Use of marine controlled‐source electromagnetic sounding for sub‐basalt exploration

The problem of imaging sedimentary structure obscured by high‐velocity layers, such as carbonate, basalt or salt, using conventional seismic techniques is well known. When this problem is encountered in offshore areas, marine electromagnetic data can provide valuable, alternative and complementary constraints on the structure. We concentrate on the use of marine controlled‐source electromagnetic (CSEM) sounding in the frequency domain. The feasibility of mapping sub‐basalt sedimentary structure using this technique is explored by means of modelling studies which mimic a type of survey which could readily be employed in practice. As a starting point the problem is addressed in terms of 1D resistivity structure. We show that sub‐basalt sediments can be detected and their depth of burial quantified to within 200 m in the examples shown. The presence of small‐scale inhomogeneities in the basalt (which cause much of the scattering in seismic data) is shown to have no appreciable effect on the ability of the CSEM data to detect the sediments. The modelling is then extended to 2.5 dimensions. Again the presence of sub‐basalt sediments can be readily detected and their properties and geometry quantified, especially if the electromagnetic data are combined with constraints from complementary geophysical techniques.