The Peak Frequency of Direct Waves for Microseismic Events

Direct seismic waves (P- or S-waves) are used to locate and further characterize microseismic events. The resolution of information obtained from direct waves depends on the peak frequencies of the waveforms. The peak frequency re- sults from combination of the source, propagation, and the receiver effects. For frequencies below the corner frequency, propagation effects control the peak frequency in observed seismograms of microseismic events. The frequency depen- dence of direct body waves can be modeled by attenuation, specifically the global attenuation factor. This model is con- sistent with observed data along surface profiles explaining the difference between the peak frequencies of P- and S-waves. In addition, the model is consistent with the peak frequencies observed on downhole monitoring arrays. This can be used to invert effective attenuation providing addi- tional unique measurement from microseismic events. The corner frequency can be estimated from the average stress drop and analytical source models such as a circular crack model. Typical stress drops for various magnitude ranges are discussed. The peak frequencies are usually be- low the corner frequencies of microseismic events smaller than moment magnitude 0.7 for surface monitoring and mo- ment magnitude −0.5 for downhole monitoring. Under- standing of the frequency dependence of the direct waves allows us to optimally design monitoring networks and mainly invert effective attenuation providing unique meas- urement from microseismic monitoring.