Vertical seismic profiling (VSP) is commonly used in the oil and gas industry for better subsurface imaging and characterisation, as well as for providing depth calibration for surface seismic. The use of VSP in mineral exploration and mine planning is not very common mostly due to the small diameter and stability of the boreholes, as well as the relatively high cost of such surveys. These issues can be mitigated by using cheap and potentially disposable borehole sensors, such as fibre-optic cables utilised in distributed acoustic sensing (DAS). The questions we want to answer in this work are how the quality of DAS data compares to other types of borehole measurements and what are the operational benefits and constraints for the use of this technology in mineral exploration settings. To this end, we have tested performance of DAS measurements in one of the boreholes of the Mineral Systems Drilling Program in South Australia and compared them to hydrophone measurements. The DAS measurements provide data quality that is much better than a hydrophone string, in particular it has consistent amplitudes at different depths, shows less cable and tube waves, and the reflections are much clearer. The acquisition of DAS data is quicker than any other borehole measurements that require multiple pulls of the receivers. The reduction of the acquisition time increases with the depth of the borehole. This case study demonstrates that DAS measurements show big potential for mineral exploration and exploitation.

Fibre-optic distributed sensing has the potential to revolutionize well and reservoir surveillance in the oil and gas industry. Benefits include the passive nature of optical fibre sensors, the potential for cost-effective installations, combined with the possibility of densely distributed measurements along the entire length of the fibre. Amongst a range of fibre-optic sensing technologies, Distributed Acoustic Sensing has the potential to provide a low cost alternative for conventional seismic technologies. To widen the geophysical application scope further, the fibre-optic sensing cable should be made more sensitive to incoming seismic waves that arrive at the cable perpendicular ("broadside") to its axial direction. We introduce the development of such cable concepts, and present results of a successful cable deployment in a surface seismic field trial. Efforts continue to realize cost-effective directionally-sensitive cables for geophysical use, for deployment down-hole and on surface. FIBRE-OPTIC SENSING IN THE OIL&GAS INDUSTRYThe passive nature and inherent long-term reliability of optical fibres, combined with the ability to string numerous individual sensing elements together in a single fibre, makes fibre-optic distributed sensing technology ideally suited for down-hole sensing along a full well path. A variety of fibre-optic sensors developed in the aerospace and defence industries are finding their way into the oil and gas industry, enabling measurement of quantities such as temperature, pressure, chemicals, strain and acoustics along the entire length of the fibre. As multiple fibres, each providing a specific distributed measurement, can be bundled in a single down-hole-deployable cable, the industry is encountering a unique opportunity for robust well and reservoir surveillance based on an abundance of measurements continuous in time and along the fibre's length 1 .In the oil and gas industry, seismic datasets provide a crucial source of information about the subsurface. The use of seismic surveys for finding new reservoirs with producible accumulations of hydrocarbons is widespread. More recently, seismic acquisition repeated during the productive lifetime of a field is becoming more common. This allows monitoring changes in the subsurface as a result of hydrocarbon production, and can help in optimising well integrity, subsurface models and production. Traditionally, electrical geophones have been used in onshore seismic surveys, either deployed on the surface or in a well-bore. Fibre-optic sensing technology provides an opportunity to replace these geophones by fibre-optic cables, offering the potential of reducing cost substantially while increasing the spatial sensor resolution dramatically.This paper focuses on the use of Distributed Acoustic Sensing for geophysical use. We will explain how the construction of fibre-optic cables confines the directional sensitivity, and we will present the development and testing of improved sensing cables.

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