Passive-seismic image-domain elastic wavefield tomography

Elastic time-reverse imaging offers a robust wavefield-based approach for locating microseismic events; however, event location accuracy greatly depends on the veracity of the elastic velocity models (i.e. V-P and V-S) used for wave propagation. In this study, we propose a methodology for microseismic image-domain wavefield tomography using the elastic wave equation and zero-lag and extended source images, the focusing of which is used as a quality control metric for velocity models. The objective function is designed to measure the focusing of time-reversed microseismic energy in zero-lag and extended event images. The function applies penalty operators to source images to highlight poorly focused residual energy caused by backpropagation through erroneous velocity models. Minimizing the objective function leads to a model optimization problem aimed at improving the image-focusing quality. P- and S-wave velocity model updates are computed using the adjoint-state method and build on the zero-lag and extended image residuals that satisfy the differential semblance optimization criterion. Synthetic experiments demonstrate that one can construct accurate elastic velocity models using the proposed method, which can significantly improve the focusing of imaged events leading to, for example, enhanced fluid-injection programs.