Underground Research Laboratory

A 15 kHz cross-hole seismic tomography survey was completed between two boreholes that cross a well-known subvertical fracture called the "Room 209 fracture" on the 240 Level of the Atomic Energy of Canada Limited Underground Research Laboratory. This survey measured in situ P- and S-wave velocity and amplitude relations across the fracture between the two boreholes. A velocity anisotropy of about 1.5% (peak-to-peak) exists in the rock mass for both P and S waves. Consistently faster velocities were observed for rays oriented roughly parallel to the Room 209 fracture. This anisotropy is consistent with geological and geotechnical models of microcrack populations for the region. Tomographic velocity images for both the P- and S-waves show a distinct change across the fracture. Higher velocities occur east of the fracture, whereas lower velocities occur to the west. The velocity differences indicate minor changes in the rock structure from one side of the fracture to the other which correlate with observed differences in alteration and fracture frequency. There is a slight reduction in velocity at the fracture itself. A study of seismic amplitudes was also completed. A weak anisotropy also exists in the amplitude data, with higher amplitudes for rays that parallel the fracture direction. The P-wave attenuation ranges between 0.03 and 0.7 dB/m across the survey panel. The region of the fracture shows a subtle increase in the P-wave attenuation of about 0.04 dB/m above the local background. These measurements provide a useful demonstration of the sensitivity of the technique. The results correlate well with geological models and simple physical models for seismic wave propagation in sparsely fractured granite.

Hot dry rock research experiments at Fjällbacka, Sweden

A number of in-situ experiments, aimed at investigating geological, hydrogeological and hydromechanical aspects of HDR reservoir development were carried out at the Fjällbacka test site in western Sweden between 1984 and 1995. By means of hydraulic stimulations, a roughly horizontal reservoir that connects two 500 m deep wells was created. An open-loop circulation test performed between the wells gave a flow recovery of about 50%. All major fluid injections have resulted in microseismic activity due to shear failures. A cross-hole seismic survey of the inflated reservoir provided some additional evidence of the distribution of hydraulically active fractures.

A 15 kHz cross-hole seismic survey across a fracture at the AECL Underground Research Laboratory

A 15 kHz cross-hole seismic survey across a fracture at the AECL Underground Research Laboratory

Research article: Crosshole seismic imaging of a fractured reservoir

Time-lapse crosshole seismic surveys have been conducted over a naturally fractured reservoir to demonstrate the feasibility of obtaining high-resolution seismic images to monitor changing fluid saturation within the reservoir. The experiment was perforrned at the Chalmers Hot Dry Rock (HDR) geothermal research site in Fjällbacka, Sweden, across a fractured reservoir. in crystalline rock at around 450 m depth. Although the survey technique being demonstrated is intended to be applied to oil reservoirs, the Fjällbacka site was selected as the geothermal reservoir there is analogous to a naturally fractured oilfield reservoir but is far more readily accessible for experimental work.

Rock engineering for Dundee Inner Ring Road

Abstract Stage XII of Dundee Inner Ring Road runs through a built-up part of the city of Dundee and includes a 145 m long cut and cover tunnel and a 355 m length of road in vertical open cut with retaining walls. Excavations extended 14 m below the level of adjacent ground. During construction, disturbance to nearby buildings and services had to be minimized; close attention was paid to the stability of the excavations and the selection of excavation techniques. This paper presents an outline of the site geology, describes the approach to geotechnical investigations and highlights certain aspects of design and construction, particularly the stabilization of the sidewalls and the role of detailed geotechnical mapping. The route runs within a Devonian age sandstone-mudstone-shale-basic lava sequence dipping at 20 to 30 degrees. Historically, stability problems have occurred in these formations in the Dundee area. Site investigations included inclined coring to clarify the disposition of faults, exploratory trenching, cross-hole seismic surveys and rock anchor trials. Particular attention was paid to elucidating the discontinuity characteristics with reference to the presence of low shear strength planes. Rock classification techniques were used to assist in excavatibility studies and to provide guidelines for rock reinforcement selection. Excavations were mapped in detail during construction, and supplementary rock testing was carried out. Updated interpretations were prepared, enabling the design for the temporary and permanent works to be kept under review and adjusted as work progressed.