A Fast Sweeping Method For Calculating QP Traveltimes&Joint Microseismic Events And Anisotropic Parameters Inversion

Anisotropy plays a key role in modern seismic exploration.Especially vertical transversely isotropic(VTI)and tilted transversely isotropic(TTI)attract more attentions.In anisotropic media,velocity varies with the angle,and this indicates the big challenge of performing an accurate raytracing approaches.Our study makes the efforts from the following two aspects:1.A high order fast sweeping method for calculating qP traveletimes in TTI media.When the conversional raytracing method is extended to the anisotropic cases,they mainly face two challenges:1)Causality problems caused by the incoherence of phase and group velocity directions;2)When complex velocity model is handled,how to guarantee the traveltimes are related to the first arrival.Here we propose a new fast sweeping method(FSM)in which a quartic equation is solved analytically to obtain some possible traveltimes.Then the causality of each possible traveltimes is tested.Through this procedure,first challenge is considered.We accept Gauss-Seidel iterations to update the traveltime field.After sufficient iterations,the traveltime field becomes stable.This guarantees the minimum of the traveltimes.2.Joint microseismic event locations and Thomsen parameters inversion in layered VTI media.In microseismic monitoring,achieving reliable velocity model and event locations helps enhance the product of oil and gas.However anisotropy has a significant effect on the event locations.Thus we come up with a joint inversion approach in which both the event locations and Thomsen parameters are inverted simultaneously.For the traveltime calculation,we employ a horizontal slowness shooting method in which the Snell's law is involved implicitly.Based on the slowness components,we derive the traveltime derivatives with respect to Thomsen parameters.Compared with the method based on the stiffness components,our approach is simple and easily understood.Finally synthetic tests are performed to test the reliability of the inversion results,and this joint method is applied to real data case.