The reflectivity of the subsurface can be precisely determined using least-squares reverse time migration (LSRTM). Since LSRTM necessitates solving the wave equation, the numerical solution method of the wavefield directly determines the quality of the migration image. The conventional LSRTM method usually uses the finite difference method based on a regular grid to calculate the wavefield. Due to the stepwise approximation of an irregular surface with a regular grid, scattering noise may occur in the propagation of the wavefield, which affects the quality of the image. In addition, the conventional LSRTM cannot effectively handle the models with rugged topography. The finite difference method generated by radial basis functions (FD-RBF) is a mesh-free method and can construct interpolation functions to solve the wave equations numerically according to arbitrarily distributed spatial coordinate points. Therefore, we use the FD-RBF method to develop a mesh-free LSRTM approach to eliminate the influence of the inherent limitation of a regular grid on the imaging. Numerical examples show that the mesh-free LSRTM method can better represent the curved or steep interface within the model and is also suitable for models with rugged topography. The LSRTM method can provide higher-quality images and effectively reduce the memory required for calculations.
The application of the pre-stack depth migration (PSDM) method using deep seismic reflection data is still a challenge. In this paper, a deep seismic reflection dataset collected from the Bohai Bay Basin is used to test the applicability of the pre-stack depth migration method. After improving the signal-to-noise ratio of the prestack gathers and building an accurate velocity model capturing lateral and vertical variations, a high-quality pre-stack depth migration profile is obtained. This profile shows detailed crustal structures and an undulating Moho. The depth difference between the east and west sides of the Moho imaged in the profile is about 6 km. The Moho is disrupted by several discontinuities. Compared with conventionally stacked time-migration profiles, the pre-stack depth migration profile provides a higher-resolution subsurface image with accurate depth information. Combining geological and geophysical data, the pre-stack depth migration profile reveals geologic structures responsible for crust thinning during the destruction of the North China Craton. The profile also provides more accurate depth domain information for further study of crust-mantle interaction in this area.
Seismic anisotropic attenuation and anisotropic velocity exist widely in the earth's interior and have a great influence on the propagation of seismic waves. Ignoring the effects of attenuation anisotropy may lead to amplitude imbalance or noise in reflection seismic imaging, thus reducing the quality of the imaging results. In order to incorporate attenuation anisotropy into imaging methods and explore its effect on imaging, based on a novel two-way pure qP wave equation in viscoacoustic vertical transversely isotropy media, we propose the corresponding reverse time migration and least-squares reverse time migration method. Both imaging methods can accurately obtain subsurface structure information, especially the least-squares reverse time migration has the potential to compute accurate subsurface reflectivity. In this paper, we first introduce the pure qP wave equation in viscoacoustic vertical transversely isotropy media. As the equation is derived from the complex dispersion relation of P wave, wave propagation can be simulated without interference of SV wave and limitation of anisotropic parameters. Then, we derive the corresponding linearized wave equation and adjoint gradient for updating the imaging result. Finally, using two synthetic models, we demonstrate the effectiveness of the imaging method and discuss the effect of attenuation anisotropy on seismic imaging.
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