Reverse VSP Research Articles

Fracture imaging using Image Point transform and midpoint imaging of RVSP data

An Image Point (IP) transform integrates along a hyperbolic path and converts data to a point in the IP domain so that the reflection events are accumulated to the image point for given sources in the IP domain. In addition, the image point by definition is uniquely determined and the midpoints between sources and the image points always lie on the reflector surface. Exploiting these two facts, the image point transform technique for Vertical Seismic Profile (VSP) to reverse VSP (RVSP) data is expanded to suggest a new method called the midpoint imaging method for imaging reflectors such as fractures. Applying the transform with synthetic data for Two-dimensional (2-D) models with single reflector and two reflectors, we confirm and demonstrate that the reflection events are clearly identifiable in the IP domain. The midpoint imaging method can successfully image the reflector surfaces and the dip information even with only two shot records in a simple case.

Fault mapping using a generalized image point transform of three dimensional reverse VSP data

Image point (IP) transform is one type of radon transform with a hyperbolic integral path and reflection waves are accumulated in an IP domain. In expanding IP transform to three dimensional reverse vertical seismic profiling (3-D RVSP), consideration of a strike of a reflector is needed. We introduce a new line that is assumed to be normal to the strike and derived a new transform to the line with all data of receivers. The transform is applicable whether receivers are deployed straight lines or not, and named as a generalized image point (GIP) transform. We apply GIP transform to 3-D synthetic data having two dipping reflectors and confirm that reflections on an IP domain are only accumulated on a transverse line to a strike of a reflector. Applying the GIP transform to a field data, we delineate faces of faults within survey area and the results are good consistency with the known geological structures. ►Expanding IP transform to 3D RVSP data. ►New transform (GIP) is proposed and is flexible where receiver lines are crooked or not. ►Showing agreeable results by applying GIP transform to field data.

Near-surface seismic traveltime tomography using a direct-push source and surface-planted geophones

Information about seismic velocity distribution in heterogeneous near-surface sedimentary deposits is essential for a variety of environmental and engineering geophysical applications. We have evaluated the suitability of the minimally invasive direct-push technology for near-surface seismic traveltime tomography. Geophones placed at the surface and a seismic source installed temporarily in the subsurface by direct-push technology quickly acquire reversed multioffset vertical seismic profiles (VSPs). The first-arrival traveltimes of these data were used to reconstruct the 2D seismic velocity distribution tomographically. After testing this approach on synthetic data, we applied it to field data collected over alluvial deposits in a former river floodplain. The resulting velocity model contains information about high- and low-velocity anomalies and offers a significantly deeper penetration depth than conventional refraction tomography using surface-planted sources and receivers at the investigated site. A combination of refraction seismic and direct-push data increases resolution capabilities in the unsaturated zone and enables reliable reconstruction of velocity variations in near-surface unconsolidated sediments. The final velocity model structurally matches the results of cone-penetration tests and natural gamma-radiation data acquired along the profile. The suitability of multiple rapidly acquired reverse VSP surveys for 2D tomographic velocity imaging of near-surface unconsolidated sediments was explored.

Techbits: Seismic-While-Drilling Technology Takes ETW Spotlight

Techbits While Seismic-While-Drilling (SWD) methods have been investigated for nearly 30 years as a complete, real-time alternative to conventional vertical-seismic-profiling (VSP) techniques, the industry has yet to accept it beyond a rather narrow scope. Expanding this scope was the focus of a recent SPE Emerging Technology Workshop (ETW) held in League City, Texas. SWD technology, until recently primarily referred to as drill-bit (DB) seismic, began as a method to detect the natural vibration from the end of the drill bit by means of surface geophones, enabling geophysicists and drillers to use these data in real time to optimize drilling efficiency, predict pore pressures ahead of the bit, and verify reservoir models. This technique has been eclipsed by another method, particularly for deepwater offshore drilling applications, which is based on traditional wireline methods that use a standard offshore air-gun seismic source at the surface with suitable receivers or sensors in a downhole tool. This, together with recording and processing capabilities, is part of an enhanced logging-while-drilling (LWD) tool. Nonetheless, proponents of DB/SWD technology believe the technique has untapped potential in certain applications. The ETW was seen as the ideal forum in which to discuss this. ETWs are interactive events designed to give participants a field's-eye view of new technologies and to expedite the adoption of these technologies by publicizing their initial successes as they move from laboratory to field. Andy Hawthorn of Schlumberger helped organize the League City event. "When I got involved, the entire focus was on theory and processing of DB seismic. But because there are still so many limitations with it, we had to include the LWD seismic [LWD/SWD], which has replaced it more successfully on an economic basis." Open Dialogue, Collaboration Promoted According to SWD ETW Chairperson Bob Radtke of Technology International, the workshop's open format encouraged lively discussion and debate on the future of the technology. "We had presentations describing the current state of the art, as well as detailed sections on surface source, downhole source, emerging technologies, and case histories," Radtke said. "In addition, this workshop was unique, in that we had a special keynote speaker, Nansen Saleri." Saleri, manager of Reservoir Management, Saudi Aramco, and a 1991 SPE Distinguished Lecturer, delivered an address titled "The Technology Factor: How It's Changing Upstream Performance." His talk described how the SWD method would be useful for increasing understanding of reservoirs in the Middle East and elsewhere, and indicated that significant producers may be ready to give this technology extensive use. Another special feature of the ETW was the presentation of two "Legends of SWD" awards to Bruce Marion and James Rector for their 1991 demonstration of the first successful use of drill-bit reverse VSP employing drill-bit-correlated data.

Seismic interferometry with a TBM source of transmitted and reflected waves

We discuss the use of autocorrelogram interferometry by using noise from the tunnel-boring machine (TBM). The TBM provides seismic signals/waves while drilling in a tunnel (TSWD). The tunnel geometry, unlike a reverse vertical seismic profile (RVSP) using a drill bit, makes it possible to record the waves reflected from the region between the tunnel face and the projected tunnel exit and those transmitted ahead of the tunnel face. We processed the waves recorded at back positions with respect to the TBM in a manner similar to a RVSP data set obtained by conventional reference-correlation techniques. We processed the waves transmitted ahead of the TBM using autocorrelogram interferometry techniques. Using these wavefields offers advantages over conventional borehole drill-bit vertical seismic profiles (VSPs). The most important advantage is getting reflections from the transmitted (front) wavefield by utilizing Kunetz’s equation and reversed-time traces. The approach also improves the analysis of the transmitted amplitudes. Finally, we improved the deconvolution of the reflected (back) waves by using the transmitted wavefields measured for interferometry purposes. In particular, by using both front (transmitted) and back (reflected) waves, it is possible to deconvolve the signature of the source extended spatially along the tunnel axis. We use a 1D model in which the interfaces are assumed subvertical. We present a case history in which TSWD data were acquired in a tunnel measuring [Formula: see text] long. We compare results from the transmitted reversed-time and back-reflected waves ([Formula: see text]-waves) with those obtained by amplitude analysis and estimation of reflection coefficients. Each approach matches the interpretation of the fractures encountered in the tunnel.

Prediction ahead of the bit by using drill‐bit pilot signals and reverse vertical seismic profiling (RVSP)

Surface drillstring axial pilot signals are used to predict reflections ahead of the drill bit. We show that part of the drill‐bit signal propagates downward in the formation, reflects upward by a seismic interface, and is then transmitted to the drillstring and the surface pilot sensors. These reflections are interpreted in drill‐bit pilot signals by means of a numerical model of the drillstring coupled to the formation at the bit–rock contact. The result is an additional, low‐cost, reverse VSP (RVSP) in the zero‐offset approximation. These while‐drilling results are integrated with conventional drill‐bit RVSP measurements and compared with other geophysical and well results.

A new approach to offshore drill‐bit reverse vertical seismic profiling (RVSP)

Seismic while drilling (SWD) is a tool that can be used to optimize drilling at offshore areas and to reduce the related risks. Different while-drilling vertical seismic profiling (VSP) technologies that use seismic receivers located in fixed positions can be considered for offshore 2-D investigations with short–medium offset and in limited water depth. We introduce a new approach using conventional towed streamers to extend the offshore SWD measurements to large-offset, 3-D, and deepwater applications, where conventional technology has limitations. This technology may not only replace other SWD methods but could also integrate them.

Acoustic velocity of seismic‐while‐drilling (SWD) borehole guided waves

Borehole guided waves, other than the extensional waves traveling through the drillstring, can be used as pilot signals to obtain seismic‐while‐drilling (SWD) seismograms and information about the drilling conditions. This occurs when there is no rotation of the drillstring while drilling and when the contacts between the drillpipe and the borehole wall preclude the propagation of the extensional wave. To obtain the velocity of the guided waves, we model the compressional‐wave velocity of drilling mud with different compositions of low‐ and high‐gravity solids, corresponding to a given drilling plan and taking into account the presence of formation cuttings and in‐situ conditions. Then we compute the velocities of the tube wave (with and without casing) and the guided wave traveling in the mud inside the drillstring (pipe wave). The results indicate the pipe wave constitutes a reliable pilot signal in the absence of drillpipe rotation. This allows us to obtain a complete depth reverse vertical seismic profile.

Imaging coal seam structure using 3-D seismic methods

Coal mining and production is a very large industry within Australia with over 100 active collieries. However, to run at a profit, the coal mines must have continuous production and fulfil large annual quotas. This is generally achieved through the use of the capital intensive, underground, longwall mining technique. This technique requires that the coal seam structure be accurately known ahead of the extraction coal face. Any unknown faulting, unstable roof conditions or gas accumulations encountered while mining can lead to equipment damage, down-time and injury to workers. Thus, the responsibility is on the mine geologist to ensure that the coal seam structure is accurately mapped ahead of mining, to provide a safe mine plan. Before the introduction of the seismic method in coal mining, all coal seam mapping was done using surface boreholes. This is an expensive method to use and provides only a low level of lateral resolution.Seismic methods are gradually replacing boreholes and becoming more common place in coal industry as an accurate and cost effective alternative method for imaging the seam structure. Burton Downs colliery in central Queensland, was one such mine to take advantage of this method as apart of an Underground Feasibility Study for their northern, Kerlong mine area. With no previous seismic work done in the area, reverse vertical seismic profiling (RVSP) and two-dimensional (2-D) surveys were initially shot. RVSP results showed that the seismic method was viable, imaging the coal seam of interest and provided acquisition parameters for the 2-D surveys. The 2-D lines gave high-resolution images of the coal seams within the mine area, and showed a complex faulting system which included low-angle reverse faulting and normal faulting. The success of the 2-D profiles prompted the acquisition of a three-dimensional (3-D) survey. After processing the 3-D depth volume, a much more complete structural interpretation and hence, mine plan could be made.

Poroelastic model to relate seismic wave attenuation and dispersion to permeability anisotropy

A transversely isotropic model with a horizontal axis of symmetry, based on the Biot and squirt‐flow mechanisms, predicts seismic waves in poroelastic media. The model estimates velocity dispersion and attenuation of waves propagating in the frequency range of crosswell and high‐resolution reverse vertical seismic profiling (VSP) (250–1250 Hz) for vertical permeability values much greater than horizontal permeability parameters. The model assumes the principal axes of the stiffness constant tensor are aligned with the axes of the permeability and squirt‐flow tensors. In addition, the unified Biot and squirt‐flow mechanism (BISQ) model is adapted to simulate cracks in permeable media. Under these conditions, the model simulations demonstrate that the preferential direction of fluid flow in a reservoir containing fluid‐filled cracks can be determined by analyzing the phase velocity and attenuation of seismic waves propagating at different azimuth and incident angles. As a result, the fast compressional wave can be related to permeability anisotropy in a reservoir. The model results demonstrate that for a fast quasi-P-wave propagating perpendicular to fluid‐filled cracks, the attenuation is greater than when the wave propagates parallel to the plane of the crack. Theoretical predictions and velocity dispersion of inter‐well seismic waves in the Kankakee Limestone Formation at the Buckhorn test site (Illinois) demonstrate that the permeable rock matrix surrounding a low‐velocity heterogeneity contains vertical cracks.

Least‐squares migration of incomplete reflection data

A least‐squares migration algorithm is presented that reduces the migration artifacts (i.e., recording footprint noise) arising from incomplete data. Instead of migrating data with the adjoint of the forward modeling operator, the normal equations are inverted by using a preconditioned linear conjugate gradient scheme that employs regularization. The modeling operator is constructed from an asymptotic acoustic integral equation, and its adjoint is the Kirchhoff migration operator. We tested the performance of the least‐squares migration on synthetic and field data in the cases of limited recording aperture, coarse sampling, and acquisition gaps in the data. Numerical results show that the least‐squares migrated sections are typically more focused than are the corresponding Kirchhoff migrated sections and their reflectivity frequency distributions are closer to those of the true model frequency distribution. Regularization helps attenuate migration artifacts and provides a sharper, better frequency distribution of estimated reflectivity. The least‐squares migrated sections can be used to predict the missing data traces and interpolate and extrapolate them according to the governing modeling equations. Several field data examples are presented. A ground‐penetrating radar data example demonstrates the suppression of the recording footprint noise due to a limited aperture, a large gap, and an undersampled receiver line. In addition, better fault resolution was achieved after applying least‐squares migration to a poststack marine data set. And a reverse vertical seismic profiling example shows that the recording footprint noise due to a coarse receiver interval can be suppressed by least‐squares migration.

Elastic reverse time migration of marine walkaway vertical seismic profiling data

Walkaway vertical seismic profiling (VSP) acquisition with three‐component geophones allows for direct measurement of compressional as well as shear energy. This makes full elastic reverse time migration an attractive alternative for imaging data. We present results from elastic reverse time migration of a marine walkaway VSP acquired offshore Norway. The reverse time migration scheme is based on a high‐order finite‐difference solution to the two‐way elastic wave equation. Depth images of the subsurface are constructed by correlation of forward‐ and back‐propagated elastic wavefields. In the walkaway VSP configuration, the number of shots is much larger than the number of geophone levels. Using processing methods operating in the shot/receiver domain, it is advantageous to use the reciprocal relationship between the walkaway VSP and the reverse VSP configurations. We do this by imaging each component of each geophone level as a reverse VSP common shot gather. The final images are constructed by stacking partial images from each level. The depth images obtained from the vertical components reveal the major characteristics of the geological structure below geophone depth. A graben in the base Cretaceous unconformity and a faulted coal layer can be identified. The horizontal components are more difficult to image. Compared to the vertical components, the horizontal component images are more corrupted by migration artifacts. This is because the horizontal component images are more sensitive to aperture effects and to the shear‐wave velocity macromodel. When converted to two‐way time, the migration results tie well with the surface seismic section. Comparison of fully elastic and acoustic reverse time migration shows that the vertical component is dominantly PP-reflected events, whereas the horizontal components get important contributions from PS-converted energy. The horizontal components also provide higher resolution because of the shorter wavelength of the shear waves.

An introduction to this special section: Advanced borehole seismology

A few years ago, a number of major oil companies essentially closed their borehole geophysical research groups; many of us thought this was the death knell for crosswell seismic and reverse vertical seismic profiling. But it turned out to indicate a change in the thrust of the efforts — from major oil‐company‐sponsored efforts to private (consortium‐sponsored) efforts and service companies.

A borehole‐to‐surface electromagnetic survey

The results of a limited field trial confirm the usefulness of borehole‐to‐surface electromagnetic (EM) measurements for monitoring fluid extraction. A vertical EM profiling experiment was done at the University of California Richmond Field Station, where we simulated a brine spill plume by creating a saline water injection zone at the depth of 30 m. The data acquisition mode was analogous to the reverse vertical seismic profiling (VSP) configuration used for seismic measurements in that the EM transmitter traversed the PVC-cased borehole used for fluid injection and extraction while the receivers were deployed on the surface. The EM measurements were made at 9.6 kHz with an accuracy of 1% in signal amplitude and 1° in signal phase. Observations were taken at 5-m intervals along two intersecting profiles that were centered on the injection well and extended for 60 m on either side of it. The presence of the injected salt water, at the expected 30 m depth, was indicated clearly by differences between the pre‐extraction and postextraction data. A limited amount of numerical modeling showed that the experimental data were consistent with the presence of two superposed saline plumes. The uppermost of these, located at 26 m depth, was 2 m thick and had an area of 30 m2. The lower plume, located at 30 m, is the major cause of the observed anomaly, as it has an areal extent of 120 m2 and a thickness of 3 m. Surprisingly, the measurements were very sensitive to the presence of the cultural surficial conductivity anomalies. These spurious effects were reduced by spatial filtering of the data prior to interpretation.

Development of an advanced vibratory source for borehole seismology

Single-well seismology, reverse vertical seismic profiles (RVSP’s) and Crosswell seismology are three new seismic techniques for obtaining much higher resolution images of oil and gas reservoirs than what is currently obtainable with surface seismic techniques. Borehole seismology involves inserting both the source and the seismic receivers in oil or gas wells. In the past these methods have been limited to short distances between source and receivers in shallow wells. In 1997 a new borehole seismic source and receiver system has been deployed with a capability of handling source–receiver spacings of more than 2000 m in rock with a Q value of more than 20, and well depths as great as 6500 m. This advanced borehole seismic data acquisition system consists of a powerful, clamped, swept-frequency, vibratory source which is nondestructive, a multilevel receiver string of clamped, three-component geophones, and an acquisition system for reverse VSP’s. The downhole vibrator produces high-quality S waves, with controlled polarization, as well as P-wave direct arrivals and reflections. This paper discusses the design and performance of this system.

Seismic traveltime analysis for azimuthally anisotropic media: Theory and experiment

Natural fractures in reservoirs, and in the caprock overlying the reservoir, play an important role in determining fluid flow during production. The density and orientation of sets of fractures is therefore of great interest. Rocks possessing an anisotropic fabric and a preferred orientation of fractures display both polar and azimuthal anisotropy. Sedimentary rocks containing several sets of vertical fractures may be approximated as having monoclinic symmetry with symmetry plane parallel to the layers if, in the absence of fractures, the rock is transversely isotropic with symmetry axis perpendicular to the bedding plane. A nonhyperbolic traveltime equation, which can be used in the presence of azimuthally anisotropic layered media, can be obtained from an expansion of the inverse‐squared ray velocity in spherical harmonics. For a single set of aligned fractures, application of this equation to traveltime data acquired at a sufficient number of azimuths allows the strike of the fractures to be estimated. Analysis of the traveltimes measured in a physical model simulation of a reverse vertical seismic profile in an azimuthally anisotropic medium shows the medium to be orthorhombic with principal axes in agreement with those given by an independent shear‐wave experiment. In contrast to previous work, no knowledge of the orientation of the symmetry planes is required. The method is therefore applicable to P‐wave data collected at multiple azimuths using multiple offset vertical seismic profiling (VSP) techniques.

Transmission‐reflection tomography: Application to reverse VSP data

A multiphase tomographic algorithm is presented that allows 2-D and 3-D slowness (inverse of velocity) and variable reflector depth to be reconstructed simultaneously from both transmission and reflection traveltimes. We analyze the ambiguity in the determination of velocity and depth in transmission and reflection data and realize that depth perturbation is more sensitive to reflection traveltime anomalies than slowness perturbation, whereas the reverse is true of transmission traveltime anomalies. Because of the constraints on velocity and depth provided by the different wave types, this algorithm reduces the ambiguity substantially between velocity and depth prevalent in reflection tomography and also avoids the undetermined problem in transmission tomography. The linearized inversion was undertaken iteratively by decoupling velocity parameters from reflector depths. A rapid 2-D and 3-D ray‐tracing algorithm is used to compute transmission and reflection traveltimes and partial derivatives with respect to slowness and reflector depth. Both depth and velocity are parameterized in terms of cubic B‐spline functions. Synthetic examples indicate the improvement in tomographic results when both transmission and reflection times are included. The method has been applied to a reverse vertical seismic profile (VSP) data set recorded on the British coal measures along a crossed‐linear array. Traveltimes were picked automatically by the simultaneous determination of time delays and stacking weights using a waveform matching technique. The tomographic inversion of the observed reverse VSP images two fault‐zones of lower velocity than the surrounding media. The location of the faults was confirmed by near‐by reflection lines. The technique can be applied to offset VSPs or reverse VSPs and coincident VSP and surface reflection data.

Triaxial seismic measurement while drilling and estimation of subsurface structure: H. Asanuma & H. Niitsuma, Geothermal Science & Technology, 5(1–2), 1995, pp 31–51

A reverse Vertical Seismic Profile (VSP), which utilizes a drill-bit as a seismic source, is promising for evaluating subsurface structure and for on-line determination of a drilling target, such as geothermal reservoir. A seismic measurement was made in a measurement well 140m from the drilling well and the associated seismic signal at bit depths from 313 m to 354 m was continuously recorded. We have found that the hodogram of the three component signal is polarized along the direction of the drill-bit and that the seismic signal shows spectral whiteness. The direction of the drill-bit was successfully estimated by a spectral matrix analysis of the 3-C seismic signal. A reverse VSP technique (TAD-VSP; Triaxial Drill-bit VSP) based on a 3-D hodogram analysis of a continuous seismic signal by a single-point observation was introduced. In TAD-VSP, a spatial-temporal cross-correlation analysis of the particle motion was made to detect reflected waves. Delay and propagating directions of reflected waves were discriminated from the extreme values of the 2-D cross-correlation coefficients. We also devised a method to project the 2-D cross-correlation coefficients into the object plane, which represents the distribution of the reflection coefficients. Distribution of reflection coefficients estimated by TAD-VSP agreed satisfactorily with the core observation and sonic log in the field. The TAD-VSP survey has potential to be effectively used for on-line evaluation of deeper subsurface structure.

Improving depth prediction accuracy of quantified drilling hazards

Under-compacted shales are often associated with over-pressured formations. These shales have excess water and tend to be mechanically weak (or are overlain by mechanically weak formations), thus the safe mud window for drilling the under-compacted interval can be quite narrow. Efficient and safe drilling operations require accurate depth predictions of these over-pressured formations as well a knowledge of the magnitude of the over-pressure. In this paper we describe a technique which combines the best aspects of conventional Vertical Seismic Profiles (VSP) and Reverse Vertical Seismic Profiles (RVSP) to detect under-compacted shales and predict formation pressures to locate drilling hazards below TD. The excess water in the under-compacted shales will have a lower acoustic impedance than expected from the compaction trend. Shales that depart from the compaction trend may indicate potential drilling hazards below. Conventional VSPs provide at discrete intervals in the well, high quality reflection data which can be used to accurately predict acoustic impedance below the bit. This acoustic impedance is then interpreted to provide both the location (in time and depth) of the drilling hazard and the mud weight necessary to contain it. The two way time estimate of the hazard location is usually quite accurate but the depth estimate is less certain due to the estimation off ormation velocities below TD. The RVSP using the drill bit as a source, provides a continuous time versus depth relationship while drilling. This time verses depth is used to continually update the conventional VSP depth prediction of the drilling hazard and thus provide the most accurate depth of the hazard prior to it's penetration.

Walk‐away VSP using drill noise as a source

We describe a method for extracting and deconvolving a signal generated by a drill bit and collected by an array of surface geophones. The drill‐noise signature is reduced to an effective impulse by means of a multichannel Wiener deconvolution technique, producing a walk‐away reverse vertical seismic profile (VSP) sampled almost continuously in depth. We show how the multichannel technique accounts for noise and for internal drill‐string reflections, automatically limiting the deconvolved data to frequencies containing significant energy. We have acquired and processed a data set from a well in Germany while drilling at a depth of almost 4000 m. The subsurface image derived from these data compares well with corresponding images from a 3-D surface seismic survey, a zero‐offset VSP survey, and a walk‐away VSP survey acquired using conventional wireline techniques. The effective bandwidth of the deconvolved drill‐noise data is comparable to the bandwidth of surface seismic data but significantly smaller than what can be achieved with wireline VSP techniques. Although the processing algorithm does not require the use of sensors mounted on the drill string, these sensors provide a very economic way to compress the data. The sensors on the drill string were also used for accurate timing of the deconvolved drill‐noise data.