Surface microseismic imaging in the presence of high-velocity lithologic layers

Abstract

Surface microseismic monitoring above regions containing high-velocity units will be influenced by a reduction in the effective array aperture and contain significant seismic coda due to an increase in reflected and converted wave energy, which can influence the capability of surface arrays to image microseismic events. We have examined the imaging difficulties due to the reduction of the effective array aperture and increased coda within microseismic data caused by the presence of a fast anhydrite layer. We have generated and used finite-difference, full-waveform microseismic synthetics derived from 1D sonic log data taken from an unconventional shale reservoir in complex anhydrite geology. Two imaging techniques are used to locate the synthetic microseismic events. We use a simplistic and easily reproducible standard diffraction imaging procedure and a more sophisticated technique, Moment Tensor Microseismic Imaging™ (MTMI™). We have confirmed that the presence of high-velocity layers, such as anhydrite, reduce the overall aperture of a surface array, which result in poor resolution of imaged events and reduction in the accuracy of event locations. More importantly, we have determined that the coda in the microseismic data can isolate itself from the direct arrival, stacked into the final imaging result, and it can be misinterpreted as separate events located directly below the true source location, which is unavoidable given the ambiguity between event location and time. Comparison of the two imaging procedures found that even though the more sophisticated MTMI was able to better cope with the effects caused by high-velocity layers, it still demonstrated a significant reduction in resolution of imaged events, a reduction in accuracy, and evidence of multiple and converted energy still present in the final imaging result.