Here, a method of passive seismic imaging using multicomponent (3-C) data is presented. It is a Beamforming/Kirchhoff type migration, which is based upon the isotropic elastic wave equation within geometrical optics theory. To account for the effects of the source mechanism, polarity corrections are applied.
Mathematically, the goal in a passive seismic survey is to characterize the source term in the elastic wave equation, given seismic velocities and measured displacements at some number of observation points. Following Haldorsen et al. (2013) approach, using Helmholtz decomposition (Muller, 2007), the source wavefield can be decomposed into a curl-free longitudinal component (L) and divergence-free transverse (T) components. They are utilized to locate and characterize the seismic event that sourced the wavefield. The method can be implemented for both surface and downhole receiver array geometries. Here we are presenting the method as it applies to downhole surveys. Both synthetic and field data examples are demonstrated.
The synthetic example proves the feasibility of the imaging technique, by producing the resulting image coincident with the modeled synthetic event. The accuracy of the approach with a real world example is validated by quality-control of the imaging procedure, by the relative position to a treatment well of the event locations, and by the match of the imaged perforation shot to its known location.
Passive seismic data recorded by surface arrays, which have large apertures, wide azimuths and high fold, are routinely used for imaging of microseismicity that occurs during hydraulic fracturing (Duncan and Eisner, 2010). Mapping of the microseismicity created during hydraulic fracturing, when tight shale formations are stimulated in order to increase permeability, is critical to understanding the well efficiency, to optimize completion processes and to maximize production. The method presented here can be used for 3-C data recorded both on/near Earth’s surface and/or in downhole deployments to characterize and locate passive microseismic events. We present the theoretical basis, from which the 3-C imaging solution is derived. We then demonstrate the method using both synthetic and real field data.