Julia Correa

The CO2CRC otway project deployment of a distributed acoustic sensing network coupled with permanent rotary sources

Summary We have deployed a novel permanent monitoring system at the Australian CO2CRC Otway Site that includes a surface and borehole distributed acoustic sensing (DAS) network with orbital vibrator (rotary) surface seismic sources. DAS is an emerging technology for performing seismic acquisition based on optical interferometric techniques, which allows for data collection with a wide spatial aperture and high temporal resolution using commercially available telecommunications fibres. DAS sensitivity currently lags behind conventional discrete geophone and hydrophone sensor technologies. Our implementation of surface rotary seismic sources is based on open-loop controlled asynchronous motors. This avoids the complexity of feedback loops for phase control, instead using deconvolution of the source function as measured by a shallow source-monitor sensor. Initial data analysis shows that the amount of energy available from long source sweeps overcomes limitations in DAS sensitivity. The combination of relatively inexpensive but powerful permanent surface sources with permanent DAS deployment in an areal array provides a new paradigm for time-lapse seismic monitoring. The methodology we describe has broad applicability for long-term reservoir surveillance, with time-lapse change sensitive to many subsurface properties.

DAS Versus Geophones: a Quantitative Comparison of a VSP Survey at a Dedicated Field Laboratory

Summary Distributed Acoustic Sensing (DAS) is a novel technology to acquire acoustic data. DAS is noticeably promising for Vertical Seismic Profiling (VSP) applications as it offers highly sampled data, acquired simultaneously at all levels, at relatively low cost. In this study, DAS data is acquired using two different cable deployments: cemented behind the casing, and deployed suspended in the well. Both datasets were compared to conventional 3-component geophone VSP. DAS acquired with cemented cable presented similar quality and signal to noise ratio as geophone data, although it suffers from loss of high frequencies. The suspended cable presented predominantly tube noise, however reflections can still be recognized. We hope the results from this study contribute to a broader use of DAS for VSP acquisition.

Amplitude and Phase Response of DAS Receivers

The emergence of distributed acoustic sensing (DAS) as a method for acquiring seismic data has inevitably resulted in comparisons between DAS and geophone data. Herein we present and analyse the differences between the amplitude and phase spectra of the two types of datasets in terms of the pulse and gauge lengths. We have shown and demonstrated using both synthetic and field data examples that the effect of the lengths on phase is considerable, leading to changes of the shape of the wavelet and, possibly more importantly, the arrival time. The presented process can easily correct the phase of DAS data, and we expect this to become a standard part of DAS data processing. We have also quantified for what wavelengths the commonly used approximation of a DAS receiver directivity pattern by cosine squared is valid – the wavelengths should be at lest four times longer that the gauge length and the pulse length.

A Comparison of DAS and Geophones for VSP Acquisition at a Dedicated Field Laboratory.

Summary Distributed Acoustic Sensing (DAS) is a novel technology that uses fibre-optic cables to detect acoustic signals. DAS provides a cost-effective solution for acquiring vertical seismic profile (VSP) surveys, but it suffers from a comparatively poor signal-to-noise ratio and uncertainties in depth. In this work, we compare the quality of a VSP survey acquired using a permanently installed fibre-optic DAS system with that acquired using a conventional 3-component geophone system. The results show that DAS is capable of producing a time-depth curve with differences of less than 0.8 ms from the geophone results but that the raw interval velocities calculated from the DAS data fluctuate wildly due to their small depth interval. The datasets themselves were consistent although the S/N of the DAS data was inferior and small differences in phase were evident. The DAS amplitudes were also found to decay with cosine-squared of the incidence angle compared to cosine for the geophone data. Corridor stacks generated from both datasets were broadly consistent but residual noise in the DAS data contaminated the stack. The results of this, and previously published surveys, should encourage the more widespread use of DAS for zero offset VSPs.