Moment tensor inversion of microseisms from the B-sand propped hydrofracture, M-site, Colorado

We apply a moment-tensor inversion algorithm to microseismic events generated by hydrofracturing. Our dataset consists of inicroseismic events associated with a hydraulic fracturing experiment conducted at the GRYDOE M-site near Rifle, Colorado. The event recordings were from a vertical string of 3-component accelerometer pods that are cemented in a borehole. The accelerograms clearly show P and S waves, which can be rotated into the SH and SV components. To invert the P and S amplitudes for moment tensors, we use just the first two peaks after the first arrival. Using this amplitude information and the average pulse width, we formed a symmetric, stylized pulse for integration and we inverted for the moment tensor. Resolution tests revealed that with the source (microseism) to receiver geometry we have at the M-site, where all receivers are in one borehole and at the same azimuth from the source, we can extract the five deviatoric components of the moment tensor using combinations of P, SH and SV amplitudes (for example, a minimum data set of 3 P-wave, 2 SV-wave, and I SH-wave amplitudes). To obtain the sixth component, which is the isotropic component, requires amplitude information from receivers at another azimuth. We studied seven high-quality events in detail and tested their results against two alternative source geometries. These microseismic events tend to have a major-double-couple nodal plane aligned parallel to the main hydrofracture plane, rather than have their tension (T) axes aligned parallel to the regional minimum principal-compressive-stress direction. The events have seismic moments (M-0) on the order of 10(-5) Nm and moment magnitudes (M-w) of about -2.5. (C) 2001 Elsevier Science B.V. All rights reserved.