Speaker: Nate Lindsey; UC Berkeley
Our understanding of Earth suffers from profound observation bias – measurement points are sparse and clustered on continents. Distributed fiber-optic acoustic sensing (DAS) is an emerging photonic tool that transforms a telecommunications fiber-optic cable into a massive, dense ground motion array with 1 sensor per meter over kilometers of fiber length. This is a new opportunity to study earth systems with continuous, long-range, regularly-sampled seismic wavefield records in large volumes (~ 1 TB / day). To-date, most DAS applications have studied signal frequencies above 10 Hz (e.g., active-source reservoir imaging, Vs30, regional to local earthquake detection and location). Here I ask, through three field experiments: How broadband is DAS?
In Fairbanks, AK, we record local seismic noise generated by cars and trucks, and apply ambient noise seismology to measure daily shear wave speed changes caused by permafrost warming beneath the road. The seismic signature of soil softening is tracked through time and space and compared with the coevolution of temperature measurements from thermistor arrays and hazardous ground subsidence observed with campaign leveling, photogrammetry and LiDAR.
In the Central Valley of California, we use teleseismic wavefields recorded by DAS and a collocated broadband seismometer to reverse engineer the DAS instrument response, which is presently unknown. The DAS is at least as broadband as a Guralp CMG-3T seismometer over the range f=0.01 – 0.5 Hz, but appears to have a strong fiber-ground coupling dependence at f>0.5 Hz.
Underneath Monterey Bay, we repurpose a submarine-cabled-observatory communication fiber as a 20-km-long, 10,000-component DAS array. We use knowledge of its broadband sensitivity to make full wavefield observations of ocean waves, ocean sea state, as well as seismic waves from a local M3.4 earthquake.
These three examples demonstrate the value of using fiber-optic seismology with existing fiber networks to interrogate a range of earth science processes, specifically in cryosphere and marine environments where geophysics has proven logistically challenging.