The initial appraisal of buried DAS system in CO2CRC Otway Project: the comparison of buried standard fibre-optic and helically wound cables using 2D imaging

Abstract

This study aims to assess the ability of shallow Distributed Acoustic Sensing (DAS) to serve as a cost-effective seismic sensor array for permanent monitoring applications. To this end, as part of the CO2CRC seismic monitoring program, a fibre-optic DAS array was deployed alongside a permanently buried geophone array at the Otway Project site (Victoria, Australia).The DAS array consisted of a standard commercially available tactical fibre-optic cable, which was deployed in 0.8 m deep trenches. A custom designed helically wound (HW) cable was also deployed into one of the DAS trenches for the comparison of the cable designs. The simultaneous acquisition of the seismic data was carried out using ~3000 vibroseis source points and geophones, DAS standard and the HW cables. In order to have the initial assessment of the seismic images acquired with DAS and to compare different cable designs, preliminary 2D seismic reflection processing is conducted on both DAS cables and geophone data along a single 2D line. The geophone data processing guided the data processing of DAS data. Several shallow structures (100 -450 ms) and some important reflectors at 450 -600 ms are observed on the final DAS images. The comparison of two different DAS cable types demonstrated that seismic imaging would benefit DAS technology. However, the benefit of utilising HW cable is modest compared to the standard cable. The workflows and the results of this study paved the way for the processing of the 3D seismic dataset acquired with the DAS array, as well as the further detailed analysis of the DAS cables and the system itself.

Modeling Subsurface Explosions Recorded on a Distributed Fiber Optic Sensor

“…The pronounced directional response of a linear fiber can be alleviated by winding the fiber in a helix around the cable. This increases sensitivity to P seismic waves that are not traveling parallel to the fiber (Kuvshinov, 2016) and field tests confirm this capability (Hornman, 2017;Yavuz et al, 2019). However, a helical winding reduces the sensitivity to S waves (Baird, 2020).…”

Using the Shallow Strain Tensor to Characterize Deep Geologic Reservoirs

“…The approach we used is intended to measure the strain tensor at several locations, instead of measuring strain components distributed at many locations, which is the approach used by other optical fiber strain sensors. This makes distributed strain sensing well suited to measure basic aspects of a transient strain field distributed in space, and applications of distributed strain sensing have capitalized on this aspect by measuring strain distributed at hundreds or more locations along optical fiber in boreholes (Becker et al., 2017; Xue & Hashimoto, 2017), or in networks of trenches (Yavuz et al., 2019). Another limitation to using low frequency to quasi‐static strain tensor data is the need to remove strains caused by earth tides, barometric pressure fluctuations, rainfall, and other processes unrelated to changes in reservoir pressure.…”

Fiber-Optic Seismology

“…Data from fiber in tubing in fluid-filled boreholes show strong tube wave reverberations. Fiber deployed on casing with a mix of cemented and uncemented zones at the Ketzin, Germany, CO 2 site yielded poorer data quality in uncemented sections than in cemented sections (Daley et al 2013(Daley et al , 2016Egorov et al 2017;Correa et al 2019;Yavuz et al 2019).…”

“…Abbreviations: DAS, distributed acoustic sensing; NRMS, normalized root mean square; OBC3D, 3D Imaging with Ocean Bottom Cable instruments; RMS, root mean square; VSP, vertical seismic profile. (Daley et al 2013, Egorov et al 2017, Correa et al 2019, Yavuz et al 2019). Also at the Otway site, researchers showed that time-lapse full-waveform inversion (FWI) imaging could be used to target a layer just 20 m thick where injection had the greatest effect (Egorov et al 2017).…”