Know Your Moment (Magnitude)
Increased public attention to hydraulic fracturing and waste water disposal has resulted in the government seeking methods to mitigate the impact of hydraulic fracturing on private citizens. In Western Canada, Moment Magnitude (Mw) has become the metric by which governments representatives may step in and issue a temporary or total work stoppage. Because of potential for expensive disruption, it is worth being absolutely sure that the values provided by the monitoring vendor are legitimate. Here I will summarize some of the possible sources of error based on my experiences as a microseismic processor.
Major Sources of Error:
Bandpass filters are often one of the first steps a geophysicist uses when processing microseismic data. If the Mw is calculated on the signal after an aggressive bandpass filter is applied the resulting Mw value can be altered by more than an order of magnitude of difference.
Sensor parameters configuration of the receiver array can also impact the calculations to varying degrees. I have observed several mistakes including 1) not knowing about the frequency dependent descaling factor of Sercel geophones 2) and improper geophone channel orientation assignment and 3) improper assignment of the geophone cartridges within the similar geophone housings (e.g. improperly applying SMC-1850 and Omni-2400 parameters). Depending on the severity of the incorrect setting, this is generally less significant than the filtering, but can be up to 0.5 Mw in error (but usually less)
Contributing instruments and signal type can also account for error. The use of both P-waves and S-wave in the source parameter calculations will often result in a different value than one which uses P-wave only or where there is little S-wave signal. Perforation shots and stringshots will often provide lower than expected Mw due to the lack of an S-wave component when compared to the events despite the apparent higher SNR observed in the waveforms moveouts.
Conclusions
There is good news. An experienced processor will minimize the impact of the bandpass filter, apply the correct receiver settings.
It is worth noting that microseismic data recorded from two different arrays with two different geophone types and/or with two different software packages can result in similar calculated Mw values (+/- 0.1) in my experience,
Surface and Downhole Arrays Which is better a quantifying Mw?
Surface deployed arrays are calibrated based on the explosive source of a known value (primacord stringshot). Downhole systems by comparison are typically not calibrated on a known source value but based on the recorded attributes. This is often because the stringshot and/or perforation shot is a P-wave source and little shear is often observed (the opposite of a microseism)
Experienced geophysicists operating independently may often obtain a similar value (+/- 0.2 Mw) despite the differences in recording and processing methods (e.g. surface and downnhole) and uncertainties associated.