Preprints
https://doi.org/10.5194/se-2021-37
https://doi.org/10.5194/se-2021-37

  15 Apr 2021

15 Apr 2021

Review status: this preprint is currently under review for the journal SE.

Distributed acoustic sensing as a tool for exploration and monitoring: a proof-of-concept

Nicola Piana Agostinetti1,2, Alberto Villa1, and Gilberto Saccorotti3 Nicola Piana Agostinetti et al.
  • 1DISAT, Universitá di Milano Bicocca, Piazza della Scienza 4, 20126, Milano, Italy
  • 2Department of Geology, University of Vienna, Vienna, Austria
  • 3Istituto Nazionale di Geofisica e Vulcanologia. Via Cesare Battisti 53, 56125 Pisa, Italy

Abstract. We use PoroTOMO experimental data to compare the performance of Distributed Acoustic Sensing (DAS) and geophone data in executing standard exploration and monitoring activities. The PoroTOMO experiment consists of two "seismic systems": (a) a 8.6 km long optical fibre cable deployed across the Brady geothermal field and covering an area of 1.5 x 0.5 km with 100 m long segments, and (b) an array of 238 co-located geophones with an average spacing of 60 m. The PoroTOMO experiment recorded continuous seismic data between March 10th and March 25th 2016. During such period, a ML 4.3 regional event occurred in the southwest, about 150 km away from the geothermal field, together with several microseismic local events related to the geothermal activity. The seismic waves generated from such seismic events have been used as input data in this study. For the exploration tasks, we compare the propagation of the ML 4.3 event across the geothermal field in both seismic systems in term of relative time-delay, for a number of configurations and segments. Defined the propagation, we analyse and compare the amplitude and the signal-to-noise ratio (SNR) of the P-wave in the two systems at high resolution. For testing the potential in monitoring local seismicity, we first perform an analysis of the geophone data for locating a microseismic event, based on expert opinion. Then, we a adopt different workflow for the automatic location of the same microseismic event using DAS data. For testing the potential in monitoring distant event, data from the regional earthquake are used for retrieving both the propagation direction and apparent velocity of the wavefield, using a standard plane-wave-fitting approach.

Our results indicate that: (1) at a local scale, the seismic P-waves propagation and their characteristics (i.e. SNR and amplitude) along a single cable segment are robustly consistent with recordings from co-located geophones (delay-times δt ∼ 0.3 over 400 m for both seismic systems) ; (2) the interpretation of seismic wave propagation across multiple separated segments is less clear, due to the heavy contamination of scattering sources and local velocity heterogeneities; nonetheless, results from the plane-wave fitting still indicate the possibility for a consistent detection and location of the event; (3) at high-resolution (10 m), large amplitude variations along the fibre cable seem to robustly correlate with near surface geology; (4) automatic monitoring of microseismicity can be performed with DAS recordings with results comparable to manual analysis of geophone recordings (i.e. maximum horizontal error on event location around 70 m for both geophones and DAS data) ; and (5) DAS data pre-conditioning (e.g., temporal sub-sampling and channel-stacking) and dedicated processing techniques are strictly necessary for making any real-time monitoring procedure feasible and trustable.

Nicola Piana Agostinetti et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on se-2021-37', Martijn van den Ende, 01 May 2021
  • CC1: 'Comment on se-2021-37', Herbert Wang, 07 May 2021
    • RC2: 'CC1 again as RC', Herbert Wang, 11 May 2021
  • RC3: 'Comment on se-2021-37', Anonymous Referee #3, 19 Jul 2021

Nicola Piana Agostinetti et al.

Nicola Piana Agostinetti et al.

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Short summary
Sensing the Earth is a fundamental operation for the future where geo-resources, like geothermal energy and CO2 underground storage, will become important tools for addressing societal challenges. The development of networks of optical fibre cables give possibility of a sensing grid with an unprecedented spatial coverage. Here we investigate the potential of using portions of a optical fibre cable as a a standard seismometer for exploring the subsurface and monitoring the geo-resources.