Inversion Of Lithology Parameter Contrasts Based On Chaos-Genetic Algorithm

Amplitude variation with offset (AVO) analysis has been used extensively for lithology prediction in many regions. In conventional AVO inversion, usual AVO Attributes are obtained and can be interpreted in terms of anomalies. More recently, there has been an increasing effort to estimate the elastic parameters of the subsurface targets by means of AVO inversion.The theory of AVO is based on the Zoeppritz equations, which are complicated for reflection and transmission coefficients. In this paper, we derive several new approximation formulas for the plane P-wave reflection coefficient. They are applicable for wider incidence angles and larger contrast of elastic parameters. These approximations are proved to be of better accuracy when compared with those linear approximations.P-wave AVO inversion is a complicated constrained nonlinear optimization problem, which is better achieved using global optimization methods. Based on the improvement of genetic algorithm and chaos optimization method, a novel chaos-genetic algorithm (CGA) was proposed. Simulation results of typical complex function optimization show that CGA is higher efficiency than the simple genetic algorithm. Applying the CGA to P-wave AVO inversion, four elastic parameter contrasts are extracted. The P-wave velocity contrast and the density contrast are better estimated, making determination of non-economic gas from economic gas a possibility.For multi-layer mediums, the Zoeppritz equations are not suitable to study the AVO response. Based on Brekhovski wave propagation theory, the reflection expression for a thin layer was given. Supposing the surrounded section known, three main elastic properties are extracted. The study is useful for the AVO analysis of a thin layer.