Journal of Geophysics and Engineering - Latest Papers

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Latest articles for Journal of Geophysics and Engineering
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Seismic rock physics characterization of anisotropic shale—a Longmaxi Shale case study

Mon, 02/12/2018 - 7:00pm
An anisotropic rock physics model based on effective medium theories is constructed for shales and applied to the Longmaxi Shale formation. In the rock physics model, intrinsic VTI (transverse isotropy with a vertical asymmetry axis) anisotropy due to preferred orientations of clay particles is quantified by introducing the clay lamination ( CL ) index in the Backus averaging method, and additional anisotropy enhanced by bed-parallel fractures is described by the Chapman model for multi-scale pore spaces. Rock physics templates are constructed based on the model in order to better understand the implicit relations between reservoir properties, elastic and mechanical properties and seismic parameters. A model-based method is proposed for inversion of the CL and aspect ratio ( AR ) of horizontal fractures, as well as VTI anisotropy parameters for the shale formation. Results indicate that the obtained CL index shows a negative correlation with clay content,...

Accuracy and sensitivity analysis on seismic anisotropy parameter estimation

Mon, 02/12/2018 - 7:00pm
There is significant uncertainty in measuring the Thomsen’s parameter δ in laboratory even though the dimensions and orientations of the rock samples are known. It is expected that more challenges will be encountered in the estimating of the seismic anisotropy parameters from field seismic data. Based on Monte Carlo simulation of vertical transversely isotropic layer cake model using the database of laboratory anisotropy measurement from the literature, we apply the commonly used quartic non-hyperbolic reflection moveout equation to estimate the seismic anisotropy parameters and test its accuracy and sensitivities to the source-receive offset, vertical interval velocity error and time picking error. The testing results show that the methodology works perfectly for noise-free synthetic data with short spread length. However, this method is extremely sensitive to the time picking error caused by mild random noises, and it requires the spread length to be greater than the de...

Experimental analysis and application of the effect of stress on continental shale reservoir brittleness

Mon, 02/12/2018 - 7:00pm
Hydraulic fracturing is an effective measure of reservoir modification for the development of shale gas. The evaluation of rock brittleness can provide a basis for the optimization of fracturing. In this paper, the effect of stress on the brittleness of shale is systematically analyzed by designing triaxial mechanics tests. The strain analysis method was used to evaluate the shale brittleness. The research indicates that, with the increase of effective confining pressure, the value of the brittleness index ( B 1 ) decreases. There is a linear and positive correlation between the average reduction ratio of B 1 and the buried depth. The stress has a significant effect on the shale brittleness. Therefore, the rock brittleness can be overestimated without considering the influence of the buried depth or the stress of formation when using the mineral composition method. Being affected by the stress, when the brittle mineral content of the shale reser...

An effective absorbing layer for the boundary condition in acoustic seismic wave simulation

Mon, 02/12/2018 - 7:00pm
Efficient numerical simulation of seismic wavefields generally involves truncating the Earth model in order to keep computing time and memory requirements down. Absorbing boundary conditions, therefore, are applied to remove the boundary reflections caused by this truncation, thereby allowing for accurate modeling of wavefields. In this paper, we derive an effective absorbing boundary condition for both acoustic and elastic wave simulation, through the simplification of the damping term of the split perfectly matched layer (SPML) boundary condition. This new boundary condition is accurate, cost-effective, and easily implemented, especially for high-performance computing. Stability analysis shows that this boundary condition is effectively as stable as normal (non-absorbing) wave equations for explicit time-stepping finite differences. We found that for full-waveform inversion (FWI), the strengths of the effective absorbing layer—a reduction of the computational and memory cost c...

Comparative study of receiver-side ghost wavefield attenuation on different marine acquisition configurations

Mon, 02/12/2018 - 7:00pm
In marine seismic exploration, the ghost energies (down-going waves), which arise from the reflection at the surface, are often treated as unwanted signals for data processing. The ghost wave fields interfere with the desired primary signals, leads to frequency notches and attenuation of low frequencies, which in turn downgrade the resolution of the recorded seismic data. There are two main categories of methods to solve the ghost or the so-called notch problem: the non-conventional acquisition configuration-based technique and a deghosting algorithm-based solution. The variable-depth streamer (VDS) acquisition solution is one of the most representative methods in the first category, which has become a popular solution for marine seismic acquisition to obtain broad data bandwidth. However, this approach is not as economic as the conventional constant depth streamer (CDS) acquisition, due to the precise control of the towing streamer. In addition, there are large quantities of co...

Sensitivity analyses of acoustic impedance inversion with full-waveform inversion

Mon, 02/12/2018 - 7:00pm
Acoustic impedance estimation has a significant importance to seismic exploration. In this paper, we use full-waveform inversion to recover the impedance from seismic data, and analyze the sensitivity of the acoustic impedance with respect to the source-receiver offset of seismic data and to the initial velocity model. We parameterize the acoustic wave equation with velocity and impedance, and demonstrate three key aspects of acoustic impedance inversion. First, short-offset data are most suitable for acoustic impedance inversion. Second, acoustic impedance inversion is more compatible with the data generated by density contrasts than velocity contrasts. Finally, acoustic impedance inversion requires the starting velocity model to be very accurate for achieving a high-quality inversion. Based upon these observations, we propose a workflow for acoustic impedance inversion as: (1) building a background velocity model with travel-time tomography or reflection waveform inversion; (2...

Improved vertical displacements induced by a refined thermal expansion model and its quantitative analysis in GPS height time series

Mon, 02/12/2018 - 7:00pm
There are apparent seasonal variations in GPS height time series, and thermal expansion is considered to be one of the potential geophysical contributors. The displacements introduced by thermal expansion are usually derived without considering the annex height and underground part of the monument (e.g. located on roof or top of the buildings), which may bias the geophysical explanation of the seasonal oscillation. In this paper, the improved vertical displacements are derived by a refined thermal expansion model where the annex height and underground depth of the monument are taken into account, and then 560 IGS stations are adopted to validate the modeled thermal expansion (MTE) displacements. In order to evaluate the impact of thermal expansion on GPS heights, the MTE displacements of 80 IGS stations with less data discontinuities are selected to compare with their observed GPS vertical (OGV) displacements with the modeled surface loading (MSL) displacements removed in advanc...

Automatic pattern identification of rock moisture based on the Staff-RF model

Thu, 02/08/2018 - 7:00pm
Studies on the moisture and damage state of rocks generally focus on the qualitative description and mechanical information of rocks. This method is not applicable to the real-time safety monitoring of rock mass. In this study, a musical staff computing model is used to quantify the acoustic emission signals of rocks with different moisture patterns. Then, the random forest (RF) method is adopted to form the staff-RF model for the real-time pattern identification of rock moisture. The entire process requires only the computing information of the AE signal and does not require the mechanical conditions of rocks.

Signal characteristics of electroseismic conversion

Thu, 02/08/2018 - 7:00pm
Electric fields applying on the fluid-filled porous materials can induce small relative pore-fluid motions due to electroseismic conversions. In order to characterize the electroseismic propagation phenomena, we have designed an experimental apparatus to acquire the electroseismic (ES) signals. The electroseismic measurements on different samples have been conducted to confirm the origin of the recorded signals. We find that a strong acoustic signal generates around the electrode and affects the identification of ES signals. To further confirm and distinguish the ES signal as well as the acoustic signal around the electrode, we have analyzed records obtained with regular movements of the receiver, the sample and the source. Analysis has been made on the characteristics of the traveltime, polarity and frequency of ES signals. Our results show that the traveltime of ES signal relates to the distance between the rock sample and the receiver, the location of the exciting electrode h...

Effect of pore structure on the dispersion and attenuation of fluid-saturated tight sandstone

Thu, 02/08/2018 - 7:00pm
The pore structure of tight sandstone has an important effect on its elastic properties, and also determines the fluid-flow-related wave dispersion and attenuation mechanisms. However, in these dispersion models, only compliant pores with a fixed aspect ratio and content have been considered, which is not completely realistic in reservoir rock. A procedure is presented to obtain the pore aspect distribution of compliant pores (cracks) from the pressure dependence of the velocities. Based on the pore aspect distribution of the compliant pores, Gurevich’s squirt-fluid model is modified to consider the complex pore structure of reservoir rocks. The extended Gurevich squirt-flow model is consistent with Gassmann’s equation at the low-frequency limit, and with the Mavk–Jizba model at high frequency. To illustrate the validation of the extended Gurevich squirt-flow model, we compare the predictions of our squirt model with the laboratory measurements of two examples of water-saturated...

Least-squares Fourier finite-difference pre-stack depth migration for VTI media

Thu, 02/08/2018 - 7:00pm
Most traditional least-squares migration (LSM) methods are based on the assumption of isotropic media, which is not consistent with the actual underground situation. The media’s anisotropy will greatly change the seismic wave travel time affecting the imaging quality. In this paper we propose a preconditioned least-squares Fourier finite-difference (PLSFFD) pre-stack depth migration for vertical transverse isotropic (VTI) media. By improving the approximated FFD operators’ orders, the one-way wave propagator is of higher precision, making convergence easier. An illumination preconditioning operator is used to balance the amplitude for different depths. On this basis, we derive a one-way wave VTI migration operator and a de-migration operator to implement the VTI-PLSFFD. The accuracy and superiority of the new method are proved via numerical experiments on a simple anisotropic trapezoid model, the strong lateral variation Hess model and an anisotropy model associated with the fie...

Nonlinear PP and PS joint inversion based on the exact Zoeppritz equations: a two-stage procedure

Thu, 02/08/2018 - 7:00pm
S-velocity and density are very important parameters in distinguishing lithology and estimating other petrophysical properties. A reliable estimate of S-velocity and density is very difficult to obtain, even from long-offset gather data. Joint inversion of PP and PS data provides a promising strategy for stabilizing and improving the results of inversion in estimating elastic parameters and density. For 2D or 3D inversion, the trace-by-trace strategy is still the most widely used method although it often suffers from a lack of clarity because of its high efficiency, which is due to parallel computing. This paper describes a two-stage inversion method for nonlinear PP and PS joint inversion based on the exact Zoeppritz equations. There are several advantages for our proposed methods as follows: (1) Thanks to the exact Zoeppritz equation, our joint inversion method is applicable for wide angle amplitude-versus-angle inversion; (2) The use of both P- and S-wave information can furt...

Effect of temperature on the permeability of gas adsorbed coal under triaxial stress conditions

Thu, 02/08/2018 - 7:00pm
The combined effects of gas sorption, stress and temperature play a significant role in the changing behavior of gas permeability in coal seams. The effect of temperature on nitrogen and methane permeability of naturally fractured coal is investigated. Coal permeability, P-wave velocity and axial strain were simultaneously measured under two effective stresses and six different temperatures. The results showed that the behavior of nitrogen and methane permeability presented nonmonotonic changes with increasing temperature. The variation in the P-wave velocity and axial strain showed a good correspondence with coal permeability. A higher effective stress limited the bigger deformation and caused the small change in permeability. Methane adsorption and desorption significantly influence the mechanical properties of coal and play an important role in the variations in coal permeability. The result of coal permeability during a complete stress–strain process showed that the variatio...

Reservoir characteristics of coal–shale sedimentary sequence in coal-bearing strata and their implications for the accumulation of unconventional gas

Thu, 02/08/2018 - 7:00pm
Shale gas and coalbed methane (CBM) are both considered unconventional natural gas and are becoming increasingly important energy resources. In coal-bearing strata, coal and shale are vertically adjacent as coal and shale are continuously deposited. Research on the reservoir characteristics of coal–shale sedimentary sequences is important for CBM and coal-bearing shale gas exploration. In this study, a total of 71 samples were collected, including coal samples (total organic carbon (TOC) content >40%), carbonaceous shale samples (TOC content: 6%–10%), and shale samples (TOC content <6%). Combining techniques of field emission scanning electron microscopy (FE-SEM), x-ray diffraction, high-pressure mercury intrusion porosimetry, and methane adsorption, experiments were employed to characterize unconventional gas reservoirs in coal-bearing strata. The results indicate that in the coal-shale sedimentary sequence, the proportion of shale is the highest at 74% and that of carbon...

Downhole microseismic signal-to-noise ratio enhancement via strip matching shearlet transform

Sun, 02/04/2018 - 7:00pm
Shearlet transform has been proved effective in noise attenuation. However, because of the low magnitude and high frequency of downhole microseismic signals, the coefficient values of valid signals and noise are similar in the shearlet domain. As a result, it is hard to suppress the noise. In this paper, we present a novel signal-to-noise ratio enhancement scheme called strip matching shearlet transform. The method takes into account the directivity of microseismic events and shearlets. Through strip matching, the matching degree in direction between them has been promoted. Then the coefficient values of valid signals are much larger than those of the noise. Consequently, we can separate them well with the help of thresholding. The experimental results on both synthetic records and field data illustrate that our proposed method preserves the useful components and attenuates the noise well.

DenInv3D: a geophysical software for three-dimensional density inversion of gravity field data

Sun, 02/04/2018 - 7:00pm
This paper presents a three-dimensional density inversion software called DenInv3D that operates on gravity and gravity gradient data. The software performs inversion modelling, kernel function calculation, and inversion calculations using the improved preconditioned conjugate gradient (PCG) algorithm. In the PCG algorithm, due to the uncertainty of empirical parameters, such as the Lagrange multiplier, we use the inflection point of the L-curve as the regularisation parameter. The software can construct unequally spaced grids and perform inversions using such grids, which enables changing the resolution of the inversion results at different depths. Through inversion of airborne gradiometry data on the Australian Kauring test site, we discovered that anomalous blocks of different sizes are present within the study area in addition to the central anomalies. The software of DenInv3D can be downloaded from [] .

pyres : a Python wrapper for electrical resistivity modeling with R2

Sun, 02/04/2018 - 7:00pm
A Python package, pyres , was written to handle common as well as specialized input and output tasks for the R2 electrical resistivity (ER) modeling program. Input steps including handling field data, creating quadrilateral or triangular meshes, and data filtering allow repeatable and flexible ER modeling within a programming environment. pyres includes non-trivial routines and functions for locating and constraining specific known or separately-parameterized regions in both quadrilateral and triangular meshes. Three basic examples of how to run forward and inverse models with pyres are provided. The importance of testing mesh convergence and model sensitivity are also addressed with higher-level examples that show how pyres can facilitate future research-grade ER analyses.

Integrated detection of fractures and caves in carbonate fractured-vuggy reservoirs based on seismic data and well data

Sun, 02/04/2018 - 7:00pm
Aiming at the prediction of carbonate fractured-vuggy reservoirs, we put forward an integrated approach based on seismic and well data. We divide a carbonate fracture-cave system into four scales for study: micro-scale fracture, meso-scale fracture, macro-scale fracture and cave. Firstly, we analyze anisotropic attributes of prestack azimuth gathers based on multi-scale rock physics forward modeling. We select the frequency attenuation gradient attribute to calculate azimuth anisotropy intensity, and we constrain the result with Formation MicroScanner image data and trial production data to predict the distribution of both micro-scale and meso-scale fracture sets. Then, poststack seismic attributes, variance, curvature and ant algorithms are used to predict the distribution of macro-scale fractures. We also constrain the results with trial production data for accuracy. Next, the distribution of caves is predicted by the amplitude corresponding to the instantaneous peak frequency...

GPR random noise reduction using BPD and EMD

Sun, 02/04/2018 - 7:00pm
Ground-penetrating radar (GPR) exploration is a new high-frequency technology that explores near-surface objects and structures accurately. The high-frequency antenna of the GPR system makes it a high-resolution method compared to other geophysical methods. The frequency range of recorded GPR is so wide that random noise recording is inevitable due to acquisition. This kind of noise comes from unknown sources and its correlation to the adjacent traces is nearly zero. This characteristic of random noise along with the higher accuracy of GPR system makes denoising very important for interpretable results. The main objective of this paper is to reduce GPR random noise based on pursuing denoising using empirical mode decomposition. Our results showed that empirical mode decomposition in combination with basis pursuit denoising (BPD) provides satisfactory outputs due to the sifting process compared to the time-domain implementation of the BPD method on both synthetic and real example...

Modeling and experiments for the time-dependent diffusion coefficient during methane desorption from coal

Wed, 01/31/2018 - 7:00pm
Statistical analysis shows that in the coal matrix, the diffusion coefficient for methane is time-varying, and its integral satisfies the formula μt κ /(1 + β κ ). Therefore, a so-called dynamic diffusion coefficient model (DDC model) is developed. To verify the suitability and accuracy of the DDC model, a series of gas diffusion experiments were conducted using coal particles of different sizes. The results show that the experimental data can be accurately described by the DDC and bidisperse models, but the fit to the DDC model is slightly better. For all coal samples, as time increases, the effective diffusion coefficient first shows a sudden drop, followed by a gradual decrease before stabilizing at longer times. The effective diffusion coefficient has a negative relationship with the size of the coal particle. Finally, the relationship between the constants of the DDC model and the effective diffusion coefficient is discuss...