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Abstract: AFMAG: Geotech’s New Airborne Audio Frequency
Electromagnetic (EM) Systemn
1BHL Earth Sciences
An airborne AFMAG (Audio Frequency electroMAGnetic) system has been developed and successfully tested by Geotech Ltd. The system uses proprietary and patentpending technology to damp vibrations and de-rotate the electromagnetic (EM) fields respectively. Three orthogonal coils are used to measure the horizontal and vertical components of these fields (Figure 1).
The EM source energy is the Earth’s natural time-varying magnetic field, in the audio frequency range of 30 to 6,000 Hz. This method is quite similar to MT or magnetotelluric methods; however, electric fields are not measured by AFMAG. Natural EM sources are nearly homogeneous, which allows deep penetration. Typical penetration depths, observed from the system tests, range from zero to hundreds of meters for small test bodies that were hundreds of meters in size. Theoretically, the AFMAG system is capable of detecting large conductive features such as salt water trapped in anticlines as deep as 10 km. Like all airborne systems, this data can be rapidly acquired over large areas, over rugged terrain and over areas where access is difficult.
The development of AFMAG was partially funded by the Ontario Minerals Exploration Technologies (OMET) program and two mining companies. Therefore initial tests, completed in August of 2004, have focused on minerals exploration applications. Airborne field tests are very encouraging and in addition to mineral targets, some large conductive features due to conductive sedimentary units have been mapped (Figure 2).
Geotech is now completing its OMET project report and is seeking suitable petroleum plays for airborne tests to define airborne
AFMAG’s performance in petroleum exploration. In addition, current development is now focusing on using the field data collected by the base station to monitor the natural EM fields and perhaps to provide a remote reference for signal processing of the EM fields. The base station data will also be assessed to determine if the number of airborne coils can be reduced to just the vertical dipole receiver coil, providing the possibility of installation onto a fixed-wing platform.
An introduction to the AFMAG method, the system, and its deployment will be presented. Results from the Sudbury test area, characterized by deep conductive targets including conductive sedimentary rocks, as well as several numerical models of typical petroleum target geometries, will also be presented.
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