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.

Strain microseismics: Radiation patterns, synthetics, and moment tensor resolvability with distributed acoustic sensing in isotropic media

Geophysics ◽  
2020 ◽  
Vol 85 (3) ◽  
pp. KS101-KS114
Author(s):  
Ismael Vera Rodriguez ◽  
Andreas Wuestefeld
Keyword(s):  
Moment Tensor ◽  
Near Field ◽  
Gauge Length ◽  
P Wave ◽  
Far Field ◽  
S Wave ◽  
Acoustic Sensing ◽  
Moment Tensors ◽  
Isotropic Media ◽  
Distributed Acoustic Sensing

We have derived analytical formulations for the strain field produced by a moment tensor source in homogeneous isotropic media. Such formulations are important for microseismic projects that increasingly are monitored with fiber-optic distributed acoustic sensing (DAS) systems. We find that the spatial derivative of displacement produces new terms in strain proportional to [Formula: see text] with [Formula: see text]. In viscoelastic media, the derivative also produces an additional far-field term that is scaled by a frequency-dependent factor. When comparing with full wavefield synthetic data, we observe that the new terms proportional to [Formula: see text] can be considered part of a near-field in strain, similar to the practice with the displacement formulation. Analyses of moment tensor resolvability show that full moment tensors are resolvable with P-wave information from two or more noncoplanar vertical DAS cable geometries if intermediate- and far-field terms are considered and that S-wave information alone cannot constrain full moment tensors using only vertical wells. These results mirror previous observations made with displacement measurements. Furthermore, the addition of the new terms proportional to [Formula: see text] in strain improves the moment tensor resolvability but only in the case of a single vertical array. In the case of a single deviated/horizontal well, we can, in theory, resolve a full moment tensor but a case-by-case analysis is necessary to identify regions of full constraint around the well and the necessary noise conditions to guarantee reliable solutions. Real DAS measurements also are affected by the gauge length and interrogator details. In the case of the gauge length, we observe that this operator does not change the resolvability of the problem but it does affect inversion stability. The results derived here represent theoretical limits or in some cases specific examples. Practical implementations require analyses of conditioning, noise, coupling, and the effect of gauge length on a case-by-case basis.

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.

Shallow velocity structure and poisson's ratio at the Tarzana, California, strong-motion accelerometer site

1996 ◽  
Vol 86 (6) ◽  
pp. 1704-1713 ◽  
Author(s):  
R. D. Catchings ◽  
W. H. K. Lee
Keyword(s):  
Urban Areas ◽  
Velocity Structure ◽  
Strong Motion ◽  
P Wave ◽  
S Wave ◽  
Near Surface ◽  
Velocity Models ◽  
Wave Velocities ◽  
Poisson's Ratios ◽  
Poisson’S Ratios

Abstract The 17 January 1994, Northridge, California, earthquake produced strong ground shaking at the Cedar Hills Nursery (referred to here as the Tarzana site) within the city of Tarzana, California, approximately 6 km from the epicenter of the mainshock. Although the Tarzana site is on a hill and is a rock site, accelerations of approximately 1.78 g horizontally and 1.2 g vertically at the Tarzana site are among the highest ever instrumentally recorded for an earthquake. To investigate possible site effects at the Tarzana site, we used explosive-source seismic refraction data to determine the shallow (<70 m) P-and S-wave velocity structure. Our seismic velocity models for the Tarzana site indicate that the local velocity structure may have contributed significantly to the observed shaking. P-wave velocities range from 0.9 to 1.65 km/sec, and S-wave velocities range from 0.20 and 0.6 km/sec for the upper 70 m. We also found evidence for a local S-wave low-velocity zone (LVZ) beneath the top of the hill. The LVZ underlies a CDMG strong-motion recording site at depths between 25 and 60 m below ground surface (BGS). Our velocity model is consistent with the near-surface (<30 m) P- and S-wave velocities and Poisson's ratios measured in a nearby (<30 m) borehole. High Poisson's ratios (0.477 to 0.494) and S-wave attenuation within the LVZ suggest that the LVZ may be composed of highly saturated shales of the Modelo Formation. Because the lateral dimensions of the LVZ approximately correspond to the areas of strongest shaking, we suggest that the highly saturated zone may have contributed to localized strong shaking. Rock sites are generally considered to be ideal locations for site response in urban areas; however, localized, highly saturated rock sites may be a hazard in urban areas that requires further investigation.

Measurement of acoustic properties of near‐surface soils using an ultrasonic waveguide

Geophysics ◽  
2004 ◽  
Vol 69 (2) ◽  
pp. 460-465 ◽  
Author(s):  
Rob Long ◽  
Thomas Vogt ◽  
Mike Lowe ◽  
Peter Cawley
Keyword(s):  
Urban Areas ◽  
Surrounding Medium ◽  
Longitudinal Mode ◽  
Acoustic Properties ◽  
P Wave ◽  
S Wave ◽  
Surface Soils ◽  
Near Surface ◽  
Experimental Laboratory ◽  
Ultrasonic Waveguide

A technique is presented that uses a circular ultrasonic waveguide to measure the bulk shear (S‐wave) and longitudinal (P‐wave) velocities of unconsolidated media, with particular application to near‐surface soils. The technique requires measuring the attenuation characteristics of the fundamental longitudinal mode that propagates along an embedded bar, from which the acoustic properties of the surrounding medium are inferred. The principles behind the technique are discussed, and the results of an experimental laboratory validation are presented, followed by details of in‐situ soil property measurements obtained at various sites in urban areas of the United Kingdom.

Distributed acoustic sensing for shallow seismic investigations and void detection

Geophysics ◽  
2021 ◽  
pp. 1-69
Author(s):  
Yarin Abukrat ◽  
Moshe Reshef
Keyword(s):  
Vertical Component ◽  
Test Site ◽  
Wave Mode ◽  
Invasive Technique ◽  
S Wave ◽  
Near Surface ◽  
Acoustic Sensing ◽  
Shallow Subsurface ◽  
Diffraction Imaging ◽  
Distributed Acoustic Sensing

During the last decade, fiber-optic-based distributed acoustic sensing (DAS) has emerged as an affordable, easy-to-deploy, reliable, and non-invasive technique for high-resolution seismic sensing. We show that fiber deployments dedicated to near-surface seismic applications, commonly employed for the detection and localization of voids, can be used effectively with conventional processing techniques. We tested a variety of small-size sources in different geological environments. These sources, operated on and below the surface, were recorded by horizontal and vertical DAS arrays. Results and comparisons to data acquired by vertical-component geophones demonstrate that DAS may be sufficient for acquiring near-surface seismic data. Furthermore, we tried to address the issue of directional sensing by DAS arrays and use it to solve the problem of wave-mode separation. Records acquired by a unique acquisition setup suggest that one can use the nature of DAS systems as uniaxial strainmeters to record separated wave modes. Lastly, we applied two seismic methods on DAS data acquired at a test site: multi-channel analysis of surface waves (MASW) and shallow diffraction imaging. These methods allowed us to determine the feasibility of using DAS systems for imaging shallow subsurface voids. MASW was used to uncover anomalies in S-wave velocity, whereas shallow diffraction imaging was applied to identify the location of the void. Results obtained illustrate that by using these methods we are able to accurately detect the true location of the void.

Detecting anisotropy using Distributed Acoustic Sensing and fibre-optic seismology in a fast-flowing glacier in Greenland

2020 ◽  
Author(s):  
Adam Booth ◽  
Poul Christoffersen ◽  
Charlotte Schoonman ◽  
Andy Clarke ◽  
Bryn Hubbard ◽  
...  
Keyword(s):  
Wave Energy ◽  
Internal Flow ◽  
P Wave ◽  
Sediment Layer ◽  
Fibre Optic ◽  
S Wave ◽  
Acoustic Sensing ◽  
Seismic Properties ◽  
Distributed Acoustic Sensing ◽  
Zero Offset

<p>Material anisotropy within a glacier both influences and is influenced by its internal flow regime. Anisotropy can be measured from surface seismic recordings, using either active sources or natural seismic emissions. In the past decade, Distributed Acoustic Sensing (DAS) has emerged as a new, and potentially transformative, seismic acquisition technology, involving determining seismic responses from the deformation of optical fibres. Although DAS has shown great potential within engineering and resources sectors, it has not yet been widely deployed in studies of glaciers and ice masses.</p><p>Here, we present results from a glaciological deployment of a DAS system. In July 2019, a Solifos BRUsens fibre optic cable was installed in a 1050 m borehole drilled on Store Glacier in West Greenland. Vertical seismic profiles (VSPs) were recorded using a Silixa iDAS interrogation unit, with seismic energy generated with a 7 kg sledgehammer striking a polyethene (UHMWPE) impact plate. A three-day sequence of zero-offset VSPs (with the source located ~1 m from the borehole top) were recorded to monitor the freezing of the cable, combined with offset-VSPs in along- and cross-flow directions, and radially at 300 m offset.</p><p>P-wave energy (frequency ~200 Hz) is detectable through the whole ice thickness, sampled at 1 m depth increments. The zero-offset reflectivity of the glacier bed is low, but reflections are detected from the apparent base of a subglacial sediment layer. S-wave energy is also detectable in the offset VSP records. The zero-offset VSPs show a mean vertical P-wave velocity of 3800 ± 140 m/s for the upper 800 m of the glacier, rising to 4080 ± 140 m/s between 900-950 m. In the deepest 50 m, velocity reduces to 3890 ± 80 m/s. This variation in vertical velocity is consistent with the development of an anisotropic ice fabric in the lowermost 10% of the glacier. The full dataset also contains natural seismic emissions, highlighting the potential of DAS as both an active and passive seismic monitoring tool.</p><p>DAS offers transformative potential for understanding the seismic properties of glaciers and ice sheets. The simplicity of the typical VSP geometry makes the interpretation of seismic travel-times less vulnerable to approximations, and thus the derivation of seismic properties more robust, than in conventional surface seismic surveys. As an addition, DAS facilitates VSP recording with unprecedented vertical and temporal resolution. Furthermore, the sensitivity of the optical-fibre to both P- and S-wave particle motion means that a comprehensive suite of acoustic and elastic properties can be inferred.</p>

Shear Wave Velocity Analysis of 2-D Multichannel Analysis of Surface Wave (MASW) to investigate subsurface Fault of Alternative Bridge Construction in Kelok Sago Jambi

2020 ◽  
Vol 25 (1) ◽  
pp. 18-20
Author(s):  
Muhammad Farhan ◽  
◽  
Gunawan Handayani ◽  
Keyword(s):  
Surface Wave ◽  
Surface Waves ◽  
Wave Velocity ◽  
Urban Areas ◽  
Signal To Noise Ratio ◽  
Body Wave ◽  
Geophysical Methods ◽  
P Wave ◽  
S Wave ◽  
Multichannel Analysis

Every geotechnical measurement requires geophysical methods to classify soil types under the ground. S-wave velocity (Vs), P-wave velocity (Vp), and density (ρ), are the most important parameters in the classification of soils. There are various methods to determine Vs, one of them is P-S logging method. However, this method is less suitable to be applied in urban areas due to the difficulties of data acquisition and high expense in operational costs. In 1999, a seismic method uses surface waves to de-termine Vs profile with a higher signal to noise ratio which was known by the name of Multichannel Analysis of Surface Waves (MASW). A surface wave, especially Rayleigh wave, creeps slowly on the surface with a larger amplitude than a body wave. The wavelengths of the surface wave will disperse in the layers system i.e. the phase velocity of the surface waves is now func-tion of frequency. MASW 2-D method is used in this paper to determine subsoil properties and to identify the fault under the bridge abutments plan (abutment 1 and abutment 2) in Kelok Sago Jambi.

Distributed acoustic sensing for near-surface imaging using submarine telecommunication cable: a case study in the Trondheimsfjord, Norway

Geophysics ◽  
2021 ◽  
pp. 1-69
Author(s):  
Kittinat Taweesintananon ◽  
Martin Landrø ◽  
Jan Kristoffer Brenne ◽  
Aksel Haukanes
Keyword(s):  
Signal To Noise Ratio ◽  
Seismic Source ◽  
Near Surface ◽  
Acoustic Sensing ◽  
Optical Cable ◽  
Distributed Acoustic Sensing ◽  
Hydrophone Array ◽  
Seismic Images ◽  
Data Processing Methods

Distributed acoustic sensing (DAS) transforms submarine telecommunication cables into densely sampled seismic receivers. To demonstrate DAS applications for seismic imaging, we use an optical cable on the seafloor in the Trondheimsfjord, Norway, to record seismic data generated by a controlled seismic source. The data are simultaneously recorded by a towed hydrophone array and the fiber optic cable. Following our data processing methods, we can produce seismic images of the seafloor and underlying geological structures from both hydrophone array and DAS data. We find that the hydrophone and DAS data have a comparable signal-to-noise ratio. Moreover, DAS images can be improved by using a seismic source that has sufficiently large energy within the frequency range matching the spatial resolution of DAS. The temporal resolution of the DAS images can be improved by minimizing the crossline offset between seismic sources and the DAS cable. The seismic images from DAS can be used to support geohazard analysis and various subsurface exploration activities.

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>

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