Objective In the exploration for hydrocarbon deposits, seismic techniques are the methods of first choice for the initial survey, both onshore and offshore.The aim of a seismic survey is to delineate the sedimentary structures under the ground or below the seabed and to use this delineation to identify potential structural traps for hydrocarbons.A seismic technique which is specifically sensitive to natural gas deposits and not just lithology would be highly advantageous.The initial objective of the British Gas project was to evaluate the feasibility of developing such a technique, which would be innovative and would facilitate the reliable location of gas deposits. If the technique were adopted generally, a saving of some 80% of exploration drilling costs could be possible.The project has been carried out in four main stages :The first stage has involved the optimisation of the repetitive source design based on shock wave technology. Both 25 mm and 50 mm internal bore seismic guns have been constructed, with their main novel design feature a shock wave valve which will permit repetitive operation. The seismic gun was tested in the laboratory and on a local site prior to field trials. This phase of the work also involved the optimisation of seismic recording equipment and the development of algorithms for computer processing of the data.The second stage was to carry out field trials with the seismic source and associated instrumentation at the borehole site at Purton in Wiltshire, with the objectives of establishing its behaviour as a compression wave source and of comparing the results with those obtained by university geosciences groups on this site. The advantage of using the Purton site was that, geologically, it is very simple, with a thick layer of Oxford clay on limestone. The performance of the source was tested out in terms of seismic reflection on the main reflector at the clay-limestone interface at ca. 130 m and on subsidiary reflectors at shallower and greater depths. The results obtained checked closely with those obtained by university groups.The third stage was to pursue field trials at Purton with geophones capable of response in three orthogonal directions in order to examine the performance of the source capable in generating both compression and shear waves simultaneously. This stage was successful in that compressive wave reflections, shear wave reflections, shear wave refractions and wave-interconversion processes could be observed and interpreted in terms of the known geological structure of the site.The fourth stage was an offshore field trial from a barge on Loch Linnhe, Fort William, Scotland. Here the source was tested with the objective of establishing that the shock wave structure was maintained down to the seabed and that the shear and compression waves were simultaneously developed in the local granite. 3-D geophone measurements established that the gun gave a compression wave of velocity 5000 m s-1 and a vertically-polarised shear wave with a velocity of 2000 m s-1, both values characteristic of the granite. It was demonstrated, moreover, that the shear wave was produced directly from the shock wave and from no other interconversion process.Two complementary lines of investigation are being followed, (i) the development of a seismic source based on new principles and (ii) the use of interconversion processes between shear waves and compressional waves which occur in sedimentary lithology.The development of the new source had to overcome the difficulty that conventional sources which generate shear waves cannot be used near the surface of the sea since shear waves cannot propagate through water.Such sources must therefore be used at seabed level. Onshore, both vibrator and explosive charge techniques are available for shear wave generation. A design for a new seismic source, based on controlled (supersonic) shock-wave generation (without the use of explosives) has therefore been developed. This source can be used also for conventional applications, but it has features in the mode of generating shock waves which will allow it to generate shear waves of known properties in sedimentary strata. The project has involvedboth new source development and advanced signal processing in both laboratory and field testing, both onshore and offshore. Programme(s) ENG-HYDROCARB 2C - Programme (EEC) of support for technological development in the hydrocarbons sector, 1986-1989 Topic(s) 1.4 - GEOPHYSICS AND PROSPECTING Call for proposal Data not available Funding Scheme DEM - Demonstration contracts Coordinator British Gas plc EU contribution No data Address London Research Station Michael Road SW6 2AD London United Kingdom See on map Total cost No data
