Relative location of microseismicity

Abstract

Applying the two-point paraxial ray tracing, we develop a technique for relative location of microseismic events. Our technique assumes the availability of a perforation shot or an already located microseismic event, termed the master, for which the paraxial ray tracing has been performed. The ray-tracing output for the master makes it possible to compute the relative locations of adjacent microseismic events, as many as a data set contains, with an efficient algorithm that requires no additional ray tracing and reduces to solving a series of simple, low-dimensional and well-behaved optimization problems. The relative event-location approach discussed in our paper is especially well suited for surface microseismic monitoring because the high accuracy of the paraxial ray approximation in the directions orthogonal to the reference rays, typically spanning the stimulated horizons for surface microseismic geometries, ensures the calculation of precise event hypocenters at appreciable distances from the master. Also, since our computations operate with differences of the recorded times of microseismic events rather than with the times themselves, inaccuracies in static corrections for surface receiver stations are largely eliminated. We test the relative location technique on synthetic and field microseismic data to demonstrate its accuracy, computational efficiency, and insensitivity to velocity errors.