One-way wave equation seismic prestack forward modeling with irregular surfaces

Mathematical geophone （MG） and equal-time stacking （ETS） principles are used to implement seismic prestack forward modeling with irregular surfaces using the oneway acoustic wave-equation. This method receives seismic primary reflections from the subsurface using a set of virtual MGs. The receivers can be located anywhere on an irregular observing surface. Moreover, the ETS method utilizes the one-way acoustic wave equation to easily and quickly image and extrapolate seismic reflection data. The method is illustrated using high single-noise ratio common shot gathers computed by numerical forward modeling of two simple models, one with a flat surface and one with an irregular surface, and a complex normal fault model. A prestack depth migration method for irregular surface topography was used to reoroduce the normal fault model with high accuracy.     

基于三角网格的有限差分法叠后逆时偏移

Compared with other migration methods, reverse-time migration is based on a precise wave equation, not an approximation, and performs extrapolation in the depth domain rather than the time domain. It is highly accurate and not affected by strong subsurface structure complexity and horizontal velocity variations. The difference method based on triangular grids maintains the simplicity of the difference method and the precision of the finite element method. It can be used directly for forward modeling on models with complex top surfaces and migration without statics preprocessing. We apply a finite difference method based on triangular grids for post-stack reverse-time migration for the first time. Tests on model data verify that the combination of the two methods can achieve near-perfect results in application.     

Prestack reverse-time depth migration of arbitrarily wide-angle wave equations

Based on arbitrarily wide-angle wave equations,a reverse-time propagation scheme is developed by substituting the partial derivatives of depth and time with central differences. The partial derivative of horizontal direction is replaced with high order difference. The imaging condition is computed by solving the eikonal equations. On the basis of above techniques,a prestack reverse-time depth migration algorithm is developed. The processing exam-ples of synthetic data show that the method can remove unwanted internal reflections and decrease the migration noise. The method also has the advantage of fidelity and is applicable of dip angle reflector imaging.     

Estimation of Thomsen＇s anisotropic parameters from walkaway VSP and applications

Estimation of Thomsen＇s anisotropic parameters is very important for accuratetime-to-depth conversion and depth migration data processing. Compared with othermethods, it is much easier and more reliable to estimate anisotropic parameters that arerequired for surface seismic depth imaging from vertical seismic profile （VSP） data, becausethe first arrivals of VSP data can be picked with much higher accuracy. In this study, wedeveloped a method for estimating Thomsen＇s P-wave anisotropic parameters in VTImedia using the first arrivals from walkaway VSP data. Model first-arrival travel times arecalculated on the basis of the near-offset normal moveout correction velocity in VTI mediaand ray tracing using Thomsen＇s P-wave velocity approximation. Then, the anisotropicparameters 0 and e are determined by minimizing the difference between the calculatedand observed travel times for the near and far offsets. Numerical forward modeling, usingthe proposed method indicates that errors between the estimated and measured anisotropicparameters are small. Using field data from an eight-azimuth walkaway VSP in TarimBasin, we estimated the parameters 0 and e and built an anisotropic depth-velocity modelfor prestack depth migration processing of surface 3D seismic data. The results showimprovement in imaging the carbonate reservoirs and minimizing the depth errors of thegeological targets.     

Simulation of the seismic response of sedimentary basins with constant-gradient velocity along arbitrary direction using boundary element method:SH case

We presented a boundary element method using the approximate analytical Green's function given by Sánchez-Sesma et al.Coordinate transform is introduced to extend the method to deal with the model with constant-gradient velocity along oblique direction.The method is validated by comparing the numerical results with other independent methods.This method provides a useful tool for analyzing local site effects.We computed seismic response for two series of models.The results in both frequency and time domains are analyzed and show complex amplification patterns.The fundamental mode of resonance is dependent not only on the velocity at the free surface but also on the velocity distribution of the whole basin.For the higher modes of vibration the heterogeneous basin also has its own characteristic.     

基于虚拟偏移距方法的各向异性转换波保幅叠前时间偏移

In this paper, we use the method of pseudo-offset migration （POM） to complete converted wave pre-stack time migration with amplitude-preservation in an anisotropic medium. The method maps the original traces into common conversion scatter point （CCSP） gathers directly by POM, which simplifies the conventional processing procedure for converted waves. The POM gather fold and SNR are high, which is favorable for velocity analysis and especially suitable for seismic data with low SNR. We used equivalent anisotropic theory to compute anisotropic parameters. Based on the scattering wave traveltime equation in a VTI medium, the POM pseudo-offset migration in anisotropic media was deduced. By amplitude-preserving POM gather mapping, velocity analysis, stack processing, and so on, the anisotropic migration results were acquired. The forward modeling computation and actual data processing demonstrate the validity of converted wave pre-stack time migration with amplitude-preservation using the anisotropic POM method.     

Focal Mechanism and Focal Depth of the May 22, 2016 MS4.6 Earthquake in Chaoyang, Liaoning

An earthquake with M_S4.6 occurred at 17:08 p.m., May 22, 2016 in Chaoyang County, Liaoning Province. We used the P-wave first motion method, TDMT method, and CAP method to determine the focal mechanisms and the PTD method and sPn-Pn method to determine the focal depth. The focal mechanism results of the three methods are consistent. The depth results of the CAP method, PTD method and sPn-Pn method are close. We used the double difference location method to relocate earthquakes in 2009-2016, and obtained the strikes and dip angles of the small earthquake distributions with the help of simulated annealing algorithm and gauss Newton algorithm fitting. According to the focal mechanism results, the depth results, the characteristics of small earthquake distributions and the structural characteristics of the source area, the seismogenic fault strike is NEE and the main pressure force direction is NNW. The earthquake focal mechanism is for a normal fault type with a little left-lateral strike slip motion.     

基于约束插值算法的井资料外推

Based on a detailed analysis of differences between seismic data and well logs, we discuss the problem of matching seismic traces and well logs and present a new matching method based on event search in instantaneous phase which greatly improves seismic resolution. The method is based on flattening events in instantaneous phase to compare the seismic traces to the well log traces with the same phase. We calculate the coefficients using the singular value decomposition method to extrapolate the well logs. As a result, the events in the seismic profile are continuous and match well with well logs. We apply this method to the Mao-2 well in Daqing Oilfield with good results.     

Rupture pattern of the Oct 23,2011 Van-Merkez,Eastern Turkey earthquake

High-frequency rupture process of the Oct 23, 2011 Van-Merkez earthquake is imaged by back-projection method using high-quality teleseismic P wave data from the US Array, and prestack Kirchhoff migration using P wave data from a subarray of global seismic networks. The rupture model with two asperities is confirmed by previous two methods. In low-frequency imaging, a large asperity derived from the migration method corresponds to the second one from the high-frequency P waves. The con- sistency of the locations of asperities from datasets with different frequency bands indicates that there is possible insignificance of the frequency-dependent feature for the earthquake. The resultant images illustrate the spatial and temporal evolution of the rupture, which mainly propa- gated WSW over a length of 33 km during the first 18 s, accompanying with bursts of two asperities at 3 and 11-13 s. The rupture direction is confirmed by the S wave comer frequency variations of strong ground accelerations. The rupture fronts are mainly located at the updip of the causative fault. Based on polarities of the P waveforms and focal mechanisms of the mainshock and aftershocks, the failure of these two asperities is determined to have occurred on a reverse fault with a dip angle of 47°. Hence, the rupture pattern of the 2011 Van-Merkez earthquakewas dominated by a unilateral rupture toward the west- southwest direction.     

Suppressing random noise to broaden the useful bandwidth of seismic reflection data

The signal-to-noise ratio （SNR） of seismic reflection data in many areas is rather poor and conventional two-dimensional filters designed to suppress noise with different moveout from the signal tend to generate artifacts. We have extended a method of multichannel filtering, based on the hypothesis that signals on adjacent channels are similar, for enhancing the SNR on stacked sections. Using only the mid-range frequencies where the SNR is highest, the event trend is found for overlapping windows on the section and the average signal vector is calculated. Then the data from the full bandwidth section are projected onto the spatially varying unit similarity vectors and the results are merged for the overlapping windows. Application of the method to synthetic data containing steeply dipping events and to a stacked section for a marine 2D line has produced good results. The modifications we have introduced carry a small overhead in computing time but they should enable the method to be used effectively even on sections containing steep dips.     

Experiment and simulation of turbulent flow in local scour around a spur dyke

The turbulent flow in the local scour hole around a single non-submerged spur dyke is investigated with both experimental and numerical methods. The experiments are conducted under clear-water scour regime with an impermeable spur dyke. The scour geometry and flow velocities are measured in details with a high-resolution laser displacement meter, electro-magnetic velocimetries and PIV （Particle image velocimetry）. A 3D non-linear k-ε model is developed to simulate the complex local flow field around the scour area. The numerical model is formulated using FVM （Finite volume method） on a collocated unstructured mesh, capable of resolving complex geometries and boundaries. It is found that the simulation results are reasonably consistent with those of the experimental measurements. Based on the study results, the nature of the flow structure around a spur dyke with local scour hole is analyzed.     

Research on 3D marine electromagnetic interferometry with synthetic sources for suppressing the airwave interference

In order to suppress the airwave noise in marine controlled-source electromagnetic （CSEM） data, we propose a 3D deconvolution （3DD） interferometry method with a synthetic aperture source and obtain the relative anomaly coefficient （RAC） of the EM field reflection responses to show the degree for suppressing the airwave. We analyze the potential of the proposed method for suppressing the airwave, and compare the proposed method with traditional methods in their effectiveness. A method to select synthetic source length is derived and the effect of the water depth on RAC is examined via numerical simulations. The results suggest that 3DD interferometry method with a synthetic source can effectively suppress the airwave and enhance the potential of marine CSEM to hydrocarbon exploration.     

Analysis and application of pseudo-offset method in the converted-wave prestack time migration

Pseudo-offset migration （POM） is a new method for prestack time migration of converted waves that improves on equivalent-offset migration （EOM）. The mapping of POM is different than EOM but the purpose of the two methods is to map the input samples to the common conversion scatter point （CCSP） gathers. This paper introduces the principles of the two migration methods and model parameter sensitivity tests for both POM and EOM. At large offset-to-depth ratios the hyperbolic approximation, the three-term approximation and the double square root （DSR） equation are used to NMO-correct the mapped POM gathers to obtain more accurate migration velocities. These equations were derived and calculated by small pseudo offset. POM is then used to image complex structure and prestack time migration.     

The application of the high-frequency electromagnetic sounding method in the exploration of underground thermal water

High-frequency electromagnetic sounding is an electromagnetic exploration method using the natural high-frequency electromagnetic field as a field source. It has higher resolution and greater depth penetration than the direct current method and is especially fit for geothermal energy exploration and low- and mid-level groundwater detection. We introduce a successful application of high-frequency electromagnetic sounding for evaluating geothermal water resources. The high frequency electromagnetic system （MT-USA with a frequency range from 10 KHz to 1 Hz） is first applied to sample field data from China. A remote reference station is used to assure sampled data quality. We then perform 2D inversion image processing with the electrical method data. The results basically indicate the spatial distribution of underground geothermal water and provide favorable clues to finding the sources of the subsurface geothermal water in this area.     

矢量波场弹性波Kirchhoff偏移

Based on Kuo and Dai＇s vectorial wave-field extrapolation equations, we derive new Kirchhoff migration equations by introducing unit vectors which represent the ray directions at the imaging points of the reflected P- and PS converted-waves. Furthermore, using the slope of the events on shot records and a ray racing procedure, mirror-image reflection points are found and the reflection data are smeared along the Fresnel zone. The migration method proposed in this paper solves two troublesome imaging problems caused by limited receiving aperture and migration artifacts resulting from wave propagation at the velocities of non original wave type. The migration method is applied successfully with model data, demonstrating that the new method is effective and correct.     

Numerical modeling of local scour at a submerged weir with a downstream slope using a coupled moving-mesh and maskedelement approach

This paper presents a model for local scour at submerged weirs with downstream slopes that uses a coupled moving-mesh and masked-element approach.In the developed model,the fluid-sediment interface is tracked using a moving-mesh technique,and the effects of the structure on the hydrodynamics and bed morphology are resolved using a masked-element technique.Compared to traditional sediment scour models,based on the moving-mesh technique,the present model has the advantage of allowing for a simpler setup of the computational grids and a larger-amplitude deformation.Laboratory experiments on local scour at a submerged weir with a downstream slope were conducted,which provided bed profiles at different time instants.The results obtained by the present model are compared to the experimental data.The comparisons demonstrate the performance of the model in satisfactorily predicting local scour at a submerged weir with a downstream slope.The model was further modified and employed to carry out additional computations to investigate the influence of various parameters and sub-models.     

Coseismic deformation of the 2020 Yutian M_W 6.4 earthquake from Sentinel-1A and the slip inversion

A remarkable earthquake struck Yutian, China on June 26~(th), 2020. Here, we use Sentinel-1 images to investigate the deformation induced by this event. We invert the In SAR observations using a two-step approach: a nonlinear inversion to constrain fault geometries with uniform slip based on the rectangular plane dislocation in an elastic half-space, followed by a linear inversion to retrieve the slip distribution on the fault plane. The results show that the maximum LOS displacement is 22.6 cm, and the fault accessed to the ruptured characteristics of normal faults with the minor left-lateral strike-slip component. The fault model indicates a 210° strike. The main rupture zone concentrates in the depth of 5–15 km, and the fault slip peaks at 0.89 m at the depth of 9 km. Then, we calculate the variation of the static Coulomb stress based on the optimal fault model, the results suggest that the Coulomb stress of the Altyn Tagh fault and other neighboring faults has increased and more attention should be paid to possible seismic risks.     

Crustal anisotropy of Taihang Mountain Range using azimuthal variation of receiver functions

We discussed the possibility of studying crust anisotropy by analyzing azimuthal variation of the receiver functions and presented a technique for computing the transmission response of a flat-layered medium with arbitrarily ori-ented hexagonally symmetric anisotropy using the reflectivity algorithm. Using this method we investigated the crust anisotropy of Taihang Mountain Range (TMR). Our result shows that there is significant anisotropy with a slow symmetry axis in the upper crust and a fast symmetry axis in the lower crust. The anisotropy magnitude of about 8%~15% is found in the upper crust and a smaller magnitude of about 3%~5% in the lower crust. Orienta-tion of the symmetry axes and the depth of anisotropy appearance as deduced from the seismic records of four individual seismic stations are different from each other. The crust anisotropy beneath the four stations may be associated with the local crustal fabrics in a small area.     

Study on Inelastic Attenuation and Source Parameters of the Shidian Swarm in 2001

Based on horizontal-component digital seismograms recorded on 6 stations of the Yunnan Regional Digital Network, we inversed the inelastic attenuation in the source region of the Shidian swarm using the Atkinson method and the site responses of the 6 stations 200km around the Shidian epicenters using the Moya method. The observational seismic waveform data were corrected by removing the propagation, instrument and site effects before the source parameters of the Shidian swarm in 2001 were determined using genetic algorithms. The results are as follows: (1) There is a linear relation between seismic moment and local magnitude. The seismic moment is between 1×10~ 12 and 10~ 14 N·m. The rupture radius of the seismic focus varies from 157m to 973m. The seismic moment and the rupture radius maintain a linear correlation. (2) The corner frequency increases as the seismic moment decreases. Based on the expression between corner frequency and seismic moment using least squares fitting, we can obtain the estimated value of the corner frequency. The time-varying value of the calculated corner frequency minus the estimated corner frequency shows that there were continuous high and low anomalies before the strong aftershocks. (3) The seismic stress drop is in the range of 0.07～1.55MPa. The stress drop seems independent of the local magnitude. The variation of stress drops is high before the occurrence of the strong aftershocks. (4) The depth of aftershocks is mostly in a range from 5km to 10km, which means that energy release of aftershocks is mainly concentrated in this range of depth.     

Doppler effect in high-speed rail seismic wavefield and its application

We demonstrate by theoretical analysis that periodically distributed viaduct piers of high-speed rail result in the Doppler effect in the seismic wavefield of high-speed rail at specific frequencies and analyze the Doppler effect’s influence on the wavefield’s spectrum feature.We further verify our theoretical prediction by using observational data of the high-speed rail seismic wavefield in Rongcheng,Hebei Province,China.We find that the wavefield component with a noticeable Doppler effect vibrates in the propagation direction and only has a unique apparent wave speed,indicating that P-wave is dominant.Furthermore,we propose a speed measurement method based on the Doppler effect and measure the wave speed of the medium along the rail.Measurement results are highly stable and consistent.