Jiubing Cheng

Local Angle Domain Elastic Reverse Time Migration in TI Media

Local angle-domain elastic reverse time migration (ERTM) can provide more opportunities for complex structure imaging and elastic parameter inversion. However, its implementation still has many challenges, especially in anisotropic media. We present an accurate local angle-domain ERTM algorithm for heterogeneous TI media on the base of the cross correlation imaging condition and vector decomposition of the elastic wave modes. At the mean time, the computational cost is significantly reduced by applying fast algorithms of wave mode separation and decomposition using low-rank approximation of the mixed-domain operators. Synthetic example shows that, compared with the Helmholtz decomposition, the efficient wave mode separation/decomposition based on polarization projection promotes the quality of angle-domain common-image gathers for TI media.

Numerical Pure Wave Source Implementation and Its Application to Elastic Reverse Time Migration in Anisotropic Media

For isotropic media, a P- or S-wave source can be easily implemented by adding a disturbance (wavelet) on its related stress components. However, it does not work for anisotropic media. Inspired by the idea of elastic wave mode decoupling, we propose a general approach to numerically simulate the pure wave source for elastic wave propagation. This is achieved by calculating the spatial counterpart of the normalized polarization for the given mode in the anisotropic media where the actual source is triggered. This approach may be helpful for seismic imaging, waveform inversion and micro-seismic monitoring when the source locates in an anisotropic rock, such as gas shale. Synthetic examples demonstrate the feasibility and application to elastic reverse time migration in a VTI medium.

Prestack scalar reverse time migration of elastic seismic data in TI media

We present new scalar wave-equations that can characterize P-, SVand SH-wave in transversely isotropic (TI) media. They can be used as the basis for migration of both acoustic and elastic data. SH wave-equation is directly separated from the 2D VTI elastic wave-equation. P and SV wave-equations are derived by projecting the wavefield of elastic equations onto phase vector and its normal in the wavenumber domain, respectively. Group velocity surfaces and radiation patterns verify the kinematics and dynamics yielded from these wave-equations. 3D TTI impulse responses and numerical examples on 2D thrust model demonstrate their application in reverse-time migration of elastic data.

Azimuth-preserved Local Angle Domain Imaging and Its Application to Prestack Time Migration

Employing 3D prestack migration in azimuthal analysis allows for some structural dip and provides for higher signal to noise ratio in the measurements. Standard common-offset migration does not preserve azimuthal information. Recent attempts to move this analysis into migrated space concern source-receiver azimuth and offset on the surface, and generally involve some form of data sectoring. In fact, surface-azimuth and offset may be poor representations of the direction of wave path in the subsurface. Sectored migration may produce noisy prestack images owing to limited fold within the sectors and often requires large sectors leading to poor statistics for analysis of anisotropic properties. To tackle these problems, we present an azimuth-preserved local angle domain imaging approach that honors the local directional characteristics of the subsurface wave path in isotropic and vertical transversely isotropic media. The example has shown the advantage of our azimuth-preserved local angle domain Kirchhoff prestack time migration algorithm for azimuthal analysis.

Automatic Shear-wave Velocity Analysis with Elastic Reverse Time Migration

Converted-wave has the potential to be a positive complement of the compressional imaging in some special cases. However, converted wave imaging is more difficult because the need of estimating the background velocity of S-wave. By applying differential semblance to measure the focusing error in the imaging domain, we automatically estimate the background S-wave velocity using reverse-time migration with a given estimated P-wave velocity. Compared to the P-wave, the S-wave velocity estimation via PS imaging gather does not need gradient from the source side. A simple synthetic result demonstrates the feasibility of the proposed method.

Local angle domain Kirchhoff prestack depth migration in TI media

To support velocity analysis and amplitude variation with incident angle analysis in transversely isotropic media, we present a ray-based local angle domain prestack depth migration algorithm to generate common-image gathers in scattering and illumination angle domain. An efficient kinematic ray tracing system is used to construct numerical tables of traveltime, geometrical spreading factor and takeoff angle of various raypath between the image-point and source or receiver on the surface. Weighted superposition of all impulse responses according to the local angular attributes of the raypath generates scattering and illumination angle domain migrated images. Synthetic examples on the SEG/HESS VTI model and the overthrut TTI model demonstrate the validity of this algorithm

Combined Bayesian AVO inversion with rock physics to predict gas carbonate reservoir

Summary A better understanding of fluid effects on carbonate rock properties is considered to be key issue in applying AVO inversion to carbonate reservoir characterization. Analysis of well log data in a gas carbonate reservoir of Southwestern China indicates that gas does influence carbonate’s elastic rock properties, and low VP/VS ratio and low acoustic impedance can be treated as calibration for the interpretation of seismic inversion result. We invert prestack seismic data using Bayesian linearized AVO inversion to estimate elastic properties and assess their uncertianty. We also show how to combine a credible seismic inversion result with rock physics analysis to identify gas carbonate reservoir.

Azimuth‐preserved local angle domain imaging in azimuthally anisotropic media

For seismic data with low fold and poor azimuthal sampling, more reliable imaging and azimuthal analysis can be achieved using azimuthal migration of full 3D data, rather than sectored isotropic migration of the azimuth-limited or common offset vector data. However, most routinely used prestack migration techniques only concern source-receiver azimuth and offset on the surface, which may be a poor representation of subsurface direction of the wavepath. To extract high-resolution angle-dependent reflectivity and detailed information from geologic targets, we present an azimuth-preserved local angle domain Kirchhoff prestack time migration approach which can tackle azimuthal anisotropy of the overburden and reservoir. Real data example is presented to prove the necessity of imaging in subsurface angle domain for properly focusing and revealing azimuthand angle-dependent variations of residual moveout and migrated amplitude.

Cross‐line common‐offset migration for narrow azimuth dataset

Based on the double square root (DSR) wave equation, cross-line common-offset (CLCO) migration approach is presented, which is suitable for the narrow azimuth dataset. When the cross-line offset becomes zero, the CLCO propagator turns into the common-azimuth propagator, which can deal with the towed streamer data effectively. Migrations of the C3 dataset of the 3D SEG/EAGE salt model and a field dataset aimed to buried hill survey using the CLCO method produce reasonable images.

P/S Separation of Multi-component Seismogram Recorded in Anisotropic Media

Summary The multi-component receiver records complete wavefields, which include both P- and S-wave modes. The combination of P- and S-wave characteristics yields better insight into rock properties in the sub-surface. We propose a new approach to decoupling different wave modes in multi-component gather in anisotropic media. By solving the quartic dispersion relation and eigenvalue problem of the Christoffel equation, the polarization vectors of different wave modes can be obtained on the acquisition surface. Accordingly, one can decouple different wave modes both into scalar and vector forms by projecting elastic wavefields onto polarization-related directions in the frequency-wavenumber domain. Synthetic examples demonstrate the validity of the proposed method.