Distributed Acoustic Sensing VSP Research Articles

Application of improved time\u2013frequency representation to local noise removal

The temporal resolution of conventional S transform (ST) is not sufficient for the separation of local coherent noise. We present a revised S transform (RST) which uses an analyzing window function with two control parameters of the scalar σ and the exponential factor γ. Selecting proper parameter values (say σ = 1.1, γ = 1.08), the time–frequency representation (TFR) acquired by our method exhibits a higher temporal resolution. Applying an appropriate filter in the time–frequency domain, we are able to remove specific local noise. Distributed acoustic sensing (DAS) VSP section may suffer from fiber cable coupling noise, hindering the subsequent data processing and geologic interpretation. The real data example shows the coupling noise occurred in the DAS VSP can be removed by the presented RST.

Analysis of signal to noise and directivity characteristics of DAS VSP at near and far offsets — A CO2CRC Otway Project data example

During the last decade, distributed acoustic sensing (DAS) has emerged as a new technology for seismic acquisition. DAS has the potential to reduce the cost of permanent monitoring operations over time as it offers long equipment survivability and requires minimum maintenance. However, broad adoption of DAS technology still faces some challenges, such as low sensitivity and high levels of noise compared to conventional seismic sensors. Recent developments in fiber-optic systems and cable designs aim to overcome these limitations. To understand how DAS can be used in monitoring applications, it is important to know how it behaves with varying offsets and incidence angles. An offset VSP survey was acquired, at the CO2CRC Otway Project, using a straight single-mode fiber, a straight “enhanced-backscatter” fiber, and a conventional three-component geophone tool. The results from this survey show that DAS has the potential to provide similar, or even superior, quality data sets as conventional geophones.

Validation of DAS data integrity against standard geophones — DAS field test at Aquistore site

Distributed acoustic sensing (DAS) using fiber-optic cables is a recent addition to seismic acquisition methods. However, a DAS “sensor” differs significantly from conventional, discrete sensing devices such as geophones or accelerometers. For one, DAS measures something akin to strain instead of particle velocity or acceleration. Other properties of the DAS system also aren't obvious at first. What is its instrument response, noise performance, and repeatability? How are DAS channels properly positioned, e.g., in case of a borehole deployment: depth calibrated? To better understand these issues and their impact on the DAS seismic method's application space, a field test was conducted in which three DAS vendors recorded the same survey using a borehole-installed fiber while recording simultaneously with a conventional downhole array. The results show that all DAS systems achieved good, repeatable signal integrity while exhibiting different noise characteristics. DAS noise can be addressed with well-established processing algorithms, but further benefits can be gained from DAS-specific algorithms. Where required, DAS seismic data can be processed to closely match the vector response of conventional geophones. DAS data converted in this way can assist in the up/down separation step without the need for dip filters. DAS VSP data can also be merged with conventional 3D and 4D seismic, adding value in situations such as undershooting of surface facilities in marine settings.

Efficient Containment Monitoring with Distributed Acoustic Sensing: Feasibility Studies for the Former Peterhead CCS Project

This paper investigates the technical feasibility of monitoring the Goldeneye storage site for containment with time-lapse Distributed Acoustic Sensing (DAS) VSP and DAS microseismic. Specifically, the study examines if the expected seismic signals are measurable with DAS, i.e. whether it would be possible to detect unintended migration of CO2 in a timely manner with sufficient resolution and spatial coverage. In the case of Goldeneye, the multiplicity of wells and diversity of their trajectories provide a favorable geometry that compensates for DAS broadside sensitivity limitations. A ray trace modelling exercise demonstrates that a multi-well DAS VSP survey with minimized source effort can provide a high fold image of the main storage site in an area of approximately 2 km2 around the platform and injectors. The feasibility of monitoring induced seismicity with DAS is evaluated in terms of the spatial distribution of minimum detectable moment magnitudes and event location errors. Provided the noise floor of DAS interrogation units will be reduced and given the Goldeneye well geometry, DAS microseismic may be a possible containment monitoring technology subject to field trial, noise characterization and further processing developments. The economics for DAS seismic solutions are advantageous given that a fiber optic cable can generally be cost-effectively installed in multiple wells and its use can be combined with other in-well monitoring applications, like Distributed Temperature Sensing (DTS). For the Goldeneye storage site, DAS VSP has been identified as a potential containment monitoring alternative to a much more costly surface seismic survey in the vicinity of the injector wells.