Elastic Wave Reverse Time Migration Research Articles

Imaging Steeply Dipping Faults Using Angle-Controlled Decoupled Elastic Reverse-Time Migration of Multicomponent Seismic Data

High-angle and steeply-dipping faults are important for geothermal fluid flow and production. However, imaging such faults is challenging for conventional seismic imaging methods using acoustic waves because of the limited illumination of compressional waves. We develop a novel imaging method to use elastic waves including both compressional-to-compressional (P-P) waves and compressional-to-shear (P-to-S) converted waves to image the high-angle faults. P-S converted waves provide additional seismic illumination for high-angle faults because they have smaller reflection angles than those of the P-P reflection waves at interfaces. Our new imaging method employs the decoupled wave equations in the context of elastic-wave reversetime migration (ERTM) to perform imaging using both P-P and P-S waves. Our decoupled ERTM does not require the polarity correction of S waves. In addition, we use the Poynting vector, measuring the wave propagation direction by computing the wave energy flux, to control the image dip angles thus improving the ability to image high-angle faults. Based on the angle-controlled P-P and P-S images, we further enhance the structural images of high-angle faults by extracting the significant structures that coexist in both P-P and P-S images. We apply our angle-controlled decoupled ERTM (ADEM) to synthetic multi-component seismic data for a simple layer model with two vertical faults and a modified Marmousi2 model with artificially embedded vertical faults. Our numerical results demonstrate that our ADEM method can effectively image high-angle faults in complex structures using both P-P and P-S waves.

Source-Free P-SV Converted-Wave Reverse-Time Migration Using First-Order Velocity-Dilatation-Rotation Equations

The cross-correlation imaging condition between source- and receiver-wavefields is often used in the elastic wave reverse-time migration (RTM) to utilize P- and S-waves. However, it cannot be applied in the absence of source information (e.g., source location, and source wavelet), which is quite common in passive source exploration. We employ a source-free P-SV converted-wave imaging condition, which only requires the back-propagating receiver-wavefield to utilize the P-SV converted waves in imaging the subsurface structures. The imaging condition is independent of source information, which can avoid the extrapolation and reconstruction of the source-wavefield. As a result, the computational cost is decreased to about one-third of conventional RTM that uses source-wavefield reconstruction strategies, e.g., random boundaries. The memory requirement could be also reduced by avoiding the calculation of source-wavefield. Because our imaging condition uses the vector P-wavefield and vector S-wavefield to utilize the P-SV waves, it is necessary to decouple P-wavefield and S-wavefield during the reverse-time extrapolation of receiver-wavefield. We use the first-order velocity-dilatation-rotation elastic wave equations to realize the reverse-time propagation of vector receiver-wavefield, where the vector P-wavefield and vector S-wavefield can be obtained directly. Based on the above methods, a source-free P-SV converted-wave RTM of multi-component seismic data is realized. The model tests show that this method can generate promising subsurface images and can be complementarily used when conventional cross-correlation imaging conditions are not suitable.

Elastic reverse-time migration in irregular tunnel environment based on polar coordinates

When seismic exploration is conducted in a special geological environment such as a tunnel space, the traditional imaging method in the Cartesian coordinate system cannot accurately discretize the air column in that environment. Thus, obtaining Thus, obtaining high-quality imaging results is difficult. Therefore, an elastic-wave reverse-time migration method based on the polar coordinate system is proposed. In this method, three boundary conditions exist: outer, inner, and corner boundaries. In the outer boundary, the polar-coordinated absorbing boundary in the radial direction is used to suppress the artificial-boundary reflection. The free-surface boundary condition is adopted in the tunnel space at the inner boundary. In the angular boundaries, we use two different boundary conditions for two cases. The air column in the tunnel space is usually not an irregular circle. Therefore, the irregular tunnelspace geological body in the polar coordinate system is meshed into curvilinear grids and transformed into a regular one in an auxiliary polar coordinate system using the mapping method. Finally, elastic reverse-time migration technology is applied into the auxiliary polar coordinate system. In the numerical examples, two typical models are used to test the proposed method, which verify that the proposed method can obtain accurate images from the datasets in the tunnel space.

Reverse Time Migration of Elastic Waves Using the Pseudospectral Time-Domain Method

Reverse time migration (RTM), especially that for elastic waves, consumes massive computation resources which limit its wide applications in industry. We suggest to use the pseudospectral time-domain (PSTD) method in elastic wave RTM. RTM using PSTD can significantly reduce the computational requirements compared with RTM using the traditional finite difference time domain method (FDTD). In addition to the advantage of low sampling rate with high accuracy, the PSTD method also eliminates the periodicity (or wraparound) limitation caused by fast Fourier transform in the conventional pseudospectral method. To achieve accurate results, the PSTD method needs only about half the spatial sampling rate of the twelfth-order FDTD method. Thus, the PSTD method can save up to 87.5% storage memory and 90% computation time over the twelfth-order FDTD method. We implement RTM using PSTD for elastic wave equations and accelerate it by Open Multi-Processing technology. To keep the computational load balance in parallel computation, we design a new PML layout which merges the PML in both ends of an axis together. The efficiency and imaging quality of the proposed RTM is verified by imaging on 2D and 3D models.

Extraction of amplitude-preserving angle gathers based on vector wavefield reverse-time migration

Angle-domain common-image gathers (ADCIGs) transformed from the shotdomain common-offset gathers are input to migration velocity analysis (MVA) and prestack inversion. ADCIGs are non-illusion prestack inversion gathers, and thus, accurate. We studied the extraction of elastic-wave ADCIGs based on amplitude-preserving elastic-wave reversetime migration for calculating the incidence angle of P-and S-waves at each image point and for different source locations. The P-and S-waves share the same incident angle, namely the incident angle of the source P-waves. The angle of incidence of the source P-wavefield was the difference between the source P-wave propagation angle and the reflector dips. The propagation angle of the source P-waves was obtained from the polarization vector of the decomposed P-waves. The reflectors’ normal direction angle was obtained using the complex wavenumber of the stacked reverse-time migration (RTM) images. The ADCIGs of P-and S-waves were obtained by rearranging the common-shot migration gathers based on the incident angle. We used a horizontally layered model, the graben medium model, and part of the Marmousi-II elastic model and field data to test the proposed algorithm. The results suggested that the proposed method can efficiently extract the P-and S-wave ADCIGs of the elastic-wave reverse-time migration, the P-and S-wave incident angle, and the angle-gather amplitude fidelity, and improve the MVA and prestack inversion.

Reverse Time Migration of Elastic Waves Using the Pseudospectral Time-Domain Method

Reverse time migration (RTM), especially that for elastic waves, consumes massive computation resources which limit its wide applications in industry. We suggest to use the pseudospectral time-domain (PSTD) method in elastic wave RTM. RTM using PSTD can significantly reduce the computational requirements compared with RTM using the traditional finite difference time domain method (FDTD). In addition to the advantage of low sampling rate with high accuracy, the PSTD method also eliminates the periodicity (or wraparround) limitation caused by fast Fourier transform in the conventional pseudospectral method. To achieve accurate results, the PSTD method needs only about half the spatial sampling rate of the twelfth-order FDTD method. Thus, the PSTD method can save up to 87.5% storage memory and 90% computation time over the twelfth-order FDTD method. We implement RTM using PSTD for elastic wave equations and accelerate it by Open Multi-Processing technology. To keep the computational load balance in parallel ...

Elastic-wave reverse-time migration based on decoupled elastic-wave equations and inner-product imaging condition

Polarity reversal in converted wave images of elastic reverse time migration destructs the reflection events after stacking multi-shot migration profile. We derive a new imaging method for elastic reverse-time migration to automatically circumvent polarity reversal. Instead of obtaining scalar P-wave and vector S-wave potentials from the wavefield by using Helmholtz decomposition as in conventional methods, we obtain vector P- and S-wave displacement wavefields based on a decoupled elastic wave equation, which denotes the displacement component along the propagation direction and perpendicular to the propagation direction, respectively. Based on this decomposition method, the vector P- and S-wavefields preserve the amplitude and phase attributes of the original wavefield. As for the vector wavefields (vector P- and S-wave displacement wavefields), the inner-product imaging condition is proposed to extract reflectivity of specified wave modes at interfaces. The analysis of the imaging kernel demonstrates this imaging condition is valid not only for pure-mode imaging (PP and SS), but also for converted wave imaging (PS and SP) of ground-based seismic exploration. With this new method, we do not have to correct the polarity reversal in converted wave images, which is an essential step in the conventional method with expensive computation costs. Numerical examples with synthetic data have shown that the inner product imaging method works and the quality of the imaging events is effectively improved.