Pre-processing

Raw electroseismic data is dominated by energy from the power grid at harmonics of 60 Hz (Figure a). We remove this noise using the sinusoid subtraction technique of () for all harmonics of 60 Hz up to the Nyquist frequency. Coseismic energy is generally lower in frequency than the interface response due to a greater distance of travel as a seismic wave, so we use a low-cut filter to begin the process of noise removal. We also employ a high-cut filter to minimize background noise that can obscure weaker arrivals (Figure b).

 

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Figure 5 (a) Raw data. (b) Same file, with 60 Hz energy removed and bandpass filter (120-500 Hz) applied. Note horizontal interface response event at 0.015 seconds.