Expanding the frequency spectrum in marine CSEM applications

In this paper we show how marine controlled source electromagnetic data interpretation can be improved if the data are acquired with an expanded frequency spectrum. Especially in the case of targets located at a wide range of depths, both high- and low-frequency data can provide useful information for improving the risk analysis associated with different prospects with variable size and depth. We discuss an application to a real data set acquired in deep water offshore Nigeria using two values of fundamental frequency: 0.05 Hz and 0.25 Hz. Both frequencies, together with higher frequency harmonics, have been used for multi-frequency and multi-dimensional modelling and inversion with excellent results.
At the end of the interpretation work, areas with different electric and magnetic anomalies were mapped and quantitatively interpreted in terms of resistivity distribution in the 3D space. Relatively high-frequency data (0.25 Hz and the first two harmonics) were useful for constraining the shallowest part of the resistivity models, including the presence of thin resistive layers, such as the gas sand recognized in correspondence of the well previously drilled in the NW portion of the block.
Low-frequency data (0.05 Hz) were useful for constraining the deepest portion of the models (from 2 km to 4 km below sea floor) and for characterizing the resistivity of the background.
From an exploration point of view, the whole workflow based on multi-frequency data analysis of this electromagnetic data set was very useful for further de-risking the different prospects previously individuated in the area using seismic information.