Saurashtra

Exploration with controlled–source electromagnetics under basalt cover in India

Prelude

This case history describes the use of long-offset time-domain electromagnetic surveying (LOTEM) to penetrate thick basaltic cover and image hydrocarbon-bearing sediments in the Saurashtra basin, India.

Special Thanks

Thanks to the Society of Exploration Geophysicists and KMS Technologies for permission to reproduce figures and adapt text from the source material. This case history was adapted from the Leading Edge article “Exploration with controlled–source electromagnetics under basalt cover in India” by [SP07].

Abstract

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Recently, there has been increased interest, mostly offshore and to some extent onshore, in the controlled-source electromagnetic (CSEM) method because of its ability to map thin resistive layers sometimes associated with hydrocarbons.

CSEM can complement other geophysical techniques in difficult areas, in particular areas with basalt cover and high seismic velocities. One CSEM method, long-offset transient electromagnetics (LOTEM), has intriguing possibilities because its acquisition/processing can be carried out in a manner similar to seismic surveys. In addition, the number of measurements per day is relatively high and the subsurface resistivity structures are reliable. Unlike marine CSEM, where we often look for a resistive reservoir in a conductive background, land EM methods are often looking for conductive sediments to infer structural or stratigraphic information.

This case history describes how LOTEM was used for sub-basalt imaging in India in the late 1980s. A well drilled in the late 1990s confirmed the LOTEM interpretation which was based on various 1D inversion methods and 3D modeling.