Reverse VSP Data Research Articles

Full‐waveform inversion for reverse vertical seismic profiling data based on a variant of the optimal transport theory

ABSTRACTFull‐waveform inversion has become a useful tool to construct the details of the geological structure. However, due to the strong nonlinearity of the inverse problem and inaccuracy of the initial model, full‐waveform inversion often faces severe cycle‐skipping problems and thus may exhibit poor inversion results. The optimal transport‐based full‐waveform inversion can mitigate the cycle‐skipping problem. For that, transformations of oscillatory seismic data to distributions need to satisfy positive and mass conservation conditions. In this work, we introduce a new data normalization method based on the Sigmoid function. The convexity of the objective function is improved compared to the traditional L2 norm by setting appropriate parameters. Combined with the adaptive quadratic Wasserstein distance, our approach presents strong adaptability to a poor initial model and can reduce cycle skipping. Numerical examples are provided to validate the method in a reverse vertical seismic profiling geometry, and the inverted models obtained by different normalization methods are also compared.

Subsurface seismic imaging using drill bit noise

SummaryAcoustic signal generated by drill bit cutting action in the borehole can be recorded passively by geophones on the surface and used to produce an image of the subsurface under the geophone array. A single pilot trace is commonly recorded on the drill string and used to convert geophone recordings to equivalent reverse vertical seismic profile data. If there is no pilot measurements on the drill string, seismic interferometry can be used to move continuous sources of energy from the bottom of the borehole to the surface and use conventional seismic methods to image the subsurface.In this study, we recorded the noise coming from drilling operation for several shallow wells over the surface of a small iron ore prospect in Pilbara region of Western Australia. As pilot measurements were not available, we used seismic interferometry to produce virtual shots on the surface. We then migrated virtual data generated along one receiver line for one deep well and four shallower wells over the line. We saw that only deep well with longer drilling time can produce a seismic image with coherent events that are comparable to 3D surface seismic data.

Shallow near-surface effects

The top 6 m of the near surface has a surprisingly large effect on the behavior of P- and S-waves. For unconsolidated sediments, the P-wave velocity gradient and attenuation can be quite large. Computer modeling should include these properties to accurately reproduce seismic effects of the near surface. We have used reverse VSP data and computer simulations to demonstrate the following effects for upgoing P-waves. Near the surface, we have observed a large time delay, indicating low velocity ([Formula: see text]), and considerable pulse broadening, indicating high attenuation ([Formula: see text]). Consequently, shallowly buried geophones have greater high-frequency bandwidth compared with surface geophones. In addition, there is a large velocity gradient in the shallow near surface (factor of 10 in 5 m), resulting in the rotation of P-waves to the vertical with progressively smaller amplitudes recorded on horizontal phones. Finally, we have found little indication of a reflection or ghost from the surface, although downgoing reflections have been observed from interfaces within the near surface. In comparison, the following have been observed for upgoing S-waves: There is a small increase in the time delay or pulse broadening near the surface, indicating a smaller velocity gradient and less change in attenuation. In addition, the surface reflection coefficient is nearly one with a prominent surface ghost.

Imaging pre‐existing natural fractures using microseismic data

ABSTRACTIn hydraulic fracturing treatments, locating not only hydraulic fractures but also any pre‐existing natural fractures and faults in a subsurface reservoir is very important. Hydraulic fractures can be tracked by locating microseismic events, but to identify the locations of natural fractures, an additional technique is required. In this paper, we present a method to image pre‐existing fractures and faults near a borehole with virtual reverse vertical seismic profiling data or virtual single‐well profiling data (limited to seismic reflection data) created from microseismic monitoring using seismic interferometry. The virtual source data contain reflections from natural fractures and faults, and these features can be imaged by applying migration to the virtual source data. However, the imaging zone of fractures in the proposed method is strongly dependent on the geographic extent of the microseismic events and the location and direction of the fracture. To verify our method, we produced virtual reverse vertical seismic profiling and single‐well profiling data from synthetic microseismic data and compared them with data from real sources in the same relative position as the virtual sources. The results show that the reflection travel times from the fractures in the virtual source data agree well with travel times in the real‐source data. By applying pre‐stack depth migration to the virtual source data, images of the natural fractures were obtained with accurate locations. However, the migrated section of the single‐well profiling data with both real and virtual sources contained spurious fracture images on the opposite side of the borehole. In the case of virtual single‐well profiling data, we could produce correct migration images of fractures by adopting directional redatuming for which the occurrence region of microseismic events is divided into several subdivisions, and fractures located only on the opposite side of the borehole are imaged for each subdivision.

Seismic while drilling experiment with diamond drilling at Brukunga, South Australia

Seismic-While-Drilling (SWD) utilises drill bit vibrations as a seismic source and receivers at the surface or in a borehole to acquire reverse VSP data. The basic processing technique is based on cross correlation to generate active shot-gather-like profiles. The successful implementation of SWD will yield time-depth information and image around the drill bit, which can aid drilling and geological understanding of the area. To study the feasibility of using diamond impregnated drill bits for seismic-while-drilling,we conducted a small pseudo 3D SWD experimentat Brukunga, South Australia. It has been used to investigate the signals generated from diamond drilling, and study the potential to use a drill bit as a seismic source. The drill bit energy for seismic imaging is influenced by the rig power setting, and the state of the drill bit (new or worn bit). The experiment shows that normally the diamond drilling frequency band is wide with strong discrete peaks,however sometimes due to changes in the drilling mode, e.g., increase or decrease the drilling power, the frequency spectrum can be smoothed. The strong peaks in the spectrum mean strong periodicity of the signal and as such, the signals do not lend themselves to standard cross correlation. Thus, we use a generalised cross correlation that produces results similar to an active shot gather-like profile.

Using reverse vertical seismic profiling (RVSP) to characterise the subsurface fracture system of the Seokmo Island geothermal field, Republic of Korea

This paper presents a case study of fracture-system interpretation using reverse vertical seismic profiling (RVSP) with seismic reflection, refraction, and borehole televiewer data on Seokmo Island, South Korea. We first extracted fracture locations from a pre-existing image obtained by prestack phase-screen migration of RVSP data, and the strike and dip of each fracture from the borehole televiewer data. We established an initial velocity model using this fracture information and then generated synthetic common-receiver gather data through forward simulation. However, the synthetic data could not sufficiently reflect the characteristics of the field data. To resolve this problem, we added an upper alluvial layer, observed in the surface reflection and refraction data, to the velocity model. To improve the quality of the migrated image of the RVSP field data and the velocity model, we not only reprocessed the RVSP data, but also applied prestack generalized-screen migration, which is more accurate than phase-screen migration in computing steep fractures. The new synthetic data generated from the improved velocity model agreed well with the RVSP field data. As a result, we could describe a markedly improved subsurface structure including fracture locations. The workflow suggested in this study will be helpful for imaging fracture systems in oil and gas reservoirs as well as in geothermal reservoirs.

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.

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.

Transforming walk‐away VSP data into reverse VSP data

Marine walk‐away vertical seismic profiling (VSP) data can be transformed into reverse VSP data using an elastic reciprocity transformation. A reciprocity transform is derived and tested using data generated with a 2-D high‐order, finite‐difference modeling scheme in a complex elastic model. First, 201 shots are generated with a walk‐away VSP experimental configuration. Both the x‐component and the z‐component of the displacement are measured. These data are collected in two common receiver data sets. Then two shots are generated in a reverse VSP configuration. We demonstrate that subtraction of the reverse VSP data from the walk‐away VSP data gives very small residuals. The transformation of walk‐away data into reverse VSP data makes prestack shot‐domain migration feasible for walk‐away data. Synthetic data from a multishot walk‐away experiment can be obtained from one or a few modeling operations with a RVSP experimental configuration. The required computer time is reduced by two orders of magnitude.

Borehole stresses created by downhole seismic sources

This paper analyzes the stresses created by downhole acoustic sources used for crosshole or reverse VSP data acquisition. The pressures created by downhole sources may be as large as [Formula: see text] bars which cannot affect intact steel casing. However, such pressures might in principle either (A) crack the cement radially through excessive hoop stress [Formula: see text], or (B) loosen the cement from the casing through an excessive value of shear stress [Formula: see text].