The hydraulic fracturing technique has become an important tool in the enhancementof hydrocarbons recovery, geothermal energy extraction, and solid waste disposal. Thecharacterization of geometry parameters and growth rate of a hydraulic fracture zoneis an important task for monitoring and assessing subsurface cracks. In this paper,we develop a location approach to determine the precise hypocenter locations for acluster of seismic events induced by hydraulic fracturing. Two techniques were usedin our location scheme: waveform correlation and grid search methods. The waveformcorrelation method allows us to obtain more accurate differential arrival times amongevents within in a cluster.. The grid search method is suitable when dealing with anonlinear location problem.We applied our method to seismic waveform data from a hydraulic fracturing experimentat the Los Alamos Hot Dry Rock geothermal site and determined hypocenterlocations for 157 induced microearthquakes. The maximum absolute and relative locationerrors were estimated to be 30-39 meters and 3-9 meters, respectively. Among the157 events, 147 microearthquakes occurred in a tight cluster of a dimension of 40 meters,roughly defining a vertical hydraulic fracture zone. The length, height, and widthof the hydraulic fracture zone were measured to be 40, 35 and 5 meters, respectively.The orientation of the fracture zone is estimated at about N400W. Analysis of thetemporal-spatial pattern of the induced microseismicity revealed that the fracture zonegrows significantly in a two-hour period during the hydraulic injection. Using seismicitydistribution in time and space in this period, we determined that the fracture zonegrows toward the northwest along the fracture zone strike with a growth rate of 0.1-0.2meters per minute.
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