Crosshole Data Research Articles

3D Reverse-Time Migration Imaging for Multiple Cross-Hole Research and Multiple Sensor Settings of Cross-Hole Seismic Exploration

The two-dimensional (2D) cross-hole seismic computed tomography (CT) imaging acquisition method has the potential to characterize the target zone optimally compared to surface seismic surveys. It has wide applications in oil and gas exploration, engineering geology, etc. Limited to 2D hole velocity profiling, this method cannot acquire three-dimensional (3D) information on lateral geological structures outside the profile. Additionally, the sensor data received by cross-hole seismic exploration constitute responses from geological bodies in 3D space and are potentially affected by objects outside the well profiles, distorting the imaging results and geological interpretation. This paper proposes a 3D cross-hole acoustic wave reverse-time migration imaging method to capture 3D cross-hole geological structures using sensor settings in multi-cross-hole seismic research. Based on the analysis of resulting 3D cross-hole images under varying sensor settings, optimizing the observation system can aid in the cost-efficient obtainment of the 3D underground structure distribution. To verify this method’s effectiveness on 3D cross-hole structure imaging, numerical simulations were conducted on four typical geological models regarding layers, local high-velocity zones, large dip angles, and faults. The results verify the model’s superiority in providing more reliable and accurate 3D geological information for cross-hole seismic exploration, presenting a theoretical basis for processing and interpreting cross-hole data.

Bayesian seismic travel-time cross-hole tomography in vertically transversely isotropic media

We apply a transdimensional, hierarchical Markov chain Monte Carlo sampling algorithm (McMC) for 2-D cross-hole travel-time tomography in transversely isotropic media with vertical symmetry axis. The McMC approach has several advantages compared to classical inversion approaches: It is a global search, the high number of tested models allows the statistical analysis including the calculation of a reference model as well as uncertainty estimation, no initial models or regularization parameters are needed, the amount of data noise is automatically determined, and the model parametrization is data dependent and self-adjusting. For the forward solution a FD Fast Marching method utilizing second-order Godunov schemes is used. The performance of the approach is first tested on synthetic datasets to evaluate the potential and possible limitation to recover anisotropic models. We have shown that the recovery of models described by 2 anisotropic parameters (Thomsen parameters) and the vertical velocity is possible for observation scenarios with good distribution of sources and receivers. For more realistic observational geometries (i.e. cross-hole experiments), the recovery of the 3 parameters is limited, but still possible for example for the elliptical anisotropic case (ε = δ) or regarding the horizontal velocity. Finally we applied the McMC approach to a well-studied real cross-hole data set from the MALLIK 2002 research program and compared the results with previous conventional inversions.

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

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

Elastic vertically transversely isotropic full-waveform inversion using cross-gradient constraints — An application toward high-level radioactive waste repository monitoring

Anisotropic seismic full-waveform inversion (FWI) is a challenging task. In the case of 2D vertically transversely isotropic (VTI) media, there are five independent model parameters. This relatively large number of different parameter types imposes significant trade-off issues and makes the inversion parameterization a challenging task. The problem is less severe in a crosshole configuration, in which a wider angular coverage of the region of interest is available. There exist many suggestions for suitable inversion parameterizations. We have determined that, for a crosshole configuration, a relatively simple velocity-based parameterization provides a good FWI reconstruction of the subsurface. Furthermore, considerable improvements of the tomographic images can be achieved by supplying structural similarity constraints using cross gradients to the inversion problem. With two synthetic data sets, we determine that a structurally constrained VTI FWI workflow produces sharper subsurface images without adversely affecting the parameter trade-off issue. With a second synthetic experiment, we find that structurally constrained VTI FWI is robust to major differences in anomaly locations for different parameter types. We successfully applied the methodology to a crosshole data set acquired to image a downscaled version of a high-level radioactive waste repository. The resulting tomograms allowed a narrow highly fractured zone to be imaged.

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

Acoustic Based Crosshole Full Waveform Slowness Inversion in the Time Domain

We develop a new full waveform inversion (FWI) method for slowness with the crosshole data based on the acoustic wave equation in the time domain. The method combines the total variation (TV) regularization with the constrained optimization together which can inverse the slowness effectively. One advantage of slowness inversion is that there is no further approximation in the gradient derivation. Moreover, a new algorithm named the skip method for solving the constrained optimization problem is proposed. The TV regularization has good ability to inverse slowness at its discontinuities while the constrained optimization can keep the inversion converging in the right direction. Numerical computations both for noise free data and noisy data show the robustness and effectiveness of our method and good inversion results are yielded.

Borehole effect in 2.5D crosshole resistivity tomography

It is well known that borehole fluid effects in crosshole electrical resistivity tomography (ERT) can produce serious image artifacts using conventional inversion algorithms that do not account for borehole effects. Using the analytical solution for an infinitely long borehole model in a half-space, we demonstrated that the borehole effects depend on the borehole radius and on the resistivity contrast between the borehole fluid and the surrounding rock formation. The results also show that the borehole effects arise mainly for the inline data where the current and potential electrodes are placed at the same borehole, and they are not severe for the crosshole data. To overcome the borehole effects in the crosshole ERT surveys, we developed a new 2.5D inversion algorithm incorporating the boreholes. The new inversion algorithm yielded an electrical resistivity image that was devoid of artifacts around the boreholes.

Evaluation of Shear Wave Velocity Correlations and Development of New Correlation Using Cross-hole Data

Seismic microzonation studies are being proposed in Pakistan. In addition, a downhole array of accelerometers was planned to be installed in the University of Engineering and Technology, Mardan Campus. Therefore, extensive geotechnical and geophysical tests were conducted. Shear wave velocity (Vs) is a fundamental parameter required for these microzontation studies. It is feasible in terms of cost and space to determine Vs from Standard Penetration Test (SPT) or Cone Penetration Test (CPT) results through empirical correlations. The purpose of this paper is to explore the applicability and predictability of these correlations to the data collected from the area where downhole array is planned. Data from these surveys was used to evaluate relevant correlations selected from literature. Predicted shear wave velocity values from these correlations were compared with measured shear wave velocity values from seismic crosshole tests. Most SPT-Vs correlations evaluated showed low predictability. The SPT-Vs correlation that showed highest degree of fitness to the dataset was the one developed for Lucknow, India, a city with similar geological setting, workmanship and SPT method employed. Site specific SPT-Vs correlation was also proposed. Most of the CPT-Vs correlations evaluated showed considerable predictive capability compared to the SPT-Vs correlations.

The Research of Reverse-Time Migration for Cross-Hole Seismic

Cross-hole seismic is the leading technology of development seismology, which is still developing and improving. With the development of down-hole acquisition equipment, cross-hole seismic acquisition technology is getting mature, providing better data for imaging. According to the features of cross-hole survey and cross-hole data, we put forward a reverse-time migration method which is suitable for the wave equation for cross-hole seismic data. We propose finite difference scheme of higher order, and then derive its stability condition in cross-hole seismic. The frequency dispersion problem in cross-hole seismic wave field extrapolation is also discussed. Cross correlation imaging condition is used to realize migration, and Laplace filter is applied to remove low-frequency noise from migration section. Thus finite-difference reverse-time migration method for cross-hole seismic is established. Finally, we build geological models with anomalous ellipsoids, and apply cross-hole seismic wave field simulation and migration to them, thus our method proves its effectiveness. When dealing with real cross-hole seismic data with this method, high-resolution migration sections can be achieved. Key words : Cross-hole seismic; Reverse-time migration; Model test

Sensing and Imaging the State of Effective Stress in a 1-g Shallow Foundation Model

Abstract This paper describes an experimental study for the imaging of effective stresses under a shallow foundation system. S-wave data were collected during several loading stages of the shallow foundation model. S-wave crosshole data were inverted to obtain effective stress distribution images by solving the tomographic imaging problem. To optimize the travel-time tomographic imaging solution of the stress-heterogeneous medium, S-wave data quality, sensor array design, ray tracing algorithm selection, and inversion parameter choice were carefully considered. The experimental results show the practical engineering potential of tomographic imaging techniques for the monitoring of in situ effective stress and the evaluation of design models and effective stress-driven processes.

Validity of the acoustic approximation in full-waveform seismic crosshole tomography

Acoustic waveform inversions are an increasingly popular tool for extracting subsurface information from seismic data. They are computationally much more efficient than elastic inversions. Naturally, an inherent disadvantage is that any elastic effects present in the recorded data are ignored in acoustic inversions. We investigate the extent to which elastic effects influence seismic crosshole data. Our numerical modeling studies reveal that in the presence of high contrast interfaces, at which P-to-S conversions occur, elastic effects can dominate the seismic sections, even for experiments involving pressure sources and pressure receivers. Comparisons of waveform inversion results using a purely acoustic algorithm on synthetic data that is either acoustic or elastic, show that subsurface models comprising small low-to-medium contrast ([Formula: see text]) structures can be successfully resolved in the acoustic approximation. However, in the presence of extended high-contrast anomalous bodies, P-to-S-conversions may substantially degrade the quality of the tomographic images. In particular, extended low-velocity zones are difficult to image. Likewise, relatively small low-velocity features are unresolved, even when advanced a priori information is included. One option for mitigating elastic effects is data windowing, which suppresses later arriving seismic arrivals, such as shear waves. Our tests of this approach found it to be inappropriate because elastic effects are also included in earlier arriving wavetrains. Furthermore, data windowing removes later arriving P-wave phases that may provide critical constraints on the tomograms. Finally, we investigated the extent to which acoustic inversions of elastic data are useful for time-lapse analyses of high contrast engineered structures, for which accurate reconstruction of the subsurface structure is not as critical as imaging differential changes between sequential experiments. Based on a realistic scenario for monitoring a radioactive waste repository, we demonstrated that acoustic inversions of elastic data yield substantial distortions of the tomograms and also unreliable information on trends in the velocity changes.

Anisotropic attenuation analysis of crosshole data generated during hydraulic fracturing

Attenuation of seismic waves is sensitive to the physical properties of the subsurface and has been observed in vertical seismic profiling (VSP) and reflection data. De et al. (1994) report measurements of the P- and S-wave quality factors from VSP surveys and sonic logs. Klimentos (1995) measures compressional and shear attenuation from sonic logs in sandstone formations with variable oil, water, and gas saturation and observes that [Formula: see text] and [Formula: see text] can be used for pore-fluid discrimination. Maultzsch et al. (2007) evaluate P-wave azimuthal attenuation anisotropy from 3D VSP data acquired over a fractured hydrocarbon reservoir and infer fracture directions from attenuation analysis. Attenuation anisotropy has also been observed in P-wave reflection data (Clark et al., 2009; Vasconcelos and Jenner, 2005). Seismic attenuation is most commonly measured using the spectral-ratio method. Zhu et al. (2007) extend the spectral-ratio method to anisotropic media and apply it to physical modeling data acquired for a transversely isotropic (TI) sample. Computing spectral ratios helps eliminate the source spectrum and can be used to obtain accurate effective and interval attenuation coefficients of P- and S-waves in layered anisotropic media (Behura and Tsvankin, 2009a; Shekar and Tsvankin, 2011).

Tomografia 2D Baseado em Dados de Gpr - Modelo Direto Via Princípio de Fermat

We have developed a 2D forward modeling of GPR - Crosshole data based on theory of graphs, Fermat's principle and Dijkstra's algorithm, to compute the electromagnetic wave behavior in the subsurface.The fundamental principle of our method obeys the Snell- Decartes and reflection's laws by using Fermat's principle. We set up an undirected graph with hexagonal symmetry in which the nodes are connected by edges. The electromagnetic waves travel through the graph's edges and it obeys Fermat's principle.To solve the singlesource shortest path problem for the graph we use Dijkstra's algorithm that finds the shortest path between a single node and every other node. In this case, the shortest path in the graph is the ray path of the wave, because of Fermat's principle.The inverse modeling is a Heuristic method called Genetic Algorithm (GA), witch simulates the natural selection and crossing-over to estimate an optimal solution.

Validity of the Refraction Microtremors (ReMi) Method for Determining Shear Wave Velocities for Different Soil Types in Egypt

The Refraction Microtremor (ReMi) method is being used around the world by the geotechnical and geophysical community to determine shear-wave velocities. This is due to its faster, less expensive and accurate determination of shear wave velocities, when compared to other methods used. Unlike standard crosshole and downhole techniques, ReMi does not require any drilling. It eliminates the problem of shear-wave source and quiet site that are pre-requisites for good seismic refraction surveys. In this paper we present refraction microtremors (ReMi) measurements done at sites underlain by different soil types in Egypt. The ReMi data were collected using standard refraction equipment employing 12, 24 or 48 channels. We used deep oceanographic noise and ambient noise including energy from power generators, pile drivers and traffic. The data were processed using the SeisOpt® ReMi? (© Optim, Inc.) software to reveal one-dimensional shear-wave velocity structures beneath the arrays. To access the validity of the method for the Egyptian soils, the shear-wave profiles obtained from the ReMi measurements were compared to downhole and crosshole data for different soils. Comparisons demonstrate the robustness of the ReMi technique for obtaining shear-wave velocities for different soil types in Egypt.

Quantitative analysis of water-content estimation errors using ground-penetrating radar data and a low-loss approximation

Expressions are derived to quantify the error when estimating permittivity that results from using the low-loss approximation under lossy conditions and to examine the repercussions on estimating water content [Formula: see text]. Values are computed under a range of porosity, clay-content, water-quality, and frequency conditions. Although in most cases the error is negligible, it can be significant for some hydrogeophysical applications involving cross-hole measurements or low-frequency surface ground-penetrating radar (GPR). For instance, when the loss tangent [Formula: see text] equals 0.5, corresponding to an effective conductivity of [Formula: see text], a dielectric constantof 11, and a frequency of [Formula: see text], the relative error on dielectric permittivity is approximately 6%. If the conductivity doubles or the frequency is halved, the loss tangentdoubles but the error grows to 21%. In addition, considering a situation where the porosity is 20% and [Formula: see text], the use of the low-loss approximation leads to a 10% deviation from [Formula: see text]. In the context of water-content estimation, we therefore suggest to perform attenuation tomography, in addition to velocity tomography for crosshole data, or estimate the quality factor [Formula: see text] for surface GPR data to compute the loss tangent over the probed area. If proven necessary, the parameters sought can then be determined more accurately using a lossy formulation. We also propose to supplement GPR measurements with electrical-resistivity tomography to constrain the borehole GPR amplitude data-processing steps required by attenuation tomography or to complement the characterization of the survey area and improve the knowledge brought by [Formula: see text] estimates alone.

A 2.5D cross-hole electromagnetic modelling and inversion method and its application to survey data from the Gudao oil field, east China

Cross-hole electromagnetic (EM) inversion has been increasingly used to study the variations in rock properties between boreholes, particularly the properties and spatial distribution of the fluids in a subterranean reservoir. In this study, a practical method for simulating and inverting cross-hole EM data to reconstruct a resistivity distribution is presented. The iterative regularized inversion method incorporates a priori information and variable roughness measures, assuming that the medium in the target area is two dimensional and the injected source is a point dipole source. A forward solver for two-and-half-dimensional (2.5D) EM modelling was developed using the finite element method (FEM) for evaluating the forward problem and the 'pseudo-forward' problem needed to construct the sensitivity matrix. The resulting linear system of equations in the forward problem is solved efficiently by the modified incomplete Cholesky biconjugate gradient iterative method. The partial derivatives forming the sensitivity matrix are computed by the adjoint equation based on the reciprocity principle. The regularized inversion scheme, which can be constrained using the available a priori information from well logging, is used to determine the inter-well resistivity distribution from synthetic EM data and the field data acquired in Gudao oil field in east China. The synthetic and field data examples indicate that our scheme is robust and stable in the presence of random noise and therefore can be used for general cross-hole EM interpretation.

Application of 2.5D cross-hole electromagnetic inversion in Gudao Oil Field, East China

In this study, we present a practical technique of transforming cross-hole EM data into the inter-well resistivity distribution. The a priori information constraint is incorporated into an iterative regularized inversion procedure and a variable roughness is added into the inversion process. Finite element approximation based on a two and a half-dimensional (2.5D) model has been developed for the forward problem and the “pseudo-forward” problem needed for constructing the sensitivity matrix and synthetic data set. The regularized least-squares inversion scheme, constrained with the a priori information obtained from well logs, was adopted to reconstruct the inter-well resistivity profile from two synthetic electromagnetic data sets and field data acquired in the Gudao Oil Field, East China. The partial derivatives of the sensitivity matrix were computed by the adjoint equation based on the reciprocity principle. Inversion results of the synthetic and field data examples suggest that our method is robust and stable in the presence of random noise in the field data and can be used for cross-hole EM field data interpretation.

Implication of seismic attenuation for gas hydrate resource characterization, Mallik, Mackenzie Delta, Canada

Wave attenuation is an important physical property of hydrate‐bearing sediments that is rarely taken into account in site characterization with seismic data. We present a field example showing improved images of hydrate‐bearing sediments on seismic data after compensation of attenuation effects. Compressional quality factors estimated from zero‐offset Vertical Seismic Profiling data acquired at Mallik, Northwest Territories, Canada, demonstrate significant wave attenuation for hydrate‐bearing sediments. These results are in agreement with previous attenuation estimates obtained from sonic logs and crosshole data at different frequency intervals. The application of an inverse Q‐filter to compensate attenuation effects of permafrost and hydrate‐bearing sediments improved the resolution of surface 3D seismic data and its correlation with log data, particularly for the shallowest gas hydrate interval. Compensation of the attenuation effects of the permafrost likely explains most of the improvements for the shallow gas hydrate zone. Our results show that characterization of the Mallik gas hydrates with seismic data not corrected for attenuation would tend to overestimate thicknesses and lateral extent of hydrate‐bearing strata and hence, the volume of hydrates in place.

Full-Waveform Inversion of Crosshole Radar Data Based on 2-D Finite-Difference Time-Domain Solutions of Maxwell's Equations

Crosshole radar techniques are important tools for a wide range of geoscientific and engineering investigations. Unfortunately, the resolution of crosshole radar images may be limited by inadequacies of the ray tomographic methods that are commonly used in inverting the data. Since ray methods are based on high-frequency approximations and only account for a small fraction of the information contained in the radar traces, they are restricted to resolving relatively large-scale features. As a consequence, the true potential of crosshole radar techniques has yet to be realized. To address this issue, we introduce a full-waveform inversion scheme that is based on a finite-difference time-domain solution of Maxwell's equations. We benchmark our new scheme on synthetic crosshole data generated from suites of increasingly complex models. The full-waveform tomographic images accurately reconstruct the following: (1) the locations, sizes, and electrical properties of isolated subwavelength objects embedded in homogeneous media; (2) the locations and sizes of adjacent subwavelength objects embedded in homogeneous media; (3) abrupt media boundaries and average and stochastic electrical property variations of heterogeneous layered models; and (4) the locations, sizes, and electrical conductivities of water-filled tunnels and closely spaced subwavelength pipes embedded in heterogeneous layered models. The new scheme is shown to be remarkably robust to the presence of uncorrelated noise in the radar data. Several limitations of the full-waveform tomographic inversion are also identified. For typical crosshole acquisition geometries and parameters, small resistive bodies and small closely spaced dielectric objects may be difficult to resolve. Furthermore, electrical property contrasts may be underestimated. Nevertheless, the full-waveform inversions usually provide substantially better results than those supplied by traditional ray methods.

High-resolution aquifer characterization using seismic cross-hole tomography: An evaluation experiment in a gravel delta

We performed field experiments at a well-studied aquifer test site to evaluate the geophysical method of seismic cross-hole tomography for high-resolution aquifer characterization. We address the challenge of finding meaningful relationships between seismic velocity and hydraulic conductivity at the field scale for hydraulic conductivity estimation. The resolution of seismic cross-hole data is limited by inversion ambiguities and diffraction effects, which are not accounted for using conventional ray-based inversion algorithms for velocity reconstruction. We investigate the impact of diffraction effects on seismic velocity inversion and implications for hydraulic modelling. Our ray-based seismic tomograms correlate well with sedimentary cores and hydraulic conductivity logs allowing us to interpret a zone model with distinct geophysical/hydraulic zones. We use this zone model to constrain seismic inversions using two approaches for forward modelling: ray tracing (infinite-frequency approximation) and wavefield modelling at a realistic frequency, which accounts for diffraction. We show that using the common ray tracing approach, seismic velocities in low-velocity zones may be significantly overestimated. Based on the available data, we observe a negative linear relationship between logarithmic hydraulic conductivity and seismic velocity. This relationship is used to parameterize hydraulic models based on seismic velocity models. Computer simulations of advective contaminant transport demonstrate that their results are extremely sensitive to an accurate determination of seismic low-velocity zones. We conclude that seismic tomography is indeed very useful for high-resolution aquifer characterization. However, an inversion approach that accounts for diffraction effects, a priori knowledge of the subsurface structure and additional well data should be applied for quantitative interpretations.