The Peak Frequency of Direct Waves for Microseismic Events

The resolution of both the location and the source mechanism is a function of the peak frequency of the observed signal. The peak frequencies control the resolution of the microseismic event locations in surface monitoring. In downhole monitoring, the peak frequency determines the picking error which also controls the resolution of the microseismic event locations. Therefore, the peak frequency of the direct body-waves provides for better design of microseismic monitoring networks. In this study we show that the peak frequency can be determined for geophone recordings (i.e., particle velocity) as from global absorption factor only and this calculation very well explains observed data. Furthermore we show that, except for unusually large events, the peak in amplitude spectra is below the corner frequency for most of the typical microseismic monitoring techniques. We also show that the spectral peak is determined by attenuation of the media and is consistent with observations. We also show that the corner frequencies approach 10 Hz for earthquakes with a moment magnitude between 1 and 2, indicating that 10 Hz geophones (velocity transducers) may not be well suited for these and earthquakes of magnitude 2 and greater.