Nonlinear travel-time inversion for 3-D complex crustal velocity structure

Complex crustal structures generally characterize global orogenic belts and Midwest China. With the deepening of mineral resources exploration and increasing detection of the deep earth, it is becoming a great challenge to use new methods to probe fine crustal structure beneath orogenic belts and basin-mountain coupling regions. Basic characteristics of crustal structure of different tectonic domains are becoming clearer and clearer in deep seismic soundings. However, it is difficult for the traditional layered structure modeling method, in which layer boundaries are smoothing and fine velocity discontinuities are often ignored, to describe complex geological models, making it difficult to meet the development requirements of fine structure imaging of crust. In view of the above difficulties, based on a newly developed block modeling scheme to describe three-dimensional complex crustal models and a corresponding segmentally iterative ray tracing (SIRT) method, we derive the travel-time partial derivatives of triangular interface depth and grid velocity, and develop a 3-D nonlinear conjugate gradient travel-time inversion method. Block modeling scheme is able to construct any complex geological models in theory and can be used to efficiently build initial models by combining various priori velocity and interface information. In the inversion process, PRP (Polak-Ribiere-Polyak) type of the conjugate gradient method is used to solve the constrained damping least squares problem. We develop the joint inversion of grid velocity and interface depth based on multi seismic phases like direct waves and reflected waves, and make a great improvement of the inversion resolution compared with the traditional method which is based on single phase. To improve the convergence accuracy of inversion results, strategies like different weighting factors for different seismic phases and normalization of travel-time partial derivatives of different parameters are introduced in the joint inversion process. Numerical examples are given and a special layered cross-cutting model is used to simply simulate the application form of our method for fine structure imaging of crust, showing that nonlinear conjugate gradient travel-time inversion method based on block modeling is suitable for complex crustal models, which has good application prospects in the field of fine crustal structure imaging based on travel-time data of artificial seismic sources.