Distributed Acoustic Sensing Using Dark Fiber for Array Detection of Regional Earthquakes

2021 ◽  
Author(s):  
Avinash Nayak ◽  
Jonathan Ajo-Franklin ◽  
Keyword(s):  
Urban Areas ◽  
Geothermal Field ◽  
Seismic Events ◽  
Broadband Seismometer ◽  
Acoustic Sensing ◽  
Array Detection ◽  
Distributed Acoustic Sensing ◽  
Back Azimuth ◽  
Ubiquitous Presence

Abstract The intrinsic array nature of distributed acoustic sensing (DAS) makes it suitable for applying beamforming techniques commonly used in traditional seismometer arrays for enhancing weak and coherent seismic phases from distant seismic events. We test the capacity of a dark-fiber DAS array in the Sacramento basin, northern California, to detect small earthquakes at The Geysers geothermal field, at a distance of ∼100  km from the DAS array, using beamforming. We use a slowness range appropriate for ∼0.5–1.0  Hz surface waves that are well recorded by the DAS array. To take advantage of the large aperture, we divide the ∼20  km DAS cable into eight subarrays of aperture ∼1.5–2.0  km each, and apply beamforming independently to each subarray using phase-weighted stacking. The presence of subarrays of different orientations provides some sensitivity to back azimuth. We apply a short-term average/long-term average detector to the beam at each subarray. Simultaneous detections over multiple subarrays, evaluated using a voting scheme, are inferred to be caused by the same earthquake, whereas false detections caused by anthropogenic noise are expected to be localized to one or two subarrays. Analyzing 45 days of continuous DAS data, we were able to detect all earthquakes with M≥2.4, while missing most of the smaller magnitude earthquakes, with no false detections due to seismic noise. In comparison, a single broadband seismometer co-located with the DAS array was unable to detect any earthquake of M<2.4, many of which were detected successfully by the DAS array. The seismometer also experienced a large number of false detections caused by spatially localized noise. We demonstrate that DAS has significant potential for local and regional detection of small seismic events using beamforming. The ubiquitous presence of dark fiber provides opportunities to extend remote earthquake monitoring to sparsely instrumented and urban areas.

Empirical Investigations of the Instrument Response for Distributed Acoustic Sensing (DAS) across 17 Octaves

2020 ◽  
Author(s):  
Patrick Paitz ◽  
Pascal Edme ◽  
Dominik Gräff ◽  
Fabian Walter ◽  
Joseph Doetsch ◽  
...  
Keyword(s):  
Ground Motion ◽  
Urban Areas ◽  
Waveform Inversion ◽  
Acoustic Sensing ◽  
Full Waveform ◽  
Environmental Properties ◽  
Instrument Response ◽  
Distributed Acoustic Sensing ◽  
Reference Measurements ◽  
Seismic Networks

ABSTRACT With the potential of high temporal and spatial sampling and the capability of utilizing existing fiber-optic infrastructure, distributed acoustic sensing (DAS) is in the process of revolutionizing geophysical ground-motion measurements, especially in remote and urban areas, where conventional seismic networks may be difficult to deploy. Yet, for DAS to become an established method, we must ensure that accurate amplitude and phase information can be obtained. Furthermore, as DAS is spreading into many different application domains, we need to understand the extent to which the instrument response depends on the local environmental properties. Based on recent DAS response research, we present a general workflow to empirically quantify the quality of DAS measurements based on the transfer function between true ground motion and observed DAS waveforms. With a variety of DAS data and reference measurements, we adapt existing instrument-response workflows typically in the frequency band from 0.01 to 10 Hz to different experiments, with signal frequencies ranging from 1/3000 to 60 Hz. These experiments include earthquake recordings in an underground rock laboratory, hydraulic injection experiments in granite, active seismics in agricultural soil, and icequake recordings in snow on a glacier. The results show that the average standard deviations of both amplitude and phase responses within the analyzed frequency ranges are in the order of 4 dB and 0.167π radians, respectively, among all experiments. Possible explanations for variations in the instrument responses include the violation of the assumption of constant phase velocities within the workflow due to dispersion and incorrect ground-motion observations from reference measurements. The results encourage further integration of DAS-based strain measurements into methods that exploit complete waveforms and not merely travel times, such as full-waveform inversion. Ultimately, our developments are intended to provide a quantitative assessment of site- and frequency-dependent DAS data that may help establish best practices for upcoming DAS surveys.

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

2021 ◽  
Author(s):  
Nicola Piana Agostinetti ◽  
Alberto Villa ◽  
Gilberto Saccorotti
Keyword(s):  
High Resolution ◽  
Plane Wave ◽  
Geothermal Field ◽  
P Wave ◽  
Apparent Velocity ◽  
Near Surface ◽  
Acoustic Sensing ◽  
Geothermal Activity ◽  
Distributed Acoustic Sensing ◽  
Microseismic Event

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.

Distributed Acoustic Sensing Using a Large-Volume Airgun Source and Internet Fiber in an Urban Area

2021 ◽  
Author(s):  
Zhenghong Song ◽  
Xiangfang Zeng ◽  
Baoshan Wang ◽  
Jun Yang ◽  
Xiaobin Li ◽  
...  
Keyword(s):  
Urban Area ◽  
Urban Areas ◽  
Signal To Noise Ratio ◽  
Gauge Length ◽  
P Wave ◽  
S Wave ◽  
Near Surface ◽  
Acoustic Sensing ◽  
Wave Signal ◽  
Distributed Acoustic Sensing

Abstract Seismological methods have been widely used to construct subsurface images in urban areas, for both seismological and engineering purposes. However, it remains a challenge to continuously operate a dense array in cities for high-resolution 4D imaging. In this study, we utilized distributed acoustic sensing (DAS) and a 5.2 km long, L-shaped, telecom, fiber-optic cable to record the wavefield from a highly repeatable airgun source located 7–10 km away. No P-wave signal was observed, but the S-wave signal emerged clearly on the shot-stacked traces, and the arrivals were consistent with collocated geophone traces. Because the signal quality is significantly affected by cable coupling and local noise, three methods can be employed to improve signal-to-noise ratio: (1) stacking contiguous, colinear channels to increase effective gauge length, (2) connecting multiple fibers within a single conduit and stacking collocated channels, and (3) using engineered fiber. In conclusion, the combination of DAS, using internet fiber and an airgun source with proven efficient signal enhancement methods, can provide frequent snapshots of the near surface across an urban area.

Continuous source system and distributed acoustic sensing for reservoir to crust monitoring

2020 ◽  
Author(s):  
Takeshi Tsuji ◽  
Tatsunori Ikeda ◽  
Koshun Yamaoka
Keyword(s):  
Monitoring System ◽  
Field Experiments ◽  
Seismic Velocity ◽  
Seismic Source ◽  
Seismic Signal ◽  
Geothermal Field ◽  
High Temporal Resolution ◽  
Acoustic Sensing ◽  
Distributed Acoustic Sensing ◽  
Continuous Source

<p><strong>We have developed a permanent seismic monitoring system using a continuous seismic source and distributed acoustic sensing (DAS). </strong><strong>The active seismic source system continuously </strong><strong>generates waveforms </strong><strong>with wide frequency range. By stacking the continuous waveforms, our monitoring system improves signal-to-noise ratio of the seismic signal. Thus, less-energy vibration using</strong><strong> s</strong><strong>mall-size source could be utilized for the exploration of deeper geological targets. Presently, we have deployed the small-size monitoring source system in the Kuju geothermal field in the northeast Kyushu Island, Japan. Although our monitoring source system is small and generates high frequency vibrations (10-20Hz), the signal propagated ></strong><strong>80 </strong><strong>km distance using two-month continuous source data. Our field experiments demonstrate that variation of seismic velocity of the crust could be identified with high accuracy (~0.01 %). </strong><strong>To record the monitoring signal from continuous source system, we need to deploy seismometers. Deployment of many seismometers increase spatial resolution of the monitoring results. Recently, we have deployed the DAS system close to the continuous seismic source system. Using DAS, dense and long seismometer network can be realized, and we succeeded to identify the temporal variation of seismic velocity. By using both continuous source and DAS, we are able to monitor wide area with lower cost. </strong><strong>Our monitoring system could accurately monitor the larger-scale crust and smaller-scale reservoir in high temporal resolution.</strong></p>

scholarly journals Distributed Acoustic Sensing from mHz to kHz: Empirical Investigations of DAS Instrument Response

2020 ◽  
Author(s):  
Patrick Paitz ◽  
Pascal Edme ◽  
Cédric Schmelzbach ◽  
Joesph Doetsch ◽  
Dominik Gräff ◽  
...  
Keyword(s):  
Urban Areas ◽  
Waveform Inversion ◽  
Geophysical Methods ◽  
Low Frequency ◽  
Acoustic Sensing ◽  
Temporal Sampling ◽  
Cable Properties ◽  
Instrument Response ◽  
Distributed Acoustic Sensing ◽  
Initial Results

<p>With the upside of high spatial and temporal sampling even in remote or urban areas using existing fiber-optic infrastructure, Distributed Acoustic Sensing (DAS) is in the process of revolutionising the way we look at seismological data acquisition. However, recent publications show variations of the quality of DAS measurements along a single cable. In addition to site- and orientation effects, data quality is strongly affected by the transfer function between the deforming medium and the fiber, which in turn depends on the fiber-ground coupling and the cable properties. Analyses of the DAS instrument response functions in a limited part of the seismological frequency band are typically based on comparisons with well-coupled conventional seismometers for which the instrument response is sufficiently well known to be removed from the signal.</p><p>In this study, we extend the common narrow-band analyses to DAS response analyses covering a frequency range of five orders of magnitude ranging from ~4000 s period to frequencies up to ~100 Hz. This is based on a series of experiments in Switzerland, including (1) active controlled-source experiments with co-located seismometers and geophones, (2) low-frequency strain induced by hydraulic injection in a borehole with co-located Fiber-Bragg-Grating (FBG) strain-meters, and (3) local to teleseismic ice- and earthquake recordings with  co-located broadband stations.</p><p>Initial results show a site-unspecific, approximately flat instrument response for all experiments.</p><p>The initial results suggest that the amplitude and phase information of DAS recordings are sufficient for conventional geophysical methods such as event localisation, full-waveform inversion, ambient noise tomography and even event magnitude estimation. Despite the promising initial results, further engagement by the DAS community is required to evaluate the DAS performance and repeatability among different interrogation units and study sites.</p>

Challenges of DAS measurements in seismic urban areas: case study at Etna volcano eastern flank

2020 ◽  
Author(s):  
Rosalba Napoli ◽  
Gilda Currenti ◽  
Athena Chalari ◽  
Camille Jestin ◽  
Danilo Contrafatto ◽  
...  
Keyword(s):  
Spatial Resolution ◽  
Urban Areas ◽  
Sampling Frequency ◽  
Gauge Length ◽  
Anthropogenic Sources ◽  
Fault System ◽  
Etna Volcano ◽  
Acoustic Sensing ◽  
Eastern Flank ◽  
Distributed Acoustic Sensing

<p>We present the use of distributed acoustic sensing of telecommunication fibre to perform seismic monitoring on the lower eastern flank of Etna volcano. Eastern flank of Etna is structurally characterized by the existence of many faults until under the sea. One of the clearest morphological feature is the Timpe Fault System (TFS) crossing highly populated urban areas. The TFS is formed by several main segments producing shallow seismicity with a dominant normal faulting style and a right-lateral component, related to WNW-ESE regional extension. This area is highly seismogenic, with occurrence of a very frequent seismic activity punctuated by destructive earthquakes with magnitude ranges 4.3≤ML≤5.1 and a mean recurrence time of about 20 years.</p><p>To monitor the seismic response of this area we deployed an “intelligent” Distributed Acoustic Sensing (iDAS) system (SILIXA) in order to interrogate a 12-km-long telecommunication fibre-optic cable, managed by TELECOM Italia internet provider. The telecom cable runs from Linera to Zafferana villages along two primary directions roughly N-S and E-W and crosses the Santa Venerina and the Fiandaca faults, both part of the TFS. The former was entirely hidden until the 2002 eruption when a ML 4.4 earthquake exposed the fault at the surface and heavily damaged Santa Venerina village. The latter has been reactivated during the 2018 Etna activity, when a ML4.8 earthquake strongly damaged the Fleri village.</p><p>The iDAS was in acquisition for three months (11 September - 9 December 2019) and recorded the strain rate from natural and anthropogenic sources at a sampling frequency of 1 kHz with 2-m spatial resolution and a gauge length of 10 m. A second fibre in the same cable, was interrogated simultaneously by a FEBUS A1 system (FEBUS OPTICS) from 2 to 9 December 2019 with a spatial resolution and a gauge length of 5 m at a sampling frequency of 200 Hz. To validate the DAS measurements, gathered by both systems, two broadband seismometers (Trillium Compact 120 s) were deployed in the vicinity of the cable. We located using hammer shots along the cable at key positions.</p><p>During the acquisition period more than 800 local seismic events occurred on Etna with ML ranging between 0.4 and 3.4. Several regional earthquakes from Greece and Albania also occurred up to ML6.1. These seismic sources allows for investigating the response of the fibre and the detectability thresholds of iDAS and FEBUS A1 in urban areas with heterogeneous installation conditions of the telecommunication cable (cased conduit, attached conduit, aerial track).  We perform data analysis to characterize DAS amplitude and frequency responses to better estimate the coupling of the fibre to the ground.</p>

scholarly journals A Study of UVER in Santiago, Chile Based on Long-Term In Situ Measurements (Five Years) and Empirical Modelling

Energies ◽  
2021 ◽  
Vol 14 (2) ◽  
pp. 368
Author(s):  
Lisdelys González-Rodríguez ◽  
Amauri Pereira de Oliveira ◽  
Lien Rodríguez-López ◽  
Jorge Rosas ◽  
David Contreras ◽  
...  
Keyword(s):  
Urban Areas ◽  
Solar Spectrum ◽  
Sun Exposure ◽  
Great Accuracy ◽  
In Situ Measurements ◽  
Uv Index ◽  
Empirical Modelling ◽  
The People

Ultraviolet radiation is a highly energetic component of the solar spectrum that needs to be monitored because is harmful to life on Earth, especially in areas where the ozone layer has been depleted, like Chile. This work is the first to address the long-term (five-year) behaviour of ultraviolet erythemal radiation (UVER) in Santiago, Chile (33.5° S, 70.7° W, 500 m) using in situ measurements and empirical modelling. Observations indicate that to alert the people on the risks of UVER overexposure, it is necessary to use, in addition to the currently available UV index (UVI), three more erythema indices: standard erythemal doses (SEDs), minimum erythemal doses (MEDs), and sun exposure time (tery). The combination of UVI, SEDs, MEDs, and tery shows that in Santiago, individuals with skin types III and IV are exposed to harmfully high UVER doses for 46% of the time that UVI indicates is safe. Empirical models predicted hourly and daily values UVER in Santiago with great accuracy and can be applied to other Chilean urban areas with similar climate. This research inspires future advances in reconstructing large datasets to analyse the UVER in Central Chile, its trends, and its changes.

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