Journal of Geophysics and Engineering - Latest Papers

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Latest articles for Journal of Geophysics and Engineering
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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 http://159.226.162.30 [http://159.226.162.30] .

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...

Fast principal component analysis for stacking seismic data

Wed, 01/31/2018 - 7:00pm
Stacking seismic data plays an indispensable role in many steps of the seismic data processing and imaging workflow. Optimal stacking of seismic data can help mitigate seismic noise and enhance the principal components to a great extent. Traditional average-based seismic stacking methods cannot obtain optimal performance when the ambient noise is extremely strong. We propose a principal component analysis (PCA) algorithm for stacking seismic data without being sensitive to noise level. Considering the computational bottleneck of the classic PCA algorithm in processing massive seismic data, we propose an efficient PCA algorithm to make the proposed method readily applicable for industrial applications. Two numerically designed examples and one real seismic data are used to demonstrate the performance of the presented method.

Estimation of brittleness indices for pay zone determination in a shale-gas reservoir by using elastic properties obtained from micromechanics

Wed, 01/31/2018 - 7:00pm
The brittleness indices (BI) of gas-shales are computed by using their effective mechanical properties obtained from micromechanical self-consistent modeling with the purpose of assisting in the identification of the more-brittle regions in shale-gas reservoirs, i.e., the so-called ‘pay zone’. The obtained BI are plotted in lambda-rho versus mu-rho ##IMG## [http://ej.iop.org/images/1742-2140/15/2/307/jgeaa9a5eieqn1.gif] {$\lambda \rho -\mu \rho $} and Young’s modulus versus Poisson’s ratio ##IMG## [http://ej.iop.org/images/1742-2140/15/2/307/jgeaa9a5eieqn2.gif] {$E-\nu $} ternary diagrams along with the estimated elastic properties from log data of three productive shale-gas wells where the pay zone is already known. A quantitative comparison between the obtained BI and the well log data allows for the delimitation of regions where BI values could indicate the best reservoir target in regions with the highest shale-gas exploitation pot...

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