Seismic attenuation significantly affects the results of downhole moment tensor inversion (MTI). Though there are many other factors that also have an impact on MTI. This study focuses on the effect attributed to seismic attenuation. Synthetic microseismic data were used to do MTI with and without seismic attenuation correction by fixing all other factors and inversion parameters. The result comparisons show that seismic attenuation significantly affects the inverted focal mechanisms and the relative percentage of volumetric (ISO), compensated linear vector dipole (CLVD), and double-couple (DC) components. The impacts on strike and dip are minimal. However, the alpha angles are significantly altered by seismic attenuation correction. The effect on the inverted seismic magnitude is also demonstrated resulting in a smaller magnitude calculation by MTI without attenuation correction than one with the correction. These observations imply that the seismic attenuation has to be incorporated in MTI calculation in order to correctly interpret the rock-failure mechanism of microseismic events.
The industry has mapped thousands of fracture treatments using microseismic mapping technology, providing information on fracture azimuth, length, height growth, complexity, diversion, zonal coverage, stimulated reservoir volume, and many more aspects of the stimulation (Warpinski 2009). All of these results are estimated by processing microseismic waveforms. A typical downhole microseismic waveform recorded by highly sensitive three-component geophones in two offset wells is illustrated in Figure 1. There are at least three seismic attributes (arrival time, seismic amplitude, and polarity) used in MTI to interpret the rock-failure mechanism of microseismic events. The arrival time of P- and S-waves are used to locate the microseismic events in 3D (Maxwell 2014). Seismic amplitude is the maximum absolute value of each seismic trace for one event. And, the polarity is the sign (positive or negative) of the amplitude. MTI uses the microseismic events location, the amplitude, and the polarity as inputs to study the geomechanical details of a reservoir rock failure occurring during the hydraulic stimulation (Baig and Urbancic 2010).
