Design, construction and testing of an EMAP system for high resolution electrical conductivity studies

Abstract

The ElectroMagnetic Array Profiling (EMAP) technique was developed by Bostick (1986) as a method of removing static shift from magnetotelluric (MT) soundings. An additional benefit of EMAP is the high resolution of near vertical electrical conductivity contrasts that the method offers. This is the primary reason for developing an EMAP system at Victoria University. The EMAP system required the development of both hardware and software. Hardware was developed on Protel98TM an electronics computer aided design program with the PCB boards being manufactured commercially. The software for processing EMAP data EDGE (Emap Data processinG Engine) was written on MatlabTM with a GUI (Graphical User Interface) for easy of use. A field test for the EMAP system was performed in Waitarere forest, approximately 10km north-west of Levin. The area has simple geology with sand and gravel overlaying basement, a previous seismic survey 6 km to the south west found an average basement depth of approximately 380 m. The data were processed using EDGE to give apparent resistivity and phase plots and a simple 1-D Bostick inversion of structure. The apparent resistivity and phase were also used in a 1-D Occam inversion scheme. The results from this were in good agreement with the previous survey, with EMAP giving a value of between 450-500 m for the depth to basement. The Waitarere forest site had a large amount of 50 Hz mains hum and associated harmonics present which had a degrading effect on the high frequency data. This led to the high frequency band data not being included in the final Occam inversion (an approach to the regularized inversion of nonlinear problems).