A QUANTITATIVE ANALYSIS METHOD FOR THE SEISMIC GEOLOGICAL COMPLEXITY OF NEAR SURFACE

AbstractComplexity of the near surface, a major factor that influences low signal‐to‐noise ratio (SNR) in seismic data, scatters and attenuates all wavefields observed on the earth's surface, and causes semi‐random and semi‐coherent near‐surface scattering background noise on shot gathers where deep reflected signals are submerged. How to study and evaluate the near‐surface scattering intensity has long been an unsolved problem in petroleum exploration because of the vague relation to the roughness of surface, the near‐surface lateral velocity variation, and the angle distribution of oblique structures. Based on previous research on wave equation numerical simulation applied to complex near‐surface structure using boundary element method, this article puts forward a complex scattering amplitude matrix method to analyze the scattering intensity near surface. First the complex near‐surface structure is discretized and collocated according to the boundary element method to form the matrix equation of the boundary integral equation. Instead of solving the matrix equations, which involves massive computation, we analyze the coefficient matrix of scattering amplitude by matrix analysis techniques to ascertain the influence of the complex near‐surface structures on wavefields at different frequencies. This strategy relies on the advantage of the boundary element method in accurately describing geometrical characteristics of arbitrary boundaries to investigate how undulating surface and non‐regular geological interface affect seismic wave propagation. The coefficient matrix of scattering amplitude is obtained by applying Gaussian numerical integration of the normal derivative of the fundamental solution on boundaries. This procedure not only describes the interaction between two arbitrary points, but also includes the effect of morphological characteristics of different boundary elements, which provides the possibility to evaluate scattering intensities of different geological structures. As a preliminary evaluation method, we use the ratio of the summation of the matrix elements to the dimension as a scalar complex coefficient to characterize the scattering property of the scattering amplitude coefficient matrix, besides, a set of effective and fast computation methods is developed through theoretical and practical model tests.