Fracture hydromechanical response measured by fiber optic distributed acoustic sensing at milliHertz frequencies

Abstract

A new method of measuring dynamic strain in boreholes was used to record fracture displacement in response to head oscillation. Fiber optic distributed acoustic sensing (DAS) was used to measure strain at mHz frequencies, rather than the Hz to kHz frequencies typical for seismic and acoustic monitoring. Fiber optic cable was mechanically coupled to the wall of a borehole drilled into fractured crystalline bedrock. Oscillating hydraulic signals were applied at a companion borehole 30 m away. The DAS instrument measured fracture displacement at frequencies of less than 1 mHz and amplitudes of less than 1 nm, in response to fluid pressure changes of less 20 Pa (2 mm H2O). Displacement was linearly related to the log of effective stress, a relationship typically explained by the effect of self‐affine fracture roughness on fracture closure. These results imply that fracture roughness affects closure even when displacement is a million times smaller than the fracture aperture.

On the Broadband Instrument Response of Fiber‐Optic DAS Arrays

“…According to our results, the broadband DAS response is validated against a high‐quality broadband seismometer over the range from 1–120 s. In terms of frequency range, DAS is found to be as broadband as the broadband seismometer used for calibration. Usable teleseismic energy falls off at periods longer than 120 s, but the DAS continues to show energy in time‐frequency analysis down to 300 s. This is more than twice as long as the longest period DAS signals previously documented by Becker et al () in a hydrogeological pump test and in earthquake studies on dark fiber DAS arrays (Ajo‐Franklin et al, ; Yu et al, ). One reason for this may be the DAS data processing approach employed here in which we stack over a window of 50 m, all falling inside of the seismic wave's long coherence length, mitigating uncorrelated channel noise and improving recovery of low frequency signals.…”

“…One demonstrated fading mitigation strategy employs multiple laser frequencies (Zhou et al, ). Reduced amplitude channel noise can also result from suboptimal fiber‐ground coupling (Ajo‐Franklin et al, ; Becker et al, ; Reinsch et al, ; Willis et al, ). Note that photonic fading noise and sensor coupling noise are difficult to distinguish except at the scale of the array, where sensor coupling is usually identifiable by its systematic pattern or from field installation information.…”

“…Elevated DAS amplitude response at short periods could be caused by the style and degree of rigid coupling through which strain is transferred from the ground to the fiber. Coupling is a necessary condition for any seismometer (Lay & Wallace, 1995), yet many DAS studies have concluded that mechanical coupling of the fiber-optic to the ground had an impact on their results (Becker et al, 2017;Jousset et al, 2018;Lindsey et al, 2017;Wang et al, 2018;Willis et al, 2017;Yu et al, 2019). Coupling is a static feature of a DAS experiment (fiber array; DAS recording parameters) that should be assumed to not change over the duration of a typical geophysical campaign (days to months).…”

“…The broadband nature of DAS has largely been overshadowed by the use of DAS in active‐source experiments at high frequencies ( Hz; s). The long‐period frequency response of the DAS method has been demonstrated in recordings of teleseismic earthquakes (Ajo‐Franklin et al, ; Jousset et al, ) and laboratory and field pump tests (Becker et al, ; Becker & Coleman, ). To date, the longest period earthquake signal observed with DAS used a dark fiber DAS array in the Mojave Desert to capture propagating surface waves down to s (Yu et al, ).…”

The Potential of DAS in Teleseismic Studies: Insights From the Goldstone Experiment

“…The great consistency of earthquake waveforms recorded by DAS and by conventional seismometers (e.g., Ajo-Franklin et al, 2019;Lindsey et al, 2017;Wang et al, 2018), the observations of broadband sensitivity on DAS (Ajo-Franklin et al, 2019;Becker et al, 2017), and the availability of existing telecommunication infrastructures (e.g., Ajo-Franklin et al, 2019;Jousset et al, 2018;Lindsey et al, 2017) motivate us to provide more in-depth analyses of DAS earthquake waveforms. Here we explore the potential applications of DAS to teleseismic studies using the Goldstone OpticaL Fiber Seismic (GOLFS) experiment in Goldstone, California.…”