THE USE OF SEISMIC EMISSION TOMOGRAPHY (SET) FOR IMAGING TECTONICALLY ACTIVE FRACTURE/FAULT NETWORKS AND EVIDENCE FOR A “FLUID PRESSURE WAVE” AS PART OF THE ROCK DEFORMATION PROCESS

The rock deformation process in the earth's brittle crust is controlled by fracturing and fluid flow. Use of SET for direct imaging of the fracture scale geometry of the rock deformation process provides a new tool for better understanding the real time behavior of this process. SET uses surface based seismic networks for imaging the complete fracture size spectrum. Using SET we present the first visual evidence supporting the empirical and theoretical work (e.g. Ziv and Rubin, 2000) indicating that the earth's crust is in a near critical state, such that small (≤ .01 bar) perturbations of the ambient stress field are sufficient to cause failure of critically oriented cracks. We find that such perturbations can be generated by a fluid pressure (Pf) “wave” initiated by any ΔPf such as that generated during a flow test or frac. The phenomena may be a manifestation of the Biot (1962) “slow wave”. Wave velocity appears to be on the order of m/sec, while its effects extend for distances of kilometers from the point of ΔPf. Because the ambient permeability field consists of critically oriented cracks, passage of the Pf wave causes their failure. SET methods are used to process the weak acoustic energy (AE) emissions of the failure events illuminating the geometry of the ambient permeability field. The SET results are presented as a sequence of time stacks of the AE energy field in a contoured voxel format. The fracture/fault networks appear as transient energy events occurring in locations geographically stable over some time interval. Energy transients are typically on the order of a few seconds to minutes indicating the time scale of the crust's dynamic equilibrium.