Long Beach experimental setup and preprocessing

In August 2000, Bryan Kerr conducted an experiment Kerr and Rickett (2001) with 60 single channel geophones laid out in an $10 \times 6$ array with 5 m receiver spacing on some unused land in Long Beach, CA. The immediate environs of the experiment could be classified as mixed light industrial/low-density residential, and possible sources of ambient seismic noise included a pair of freeways about 500 m away, some overhead power cables running above the experiment, and a drill truck about 50 m away. Kerr collected 1020 (10 second) records of passive data totaling 170 minutes.

As a preprocessing step, I notch-filtered the data to remove monochromatic spikes (e.g. 60 Hz noise). Although I later low-pass filtered the data to 30 Hz, high amplitude spikes need to be removed before crosscorrelation/factorization to avoid numerical underflow. Figure  shows 5 seconds of passive data after notch-filtering. Two types of event are clearly visible in Figure : slow linear events that appear to be traveling in the direction of decreasing channel number, and hyperbolic events with a higher apparent velocity. I speculate that these events were generated by the drill truck and power lines, respectively.

Following the processing steps applied to the helioseismic data, I crosscorrelated (or spectrally factorized) the data, stacked over the record axis, and then stacked over radius (distance from zero spatial lag). I then applied a (30 Hz) low-pass filter to the data to remove aliased ground-roll energy, and finally applied an offset dependent gain correction xpow=1 for display purposes.

 

strip Figure 7 Five seconds of passive seismic data from the Long Beach experiment after notch-filtering.