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Summary Airborne electromagnetic (AEM) methods are widely used for mineral exploration. However, until recently AEM data acquired for geophysical exploration have been interpreted using 1D layered-earth models calculated for each transmitter-receiver pairs. These 1D resistivity models are often stitched or interpolated in order to produce a pseudo- 3D model over the survey area. However, the geological structures of particular interest, such as mineral deposits, are poorly resolved with these various 1D methods. Exploiting the fact that an AEM system’s sensitivity domain is significantly smaller than the area of an AEM survey, we have developed a practical 3D inversion methodology based on the moving sensitivity domain approach. This approach has resulted in a paradigm change for AEM data interpretation by making it possible to invert entire AEM surveys with no approximations into high-resolution 3D geoelectrical earth models.

We introduce a 3D inversion methodology for towed streamer EM data that takes into account anisotropy and includes a moving sensitivity domain. Our implementation is based on 3D IE method for computing the responses and Frechet derivatives, and uses re-weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. Interpretation of the towed streamer EM data is a difficult problem because the data are acquired over large areas with huge number of moving towed streamer EM system positions. We overcome this problem by exploiting the concept of moving sensitivity domain, which is implemented using the IE method. In the framework of this concept, for a given transmitter-receiver pair, the responses and Frechet derivatives are computed from a 3D earth model that encapsulates the towed EM system’s sensitivity domain. The Frechet matrix for the entire 3D earth model is then constructed as the superposition of Frechet derivatives from all transmitter-receiver pairs over the entire 3D earth model. This makes large-scale 3D inversion a tractable problem with moderate cluster resources. We present a case study of 3D anisotropic inversion of towed streamer EM data from the Troll West Oil Province.

Following recent advances in SQUID technology, airborne full tensor magnetic gradiometry (FTMG) is emerging as a practical mineral exploration method that is intended to recover information about remanent magnetization. In this paper, we introduce 3D regularized inversion of FTMG data that recovers the total magnetization vector in each cell of the 3D earth model. If a priori information about the susceptibility or remanent magnetization is available, the 3D inversion can be constrained to recover the remanent magnetization vector. If a priori information is not available, it is possible to recover attributes of remanent magnetization such as the amplitude and angle of the magnetization vector relative to the inducing field. We present a case study for data acquired over a dyke swarm in South Africa that compares our 3D FTMG inversion for magnetization with a 3D total magnetic intensity (TMI) inversion for a positively-constrained susceptibility distribution. Given the significant remanent magnetization present, the 3D FTMG inversion for magnetization recovers results that are most consistent with the known geology.

A towed streamer electromagnetic (EM) system capable of simultaneous seismic and EM data acquisition has recently been developed and tested in the North Sea. We introduce a 3D inversion methodology for towed streamer EM data that includes a moving sensitivity domain. Our implementation is based on the 3D integral equation method for computing the responses and Frchet derivatives, and uses the re-weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. We present a model study relevant to hydrocarbon exploration in the North Sea. We demonstrate the ability of a towed streamer EM system to detect and characterize the Peon discovery, which is representative of an infrastructure-led shallow gas play in the North Sea. We conclude that 3D inversion of towed streamer EM can adequately recover hydrocarbon-bearing formations to depths of approximately 2 km. Obviating the need for ocean bottom receivers, the towed-streamer EM system enables EM data can be acquired over very large areas in frontier and mature basins for higher production rates and relatively lower cost than conventional CSEM methods.