Pand S-wave Research Articles

Understanding the behaviour of seismically derived Poisson's ratio in near-surface characterization

Near-surface characterization is often challenging using a single geophysical technique. We show that seismic refraction is an effective tool by combining P- and S-wave velocity models in the form of V P / V S and Poisson's ratio ( σ ) to detect buried targets along with electrical resistivity imaging (ERI). We acquired vertical and horizontal component seismic data along a 2D profile over two known targets. The first is a water pipe of c . 0.8 m diameter located c . 1.5 m below the surface and the second is a storm drainage pipe (SDP) of c . 1 m diameter at c . 2 m depth. The first arrival times of vertical and horizontal component data were picked and inverted using grid-based tomography. The resulting velocity models have smoothly varying structures and could not identify any of the target features. However, when combined as V P / V S and Poisson's ratio models, the features clearly appeared as anomalous zones. A collocated 2D electrical resistivity model shows the presence of anomalies around the same locations. Our study suggests that anomalous V P / V S and Poisson's ratio values derived from seismic velocities are indicative of the relative changes owing to the disturbances in the soil because of construction with respect to the background instead of the anomaly.

GST-based polarization approach to picking P-and S-waves arrival time for high-speed rail early warning system

The first arrival time of seismic P- and S- waves affects the determination of the beginning of an earthquake by the Earthquake Early Warning System for High-speed Rail (HSREEWs). Accordingly, we propose an approach to picking P- and S-waves arrival time for HSREEWs. This approach can treat the continuous seismic signals in real-time without prior knowledge of whether an earthquake occurs. Firstly, we replace the fixed time window of the S transform (ST) with the dynamic window of the generalized S transform (GST) to make the necessary time of signal processing more flexible. Meanwhile, the GST extends the sequential signal to the time-frequency domain (TF), and combines the TF polarization to construct four three-component polarization filters. Accordingly, the P-wave and S-wave can be separated from the seismic waveform by those filters, which effectively avoids the phenomenon that the S-wave phase is difficult to determine because of the interference of S-wave overlapping in the coda region. Secondly, based on the instantaneous frequency of seismic signal, we construct a new STA/LTA (GSPA-STA/LTA) with the characteristic function (CF) of constrained TF spectrum to pick the first arrivals of the P- and S-waves, and provide the corresponding automatic picking algorithm. Finally, we analyze 504 seismic events larger than Ms40 near the Japanese Tohoku Shinkansen to comprehensively verify the effectiveness of our approach. Since the P-wave and S-wave are first separated, the subsequent process just has to identify the first arrival times of their respective components. The approach has a good practical value in maintaining the safe operation of high-speed rail under earthquake, which meets the industry demands for speed, scale and accuracy in HSREEWs.

ANALYSIS OF FREQUENCY CONTENT AND STATISTICAL RELATIONSHIP AMONG EARTHQUAKE PARAMETERS OF SEISMIC DATA

The frequency content of the strong-motion seismic data collected from seismic stations by the Bangladesh Meteorological Department (BMD) using GL-S60/120 broadband seismometers of earthquakes in and around Bangladesh is examined. In addition, the statistical relationships among various earthquake parameters, such as the peak ground velocity (PGV), magnitude, epicentral distance, and time of arrival of p-and s-waves of the collected data, are analyzed. Approximate seismic wave velocities are also obtained. The Fourier and wavelet analysis results reveal delicate differences in the frequency content of the p-and s-waves occurring during earthquakes. These results are compared to those generated from the STEAD dataset, which contains seismic waveforms collected in the U.S., France, and Greece regions. Regression analysis on the BMD data derives relationships among the aforementioned parameters. Collected data from some stations show perfect linear relationships among PGV and the travel time of seismic waves with magnitude and epicentral distance, whereas, in other stations, such relationships are not significant. Plausible explanations for these disparities are provided along with similar regression analysis using the STEAD data. The findings are useful for earthquake early-warning systems, geology research, seismic risk zone delineation, and seismic hazard forecasts.

SOME CHARACTERISTICS OF THE EARTHQUAKE CATALOG AND THE SEISMIC PROCESS ACCORDING TO THE KNET NETWORK

Some characteristics of the earthquake catalog and seismic process are considered. The catalog is obtained according to the KNET network (KN-code in FSDN, International Federation of Digital Seismograph Networks, that is operated by Research Station of RAS, RS RAS) and contains more than 10000 earthquake parameters from 1994 to 2020. The catalog and seismicity characteristics were determined the whole catalog as well as the catalog that is limited by the coordinates of the Bishkek geodynamic polygon (BGP). Statistical analysis of arrival times of direct P- and S-waves registered at KNET network stations was carried out. The maximum P- and S-waves were registered at AAK station, the minimum – at ULHL station. The territory of the BGP is covered by great P-traces density. We analyzed earthquake localization errors, i.e. the difference between the observed and calculated arrival times of P-and S-wave fronts (RMS), horizontal error (ERH, epicenter) and vertical error (ERZ, depth). Occurred on the BGP territory earthquakes have minimum values of the considered errors. The representative sample is defined: for the whole catalog it includes K≥7.2 earthquakes and for the limited by BGP coordinates catalog – K≥6.7. Statistical characteristics of the representative part of the catalog in terms of time and depth are determined. The temporal distribution of earthquakes by energy classes is constructed and the absence of positive or negative trends in the number of events is noted. Spatial distribution of earthquakes by depth – 0–5, 5–10, 10–15 and more than 15 km is constructed. The smallest errors of earthquake depth determination have the events that occurred on the BGP territory. During the study 46 moderate earthquakes with K≥12 occurred. The most part of these events happened in the North Tien Shan seismgenic zone. 22 events were determined with K≥10 followed by aftershock sequences and give some aftershock characteristics. The most events with aftershocks occurred in the eastern part of the Kyrgyz ridge. The distribution of earthquake numbers and STD intensity is constructed. Zones of seismic activity and maximum intense Earth’s crust deformation are identified. The Gutenberg – Richter law and provisions of nonextensive statistical physics were used to describe the energy distribution function of earthquakes.

Spectral characteristics of P-and S-wave quarry blasts in Aswan region, South Egypt

The Aswan broadband seismic network, consisting of high-sensitivity sensors with dense stations coverage, provided the opportunity to study and analysed the spectral properties of the quarry blasts in the Aswan region, southern Egypt. We comprehensively analysed 757 digital seismograms from 128 quarry blasts with local magnitude (ML) between 1.7 and 3.1 during time span from 2010 to 2021. A Generalized Inversion Technique (GIT) constrained by a reference site is applied to isolate the attenuation, site responses of the seismic stations, and the source spectra from the observed seismograms by means of iterative least square analysis. The displacement source spectra for P-and S-waves are computed for all quarry blasts based on the theoretical omega-square seismic source model (Brune's model). The spectral analysis demonstrated that both P-and S-waves displacement source spectra of the quarry blasts used in this study have low corner frequency values. The corner frequency of the quarry blasts used in this study ranged from 1.0 to 2.6 Hz and from 1.5 to 4.6 Hz for P-and S-waves, respectively. The P-and S-wave corner frequency ratio (fCP/fCS) is computed for each event. We obtained that the fCP/fCS ratio is less than 1.0 Hz for 106 events (94%) and equal to 1.0 Hz and less than 1.2 Hz for only six selected data events (6%). The analysis indicates that the observed displacement source spectra show discrepancies with the omega-square type spectrum, and they generally have a steep fall off at high frequencies. We also found that the selected quarry blasts have a narrow seismic moment range (from 2.03E+11 to 1.35E+12 Nm), and stress drop values range from 0.001 to 0.023 MPa. The distribution of the used quarry blast stress drop demonstrated a self-similar character.

Effect of thermal shock on the permeability and seismic wave velocity of the caprock and reservoir during CO2 injection

The injection of relatively cold CO2 into warmer reservoirs causes rapid cooling of the host reservoir and the overlying caprock in the near wellbore region. This rapid cooling may cause thermal fracturing, which may potentially lead to escape of the CO2. We have here investigated the effect of thermal fracturing, due to rapid quenching, on the permeability and P-and S-wave velocities of caprock and reservoir samples from the In Salah CO2 storage site, Krechba field, Algeria. Injection of CO2 may have cooled the caprock and reservoir by up to 65°C. Laboratory experiments were conducted where caprock and reservoir samples were heated to various temperatures (50, 150, 250, 350 and 500°C) and rapidly quenched to room temperature (20°C). Permeability and P- and S-wave velocities were measured pre- and post-thermal treatment at effective pressures up to 60 MPa. Backscattered scanning electron microscopy analysis revealed that most of the fracturing is mainly within siderite (Fe carbonate). X-ray diffraction analysis showed that the minerals chlorite (Fe-clay) and siderite were destroyed in specimens heated to 500°C. Specimens that were rapidly quenched by 230°C experienced less than half an order of magnitude increase in permeability and very small to negligible changes in the P- and S-wave velocities. We conclude that the effect of thermal shock fracturing in the Krechba field due to CO2 injection is negligible because the change in the permeability and velocities are insignificant for the predicted maximum degree of cooling (approximately 65°C) of the formation due to CO2 injection. The conclusion of the lack of thermal quenching effects due to CO2 injection at In Salah may be applicable to all global Carbon Capture and Storage prospects.

Unexpected Shallow Earthquake of August 1st, 2020 in the North of Indramayu, West Java, Indonesia

During the last one hundred years, there are no shallow seismicity in the north of Java. This area is dominated by intermediate and deep focus earthquakes due to the subducted Indo-Australian slab. An earthquake with magnitude ML 4.5 struck Indramayu, north of West Java on August 1, 2020. According to the Agency for Meteorology, Climatology, and Geophysics (BMKG), the earthquake was felt III MMI scale in Indramayu and its vicinity. We used waveform data from BMKG seismic station in West Java, then we picked P-and S-waves arrival times from each station and hypocenter location was determined by Geiger method. We have detected Pn before Pg phase on four BMKG seismic stations, indicating a shallow crustal earthquake. Our inversion show that the earthquake occurred in 6.1805° S, 108.2612° E with 5 km focus depth at 16:24:38 GMT+7. Our focal mechanism solution was determined by using moment tensor inversion shows a strike-slip faulting, which corresponds to the active fault in the north of Indramayu.

TO STUDY THE POSSIBILITIES OF USING THE SEISMOLOGICAL NETWORK OF THE ALTAI-SAYAN REGION FOR REGIME VIBRO-SEISMIC OBSERVATIONS

The results of deep seismic studies based on P - and S-wave data on the East-Stanov fragment of the reference 700-kilometer geophysical profile 8-DV are presented. Deep seismic sections of the upper crust (up to a depth of 20 km) with the distribution of the velocities of longitudinal and transverse waves are constructed. The P - wave velocities in the upper part of the section vary from 4-5 km / s within the Upper Zeya and Amur-Zeya depressions to 5.5-6.0 km/s within mountain ranges and plateaus; at depths of 10-20 km, lenses of high-velocity rocks up to 6.7-7.0 km/s are distinguished in the profile alignment. According to the S - waves in the upper part of the section, the velocity values are generally 3.0-3.2 km/s; reduced velocity values of 2.5-2.6 km / s are observed in the Upper Zey depression. At depths of 5-20 km within the section, according to the transverse wave data, a number of sections with reduced and increased velocity values are distinguished, respectively, up to 3.4-3.5 km/s and 3.75-3.8 km/s. The correlation of the selected anomalies according to the data of P-and S-waves is carried out.

Prestack waveform inversion based on analytical solution of the viscoelastic wave equation

Amplitude-variation-with-offset (AVO) inversion is based on single interface reflectivity equations. It involves some restrictions, such as the small-angle approximation, including only primary reflections, and ignoring attenuation. To address these shortcomings, the analytical solution of the 1D viscoelastic wave equation is used as the forward modeling engine for prestack inversion. This method can conveniently handle the attenuation and generate the full wavefield response of a layered medium. To avoid numerical difficulties in the analytical solution, the compound matrix method is applied to rapidly obtain the analytical solution by loop vectorization. Unlike full-waveform inversion, the proposed prestack waveform inversion (PWI) can be performed in a target-oriented way and can be applied in reservoir study. Assuming that a Q value is known, PWI is applied to synthetic data to estimate elastic parameters including compressional wave (P-wave) and shear wave (S-wave) velocities and density. After validating our method on synthetic data, this method is applied to a reservoir characterization case study. The results indicate that the reflectivity calculated by our approach is more realistic than that computed by using single interface reflectivity equations. Attenuation is an integral effect on seismic reflection; therefore, the sensitivity of seismic reflection to P-and S-wave velocities and density is significantly greater than that to Q, and the seismic records are sensitive to the low-frequency trend of Q. Thus, we can invert for the three elastic parameters by applying the fixed low-frequency trend of Q. In terms of resolution and accuracy of synthetic and real inversion results, our approach performs superiorly compared to AVO inversion.

The Features of the Deep Velocity Structure of the Trans-Baikal Section of the Central Asian Fold Belt from the Seismological and DSS Data (Alignment of the 1-SB Reference Profile)

The materials of deep seismic studies carried out on the 1-SB reference geophysical profile and the areal interpretation of the seismological data obtained in the Trans-Baikal part of the Central Asian Fold Belt are presented. It is shown that longitudinal and transverse waves were recorded from the reference boundaries within the crust and the Moho surface in the Trans-Baikal region using explosions and powerful vibration sources on the 1200-km long 1-SB geophysical profile. The inhomogeneous upper crust velocity structure down to 15–20-km depths and the Moho surface was revealed from P- and S-waves. The anomalous mantle, which exhibited decreased boundary velocity values of P- and S-waves at 7.8–7.95 km/s and 4.3–4.5 km/s, respectively, was reliably traced within the Baikal Rift Zone from the DSS and seismological data. In the southeastern part of the area (the central Mongol-Okhotsk Orogenic Belt and adjacent folded structures of the Trans-Baikal region) a layer of increased P-and S-wave velocity values of 8.4 ± 0.15 km/s and 4.9 ± 0.05 km/s, accordingly, was detected at the mantle surface. A comparison was made with the materials of other studies.

A staggered-grid lowrank finite-difference method for elastic wave extrapolation

Elastic wave extrapolation in the time domain is significant for an elastic wave equation-based processing. To improve the simulation reliability and accuracy of decoupled elastic P- and S- waves, we propose the staggered-grid lowrank finite-difference method based on the elastic wave decomposition. For elastic wave propagation, a lowrank finite-difference method based on the staggered grid is derived to improve the accuracy. Regarding the application of the decoupled elastic wave equation, we derive the finite-difference scheme coefficients which are dependent on velocity. Based on the elastic wave decomposition and plane wave theories, we formulate the elastic wave-extrapolation operators, which contain trigonometric adjustment factors. Accordingly, by applying the lowrank method to approximating the operators, the finite- difference scheme is designed to discretize the decoupled wave equation. The derivation processing implies the combination of elastic wave-mode decomposition and extrapolation. The proposed method enables elastic P- and S-waves to extrapolate in the time-space domain separately and produces accurate P-and S-wave components simultaneously. Dispersion analysis suggests that our proposed method is reliable and accurate in a wide range of wavenumber. Numerical simulation tests on a simple model and the Marmousi2 model validate the accuracy and effectiveness of the method, showing its ability in handling complex structures. Although the operators are accurate only when the medium is homogeneous, they are of high accuracy when the velocity gradient is quite small and are applicable when the velocity gradient is large. The subsequent results of reverse time migration for the Marmousi2 model also suggest that the proposed method is enough to serve as an extrapolator in elastic reverse time migration.

Improvement of petrophysical workflow for shear wave velocity prediction based on machine learning methods for complex carbonate reservoirs

Shear wave velocity (S-wave velocity or Vs) is one of the most critical issues for carbonate reservoirs characterization, because of its complexity of rock compositions and pore structures. The Xu-Payne petrophysical model is a commonly used method to predict S-wave velocity. However, the model excessively depends on the interpretation accuracy of rock compositions, pore structures, fluid properties and fluid saturation, which makes the prediction result potentially uncertain. With the development of intelligent algorithms, a machine learning method (Long-short Term Memory Neural Network, LSTM) is proposed to improve the traditional petrophysical workflow. With the capabilities of deep learning and data mining, the LSTM model can deeply mine the rich information in the wireline logs and then establish the relationships between S-wave velocity and wireline logs. In order to illustrate the prediction effect, the carbonate reservoir in the Majiagou Formation Member 5(Ma 5 Member) which belongs to the Ordovician system in the D area of Sulige gas field is taken as an example. Six kinds of sensitive wireline logs, including compensated acoustic log (AC), natural gamma ray log(GR), photoelectric absorption cross-section log (PE), density log (DEN), deep lateral resistivity log(RLLD), neutron log(CNL), are selected as the input data of the LSTM model. The results reveal that the accuracy of LSTM method is up to 98.9%, whereas the Xu-Payne model is only 73%. With the predicted result of all wells in the areas using LSTM, the sensitive elastic properties, including P-and S-wave velocity, P- and S-wave impendences, ratio of P- and S-wave velocity, Poisson's ratio, lame coefficient and fluid sensitivity factor, are also concluded. Compared with the Xu-Payne model, the newly proposed workflow is proved to be convenient and suitable for the prediction of S-wave velocity in carbonate reservoirs.

ОСОБЕННОСТИ ГЛУБИННОГО СКОРОСТНОГО СТРОЕНИЯ ЗАБАЙКАЛЬСКОГО УЧАСТКА ЦЕНТРАЛЬНО-АЗИАТСКОГО СКЛАДЧАТОГО ПОЯСА ПО ДАННЫМ СЕЙСМОЛОГИИ И ГСЗ (В СТВОРЕ ОПОРНОГО ПРОФИЛЯ 1-СБ)

The materials of deep seismic studies on the reference geophysical profile 1-SB and areal interpretation of seismological data in the Trans-Baikal part of the Central Asian folded belt are presented. It is shown that longitudinal and transverse waves from the reference boundaries in the Earth's crust and Moho surface were registered using explosions and powerful vibration sources on the 1200-km geophysical profile 1-SB in Trans-Baikal region. The inhomogeneous velocity structure (according to P - and S - waves) of the upper part of the Earth's crust to the depths of 15–20 km and Moho surface was established. The anomalous mantle with reduced values of the boundary velocity of P - and S-waves (7.8–7.95 km/s and 4.3–4.5 km/s, respectively) was reliably traced within the Baikal rift zone using GSS and seismology data. In the southeastern part of the area (in the central Mongol-Okhotsk orogenic belt and adjacent folded structures of the Trans-Baikal region), a layer of increased P-and S-wave velocity values on was detected the mantle surface at 8.4 ± 0.15 km/s and 4.9 ± 0.05 km/s, respectively. A comparison was made with the materials of other studies.

Imperialist competitive algorithm optimization method for nonlinear amplitude variation with angle inversion

Amplitude variation with angle (AVA) inversion is one of the most effective techniques in hydrocarbon exploration and estimating subsurface petrophysical properties. The inversion problem as a nonlinear, multiparameter, and multimodal optimization problem is conventionally solved through linearized optimization methods, but with the cost of smoothing important geologic interfaces. In addition, the results obtained by these methods are more possible to be trapped in a local minimum, while global-optimization methods can produce more accurate results and preserve the interfaces of geologic structures. A Bayesian framework is used to formulate the AVA inversion problem, which incorporates a novel prior constraint included by two regularization functions, one for sparsity of the coefficients as well as recovering discontinuities and another one for enhancing the lateral continuity. The imperialist competitive algorithm as an efficient evolutionary algorithm is then used to optimize the resulted objective function, to invert the P-and S-wave velocities as well as the density. We compare our algorithm with a commonly used Bayesian linearized inversion method by applying both methods on synthetic data and real seismic data from Gulf of Mexico. Our results reveal the practicability and stability of the presented method for the AVA inversion problem.

Nonlinear elastic impedance inversion in the complex frequency domain based on an exact reflection coefficient

The low-frequency information is extremely crucial to the stability, reliability, and enhanced resolution of the elastic impedance (EI) inversion. Bandwidth of seismic data has been extended during the development of seismic acquisition technology (especially for the acquisition of low-frequency components). In new exploration areas without sufficient prior geological information and drilling, low-frequency initial models are difficult to be established. In order to overcome the model dependence problem in conventional EI inversion, the Bayesian framework, in the complex frequency domain, is utilized to estimate the low-frequency information. The inverted parameters (EI) and likelihood function are considered to obey the Cauchy and Gaussian probability distribution, respectively. Linear EI equations has two disadvantages: low accuracy of reflection coefficient and poor applicability when the elastic parameters of the upper and lower media vary dramatically (high contrast situations), so we derived a novel nonlinear EI equation in terms of compressional wave (P-wave) impedances, shear wave (S-wave) impedances, and density. Its accuracy is basically consistent with the exact Zoeppritz equation. The P-and S-wave impedances and density can be extracted from the direct inversion algorithm Artificial Neural Network Inversion Tool (ANNIT) and EI data, and the sensitivity analysis of the novel EI equation indicates that the inversion results of impedances will be better than density. The feasibility of this method is verified by the model tests and field data examples.

Change of Elastic Wave Velocity in Granite after Radiation Exposure and Prospects for Energy Consumption Reduction in Ore Pretreatment

The regular patterns are revealed in the influence exerted by the absorbed dose on the elastic wave velocity in granite after radiation exposure. It is shown that the major change in velocity of P-and S-waves in the treated cores of granite is observed when the absorbed dose is 10 kGy. The energy consumption in breaking and crushing can be estimated in the tests of uniaxial compression up to failure. Radiation exposure of granite cores to the dose of 10 kGy reduces energy consumption from 7.68 to 3.06 J in uniaxial compression up to failure and from 700.4 to 470.88 J in crushing. The obtained result is important for improvement of ore pretreatment processes.

Numerical Simulation of M9 Megathrust Earthquakes in the Cascadia Subduction Zone

We estimate ground motions in the Pacific Northwest urban areas during M9 subduction scenario earthquakes on the Cascadia megathrust by simulating wave propagation from an ensemble of kinematic source descriptions. Velocities and densities in our computational mesh are defined by integrating the regional Cascadia Community Velocity Model (CVM) v1.6 (Stephenson et al. P-and S-wave velocity models incorporating the Cascadia subduction zone for 3D earthquake ground motion simulations—update for open-file report 2007–1348, US Geological Survey, 2017) including the ocean water layer with a local velocity model of the Georgia basin (Molnar, Predicting earthquake ground shaking due to 1D soil layering and 3D basin structure in SW British Columbia, Canada, 2011), including additional near-surface velocity information. We generate six source realizations, each consisting of a background slip distribution with correlation lengths, rise times and rupture velocities consistent with data from previous megathrust earthquakes (e.g., 2011 M 9 Tohoku or 2010 M 8.8 Maule). We then superimpose M ~ 8 subevents, characterized by short rise times and high stress drops on the background slip model to mimic high-frequency strong ground motion generation areas in the deeper portion of the rupture (Frankel, Bull Seismol Soc Am 107(1):372–386, 2017). The wave propagation is simulated using the discontinuous mesh (DM) version of the AWP finite difference code. We simulate frequencies up to 1.25 Hz, using a spatial discretization of 100 m in the fine grid, resulting in surface grid dimensions of 6540 × 10,728 mesh points. At depths below 8 km, the grid step increases to 300 m. We obtain stable and accurate results for the DM method throughout the simulation time of 7.5 min as verified against a solution obtained with a uniform 100 m grid spacing. Peak ground velocities (PGVs) range between 0.57 and 1.0 m/s in downtown Seattle and between 0.25 and 0.54 m/s in downtown Vancouver, while spectral accelerations at 2 s range between 1.7 and 3.6 m/s2 and 1.0 and 1.3 m/s2, respectively. These long-period ground motions are not significantly reduced if plastic Drucker-Prager yielding in shallow cohesionless sediments is taken into account. Effects of rupture directivity are significant at periods of ~ 10 s, but almost absent at shorter periods. We find that increasing the depth extent of the subducting slab from the truncation at 60 km in the Cascadia CVM version 1.6 to ~ 100 km increases the PGVs by 15% in Seattle and by 40% in Vancouver.

Significance and behaviour of the homogeneous and inhomogeneous components of linearized viscoelastic reflection coefficients

In a recent paper, seismic amplitude-variation-with-offset (AVO) equations describing P-to-P and P-to-S reflections from boundaries separating low-loss viscoelastic media, taking into account variations in the attenuation angle, have been derived. We find that opportunities now present themselves to use these equations to expose a range of relationships between measured amplitudes and subsurface elastic and anelastic properties. This has significant applicability in the quantitative interpretation of seismic data in, for instance, reservoir characterization. To facilitate the analysis, we decompose the equations into three parts: elastic, homogeneous and inhomogeneous. We show that, for PP modes, the elastic part is sensitive to changes across a reflecting boundary in density and P- and S-wave velocities; the homogeneous part is sensitive to changes in density, S-wave velocity and the P- and S-wave quality factors; and the inhomogeneous part is sensitive to changes in density, and P-and S-wave velocities. The latter term is seen to vanish when the attenuation angle vanishes. For PP modes, elastic and homogeneous terms are linear with respect to , where is the P-wave incidence angle; however, the inhomogeneous term is similarly linear only if normalized by dividing by tan θ P. For PS modes, the elastic part is sensitive to changes in density and S-wave velocity; the homogeneous part is sensitive to changes in density, S-wave velocity and the S-wave quality factor; and the inhomogeneous part is sensitive to changes in density and S-wave velocity. This term also vanishes for the zero attenuation angle, i.e. in the homogenous limit. For PS modes, the inhomogeneous terms are linear with respect to ; however the elastic and homogeneous terms are first and third order in . A further and key result of this expansion of the wave types allowable in AVO analysis is that, for inhomogeneous PS scattering, the viscoelastic AVO equations predict a non-zero reflectivity at normal incidence. This is a significant deviation from common models of converted wave amplitude analysis.

Geomechanical Rock Properties Using Pressure and Temperature Dependence of Elastic P- and S-Wave Velocities

It is important and meaningful for understanding the geomechanical properties of rock and providing guidance on analyzing and simulating the formation processes of engineering, geophysical, geothermal, civil and underground engineering projects. Variation in geomechanical properties of metamorphic rock, such as, seismic velocities (P-and S-wave) density deformation, is observed with the increase in pressure and temperature, Poisson’s ratio, elastic modulus, bulk modulus and shear modulus, are analyzed herein. A simple laboratory method, which measure seismic velocities has increasingly been conducted to determine the geomechanical properties of rock materials. Three metamorphic rock samples were selected for laboratory testing. Laboratory testing were done on cube samples of dry rocks in a true triaxial apparatus. First, P and S wave velocities were measured at 12–100 MPa pressure and at 20–600 °C temperature. Measurement showed that P and S wave velocities increase with increasing pressure and decrease with increasing temperature. Second, the wave velocities are used to calculate the geomechanical properties at lower to higher pressure and temperature. Furthermore, it was found that pressure and temperature have a significant effect on the change of geomechanical properties of rocks. These results contribute to a reliable estimate of geomechanical properties of rocks.

Estimation of Stress/Strain State of Tectonic Structures Using V P /V S Ratios: A Case Study of Seismically Active Zones of the Greater Caucasus, Kura Depression, Transcaucasia, and the Western Caspian Region

Earthquakes, which result from tectonic movements within the bowels of the Earth, reflect spatiotemporal changes in the stress field of the geologic medium. Weak regional earthquakes indicate the activation of some elements of seismotectonic zones and preparation of strong earthquakes. The travel times of Pand S-waves and V P /V S ratios are also a source of information on the regional tectonic setting; sometimes this information helps to reveal blind faults. The paper analyzes local earthquakes in a seismically active region, which comprises the Greater Caucasus, the Kura Depression, Transcaucasia, and the western Caspian Region. Based on a large database of hypocenter parameters, P- and S-wave travel times, and areal V P /V S ratio distributions, we identified seismotectonic zones with characteristic anomalous features and outlined gradient zones where V P /V S anomalies reverse sign. It is found that the V P /V S ratio field complies with the regional stress field and deep seismotectonic setting. Zones where anomalies reverse sign (gradient zones) are most likely confined to blind faults. Areas of the high gradient of the field can be put down to blind tectonic structures (faults) or the incipient sources of future strong earthquakes. V P /V S ratio monitoring and mapping will make it possible to identify the source zones of forthcoming earthquakes.