HR: 1340h
AN: T23B-0552    [Abstracts]
TI: Precise Automatic Relocation Of Microearthquakes Using Multiplet Analysis
AU: * Michelet, S
EM: sophiem@mit.edu
AF: MIT - Earth Resources Laboratory, 42 Carleton Street, CAMBRIDGE, MA 02142 United States
AU: Toksoz, N
EM: toksoz@mit.edu
AF: MIT - Earth Resources Laboratory, 42 Carleton Street, CAMBRIDGE, MA 02142 United States
AB: During a massive hydraulic fracturing experiment in the crystalline basement of the Soultz geothermal field at 5 km depth, around 90,000 micro-earthquakes were induced by the injection of water. Among these, 21,000 events were located with standard techniques to characterize the extent of the stimulated zones and ultimately of the fracture network. In order to resolve smaller spatial patterns, we undertook a more precise relocation by performing a multiplet analysis. This technique allows identifying microearthquakes having similar waveforms but different origin times that can then be located precisely relative to one another. In order to process this large amount of data, we had to automate the procedure. For this purpose, we calculated the time delay between two events of a doublet using the phase of the wavelet transform. We decided to select all the pairs showing a coherency higher than 80 %. We ended up with 7,463 events found to be part of the multiplet catalog, representing 35% of the original catalog. Furthermore, the number of multiplets as a function of time was found to follow the rate of the induced seismicity during the experiment. After having computed the time delays between pairs, we relocated them using a grid search algorithm. Thus, we were able to get a precision of about 15m horizontally and 5m vertically. These multiplets are located at the openhole section of the injection well and extend in the direction of the production well. They belong to large-scale structures, likely permeable fractures/faults stimulated by the injection. These clusters highlight potential seismic zones that were not apparent before relocation.
DE: 3280 Wavelet transform (3255, 4455)
DE: 5104 Fracture and flow
DE: 7200 SEISMOLOGY
SC: Tectonophysics [T]
MN: Fall Meeting 2005