Advances in aspects of the application of magnetotellurics for mineral exploration

The high-frequency magnetotelluric method, audio-MT (AMT). ii currently being widely used for mining exploration, especially in Canada. However, there are still some aspects regarding its implementation that need to be considered. These range from signal detection and processing to response funcrbn analysis to appreciation of three-dimensional effects. The main natural electromagnetic source at the range of frequencies covered by mining scale MT, audio- frequencies of 10 Hz - 20 kHz. is the global system of lightning. Due to the physical characteristics of the Earth's ionosphere and atmosphere, there is a minimum in the electromagnetic spectrum around 1.000-3.000 Hz. which is exactly the frequency range that is first sensitive to the presence of a typical conducting body. Some ore deposit exploration is being carried out in areas where there is existing mining activity, thus the data can be seriously affected by noise. The classical processing schemes are based on either the Fourier or the windowed Fourier transforms, and these methods do not readily separate noise from signal. The application of the wavelet transform offers an analysis of the time series at the frequency and time domains simultaneously. One of the main problems during the interpretation stage of MT data is the detection and removal of galvanic distortion effects caused by near-surface mhoinogeneities. In mining exploration there is the additional problem that the targets are complicated 3D structures, and thus the classical 3D;2D decomposition schemes fail For this reason a new 3D 3D algorithm has been designed. Target bodies are usually complex in geometry and are strongly conductive, requiring full 3D interpretation of the data. Different structures can be inductively coupled adding a new difficulty to this geophysical method. We have undertaken analyses to check the validity of 2D interpretations over 3D regional structures. hi this paper we describe our efforts in these four aspects of MT exploration.