How Broadband is DAS? Two Empirical Evaluations of Instrument Response

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

Distributed Acoustic Sensing (DAS) is a novel form of array seismology that has been shown to resolve decameter-scale ground motions over tens of kilometers. DAS measures the optical phase change of laser pulses traveling inside of a fiber-optic cable, which can be related to the strain acting along a portion of the fiber. The combination of DAS and excess telecommunications infrastructure – so-called “dark fiber seismology” – can be used to address a diversity of earth science questions where access, security, field logistics, and cost have historically hindered seismic observation. However, DAS instrument response has not been disclosed with the commercial instruments and thus is poorly understood. Here we probe the instrument response of one instrument, the Silixa iDAS, in the field and lab. In the field experiment, an iDAS was connected to one single-mode fiber inside of an unused telecommunications fiber-optic cable. This experiment measured horizontal strains over 20-km at a 2-m channel spacing, and the volumetric flow rate of data was ~8 TB/week. A Guralp CMG-3T broadband inertial seismometer with a well-characterized instrument response function was installed near a portion of the fiber, presenting an opportunity to compare DAS ground motion estimates with a classic inertial seismometer. Using data from five different M>7 teleseismic earthquakes, we investigated instrument response in the 0.01 – 0.5 Hz frequency range. In a second experiment, we used a mechanical fiber cable stretcher in the laboratory to simulate strain with 0.01 - 0.1 Hz signal frequency, and measured this strain with the iDAS. We explain results in terms of the photonic measurement, azimuthal sensitivity to particle motion, and fiber-soil coupling.

Presenting Author: Nathaniel J. Lindsey

Authors