VOLUMETRIC AND SPATIAL SEGMENTATION OF THE TIEN SHAN LITHOSPHERE ACCORDING TO GEOPHYSICAL DATA

This article consolidates the results of studying the deep structure of the lithosphere of the Central Tien Shan, which aimed to identify the main tectonic elements in its geophysical models. We have compared the structural and geological data with the information on the deep structure obtained by geophysical methods and from the positions of earthquake hypocenters in the study area. According to geological concepts, the Tien Shan orogenic belt is characterized by longitudinal and transverse segmentation. The boundaries of the Northern, Middle, Southern Western and Eastern segments of the Tien Shan are deep-seated fault structures. In deep faults and channels of heat and mass transfer, endogenous processes are localized. High-velocity, geoelectrical and thermal models consider such faults and channels as contrasting objects that can be referred to as indicators of these processes.Our analysis of the locations of earthquake hypocenters from NNC, KNET, CAIIG, KRNET, SOME catalogues shows that seismic events are strongly confined to the fault zones and the boundaries of large blocks. A correlation between the anomalies of geophysical fields suggests the degree of inheritance of tectonic structures and the boundaries of the main tectonic segments of the Tien Shan. To compare the crustal and upper mantle heterogeneities reflected in different geophysical fields, we have analyzed seismic tomographic sections based on volumetric seismotomographic models geoelectric and velocity sections along profiles across the main tectonic elements of the study area. The sections are used to identify the zones with relatively low (i.e. reduced) seismic wave velocities and detect the deep-seated longitudinal segmentation of the folded belt. Objects showing anomalous seismic wave velocities are found in the seismotomographic sections at all the depths under consideration. The most contrasting differences in the velocities of P- and S-waves are typical of the depths of 0-5 km and 50-65 km, showing the most clearly observed Northern, Southern and Western segments of the Tien Shan. In general, the velocities of P- and S-waves at the Northern Tien Shan are higher than those at the Middle and Southern segments. We have analyzed the distribution of geoelectric heterogeneities identified from magnetotelluric sounding data in order to determine the boundaries of the main tectonic elements that are considered as the zones of increased electrical conductivity confined to the boundaries of the fault structures. The distribution of earthquake epicenters clearly reflects the segmentation of the Tien Shan into the Northern, Middle and Southern segments and shows the Western and Eastern Tien Shan relative to the Talas-Fergana fault. Ourstudies of the crust and the upper mantle of the Tien Shan have confirmed that the abovementioned tectonic segments have differences in their deep structures Based on a comprehensive analysis of the study results, we can qualitatively identify a relationship between the distribution of the velocity and geoelectric heterogeneities in the crust and upper mantle, seismicity and the stress-strain state of the crust.

A method of phase identification for seismic data acquired with the controlled accurate seismic source (CASS)

SUMMARY With the advantages of the little destruction to the deployment site and high repeatability compared with explosive sources, the controlled accurate seismic source (CASS) has many potential applications with respect to the investigation of the crustal structure and seismic wave velocities. However, the signal generated by the CASS rapidly attenuates with the increasing distance because of its poor signal-to-noise ratio (SNR). Consequently, the difficulties in identifying specific seismic phases from the CASS data limit its application and popularization. The aim of this study is to present a new method to improve the accuracy of traveltime estimation and to identify more seismic phases travelling through the crust. We adopt the global seismic phase scanning algorithms (GSPSA) combined with an optimized narrowband time-varying filter, whose central frequency corresponds to the instantaneous frequency of the linear frequency modulation (LFM) signals produced by the CASS. Using the seismic data from the 40-ton CASS in a field experiment around Xinfengjiang reservoir in southeast China, we attain the seismic phases such as Pg, Sg, PmP and SmS at epicentral distances of more than 200 km with GSPSA. To identify and verify these seismic phases information, we also calculate synthetic waveforms. The results demonstrate that the GSPSA method is an effective tool for seismic phase identification of CASS data.

Elastic properties of rocks: Why shouldn’t they be constant?

<pre><span lang="EN-US">         Over past decades, different approaches have been suggested for assessing the elastic constants of materials. In mechanics, the elastic properties are evaluated according to Hooke’s law from the static stress-strain curves, in the strain range before the material failure. In rock mechanics, this approach is used as well for characterizing elastic constants of rocks. Moreover, thanks to development of seismology and applied geophysics, seismic wave velocities were found to allow evaluating rock elastic properties. This approach has been largely developed by the rock physics/petrophysics community as a simple and non-destructive  mean of characterization of rock elastic constants.</span></pre> <pre><span lang="EN-US">         Ideally, being expected to probe the same material constants, the two approaches should yield the same results. However, in practice, the results seldom compare for a number of potential reasons, such as strains rate and amplitude. </span></pre> <pre><span lang="EN-US">This work aims to investigate, discuss and – possibly – reconcile these two approaches. Different igneous and sedimentary rocks are tested in the laboratory to investigate the influence of different potential factors. Three measuring methods are used: i) static stress-strains, ii) ultrasonic waves velocities, iii) stress-strains oscillations of varying amplitude. The experimental results are then discussed on the basis of existing theories.  </span></pre>

Near-real-time near-surface 3D seismic velocity and uncertainty models by wavefield gradiometry and neural network inversion of ambient seismic noise

With the advent of large and dense seismic arrays, novel, cheap, and fast imaging and inversion methods are needed to exploit the information captured by stations in close proximity to each other and produce results in near real time. We have developed a sequence of fast seismic acquisition for dispersion curve extraction and inversion for 3D seismic models, based on wavefield gradiometry, wave equation inversion, and machine-learning technology. The seismic array method that we use is Helmholtz wave equation inversion using measured wavefield gradients, and the dispersion curve inversions are based on a mixture of density neural networks (NNs). For our approach, we assume that a single surface wave mode dominates the data. We derive a nonlinear relationship among the unknown true seismic wave velocities, the measured seismic wave velocities, the interstation spacing, and the noise level in the signal. First with synthetic and then with the field data, we find that this relationship can be solved for unknown true seismic wave velocities using fixed point iterations. To estimate the noise level in the data, we need to assume that the effect of noise varies weakly with the frequency and we need to be able to calibrate the retrieved average dispersion curves with an alternate method (e.g., frequency wavenumber analysis). The method is otherwise self-contained and produces phase velocity estimates with tens of minutes of noise recordings. We use NNs, specifically a mixture density network, to approximate the nonlinear mapping between dispersion curves and their underlying 1D velocity profiles. The networks turn the retrieved dispersion model into a 3D seismic velocity model in a matter of seconds. This opens the prospect of near-real-time near-surface seismic velocity estimation using dense (and potentially rolling) arrays and only ambient seismic energy.

ANOMALOUS ERRORS IN DETERMINING FOCUS COORDINATES EARTHQUAKES AND SUGGESTIONS FOR THEIR ELIMINATION

Objectives The aim of the study is to develop a method for estimating the speed of seismic waves in different directions of propagation and by taking into account the dimensions of the focus, reducing the error in determining the coordinates of the hypocenter. Method To find the hypocenter of the earthquake, the data of the seismic wave velocities, the differences in the times of arrival of seismic waves on seismic sensors and the error in determining the time difference are used. The data with an error determine the coordinates of the hypocenter using information from various combinations of seismic sensors. Processing the resulting array of coordinates, estimates the seismic wave velocities / or determines the spatial shape of the earthquake source and the coordinates of the hypocenter. According to the coordinates of the cinema center, the differences in the travel time of seismic waves are corrected and the distances to the seismic sensors are refined. Results After preliminary determination of the coordinates and shape of the earthquake source, if there are a large number of seismic sensors, it is possible to clarify the coordinates of the earthquake hypocenter taking into account the recommendations given in the works. Conclusion Using the proposed method implies the presence of a large number of sensors to determine the complex shape, the earthquake source.

The small strain stiffness from bender elements tests for clayey soils

The shear modulus of soils at small strain (G0) is one of the input parameters in a finite element analysis with the hardening soil model with small strain stiffness, required in the advanced numerical analyses of geotechnical engineering problems. The small strain stiffness can be determined based on the seismic wave velocities measured in the laboratory and field tests, but the interpretation of test results is still under discussion because of many different factors affecting the measurements of the wave travel time. The recommendations and proposed solutions found in the literature are helpful as a guide, but ought to be adopted with a certain measure of care and caution on a case-by-case basis. The equipment, procedures, tests results and interpretation analyses of bender elements (BE) tests performed on natural overconsolidated cohesive soils are presented.

On the application of the Eshelby-Cheng effective model in a porous cracked medium with background anisotropy: An experimental approach

Understanding the effect of cracks in elastic media is important for hydrocarbon recovery, especially in nonconventional reservoirs. Consequently, due to the presence of oriented cracks on these types of reservoirs, an anisotropic behavior can be induced. In terms of seismic or ultrasonic velocities, this means that elastic waves propagating on regions with oriented cracks have their velocities varying with the propagation and polarization directions. Thus, the analysis of the seismic wave velocities in a cracked medium can be used as a tool for reservoir characterization. For this reason, there is a variety of mathematical models to describe transversely isotropic cracked medium as well as the design of several experiments to test these models. We have experimentally analyzed the theoretical predictions of Eshelby-Cheng’s first-order model. For this proposal, we measured P- and S-wave ultrasonic velocities in 17 anisotropic samples. All samples indicate weak background anisotropy due to layering deposition; i.e., they are vertical transversely isotropic (VTI) media. Sixteen synthetic anisotropic samples with different crack densities and aspect ratios were simulated by penny-shaped void inclusions in a homogeneous porous matrix made with cement and sand. An uncracked sample, with weak VTI anisotropy, was constructed for reference. The crack densities and aspect ratios ranged from 0 to 0.102 and from 0 to 0.52, respectively. All measurements were performed in a dry condition. From the experimental and theoretical velocities, we calculated the Thomsen’s parameters and correlated them with the crack density. An efficient flowchart was developed to make feasible and clear the inversion of the output Eshelby-Cheng’s effective elastic coefficients in effective velocities. Our results suggest that the anisotropy increases with crack density. In general, we noted that the best fit between the Eshelby-Cheng’s model and the experimental results occurs when the crack density and aspect ratio were lower than 0.1 and 0.32, respectively, and it is largely dependent on the type of crack porosity’s equation used in the inversion of effective stiffness coefficients in the elastic effective velocities.