Borehole Seismic Methods Research Articles

3D DAS VSP for Coal Seam Exploration: A Case Study from Queensland, Australia.

Seismic methods are extensively used in coal mining for expanding resource discoveries and definition as well as for mine monitoring. However, the use of borehole seismic methods is relatively uncommon due to the high cost of borehole seismic acquisition using conventional downhole tools. The introduction of distributed acoustic sensing (DAS), which uses optical fibres to record seismic data, has dramatically increased the cost-effectiveness of borehole seismic methods. Fibre-optic cables are inexpensive and, once deployed in a borehole, can be abandoned or used later for further monitoring of the subsurface. The case study presented here concerns the use of DAS to record a 3D VSP (vertical seismic profiling) for coal seam exploration in Queensland, Australia. This study trialled effective strategies for deploying cables into boreholes and demonstrated how this technology could be incorporated into the standard coal exploration process. The final processing results produced a high-resolution 3D seismic cube where the coal seams below the basalt cover are clearly identifiable around the boreholes. Permanent installation of the fibre-optic cables into a set of boreholes provides immediate benefits of 3D seismic imaging and can create additional value in utilising these sensors for further discrete or continuous subsurface measurements, including stability monitoring of underground workings and detection of methane accumulations.

Field Evaluation of Two Impulsive Downhole Seismic Sources in Crosswell and Reverse VSP Geometries and High-Resolution Characterization of Near-Surface Texas Gulf Coast Sediments

Borehole seismic methods have been widely used for characterizing the shallow subsurface. Accurate analysis of their data is aided by a solid understanding of the borehole sources' characteristics. This study presents a field evaluation of two impulsive borehole seismic sources (Trident's Scorpion sparker and RT Clark's Ballard weight drop) in crosswell and reverse vertical seismic profile (RVSP) geometries at a Gulf of Mexico coastal site with two shallow vertical wells. The acquired data is then utilized to characterize the near-surface coastal sediments. The Scorpion source generated P-wave dominant frequencies that were recorded as 650 Hz and 250 Hz in the crosswell and RVSP geometries, respectively. For the Ballard source in the two geometries, the P-wave dominant frequencies were 1100 Hz and 250 Hz. We were also able to pick direct S-wave arrivals with the Ballard source, and their dominant frequencies were 100 Hz and 40 Hz for in situ and surface recordings, respectively. The average signal-to-noise ratio (SNR) recorded with the Scorpion data for the crosswell geometry and RVSP, respectively, is 13 and 6, and for the Ballard source, 62 and 30. We also investigated the source radiation patterns and signature wavelets. Seismic tomography was performed for the area between the two wells. Low P-wave and S-wave velocities correspond to three fresh water-saturated sand zones identified from drilling cuttings and previous well log data. Also, the plot of velocity of P-wave (Vp) versus S-wave (Vs) fits reasonably to the Mudrock Line. Both sources can excite repeatable energetic seismic signals up to 150 m away and could be useful in many geotechnical settings and even single-well imaging.

Borehole seismic methods for geologic CO2storage monitoring

Borehole geophysical methods are a key component of subsurface monitoring of geologic CO2storage sites because boreholes form a locus where geophysical measurements can be compared directly with the controlling geology. Borehole seismic methods, including intrawell, crosswell, and surface-to-borehole acquisition, are useful for site characterization, surface seismic calibration, 2D/3D time-lapse imaging, and microseismic monitoring. Here, we review the most common applications of borehole seismic methods in the context of storage monitoring and consider the role that detailed geophysical simulations can play in answering questions that arise when designing monitoring plans. Case study examples are included from the multitude of CO2monitoring projects that have demonstrated the utility of borehole seismic methods for this purpose over the last 20 years.

Characterization and imaging of a hydrothermally active near-vertical fault zone in crystalline rocks based on hydrophone VSP data

A shallow, hydrothermally active, fault zone embedded in fractured crystalline rocks of the Central Swiss Alps has been drilled and geophysically explored in view of its potential analogies to planned petrothermal reservoirs in the Alpine Foreland. Vertical seismic profiling (VSP) experiments have been performed using a 24-channel borehole hydrophone array with a sensor spacing of 1 m in order to image the fault core and to detect hydraulically open fractures. The dense sensor spacing allows for an accurate sampling of the wavefields. This, in turn, enables their separation through standard seismic processing methodologies, thus allowing for the generation of seismic body wave VSP profiles and a tube wave corridor stack, which maps brittle deformation in the surrounding rock mass.Multiple offset hydrophone VSP data were acquired with a crooked-line survey geometry and processed with 3D methods. Hereby, a laterally changing velocity cube, representative of the vertical structural variations associated with the fault zone, is generated and utilized for Kirchhoff pre-stack-depth-migration (PSDM) imaging. The resulting seismic image delineates various vertical structures, notably the target fault zone. The interpretation of the VSP data is complemented by and corroborated through a suite of geophysical borehole logs, which characterize the fine-scale petrophysical variations in response to brittle deformation in the fault core and the surrounding damage zones.This study exemplifies typical targets of hardrock seismic exploration, where the structures are frequently near-vertical, discontinuous and where acoustic impedance contrasts are commonly associated with deformation. As such, it illustrates the benefits of using borehole seismic methods in such environments and the use of borehole hydrophone arrays for targeting fractures and shear zones.

Critical Depths for Higher Modes by Minimally-invasive Multimodal Surface Wave (MMSW) Method: Simulations and Field Test

A Minimally-invasive Multimodal Surface Wave (MMSW) geophysical testing method, which is a hybrid of surface and borehole seismic methods, was developed recently by the authors to measure more extensive multi-mode dispersion data and thus improve the accuracy of inversion profiles. The new MMSW method employs a borehole geophone at selected depths to record seismic waves from different source offsets on the soil surface. Presented in this paper is a procedure for estimating a range of optimum geophone depths to capture a given higher mode by the MMSW method. Stiffness matrix and finite-element-based numerical simulations of the MMSW method are performed to identify the relationships between critical geophone depths and apparent cutoff frequencies of higher modes. Specifically, it is shown for increasing velocity profiles that 1) at a given borehole sensor measurement depth, the apparent cutoff frequencies of higher modes increase with mode number, 2) at a given frequency, the critical geophone depth at which a higher mode will first become dominant increases with mode number, and 3) for a given higher mode, the apparent cutoff frequency decreases as measurement depth increases. A preliminary field test is conducted using a vertical geophone placed at five different depths while impacts are applied to the soil surface from 3.66 to 43.89 m from the borehole, with an impact spacing of 3.66 m. Dispersion images from the five geophone depths are superimposed to produce a dispersion image having three modes with improved clarity relative to the surface-only Multichannel Analysis of Surface Waves (MASW) method. A comparison of the experimental and theoretical apparent cutoff frequencies for higher modes is used to validate the theoretical prediction of critical depths by the stiffness matrix method. Matching of experimental and theoretical apparent cutoff frequencies could provide additional optimization constraints to reduce the uncertainty of final inversion profiles.

Borehole seismic quantitative diagnosis of a seismic velocity model for 3D seismic imaging of subsurface structures

ABSTRACTQuantifying uncertainties of surface seismic velocity models is a difficult task. The use of multifarious vertical seismic profiling data provides quantification of velocity model uncertainties. Two borehole seismic diagnosis methods for velocity model uncertainties are developed. The first method is performed with 3D ray‐traced first arrival times through a surface seismic velocity model and compared with measured first arrival times from downhole receivers. It generates a profile of absolute and relative misfits, indicating the velocity uncertainty as a function of depth. The second approach directly compares the differences between the first arrival incident angles measured from 3‐C borehole seismic data and the calculated first arrival incident angles of seismic rays through the velocity model to quantify the model uncertainties. Five seismic velocity models in the Gulf of Mexico are diagnosed using the diagnosis methods with up to six sets of borehole seismic data recorded in three wells. The uncertainty profiles vary with depths and azimuths, revealing the complexity of velocity model uncertainties and providing some clues for further velocity updating. The diagnosis methods are used as velocity updating QC tools and integrated with the conventional seismic velocity updating flow to complement the criteria of flat common image gathers.

Ore-body delineation using borehole seismic techniques for hard rock exploration

SUMMARY Over recent years, seismic methods have emerged as a potential imaging technique for delineation of ore-bodies and for mine planning. The application of surface seismic methods in hard rock environments is however challenging due to various effects such as energy attenuation and scattering. Borehole seismic methods can be used to reduce these effects. The methods offer higher resolution at target depths, thus allowing better delineation and understanding of reflections from ore deposits. We present a synthetic study to understand the ability of the cross-hole seismic method to delineate ore bodies. Three variations of a simple scenario typical of nickel deposits found in the Yilgarn Craton were considered. Of the three models, two consist of volcanics overlying a granite body and a thin sulphide mineralized zone along the contact but at different locations relative to the source and receiver boreholes. The third consists of only the rock units with no sulphide mineralized zone along the contact. Synthetic shot records were produced and wavefield separated. Up-going wavefields were then used to create depth migrated images. The resulting images correlate well with the volcanic-granite contact and massive sulphide lens, showing the potential of using the cross-hole seismic method to delineate ore bodies.

Applying surface and borehole seismic methods to trenchless construction problems

Within cities and urban areas trenchless construction and tunnelling are preferred methods for providing new infrastructure mainly because of their social and environmental benefits. However, problems can occur when unexpected subsurface conditions are encountered or when relatively 'minor' geotechnical factors interact with the construction process in unexpected ways that create adverse outcomes. These can cause extensive project delays, major cost over-runs and legal disputation. Advanced geophysical technologies using combinations of marine, land and borehole seismic methods can assist with identifying and overcoming geotechnical problems encountered during trenchless construction. Case studies from recent civil infrastructure projects in Australia and China demonstrate the application of various seismic methods to a range of trenchless construction problems. These projects involve horizontal directional boring, pipe-jacking and micro-tunnelling in soils, mixed material and rock for pipeline and buried power cable installations. The case studies clearly show that appropriate seismic methods can assist in the solution of construction problems of a geological or geotechnical nature. This provides a new application area for engineering geophysics.

Onshore Time-Lapse Borehole Seismic Project for CO2 Injection Monitoring

An innovative time-lapse borehole seismic project for onshore CO2 injection monitoring was conducted in 2010 for ZeroGen in Australia. The goal of the seismic project was to investigate borehole seismic methods to monitor the effects of supercritical CO2 injection in tight, saline reservoir rocks. Results from processing data acquired before and after injection show that a time-lapse, pseudo-4D VSP approach detected likely changes in subsurface acoustic behaviour. A comprehensive borehole seismic program, consisting of two rig source VSPs, an offset VSP, and four Walkaway VSP surveys, was acquired pre-CO2 Injection in the vertical well and then repeated post-injection. Data were acquired using an eight-shuttle VSI seismic tool down-hole and a vibroseis mini-buggy as source. Data processing focused on achieving a repeatable workflow to ensure that time-lapse effects can be effectively analysed. A near surface basalt layer significantly affected both measured time and recorded signatures. This required special handling of surface statics and of source signature variations. The pre- and post injection survey results were analysed for acoustic changes related to the injection program. A time-lapse change in acoustic response in the injection interval was observed on all available borehole seismic datasets. The processed offset and Walkaway VSP images also provided estimates of lateral extent and orientation of these anomalies. Detection of time lapse changes in such a small injection test requires an investigation into the underlying causes such as fluid and pressure effects and the possibility that such techniques might be used to monitor pressure evolution in similar injection sites.

Seismic methods in mineral exploration and mine planning — Introduction

Across the globe, the mineral industry is seeking technology to improve exploration efficiency at depth and to help design safer and more productive mines. Seismic methods are increasingly used by this industry for a wide range of commodities including base metals, uranium, diamonds, and precious metals. Seismic methods provide high-resolution images of geologic structures hosting mineral deposits and, in a few cases, can be used for direct targeting of mineral deposits. Applications are not limited to only surface seismic surveys, but also include borehole seismic methods such as VSP and crosshole imaging. To date, tens of 2D and 3D surface seismic surveys have been acquired in Canada, Europe, Australia, and South Africa (see Malehmir et al., 2012) to help in targeting mineral deposits at depth or for designing deep mines. The steadily increasing usage of reflection seismic methods demonstrates that they are finally becoming recognized and established within the mining sector. This brings new opportunities for geophysicists, but also new challenges. Some of these challenges and opportunities are presented and discussed in the special section. This special issue contains a wide range of topics, from petrophysical studies to data acquisition, processing and imaging, as well as 2D and 3D seismic modeling of mineral deposits and their host-rock structures. Papers from both industry and academia are presented, which illustrate the importance of seismic methods not only in the hydrocarbon industry, but also in the mineral industry. Malehmir et al. review important contributions that have been made in developing seismic techniques for the mining industry with focus on four main regions: Australia, Europe, Canada, and South …

Using time-lapse seismic to monitor injection of CO2 into a depleted gas reservoir—Otway pilot project

The CO2CRC Otway Pilot project involves the injection of over 65,000 tonnes of CO2/CH4 gas mixture (80% of carbon dioxide and 20% of methane) into a deep depleted gas reservoir. The test site is located onshore, some 300 km west of Melbourne. CO2 injection into a depleted gas reservoir will provide important experience for monitoring and verification under these conditions. In this study we address the application of time-lapse seismic for assurance monitoring, which aims to verify that the injected gas remains confined to the target formation. Moreover, we also attempt to image CO2 plume within the reservoir. This is particularly challenging due to the small size of the reservoir and the associated CO2 plume, the presence of residual gas saturation, as well as the significant depth and complexity of the reservoir. In order to meet these challenges we developed a comprehensive seismic monitoring program, which included both time-lapse surface and borehole seismic methods. Prior to the base line survey, a series of 2D test surveys was conducted that aimed to investigate seismic repeatability at the site. It was crucial to keep the repeatability at the highest level considering that the time-lapse seismic signal was expected to be quite small. In fact, it is widely adopted that injection of CO2 into a depleted gas reservoir or equivalently enhanced methane recovery is not traceable by repeated seismic measurements. Careful planning of 3D surveys allowed us to achieve excellent repeatability, which is at the reservoir level represented by normalised RMS difference values of about 20%. Single, coherent time-lapse anomaly at the reservoir level was observed; its location and amplitude agree with the position of CO2 plume predicted by reservoir simulations. Measured amplitude differences agreed with the one predicted by seismic modelling.

Seismic Monitoring and Verification for the CO2CRC Otway Basin Project, Part 2: acquisition and analysis of borehole seismic data

Abstract The Otway Project conducted under the Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO 2 CRC) is the first of its kind, where CO 2 is injected into a depleted gas reservoir. The use of depleted fields for CO 2 storage is likely to become widely adopted globally and, therefore, the project will provide important experience for monitoring under these conditions. However, such scenario is not favorable for the application of geophysical techniques for the purpose of CO 2 monitoring and verification (M&V) because the injection of CO 2 into a CH4 depleted reservoir is modeled to produce very subtle changes in elastic properties of the reservoir rock which may be very difficult to measure. Consequently geophysical program for the Otway site was design according to the expected time-lapse effects. It combines both surface and borehole seismic methods. Surface seismic should provide a global vision of the underground and an indirect confirmation of the CO 2 containment by recording no differences between the successive time-lapse experiments. Vertical Seismic Profile (VSP) surveys are expected to provide an improved characterization of the reservoir and hopefully a direct indication of the fluid distribution and/or its potential upward migration along the reservoir bounding fault pattern. Indeed the results of the current analysis of both pre-base line (test) and base-line 2D and 3D VSP data are encouraging. The availability of vector wave field (three-component) data recorded in VSP surveys should significantly improve the outcomes of M&V program at Naylor site.

Feasibility of the application of borehole seismology for hard rock exploration

Mineralisation of economic importance typically occurs in hard rock environments that tests the resolution limits of surface seismic techniques. Borehole seismic methods reduce the effects of wavefield scattering by placing either the receiver or source within the bedrock. To test the viability of borehole seismology we have conducted a number of modelling tests. The tests are aimed to demonstrate the potential of recording and producing high resolution images from borehold seismic measurements. We also offer potential Vertical Seismic Profile methodologies to assist in hard rock seismic exploration.

Cost-effective imaging of CO2 injection with borehole seismic methods

Currently there is a critical need to increase oil and gas recovery from existing and new reservoirs. In addition, the ever-increasing need to sequester CO2 in the subsurface places further emphasis on accurate imaging methods to validate CO2 injection strategies. Two obstacles to increased efficiency are (1) a thorough understanding of the geologic complexity and fluid distribution and (2) the scaling relationships between fine scale/point measurements and larger scale/volumetric measurements. Although initially expensive, borehole methods may offer a cost-effective solution when integrated into a drilling and development program. New technology such as fiber-optic sensors emplaced during drilling and completion, microhole drilling, and other advances in sensors will make borehole technology much more cost effective when used over the long run. If deployed in a multicomponent and time-lapse fashion, seismic methods also offer the ability to define contrasts in properties, detecting subtle changes in prop...

Surface and borehole seismic methods to delineate kimberlite pipes in Australia

Gravity, magnetic, and electromagnetic methods are primary exploration techniques used in kimberlite exploration to define a kimberlite pipe's location and shape. Sometimes these methods produce nonunique results because of weathering of the upper part of a kimberlite pipe and its considerable inhomogeneity. Complex pipe geometries and associated faulting and fracturing may be very difficult to assess by potential-field methods. Application of seismic methods in kimberlite exploration has been limited. In sedimentary basins, however, kimberlite pipes are likely to be a suitable target for the application of seismic reflection methods because of the difference in elastic properties between the pipe material and the host rock. The drawback of surface seismic methods is that they are not designed for direct detection of vertical bodies such as a pipe. However, a kimberlite pipe may be inferred from seismic data indirectly by observing the termination of reflections against its flanks. Such an approach can produce good results if the sedimentary sequence is of moderate to high reflectivity. Unfortunately, McArthur Basin sediments of the Northern Territory in Australia are quite poor reflectors (Figure 1). The only reflecting horizon would probably be the sandstone/black shale interface at 140 m. Near-surface conditions are also unfavorable for surface seismic because of a thick layer of bull dust, which causes poor geophone coupling. Hence, very poor seismic data quality is typical for this area. In this study, instead of attempting a conventional surface seismic reflection approach, we used modes generated by a kimberlite pipe to precisely determine its location. The single borehole imaging (SBI) method also was used for delineation of the kimberlite pipe shape at depth. By a combined analysis of P - and S -wave data recorded on the surface and in downhole experiments, the pipe's location, shape, …

Mapping dykes using surface and downhole seismic methods

Seismic methods are commonly used for the detection of faults in the exploration and production of coal. Surface seismic methods are not used in industry for the delineation of vertical structure, such as the imaging of dykes. This is because seismic waves transmit from the surface down to horizontal reflection surfaces, and reflect back up to the surface. Consequently, where sub-vertical structure such as dykes occurs, the surface seismic method fails.Seismic methods can use different source-receiver geometries. Their ability to image dykes may therefore depend upon on the geometry used, the dyke thickness and the seismic wave propagation mode in relation to dyke composition and internal structure. Surface seismic methods have difficulty distinguishing between faults or fractures and very thin dykes (l–2m in thickness) when a dyke’s thickness is less than the seismic wavelength. Borehole seismic methods have to be used to detect the presence of dykes and avoid these problems.This paper presents some results from a research project that is attempting to use both surface and downhole seismic methods to detect dykes. The paper shows how surface seismic methods imaged a sub-vertical dyke of some 40 m thickness and its associated faulting. The alternative approach of using downhole seismic sources and receivers (single borehole seismic profiling), showed that dyke sides can also be successfully imaged at depth. In the future it should be possible to produce an image of both sides of a dyke, in its correct orientation, using existing boreholes.

Review of borehole seismic methods developed for opencast coal exploration

Review of borehole seismic methods developed for opencast coal exploration

Review of borehole seismic methods developed for opencast coal exploration

Abstract Evaluation of opencast coal reserves in the UK is accomplished by drilling boreholes on a grid spacing of 40–60 m, with even closer spacings across major fault zones and in areas of old mine workings. In spite of this intensive drilling effort, it is difficult to detect small faults with throws of less than about 2 m, which can still present a serious stability hazard at site boundaries. Borehole seismic methods could be used to detect small faults, to give structural detail across major fault zones and to locate old mine workings. Two techniques have been developed for this purpose: cross-hole seismic reflection surveying and hole to surface seismic reflection surveying. Although the cross-hole method produces sections of extremely high resolution, very close source and receiver element spacings are required to image close to the boreholes. Also the borehole separation should be less than half the length of the source and receiver arrays if complete coverage is required between the arrays. The hole to surface method produces high resolution sections and could be extended to three-dimensional surveys, but imaging is limited to a bell-shaped cross-section with its apex just below the water-table. Both methods can be used to yield continuous sections along lines of boreholes and are potentially of value in other applications.

In situ evaluation of rock mass stiffness variations for underground openings using surface wave and borehole seismic methods : Nelson, P P; Lopez, M; Stokoe, K H; Roblee, C J Proc 7th Annual Workshop Generic Mineral Technology Center Mine System Design and Ground Control, Blacksburg, 22–24 October 1989P35–46. Publ Blacksburg: Virginia Polytechnic Institute and State University, 1989

In situ evaluation of rock mass stiffness variations for underground openings using surface wave and borehole seismic methods : Nelson, P P; Lopez, M; Stokoe, K H; Roblee, C J Proc 7th Annual Workshop Generic Mineral Technology Center Mine System Design and Ground Control, Blacksburg, 22–24 October 1989P35–46. Publ Blacksburg: Virginia Polytechnic Institute and State University, 1989