A procedure of automatic evaluation of residual statistic adjustments for increasing information value of exploration seismology data

In present-day seismic exploration the quality of the observed wave field that guarantees obtaining the most exact and complete information on the structure of the studied area plays an important role. Therefore much attention is paid to elaboration of procedures for elimination of different noises and distortions present in the registered data. They include correction of statics or calculation and maintenance of static adjustments with taking into account the influence of small velocities zone (SVZ) and locality relief at each station of reception and explosion along the profile or observation area to the form of hodograph. A procedure of automatic finding of residual statistic corrections based on usage of seismograms of equal distances and first wave arrivals without conducting their correlations that gives a possibility to exclude the effect of operator mistakes to the result has been considered. A theory has been proposed with algorithm consisting of putting in the observed wave fields, assortment of paths, finding temporal shifts by equidistant paths and computation of correcting adjustments for all the sources and receivers and the programming realization of finding the residual statistic corrections as a new interactive program corst3D, aimed at increasing the level of studies of structure of geological medium of different complexity by the data of both 2D and 3D seismic exploration. Effectiveness of the given procedure at the real data of seismic exploration for improvement of their quality and as a result for rising information value of their processing and interpretation has been shown together with colleagues of «GEOUNIT» company.

The study of dynamical processes in problems of mesofracture layers exploration seismology by methods of mathematical and physical simulation

The article is devoted to the study of the propagation of elastic waves in a fractured seismic medium by methods of mathematical modeling. The results obtained during it are compared with the results of physical modeling on similar models. For mathematical modeling, the grid-characteristic method with hybrid schemes of 1-3 orders with approximation on structural rectangular grids is used. The ability to specify inhomogeneities (fractures) of various complex shapes and spatial orientations has been implemented. The description of the developed mathematical models of fractures, which can be used for the numerical solution of exploration seismology problems, is given. The developed models are based on the concept of an infinitely thin fracture, the size of the opening of which does not affect the wave processes in the fracture area. In this model, fractures are represented by boundaries and contact boundaries with different conditions on their surfaces. This approach significantly reduces the need for computational resources by eliminating the need to define a mesh inside the fracture. On the other hand, it allows you to specify in detail the shape of fractures in the integration domain, therefore, using the considered approach, one can observe qualitatively new effects, such as the formation of diffracted waves and a multiphase wavefront due to multiple reflections between the surfaces, which are inaccessible for observation when using effective fracture models actively used in computational seismic. The obtained results of mathematical modeling were verified by physical modeling methods, and a good agreement was obtained.

Application of the Generalized Finite Element Method to the acoustic wave simulation in exploration seismology

The Generalized Finite Element Method (GFEM) has been applied frequently to solve harmonic wave equations, but its use in the simulation of transient wave propagation is still limited. We apply GFEM to the simulation of the acoustic wave equation in models relevant to exploration seismology. We also perform an assessment of its accuracy and efficiency. The main advantage of GFEM is that it provides an enhanced solution accuracy in comparison to the Standard Finite Element Method (FEM). This is attained by adding user-defined enrichment functions to standard FEM approximations. For the acoustic wave equation,we consider plane waves oriented in different directions as the enrichments, whose argument include the largest wavenumber of the wavefield. We combine GFEM with an unconditionally stable time integration scheme with constant time step. To assess the accuracy and efficiency of GFEM, we present a comparison of GFEM simulation results against those obtained with the Spectral Element Method (SEM). We use SEM because it is the method of choice for wave propagation simulation due to its proven accuracy and efficiency. In the numerical examples, we perform first a convergence study in space and time,evaluating the accuracy of both methods against a semi-analytical solution. Then, we consider two simulations of relevant models in exploration seismology that include low-velocity features, an inclusion with a complex geometrical boundary and topography. Results using these models show that the GFEM presents a comparable accuracy and efficiency to the ones based on SEM. For the given examples, GFEM efficiency stems from the combined effect of local mesh refinement, non-conforming or unstructured, and the unconditionally stable time integration scheme with constant time step. Moreover, these features providegreat flexibility to the GFEM implementations, proving to be advantageous when using, for example, unstructured grids that impose severe time step size restrictions in SEM.

Microseismic Signal Denoising and Separation Based on Fully Convolutional Encoder–Decoder Network

Denoising methods are a highly desired component of signal processing, and they can separate the signal of interest from noise to improve the subsequent signal analyses. In this paper, an advanced denoising method based on a fully convolutional encoder–decoder neural network is proposed. The method simultaneously learns the sparse features in the time–frequency domain, and the mask-related mapping function for signal separation. The results show that the proposed method has an impressive performance on denoising microseismic signals containing various types and intensities of noise. Furthermore, the method works well even when a similar frequency band is shared between the microseismic signals and the noises. The proposed method, compared to the existing methods, significantly improves the signal–noise ratio thanks to minor changes of the microseismic signal (less distortion in the waveform). Additionally, the proposed methods preserve the shape and amplitude characteristics so that it allows better recovery of the real waveform. This method is exceedingly useful for the automatic processing of the microseismic signal. Further, it has excellent potential to be extended to the study of exploration seismology and earthquakes.

A new acoustic assumption for orthorhombic media

SUMMARY In exploration seismology, the acquisition, processing and inversion of P-wave data is a routine. However, in orthorhombic anisotropic media, the governing equations that describe the P-wave propagation are coupled with two S waves that are considered as redundant noise. The main approach to free the P-wave signal from the S-wave noise is the acoustic assumption on the wave propagation. The conventional acoustic assumption for orthorhombic media zeros out the S-wave velocities along three orthogonal axes, but leaves significant S-wave artefacts in all other directions. The new acoustic assumption that we propose mitigates the S-wave artefacts by zeroing out their velocities along the three orthogonal symmetry planes of orthorhombic media. Similar to the conventional approach, our method reduces the number of required model parameters from nine to six. As numerical experiments on multiple orthorhombic models show, the accuracy of the new acoustic assumption also compares well to the conventional approach. On the other hand, while the conventional acoustic assumption simplifies the governing equations, the new acoustic assumption further complicates them—an issue that emphasizes the necessity of simple approximate equations. Accordingly, we also propose simpler rational approximate phase-velocity and eikonal equations for the new acoustic orthorhombic media. We show a simple ray tracing example and find out that the proposed approximate equations are still highly accurate.

Reviews

Numerical Methods of Exploration Seismology: With Algorithms in MATLAB, by Gary F. Margrave and Michael P. Lamoureux, ISBN 978-1-107-17014-8, 2019, Cambridge University Press, 450 p., US$79.99 (print), US$64 (eBook). Plate Tectonics and Great Earthquakes: 50 Years of Earth-shaking Events, by Lynn R. Sykes, ISBN 978-0-231-18688-9, 2019, Colombia University Press, 272 p., US$35 (print), US$35 (eBook). Geologic Structures of the Arctic Basin, by Alexey Piskarev, Victor Poselov, and Valery Kaminsky, ISBN 978-3-319-77741-2, 2019, Springer, 376 p., US$139.99 (print), US$109 (eBook).