Publications

Multi-transient electromagnetic (MTEM) is a dipole-dipole time-domain galvanic electromagnetic technology, implemented in land, transition zone and shallow marine environments, suitable for mapping subsurface resistors, such as petroleum reservoirs across multiple terrains. Because MTEM is a full bandwidth, high resolution, field-optimized technology it is an ideal complement to seismic data. Most previous electromagnetic studies considered only isotropic resistivity. This new study evaluates the implications of resistivity anisotropy that when included in MTEM models results in more accurate depth inversions. The effects of microscopic (intrinsic) and macroscopic (effective) anisotropy on the VTI or polar anisotropy are evaluated in this study. Intrinsic anisotropy arises from rocks having flat minerals layered parallel with the bedding, whereas effective anisotropy arises when lithologies of different resistivity are inter-bedded. Earth electromagnetic responses to a transient source are greatly dependent on resistivity anisotropy. There is no isotropic solution to an anisotropic halfspace, and a multi-trace isotropic inversion of synthetic data arising from an anisotropic model will lead to misleading results, such as target depth estimated too shallow, exaggerated transverse resistivity for the target zone, and underestimated background resistivities. High levels of background anisotropy can obscure the target at a fixed offset which can be solved by increasing offset distance. There is a tradeoff in acquisition cost though, since signal strength decreases with increased offset. Anisotropy must therefore be accounted for and included in inversion schemes. A practical way to estimate the effective anisotropy directly from the data is shown based on the early and late parts of the step response providing values for the horizontal resistivity and the geometric mean resistivity.