Response characteristics of gradient data from the frequency-domain controlled-source electromagnetic method

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Studying how to improve the boundary recognition ability and resolution of anomalies is of great significance in the electromagnetic prospecting method. This paper analyzes the response characteristics of gradient data from the frequency-domain controlled-source electromagnetic method. A survey geometry that has one transmitter and many receiver locations, either along a line or over a grid, is used to obtain the electromagnetic field components. This kind of relatively dense coverage of receivers is needed in order to be able to determine the gradients. Spatial gradients in the two horizontal directions when measuring over a grid or in one horizontal direction when measuring along a line, and a frequency gradient which can give depth resolution, are considered in this paper. Forward modelling was carried out on a number of 3-D earth models to assess the capabilities and usefulness of the gradient data. The source–receiver geometry with survey lines arranged parallel to the direction of the grounded electric line source with measurements of the along-line component of the electric field, which is similar to the broadside geometry of marine CSEM, was found to be the best. The spatial gradients are able to indicate accurately the horizontal extents of the target, with the boundary identification ability of the spatial gradients being stronger than that of the original electromagnetic field components. Also, the frequency gradient is sensitive to the vertical extents of a target, with the main influence on the frequency gradient being the source–receiver distance, and the burial depth, resistivity and thickness of target. It is clear that the spatial and frequency gradients of electromagnetic data can provide a quick and reasonably accurate indication of the location and lateral extent of a conductivity anomaly in the subsurface, as well as an approximate depth and an indication of whether the anomaly is more or less conductive than the background.