Microseismic Source Inversion in Anisotropic Media

Summary Microseismic source mechanisms are of interest in hydraulic stimulation because of what they may reveal about the induced or enhanced fracture network. Since rocks are in general anisotropic, particularly shales, it is of interest to consider what impact anisotropy may have on microseismic sources and their inversion. The strongest and most prevalent type of anisotropy in sedimentary rocks is transverse isotropy with a symmetry axis normal to bedding or vertical (so-called VTI). While the theory used is valid for general anisotropy we consider presently only layered VTI models. An earthquake may be represented as a displacement discontinuity across a plane, composed of a double couple (DC) or shear slip component and an opening (or closing) component. While this type of source description seems physically intuitive for microseismic sources generated by hydraulic stimulation, it is an incomplete description of a moment tensor; the addition of a pure explosion (or implosion) component is necessary. We are calling such a three part Explosion + Opening + Slip source an “EOS” source, from which the moment tensor can be constructed. A theory has been developed to decompose any moment tensor