Distributed Acoustic Sensing - A New Tool for Seismic Applications

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A modular borehole monitoring concept has been implemented to provide a suite of well‐based monitoring tools that can be deployed cost effectively in a flexible and robust package. The initial modular borehole monitoring system was deployed as part of a CO2 injection test operated by the Southeast Regional Carbon Sequestration Partnership near Citronelle, Alabama. The Citronelle modular monitoring system transmits electrical power and signals, fibre‐optic light pulses, and fluids between the surface and a reservoir. Additionally, a separate multi‐conductor tubing‐encapsulated line was used for borehole geophones, including a specialized clamp for casing clamping with tubing deployment. The deployment of geophones and fibre‐optic cables allowed comparison testing of distributed acoustic sensing. We designed a large source effort (>64 sweeps per source point) to test fibre‐optic vertical seismic profile and acquired data in 2013. The native measurement in the specific distributed acoustic sensing unit used (an iDAS from Silixa Ltd) is described as a localized strain rate. Following a processing flow of adaptive noise reduction and rebalancing the signal to dimensionless strain, improvement from repeated stacking of the source was observed. Conversion of the rebalanced strain signal to equivalent velocity units, via a scaling by local apparent velocity, allows quantitative comparison of distributed acoustic sensing and geophone data in units of velocity. We see a very good match of uncorrelated time series in both amplitude and phase, demonstrating that velocity‐converted distributed acoustic sensing data can be analyzed equivalent to vertical geophones. We show that distributed acoustic sensing data, when averaged over an interval comparable to typical geophone spacing, can obtain signal‐to‐noise ratios of 18 dB to 24 dB below clamped geophones, a result that is variable with noise spectral amplitude because the noise characteristics are not identical. With vertical seismic profile processing, we demonstrate the effectiveness of downgoing deconvolution from the large spatial sampling of distributed acoustic sensing data, along with improved upgoing reflection quality. We conclude that the extra source effort currently needed for tubing‐deployed distributed acoustic sensing vertical seismic profile, as part of a modular monitoring system, is well compensated by the extra spatial sampling and lower deployment cost as compared with conventional borehole geophones.

Section: Intelligent Distributed Acoustic Sensor (Idas) Propertiesmentioning

confidence: 99%

Field testing of modular borehole monitoring with simultaneous distributed acoustic sensing and geophone vertical seismic profiles at Citronelle, Alabama

Daley

Miller

Dodds³

et al. 2015

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148

“…This system is compact and ideal for downscaled experiments (Papp 2013). It requires only a very simple data processing workflow to compute the seismic strain applied on the cable and uses the same type of fibre optic cable as the distributed optical fibre acoustic sensor (DAS) for sensing (Mateeva et al 2014;Parker, Shatalin and Farhadiroushan 2014). In this section, we describe the optical fibre interferometer, and we present the processing workflow necessary to extract the seismic signal.…”

Section: E S C R I P T I O N O F T H E F I B R E a C Q U I S I T I mentioning

confidence: 99%

“…The reference arm of the system is mechanically isolated from vibration, but the sensing arm is coupled to the medium where we wish to measure acoustic/seismic signals. Note that the entire length of the sensing arm is sensitive to vibration; therefore, this is a distributed acquisition system, similar to the fully distributed acoustic sensors that are denoted by DAS or DVS (Parker et al 2014). However, the fibre interferometer method used here outputs a single spatial measurement that is similar to a single trace in a seismic acquisition.…”

Section: Optical Fibre Interferometermentioning

confidence: 99%

A study of the geophysical response of distributed fibre optic acoustic sensors through laboratory‐scale experiments

Papp

Donno

Martin

et al. 2016

“…Distributed acoustic sensing (DAS) is a technology of strain sensing based on Rayleigh scattering of laser pulses in fibreoptic cables (see, e.g., Posey Jr., Johnson, and Vohra 2000). DAS provides a less costly alternative to conventional seismic acquisition systems, and its use for seismic recording finds increasing attention (Gaston and Bostick 2011;Parker et al 2012;Hartog et al 2013). In particular, there is a focus on applying DAS for reservoir monitoring with vertical seismic profiling (VSP) (Madsen, Parker, and Gaston 2012).…”

Section: Introductionmentioning

confidence: 99%

Interaction of helically wound fibre‐optic cables with plane seismic waves

Kuvshinov

2015

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Distributed acoustic sensing is a novel technology for seismic acquisition. In this technology, strain changes induced by seismic waves impinging on an optical fibre are monitored. Due to the fact that glass is relatively rigid, straight glass fibres are not sensitive to broadside waves. We suggest using distributed acoustic sensing systems with fibres helically wound around cables. One increases the fibre sensitivity to broadside waves by decreasing the fibre wrapping angle (the angle between the fibre axis and the plane normal to the cable axis). The optimal wrapping angle is chosen to minimize the impact of Rayleigh waves on the signal measured. This angle depends on the cable Poisson ratio, and it is approximately equal to 30° for cables composed of plastic. For reliable detection of seismic waves, one needs a good mechanical contact between the cable and the surrounding medium. On the other hand, the sensitivity of distributed acoustic sensing systems to primary waves can be significantly reduced if the cable is placed in a cemented borehole.