Reflection seismic for Minerals with DAS, why and where?

Milovan Urosevic

Andrej Bona

Sasha Ziramov

Robert Martin

During the last decade the Distributed Acoustic Sensing (DAS) data acquisition has been tested in boreholes in a soft rock environment with a variable success. The technology
is now approaching its maturity and it is showing a high potential. In hard rock environments however, its use is much more challenging. In this study we show the first attempt to use this technology for surface reflection seismic rather than more commonly used borehole seismic. The principal reason is that in some specific environments such as hyper-saline lakes, where significant mineral reserves are found at depth in Australia, the use of conventional equipment is limited and prone to significant hardware issues and equipment salt damage. However, such an environment seems appropriate for the application of the DAS technology. This study reports on the results from a comparative DAS-geophone surface reflection experimental survey conducted along salt lakes in Western Australia and shows that DAS has the potential to replace surface geophones in such environment.

Introduction

The application of the Distributed Acoustic Sensing (DAS) technology is becoming widespread. It is presently implemented across diverse disciplines and measurement scales covering from geotechnical, infrastructure and industrial issues to exploration of natural resources, monitoring of CO2 sequestration and even for deep crustal observations including earthquake monitoring. While there are many reasons for this, perhaps the main one is the ability to acquire the data along the entire length of the cable simultaneously, with a dense
spatial sampling. The low cost of the optical cable compared to a conventional point sensor is yet additional motivation for its utilisation in the exploration of natural resources. While
the overall signal to noise ratio (SNR) measure on a single shot gathers may be smaller for DAS than for traditional geophones, the ability to have practically continuous sensor overcomes this limitation (Urosevic et al., 2018). The main disadvantage of DAS system is in its limited directional sensitivity that is highest along the fibre, decreasing for compressional waves, approximately as a cosine squared function, from that direction. There are many other parameters that enter into this analysis such as the fibre properties, pulse frequency, pulse width, gauge length, post-processing algorithms, etc. Overall directional sensitivity coupled with low SNR has not inspired utilisation of DAS for surface seismic recording.

While the directivity issues of DAS acquisition can be to some extent mitigated by utilising helically wound fibres the penaltycomes with the increased attenuation due to the longer length of the fibre and hence lowering of the SNR. Enhanced backscatter fibres are now coming to an aid but there are other hurdles to be resolved related to the cost. There are, however, situations where these issues become of a secondary importance and the use of fibre is preferred in comparison to the conventional recording sensors. This is the case of surface surveys on marshy/ soft surfaces, or on salt lakes, where good coupling of fibre cable to the ground provides a better SNR. At the same time the application of conventional electronic instruments often result in damaged or permanently lost equipment. We have now tested the
applicability of DAS for reflection seismic over two salt lakes in Yilgarn Craton of Western Australia. There, vast mineral reserves are often found but conventional seismic data acquisition is severely restricted by the hyper saline conditions

Very promising results obtained over two salt lakes inspired us to develop DAS reflection seismic over hard ground. The outcomes of this experiment far exceeded our expectations
and paved the way for a new era of the DAS application of the reflection seismic mode, aimed at investigation of worldwide mineral resources.

Reflection surveys with DAS

The results of the first reflection seismic DAS feasibility study conducted over LeFroy salt lake in WA were reported at the 2018 NSGC in Porto (Urosevic et al., 2018). In this study we show
two more case studies of DAS implementation for reflection experiments over the salt lake: Lake Carey and a dry land area of the Frazer Range of WA. In both cases we compared DAS data to conventional geophones seismic lines. For the dry, hard soil of the land site we developed a plough attachment to the skid steerer (of a Bobcat) that enables rapid deployment of DAS cable below the ground surface. Some 1.4 Km of 200-pitched cable were deployed in just 15 minutes. The cost of the cable was approximately $1.5 AUS/m. The deployment of a 428 geophone system over the same length took over 3 hours, while the cost of the conventional equipment was several orders of magnitude higher than the DAS. Is the cost of conventional equipment several orders of magnitude higher than DAS if an interrogator costs 250,000?

Lake Lefroy case study

The first DAS experiment in the reflection mode took place along salt lake Lefroy (Urosevic et al., 2018). Conventional geophones were deployed along the shore of a salt lake at 15 m spacing; with parallel to short helically wound optical cables deployed on the salt lake surface. We used a Silixa iDAS interrogator with 1 m spatial sampling rate, 10 m gauge length and 50 ns laser pulse length. The source was a 60,000 lb seismic vibrator at 70% peak force. DAS produced superior results in the shallow (first kilometre depth) compared to geophones (Figure 1).

Lake Carey case study

Encouraged by the first results obtained at lake Lefroy, we conducted another experiment at lake Carey. This research was conducted within MRIWA grant 514, sponsored by Matsa Resources Ltd and HiSeis P/L. Lake Carey is characterised by generally very soft surface conditions and noise that can rapidly change with wind direction and strength. For this reflection experiment, alongside 2 km long fibre optic cable, we used a NuSeis nodal system that is possibly the best suited “conventional” geophone system for the soft salty ground such as found at Lake Carey (Figure 2). Geophones were spaced at 10m. For DAS we used this time, a Fotech interrogator with the same parameters as used before in the Lake Lefroy experiment. For the seismic energy source, this time we enrused a Betsy gun with 12 gauge blank cartridges. The gun was simply pushed into the soft surface and discharged, resulting
in very efficient data acquisition. First day the ambient conditions were calm, optimal for seismic acquisition. Unfortunately strong wind started the next day and kept on gaining strength until the acquisition was completed. Strong wing dried the ground while rapid change in temperature (morning to mid-day) resulted in stiffening and softening of the cable, respectively, producing daily variations of coupling quality. This can be clearly seen from Figure 3 where DAS data recorded during the calm and very windy

Hard surface DAS deployment
and recording

The ever-present Matsa Resources drive for innovation and implementation of new ideas led to another DAS reflection experiment, this time conducted at the hard ground site. Considering often-windy conditions in the Australian desert and our experience the Lake Carey acquisition, we developed a plough for deployment of fibre optic cable below the surface but with a minimal disturbance to the ground. The plough was made as a standard Bobcat attachment (Figure 6). As before, raw DAS shot records are displayed against geophone records (Figure 7). Some noisy sections can be seen in the DAS data but we expect that high spatial sampling will overcome these issues. The data is currently being processed.

Conclusions

The results presented in this article, demonstrate the potential of DAS for reflection investigation in diverse conditions. Salt lakes of WA are of particular exploration interest and fibre optic seems to be the sensor of choice. Hard ground requires specific deployment of DAS, but initial results are encouraging enough to pursue future developments with this deployment approach,

leading possibly to a break true in the implementation of seismic for exploration of diverse mineral resources.

Acknowledgements

We would like to thank Silixa, Fotech and NuSeis for the use of their equipment. We are thank Gold Fields and are grateful to Matsa Resources and HiSeis P/L for their continuous support and everlasting drive for innovations. We thank MRIWA for their support. We are grateful to Halliburton Company for the donation of the SeisSpace-ProMAX software used for data processing and analysis.

References

Urosevic, M., Bona, A., Ziramov, S., Martin, R., Dwyer, J., and
Foley, A., 2018 – Reflection seismic with DAS, why and where?:
Near Surface Geoscience Conference and Exhibition, 2018,
Porto, Portugal.

Author Bio

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Milovan Urosevic received BSc (Hons) in geophysics from the University of Belgrade in 1980, MSc in geophysics from the University of Houston in 1985 and PhD in geophysics from the Curtin University of Technology in 2000. He acquired over ten years of industry experience working in areas of seismic data processing, AVO, inversion, multi-component seismology and seismic anisotropy. After joining Curtin University in 1991 he has taken part in various industry projects related to the oil, coal and mineral exploration. His main interest is in the utilisation of new technologies to advance exploration of natural resources. He is currently involved in two major Australian corporative research centres (CO2CRC and DETCRC). He is also leading a large ANLEC R&D (Australian National Low Emissions Coal Research and Development) project that is investigating and evaluating the applicability of novel, alternative seismic methodologies for rock characterisation. Milovan is associate editor of the Exploration Geophysics Journal. He is in the advisory board of EU “Smart Exploration” project. He is Professor at the Dept. Of Exploration Geophysics and co-founder and principal advisor to HiSeis P/L, the world leading geophysical company dedicated to the application
of seismic methods for exploration of mineral resources. He is a member of SEG, ASEG, EAGE and AGES.

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Robert Martin has a Bachelor of Science, BSc (Hons) in Geophysics from Curtin University. Robert joined HiSeis in 2012 as a graduate geophysicist and has been involved with seismic survey design, acquisition and processing of 2D and 3D hard rock datasets throughout Australia and the world. Robert is now Technical Operations Manager and his history at HiSeis has seen him design and execute domestic and international seismic programs developing a depth of experience in Vibroseis, Explosive and Nodal seismic techniques.

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