HR: 15:25h
AN: NS53A-06 INVITED     [Abstracts]
TI: Radiomagnetotellurics in Near-Surface Studies of Azimuthal Electrical Anisotropy
AU: * Linde, N
EM: linde@aug.ig.erdw.ethz.ch
AF: ETH-Zurich, Institute of Geophysics, HPP O2, ETH Hönggerberg, Zurich, 8093, Switzerland
AU: Pedersen, L B
EM: laust.pedersen@geo.uu.se
AF: Uppsala University, Department of Earth Sciences/Geophysics, Villav. 16, Uppsala, 752 36, Sweden
AB: Electrical and electromagnetic methods are often used to study azimuthal electrical anisotropy, characterized by different resistivities in the two principal orthogonal horizontal directions. A large fraction of these studies focus on determining the dominant directions of fracturing and in some cases even to estimate secondary porosity and possible anisotropy in hydraulic transmissivity. The azimuthal resistivity method has the following limitations: anisotropy must be suspected beforehand since a specialized electrode configuration is needed; the electrode configuration needs to be rotated with small increments at each sounding location; the soundings must be repeated with a small offset to distinguish anisotropy from heterogeneity. Consequently, the time-consuming character of such surveys results in only a few isolated soundings and an interpretation that is based on the inspection of the apparent resistivity data only. Tensor radio magnetotelluric (RMT) data do not suffer from these limitations: data collected over heterogeneous or homogeneous anisotropic structures can be distinguished by simple analysis of the measured transfer functions; no rotation of the measurement system is necessary in the field; tensor estimates obtained at each station can be inverted for a layered or smoothly varying electrical anisotropy model. As an illustration, we present tensor RMT data (12.7-243 kHz) collected along a 380-m long profile where limestones overlie shale. The data display a clear and consistent electrical anisotropy signature and were therefore inverted for a layered 1D model with azimuthal anisotropy. The resulting models indicate that the limestone formation has an anisotropy ratio of 12 and that the underlying shales are isotropic. The resulting models provide more regional and depth-dependent information than azimuthal electrical resistivity surveys presented in the literature.
DE: 0619 Electromagnetic theory
DE: 1859 Rocks: physical properties
DE: 3260 Inverse theory
DE: 5104 Fracture and flow
DE: 5109 Magnetic and electrical properties (0925)
SC: Near-Surface Geophysics [NS]
MN: 2007 Joint Assembly