Eileen R. Martin

Distributed Acoustic Sensing for Seismic Monitoring of The Near Surface: A Traffic-Noise Interferometry Case Study

Ambient-noise-based seismic monitoring of the near surface often has limited spatiotemporal resolutions because dense seismic arrays are rarely sufficiently affordable for such applications. In recent years, however, distributed acoustic sensing (DAS) techniques have emerged to transform telecommunication fiber-optic cables into dense seismic arrays that are cost effective. With DAS enabling both high sensor counts (“large N”) and long-term operations (“large T”), time-lapse imaging of shear-wave velocity (VS) structures is now possible by combining ambient noise interferometry and multichannel analysis of surface waves (MASW). Here we report the first end-to-end study of time-lapse VS imaging that uses traffic noise continuously recorded on linear DAS arrays over a three-week period. Our results illustrate that for the top 20 meters the VS models that is well constrained by the data, we obtain time-lapse repeatability of about 2% in the model domain—a threshold that is low enough for observing subtle near-surface changes such as water content variations and permafrost alteration. This study demonstrates the efficacy of near-surface seismic monitoring using DAS-recorded ambient noise.

Continuous Subsurface Monitoring by Passive Seismic with Distributed Acoustic Sensors - The “Stanford Array” Experiment

Continuous seismic monitoring can be a crucial tool to optimize hydrocarbon production as well as to provide early warning of potentially hazardous conditions developing in the subsurface. However, the cost of continuous monitoring is a significant obstacle to its widespread application. Distributed Acoustic Sensors (DAS) recording systems hold the promise to enable the recording of seismic data at much lower cost. We deployed a 2.45 km long DAS array with 610 virtual receivers under Stanford University campus and started to record passive seismic data continuously in September 2016. Preliminary analysis show that the data recorded by our DAS array can be used to monitor dynamic processes in the subsurface thanks to sufficient sensitivity to low-amplitude wavefields in the frequency band between .5 and 10 Hz. Our conclusion is supported by the coherency and the frequency content of recorded events corresponding to teleseismic and regional earthquakes as well as of the virtual sources synthesized by using interferometry.

Fiber-Optic Network Observations of Earthquake Wavefields: FIBER-OPTIC EARTHQUAKE OBSERVATIONS

Author(s): Lindsey, NJ; Martin, ER; Dreger, DS; Freifeld, B; Cole, S; James, SR; Biondi, BL; Ajo-Franklin, JB | Abstract: ©2017. American Geophysical Union. All Rights Reserved. Our understanding of subsurface processes suffers from a profound observation bias: seismometers are sparse and clustered on continents. A new seismic recording approach, distributed acoustic sensing (DAS), transforms telecommunication fiber-optic cables into sensor arrays enabling meter-scale recording over tens of kilometers of linear fiber length. We analyze cataloged earthquake observations from three DAS arrays with different horizontal geometries to demonstrate some possibilities using this technology. In Fairbanks, Alaska, we find that stacking ground motion records along 20 m of fiber yield a waveform that shows a high degree of correlation in amplitude and phase with a colocated inertial seismometer record at 0.8–1.6 Hz. Using an L-shaped DAS array in Northern California, we record the nearly vertically incident arrival of an earthquake from The Geysers Geothermal Field and estimate its backazimuth and slowness via beamforming for different phases of the seismic wavefield. Lastly, we install a fiber in existing telecommunications conduits below Stanford University and show that little cable-to-soil coupling is required for teleseismic P and S phase arrival detection.

Ambient Noise Interferometry from DAS Array in Underground Telecommunications Conduits

Summary Ambient noise interferometry allows us to characterize the near surface without the cost and regulatory constraints of controlled seismic sources. Geophone arrays can be expensive to install and maintain, but trenched distributed acoustic sensing (DAS) arrays have shown promise for reducing the maintenance cost of ambient noise acquisition. In this paper we test a novel, easily installed acquisition system using fibre in existing underground telecommunications conduits for ambient noise collection on the Stanford University campus. This installation only uses gravity to hold the fibre in place in the air-filled conduits, and the conduit installation method varies some throughout the site. We outline the basic theory of ambient noise interferometry for DAS measurements, present a method of using nearby earthquake response as a sensor coupling proxy, and present signal processing methods for coping with a variety of noise sources throughout the array. We show cross-correlations and dispersion calculations along a subset of the array expected to primarily yield Rayleigh waves.