Three Dimensional Attenuation Tomography （三维衰减层析成像）.pdf

Three Dimensional Attenuation Tomography from microseismicity Frank J. Calixto, University of Alberta calixtom@ualberta.ca Mirko van der Baan, University of Alberta Summary Attenuation images are obtained from 6287 t-star spectral measurements of P-wave microseismic events in a mine setting. The resulting three-dimensional quality factor (Q) model reveals that most of the study region is characterized by low Q values or high attenuation (Q25), probably caused by cavities and fractures due to mining activities. Q values between 25 and 50 show a high spatial correlation with the location of an ore deposit. Furthermore, high quality ore deposits correlate with Q values of about 45-50 whereas the low quality ore correlates with Q values of around 30. Q values of less than 10 correlate with the observed seismic cloud epicenters. These features were previously partially resolved by velocity tomography. Therefore a joint interpretation is very important in determining the lithological properties and physical state of both, the region containing the seismic epicenters and the region with the ore deposits. The same methodology is applicable to microseismic data recorded during hydraulic fracturing treatment and may give more insight into fracture density of hydrocarbon reservoirs. Introduction The seismic quality factor (or inverse of attenuation) can provide valuable information on the lithology, fracture orientation and fluid presence of a region. The usefulness of attenuation tomography is sometimes limited by the azimuthal and depth coverage of the seismic stations. However, in the present study we have the advantage to work with microseismic events recorded by a total of 28 geophones deployed in seven boreholes. This array gives us a remarkable verti