Comparison of 4.5 Hz Geophones and a Broadband Seismometer in a Real Field Deployment

An analysis of waveforms, power spectral density and array responses was performed using geophones and broadband seismometers, co-deployed as part of a geologically motivated study. Broadband seismometers record excellent waveforms but, due to cost and deployment effort, wavefields are usually spatially aliased above ~0.1 Hz. Industry rapidly deploys many thousands of inexpensive, passive geophones to record full, unaliased seismic wavefields; however, waveform quality is limited below the instrument's natural frequency of ≥2 Hz. In 2012, coincident passive and controlled-source seismic surveys were deployed to investigate tectonics in Idaho and Oregon. Broadband stations were deployed at quiet sites every 15 km, taking experienced professionals >1 person-days per station. Fifty 4.5 Hz geophones and "Texan" seismographs at 200-m spacing were deployed per person-day by inexperienced students. Geophone data were continuously recorded for 3 nights and 1 day, while broadband seismometers were deployed for ~2 years. The spectral and array responses of these real deployments were compared. For a M7.7 teleseismic event, the broadband seismometer and geophone recorded nearly identical waveforms down to <0.03 Hz (32 s) and matching power spectral density down to 0.02 Hz (50 s). For quiet ambient noise, the waveforms strongly correlate down to <0.25 Hz (4 s) and the power spectral density match to the low-frequency side of the microseismic peak at ~0.15 Hz (~7 s). By deploying a much larger number of geophones, waveforms can be stacked to reduce instrument self-noise and beamforming can be used to identify wavefield azimuth and apparent velocity. Geophones can be an effective tool in ambient noise seismology down to ~7 seconds and can be used to record large seismic events effectively down to tens of seconds, well below the natural frequency of the instruments. A well-designed deployment of broadbands and geophones can enable full wavefield studies from long period to short period. Scientific and societal applications that could benefit from the improved unaliased wavefield bandwidth include local to regional seismicity, strong ground motion, magma migration, nuclear source discrimination, and crustal studies.