COWLEY, JOHN, ELWYN JONES, HELMUT JAKUBOWICZ, GARETH WILLIAMS and RICHARD WOMBELL*,Veritas DGC Ltd, Crawley, West Sussex, UK

Abstract: Results from a Long Offset Experiment for Sub-Basalt Imaging

Imaging through basalt can be difficult using standard seismic techniques. Modeling suggests that the large velocity contrast at the top basalt should be an efficient generator of mode-converted energy. It has been proposed that this might provide a suitable route for successful sub-basalt imaging. In the summer of 1998,Veritas acquired a non-proprietary long offset survey (0-16km) in order to evaluate the usefulness of such data.

Long offsets are required to record such energy and traditionally this has been done using multi-boat acquisition.As well as being expensive, there can be difficulties in merging the data from the different vessels into a single data set. For this survey, a single streamer was used to record continuous offsets up to 12 km.Typically, feathering of around 10 degrees was experienced, corresponding to a crossline smear of up to 2 km.While this was greater than would be experienced with a multiboat operation, it was outweighed by the benefits of acquiring a continuous data set.To help assess data at offsets over 12 km, a second vessel was used to record offsets up to 16.5 km.

Processing of such data requires the identification and separation of mode-converted primaries from multiples, aided by suitable modeling.This is combined with techniques ranging from simple corridor stacks to image events on far-offsets, to depth migration using P and S-wave velocities.

In this paper we describe the acquisition technique employed and present results from the data.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England