Application of Conventional 3D Data to Geohazard Assessment

Abstract

Summary The 3D seismic method has now seen over ten years application as an exploration tool in N.W. Europe. During this period the technology employed, volumes of data and cost per square kilometre have all been subject to significant changes. As a consequence, data quatity and the ability to interpret subtle and complex features have seen tremendous advances and it is now generally agreed that the 3D seismic technique represents one of the most significant advancesseen in upstream technology in recent times. Whilst it is too early to classify the 3D method as ‘mature’ the technique is now sufficiently well-established to encourage geophysicists to investigate other uses for the 3D data volume. Its application to geohazard assessment is one area which has received considerable attention in recent years. The manner in which this eon be achieved, with particular reference to the assessment of shallow gas risk, is presented in this paper. The technical and commercial benefits which can result from utilising good quality 3D exploration data are illustrated. The limitations and pitfalls of the method are also discussed and illustrated by comparison of 3D data withconventional high resolution imaging techniques. Finally, a methodology is defined which shows how high resolution 2D and exploration 3D data can be combined within a regional framework to provide a comprehensive and cost-effective geohazard assessment scheme. Introduction The use of conventional exploration 3D seismic data for geohazard assessment has been the subject of much discussion over the last 5 years or so. This initiative has come at a time when conventional high resolution technology for geohazard applications is reaching a mature phase in its development with very significant advances having been made in both acquisition and interpretation during the 1980s and early 1990s. Increased availability of exploration 3D data now often acquired at the exploration rather than development phase in an offshore project coupled with the dense spatial sampling and relatively large areal extent of such surveys makes this data a valuable asset from which operators naturally wish toobtain maximum benefit. At a time when cost saving initiatives in the industry are prevalent there are a number of incentives to utilise existing 3D exploration data as an integral pad of an overall geohazard assessment scheme. Taking an extreme view, there is a temptation to uw such data as a substitute for high resolution data which has traditionally formed the primary geohazard assessment database. The factors involved in assessing the value of 3D exploration seismic data for geohazard purposes are reviewed in this paper. Both theoretical and practical aspects are considered and the conclusions are supported by examples from 3D exploration and conventional 2D high resolution data sets. Availability of 3D Seismic Data The rapid growth in the volume of marine 3D seismic data squired for exploration and development purposes (Figure 1) and the general increase in the area of individual .survey to make available vast volumes of 3D seismic data to operating oil companies and partner groups worldwide.