Applied Geophysics最新文献

When simulating the propagation of seismic waves in some special structures, such as tunnels and boreholes, finite difference forward modeling in the polar system has higher accuracy than the traditional Cartesian system. In actual situations, the polar space is the most irregular. To solve this problem, a forward modeling method for an irregular polar coordinate system is proposed to improve the simulation accuracy. First, an irregular surface of the polar space was meshed into an irregular polar system. After the transformation, the undulating surface was mapped into a plane one, and the wavefield was then computed in an irregular polar system. The Lebedev staggered grid was used to solve the wave equations in the irregular polar system. In addition, the artificial absorption boundary, cylindrical free boundary, and circumferential boundary conditions were used to absorb the boundary reflection. We selected three polar space models to demonstrate the new method in this study. The results show that the proposed elastic simulation method in an irregular polar coordinate system can produce more accurate and stable simulation results when modeling seismic wave propagation in an irregular polar space. Elastic full waveform inversion further shows that the irregular polar system elastic simulation method can accurately simulate the wavefield in an undulating polar space.

This study investigates the spatial and temporal variation of fractal dimension and b-value for the eastern part of the Himalaya and adjoining area (26°N–31°N and 87°E–98°E). The analysis is carried out on the earthquake dataset of 1373 events (Mc = 4.0) by sliding window technique for the period 1964 to 2020. The region is divided into three sub regions A (87°E–92°E), B (92°E–94°E) and C (94°E–98°E). The b-value computed for the region A comprising eastern Nepal is smaller compared to other two regions which infers the possible high stress and asperities in the region. High spatial fractal dimension (D_c > 1.5) and low temporal fractal dimension (D_t < 0.31) are computed for the regions. High spatial fractal dimension may indicate that fractures generating earthquakes are approaching a 2D structure and low temporal fractal dimension implies high clustering of earthquake’s epicenters. The b value shows a weak negative correlation with Dc for regions A and C while a weak positive correlation is observed for the region B. Based on b-value and fractal dimension, this study explains the frequency of earthquakes and heterogeneity of the seismogenic structure in this part of the Himalaya.

The high-frequency electromagnetic waves of ground-penetrating radar (GPR) attenuate severely when propagated in an underground attenuating medium owing to the influence of resistivity, which remarkably decreases the resolution of reverse time migration (RTM). As an effective high-resolution imaging method, attenuation-compensated RTM (ACRTM) can effectively compensate for the energy loss caused by the attenuation related to media absorption under the influence of resistivity. Therefore, constructing an accurate resistivity-media model to compensate for the attenuation of electromagnetic wave energy is crucial for realizing the ACRTM imaging of GPR data. This study proposes a resistivity-constrained ACRTM imaging method for the imaging of GPR data by adding high-density resistivity detection along the GPR survey line and combining it with its resistivity inversion profile. The proposed method uses the inversion result of apparent resistivity data as the GPR RTM-resistivity model for imposing resistivity constraints. Moreover, the hybrid method involving image minimum entropy and RTM is used to estimate the medium velocity at the diffraction position, and combined with the distribution characteristics of the reflection in the GPR profile, a highly accurate velocity model is built to improve the imaging resolution of the ACRTM. The accuracy and effectiveness of the proposed method are verified using the ACRTM test of the GPR simulated data of a typical attenuating media model. On this basis, the GPR and apparent resistivity data were observed on a field survey line, and use the GPR resistivity-constrained ACRTM method to image the observed data. A comparison of the proposed method with the conventional ACRTM method shows that the proposed method has better imaging depth, stronger energy, and higher resolution, and the obtained results are more conducive for subsequent data analysis and interpretation.

The depth from extreme points (DEXP) method can be used for estimating source depths and providing a rough image as a starting model for inversion. However, the application of the DEXP method is limited by the lack of prior information regarding the structural index. Herein, we describe an automatic DEXP method derived from Euler’s Homogeneity equation, and we call it the Euler–DEXP method. We prove that its scaling field is independent of structural indices, and the scaling exponent is a constant for any potential field or its derivative. Therefore, we can simultaneously estimate source depths with different geometries in one DEXP image. The implementation of the Euler–DEXP method is fully automatic. The structural index can be subsequently determined by utilizing the estimated depth. This method has been tested using synthetic cases with single and multiple sources. All estimated solutions are in accordance with theoretical source parameters. We demonstrate the practicability of the Euler–DEXP method with the gravity field data of the Hastings Salt Dome. The results ultimately represent a better understanding of the geometry and depth of the salt dome.

Differences are found in the attributes of microseismic events caused by coal seam rupture, underground structure activation, and groundwater movement in coal mine production. Based on these differences, accurate classification and analysis of microseismic events are important for the water inrush warning of the coal mine working face floor. Cluster analysis, which classifies samples according to data similarity, has remarkable advantages in nonlinear classification. A water inrush early warning method for coal mine floors is proposed in this paper. First, the short time average over long time average (STA/LTA) method is used to identify effective events from continuous microseismic records to realize the identification of microseismic events in coal mines. Then, ten attributes of microseismic events are extracted, and cluster analysis is conducted in the attribute domain to realize unsupervised classification of microseismic events. Clustering results of synthetic and field data demonstrate the effectiveness of the proposed method. The analysis of field data clustering results shows that the first kind of events with time change rules is of considerable importance to the early warning of water inrush from the coal mine working face floor.

The mediate-low maturity continental shale oil reservoir in the 7th member of the Triassic Yanchang Formation is one of the main zones for shale oil exploration and development in Ordos Basin, China, but the seismic response mechanism of the reservoir remains unclear. Therefore, developing a new method for “sweet spot” seismic prediction combined with rock physics is necessary. To determine the petrophysical characteristics of continental shale in the 7th member of the Yanchang Formation in the study area, a series of tests, such as a systematic acoustic test on shale oil samples in the target segment, X-ray diffraction analyses, analysis of thin optical sections, and scanning electron microscopy, were conducted to summarize the patterns of seismic elastic property changes. Results show that the shale oil samples of the 7th member of the Yanchang Formation are primarily composed of lithic feldspar sandstone with feldspar dissolution and intergranular pores, widespread micron pores, and throats. Such composition indicates a positive correlation between porosity and permeability. The velocities of the samples are affected by their porosity and mineral composition. Velocity increases with the increase of the calcium content and decreases with the increase of the clay content, indicating a negative correlation with the porosity and total organic carbon (TOC) content on the condition of the same rock structure (quartz skeleton or clay skeleton). The elastic properties of the rock are horizontally isotropic (T1 medium), and the velocity is vertically anisotropic. In addition, the directional arrangement of clay controls the anisotropy of rock velocity. TOC is mainly distributed in the primary intergranular pores, and it has no contribution to the anisotropy of rock velocity. The results of this paper can provide a reference for the seismic prediction of continental shale oil reservoirs in the 7th member of the Yanchang Formation.

Most sedimentary formations with fine layers can be characterized as transversely isotropic media. The evaluation of shear-wave anisotropy is critical in logging-while-drilling (LWD) applications. We developed a joint method to simultaneously invert formation shear-wave anisotropy and vertical shear velocity using LWD monopole and dipole dispersion data. Theoretical analysis demonstrates that formation shear-wave anisotropy significantly affects the dispersion characteristics of Stoneley and formation flexural waves. The inversion objective function was constructed based on the change in dispersion characteristics and was weighted by the spectra of multipole waves. Numerical results using synthetic examples demonstrate that the joint inversion method can not only alleviate the non-uniqueness problem but also help improve the accuracy of the inversion results. The comparison of different signal-to-noise ratio inversion results proved that the weighted inversion method is more accurate and stable.

Acoustic velocity varies in deep-water environments. To obtain accurate inversion interpretations, it is necessary to develop a horizontally layered seawater—seabed (HLSS) model with continuously varying velocities. In this work, we used an HLSS model based on wave theory to deduce the Scholte wave dispersion equations and established an HLSS model based on the acoustic velocity profile and the submarine medium parameters of the South China Sea. We studied the dispersion characteristics of Scholte waves and theoretically calculated the amplitude—depth distribution. We also examined the influence of deep-water environments on the dispersion characteristics of Scholte waves. Using the real geological parameters of the Dongsha Islands in the South China Sea, we exploited the spectral element method to simulate seismic wave propagation in the fluid—solid interface and extracted the Scholte wave dispersion curves using multichannel analysis of surface waves (MASW). The consistent theoretical and extracted dispersion curve results verified the accuracy of our method. Numerical experiments showed that the dispersion characteristics of Scholte waves in deep water are weaker than those in shallow water. In addition to the seawater depth and the physical parameters of seabed sediments, the seawater’s variable velocity also influences Scholte wave dispersion characteristics.

In recent years, the Yanchang shale-oil formations of the Ordos Basin are rich in reserves with complex lithology and structure characteristics, low porosity and low permeability, and weak anomalies for oil and water discriminations, have been the key targets of unconventional oil/gas resource exploration and development in the relevant areas. The joint acoustic-electrical (AE) properties can be used to interpret reservoir lithology, mineralogy, pore structure, and fluid saturation. To conduct tests of thin section analysis, X-ray diffraction, and ultrasonic and electrical experiments at different pressures and saturation degrees, cores from the shale-oil formations in the Q area of the basin are collected. The variations in AE properties with respect to clay content, porosity, pressure (microfracture), and saturation are analyzed. The experimental results indicate that the rock physics behaviors of sandstones with different clay contents vary significantly. The AE properties of clean sandstones are basically dependent on the microfractures (pressure), while for muddy sandstones, the clay content is an important factor affecting the responses. The target reservoir consists of interbedded sandstone and shale layers. The AE equivalent medium equations and the Gurevich theory are applied to establish the joint models for the different lithologies and simulate the variations in AE properties with respect to fluid type, pore structure, and mineral components. The three-dimensional joint templates of clean and muddy sandstones, as well as shale, are developed based on the elastic and electrical attributes and then calibrated using the experimental and well-log data. The reservoir properties are estimated with the templates and validated by the log data. The results indicate that the joint templates based on lithology characteristics can effectively characterize the properties of interbedded sandstone and shale layers. Furthermore, the combined application of AE data provides more beneficial information for the assessment of rock properties, leading to precise estimates that conform with the actual formation conditions.