Seeing through a glass, darkly: strategies for imaging through basalt

Abstract

It is a well known problem that thick sequences of lava flows make it difficult to image structure beneath them using seismic methods. This is of particular relevance to oil companies seeking to explore potentially hydrocarbon-bearing sediments in basins with voluminous magmatism. Such basins abound in many current frontier exploration areas, including the Atlantic margin of NW Europe, the South American and African margins of the South Atlantic and both onshore and offshore India. In this paper we summarise recent measurements of the effective attenuation of layered lava sequences from Vertical Seismic Profiles (VSPs) that have been acquired in boreholes penetrating basalts. We then show theoretical modelling results which suggest that much of the effective seismic attenuation is caused by impedance contrasts and hence scattering at the tops and bases of the multiple thin lava flows from which the basalt section is built. Finally we suggest ways in which marine sources and receivers can be optimised in order to enhance the low-frequency response which is required for imaging through sections of stacked basalt flows.