Tim Dean

The Spectral Characteristics of Seismic Noise Recorded in an Urban Environment

Summary “This paper describes a test conducted to identify the characteristics of the major sources of noise for urban seismic surveys. The strongest source of noise was mechanical equipment, although it was intermittent and restricted to a relatively small bandwidth centred on 75 Hz. Traffic-related noise was also high, with a bandwidth (10-25 Hz) within that typically of interest for seismic surveys, although it did decrease during the late evening/early morning. Aircraft resulted in short (∼1 minute) wide bandwidth (30-200 Hz) bursts of noise. We found no relationship with wind, but this is probably a result of the strength of the other sources and the relatively low wind speeds observed. Electrical noise was observed with the amplitude and frequency (∼50 Hz) varying during the study period. Based on these results we recommend that: acquisition should be carried out during the late night/early morning; receiver locations should be chosen to avoid sources of mechanical and electrical noise; cables should be as short as possible to avoid electrical noise. Although nodal systems for urban surveys are logistically advantageous some degree of real-time quality control must be in place to ensure that records are not acquired when high-amplitude, short-duration noise (e.g. from aircraft).”

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 New, Simple, Low-cost, Portable Vibroseis for Research and Near-surface Surveys

The desirable characteristics of vibrators such as a wide, controllable bandwidth and high repeatability has led to them becoming the most popular land seismic sources. The hydraulic vibrators typically used for land seismic acquisition, however, are large, costly to operate and expensive to purchase. This inhibits their use for small-scale surveys, such as those of the near surface and vertical seismic profiles, as well as Vibroseis research. In this paper we describe the construction of a simple portable vibrator from commercially available components, principally a number of low-frequency actuators, which can be assembled in days at a cost of less than

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

5,000. The peak-force of the vibrator is greater than 70 kg (686 N) and could easily be increased by adding additional actuators. Initial field results show that the vibrator is superior to a sledgehammer and is capable of transmitting sweeps with optimised spectral content. We hope that the ease with which such units can be built and operated will inspire others to use portable vibrators for near-surface work and foster further Vibroseis research.

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

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.