Elastic Full Waveform Inversion And Microseismic Time-reverse Imaging For Metallic Deposit Exploration

The weak and discontinuous reflections and scattering features in complex environment make it difficult to construct velocity model suitable for imaging in conventional P-wave seismic exploration for mineral deposits.Multi-wave and multicomponent seismic exploration technology,which utilizes information from full seismic wavefields,may have the advantages in high-resolution mineral seismic migration imaging.For the multi-wave and multi-component metallic deposit seismic exploration,the paper studied the contents of three aspects: elastic wave forward modelling and illumination analysis in isotropic and anisotropic media,elastic full waveform inversion(EFWI)in time domain and microseismic time-reversal imaging in anisotropic media.The elastic forward modelling in isotropic and anisotropic media is the foundation of high precision seismic data processing,inversion and imaging for metallic deposit exploration.This paper studied the standard staggered grid finite differences method,Convolutional Perfectly Matched Layer(CPML)boundary and the parallel finitedifference method with Message Passing Interface(MPI)were used to solve the elactic wave equation.Elastic wave equation based illumination analysis,which describes the energy distribution of seismic waves in subsurface elastic media,can improve the accuracy of seismicn exploration by optimizing seismic imaging section and acquisition geometry.Based on seismic illumination,the paper proposed the target-oriented visibility analysis method.For a specific seismicn acquisition geometry,only signals from sources and receivers located within a certain extent have effect on the inversion and imaging of exploation objective body.According to the contribution of a single shot-receiver pair to the imaging of objective body,the statistical analysis method is used to obtain the whole illumination intensity of target zone,which is defined as the visibility of the single shot-receiver pair.The quantitative optimization method with visibility can optimize the 3D seismic survey and improve the illumination without increasing the exploration costs.Elastic full waveform inversion exploiting the multi-components of pre-stack seismic data can accurately reconstruct multi-parameter models for imaging deep and complex mineral deposits.The paper studied EFWI in time domain.LBFGS optimization method was used to update the initial models and obtain the multiparameters(density,P-wave and S-wave velocity)model.To utilize the prior information more adequately and improve the accuracy of inversion in deep and complex media,the paper proposed adaptive energy compensation and visibility analysis in full waveform inversion based on the seismic illumination.Firstly,compared with the reflections form shallow part of model,wavefields from deep zone with weaker energy make less contribution to the misfit of objective function even though the velocity contrast between the initial and true model is bigger.The uneven distribution of energy could reduce the inversion accuracy of deep and complex structure.Seismic illumination can express the uneven energy distribution due to many cases overall.Therefore,we used the two-way illumination intensity of elastic wavefield as weighting factor to adaptively optimize the gradients of EFWI and balance the uneven energy distribution.Zoeppritz equations describe the energy distribution of seismic waves across the interface of impedance so the transmission and reflection coefficients between two layers can be used to control the process of automatic compensation.Secondly,visibility of geometry can describe the imaging contribution of single sourcerecever pair to target.Therefore,to improve the inversion accuracy without increasing computation and acquisition cost,we can increase the proportion of residuals related to target zone during wavefields matching of EFWI by introducing the visibility analysis to the objective function.In this way,we can utilize the seismic data related to target zone as much as possible and thus improve the inversion accuracy of target zone.Finally,the paper studies the microseismic time-reverse imaging in anisotropic media.As the sedimentary rocks are generally anisotropic,accounting for anisotropy reduces uncertainties in source location compared to those obtained on purely isotropic models.The interferometric imaging condition can attenuate random fluctuations and highlight the source position.The paper proposed a new source location method with time-reversal illumination.Based on the reversibility of wave equation,we calculated the time-reverse illumination during the procedure of time-reverse modelling and considered it as the illumination of unknown subsurface sources.Because of the energy attenuation,the maximum illumination intensity must be located at source.Therefore,we can achieve the source location by finding the maximum time-reversal illumination intensity.The numerical experiment of anisotropic model has illustrated its higher location accuracy compared with conventional time-reversal methods and ability to locate more than one source with almost zero error in microseismic monitoring for anisotropic regions.