Accurate Location of Synthetic Acoustic Emissions and Location Sensitivity to Relocation Methods, Velocity Perturbations, and Seismic Anisotropy
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Acoustic emission (AE) monitoring is a non-invasive method of monitoring fracturing both in situ, and in experimental rock deformation studies. Until recently, the major impediment for imaging brittle failure within a rock mass is the accuracy at which the hypocenters may be located. However, recent advances in the location of regional scale earthquakes have successfully reduced hypocentral uncertainties by an order of magnitude. The least-squares Geiger, master event relocation, and double difference methods have been considered in a series of synthetic experiments which investigate their ability to resolve AE hypocentral locations. The effect of AE hypocenter location accuracy due to seismic velocity perturbations, uncertainty in the first arrival pick, array geometry and the inversion of a seismically anisotropic structure with an isotropic velocity model were tested. Hypocenters determined using the Geiger procedure for a homogeneous, isotropic sample with a known velocity model gave a RMS error for the hypocenter locations of 2.6 mm; in contrast the double difference method is capable of reducing the location error of these hypocenters by an order of magnitude. We test uncertainties in velocity model of up to ±10% and show that the double difference method can attain the same RMS error as using the standard Geiger procedure with a known velocity model. The double difference method is also capable of precise locations even in a 40% anisotropic velocity structure using an isotropic model for location and attains a RMS mislocation error of 2.6 mm that is comparable to a RMS mislocation error produced with an isotropic known velocity model using the Geiger approach. We test the effect of sensor geometry on location accuracy and find that, even when sensors are missing, the double difference method is capable of a 1.43 mm total RMS mislocation compared to 4.58 mm for the Geiger method. The accuracy of automatic picking algorithms used for AE studies is ±0.5 µs (1 time sample when the sampling rate is 0.2 µs). We investigate how AE locations are effected by the accuracy of first arrival picking by randomly delaying the actual first arrival by up to 5 time samples. We find that even when noise levels are set to 5 time samples the double difference method successfully relocates the synthetic AE.
KeywordsAcoustic emission earthquake relocation seismic anisotropy
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