Low-Noise Optical Accelerometers: Bridging the Gaps among Geophones, Accelerometers, and Broadbands in a Deep Borehole

Abstract

Earthquakes and other seismic sources produce waves with frequency content spanning many orders of magnitude. Recording a broader frequency band of interest has historically required deploying multiple instruments designed to work the best within limited, overlapping frequency ranges. Here, we detail a 300 m deep borehole deployment of a sensor package, including three new optical accelerometers that can potentially replace many dedicated instruments with a single, low-noise sensor. These instruments are designed with a flat frequency response from 0.005 to 1500 Hz, spanning the flat response segments of broadband sensors and geophones, as well as a low-noise floor and high sensitivity. The sensors have been functioning normally for over four years, fully grouted at depths of >100 m. Year-long power spectral density (PSD) profiles show that the optical accelerometers have a lower noise floor than a colocated geophone for all frequencies, with 20 dB noise reduction at 250 Hz. PSD comparisons to a broadband sensor installed at the surface show a 5–30 dB noise reduction for the optical accelerometer above 1 Hz, although this is likely due, in part, to the broadband sensor being subjected to much higher surface noise. At periods >5 s, the broadband sensor shows up to 20 dB lower noise than the optical accelerometer, which in turn has up to 50 dB lower noise floor than the colocated geophone. Finally, modeling the Brune displacement spectrum for theoretical seismicity within 1 km of the borehole shows that the optical accelerometers could potentially deliver a detection threshold improvement of one magnitude unit relative to the colocated geophone.

Optical Interferometric MEMS Accelerometers

“…They deployed it for seismic testing in a downhole installation at the North Saanich Marine Technology Center on Vancouver Island in Sydney, British Columbia, Canada, where data demonstrated that theoretical microvibration signals could be measured within a distance of 1 km (Figure 36a). [135] Francesco Bruno et al have captured the ring-breaking earthquake that struck Norcia, Italy, on October 30, 2016, with an optical accelerometer, despite the optical accelerometer being situated a few kilometers from the epicenter (Figure 36a). [136] Unlike terrestrial earthquakes, undersea earthquakes may incur tsunamis and sea whirlpools, causing the requirements of other parameters measurement in the ocean flow direction, ocean velocity, and sea temperature to ensure the safe passage of sea routes.…”

“…Seismometers based on optical interferometric accelerometers are constantly employed in oil and gas exploration. The seismic exploration gets the artificial seismic waves containing media layer information by applying seismometers and matching [135] Copyright 2022, Seismological Society of America. b) Optical Interferometric Accelerometers Monitored the October 30, 2016, Earthquake in Norcia, Italy.…”

“…They deployed it for seismic testing in a downhole installation at the North Saanich Marine Technology Center on Vancouver Island in Sydney, British Columbia, Canada, where data demonstrated that theoretical microvibration signals could be measured within a distance of 1 km ( Figure a). [ 135 ] Francesco Bruno et al. have captured the ring‐breaking earthquake that struck Norcia, Italy, on October 30, 2016, with an optical accelerometer, despite the optical accelerometer being situated a few kilometers from the epicenter (Figure 36a).…”

“…Reproduced with permission. [ 135 ] Copyright 2022, Seismological Society of America. b) Optical Interferometric Accelerometers Monitored the October 30, 2016, Earthquake in Norcia, Italy.…”

“…Figure 36. a) Installation of optical accelerometers in deep boreholes in Sydney, British Columbia, Canada. Reproduced with permission [135]. Copyright 2022, Seismological Society of America.…”