Influence of a velocity model and source frequency on microseismic waveforms: some implications for microseismic locations

In this paper, we examine the influence of a velocity model and microseismic source frequency on microseismic waveforms and event locations. Finite-difference waveform synthetics are generated based on the Cotton Valley hydraulic fracture experiment, where we vary the vertical heterogeneity of the velocity models as well as the microseismic source frequencies. We find that differences between plausible velocity models lead to changes in arrival times of approximately 0.0035 seconds for P-waves and 0.0085 seconds for S-waves. Based on the average P- and S-wave velocities, the difference in the P- and S-wave traveltimes is equivalent to approximately 20 m in location difference. Significant increases in the waveform coda develop with increasing model heterogeneity and increasing source frequency. The presence of signal noise as well as other sources of error (e.g., uncertainty in geophone location) will likely lead to further increase in uncertainty in location error estimates. Thus we note that location error due to incorrect velocity models cannot be ignored.