The location of Microseismic events and the Propagation of Ray path and gridded traveltimes for depth migration using locally spherical wavefronts

Abstract

An efficient algorithm is presented that estimates the apparent source of a microseismic event from the first arrival clock-times at four receiver locations in a 3D volume. Wavefronts are assumed to be locally spherical in a constant velocity medium. Applications for identifying the apparent source range from monitoring hazardous geological sites, estimating the distribution of well fraccing material, the monitoring of sequestered CO2, or global positioning from satellite data. The clock-time of the source may also be estimated, extending applications to Kirchhoff depth migrations in which traveltimes on a grid may be computed directly, or may be estimated from traveltimes computed along raypaths. The traveltimes at additional grid points can be computed from the apparent source. In heterogeneous media, wavefronts may have an arbitrary shape, but can be considered to be circular over a small region in the neighbourhood of the known points. The velocity in this region is assumed to be constant and may be extended, without error, to enclose the apparent source point.