Source Physics Experiment Data Quality Using Background Noise
We computed time-domain root-mean-square (TRMS) amplitudes and power spectral densities (PSD) from the Source Physics Experiment (SPE) seismic data using the 2018 version 1.4 of the instrument response metadata. The geophone instrument gains were set lower about a few weeks prior to the SPE shots to record on-scale and then reset soon afterward to record background noise and seismicity. When the gains were set lower, we determined that the 1-2 Hz bandpass filtered geophone amplitudes increased 20 decibels (dB) or factor of 10 relative to the geophone baseline or background noise levels from nearby broadband sensors. This indicates an inconsistency between the released metadata reporting a decrease in gain factor of 32 and the likely true geophone gain setting of 3.2. After the shots, the geophone gains were reset and recorded background noise levels similar to nearby broadbands consistent with the metadata for those time intervals. There was also a similar issue with line-3, line-4 and AF Trillium broadband horizontals gains during SPE-5 where there is a factor of 32 difference relative to baseline levels. The other Guralp and Trillium broadbands performed very well recording a 1-2 Hz noise level of -150 to -160 dB similar to the nearby station at Tonopah Springs, Nevada (US.TPNV) and compared with -170 dB from the new low noise model (NLNM) for the continental US (e.g., McNamara and Buland, 2004). The geophones were only 5-10 dB higher but performed as well as broadbands above 6 Hz. The accelerometers have poor low frequency response and were 40-50 dB higher in the 1-2 Hz band relative to broadbands.