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Resolution Of 3D Earth Structures - Detection And Measurement Configuration Using Different Sources.Normal access

Authors: E. Auken and G.M. Hoversten
Event name: 9th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
Session: Innovations In Electrical And Electromagnetic Methods
Publication date: 28 April 1996
Organisations: EEGS
Language: English
Info: Extended abstract, PDF ( 1.09Mb )

Summary:
In this paper it is shown that a plane wave source, an infinite line source and a vertical magnetic dipole source
generate approximately equal peak responses from a thin three dimensional (3-D) conductor. The main difference between
source types come in the layered response. In general the plane wave source is greater than the infinite line source layered
response which in turn is larger than the dipole layered response. For the anomaly index, as measured by the ratio of
secondary to layered response, the magnetic field component is largest for the dipole source and smallest for the plane wave
source whereas for the electric field the magnetic dipole has the smallest anomaly index. This is true both when the dominant
mode of excitation is current gathered into the scatter from the surroundings (channeling currents) and current induced in
the scatter (vortex currents).
We consider large and small transmitter loops with in loop and out of loop receiver configuration in both the
frequency and the time domain, Most of the currently used prospecting system configurations in either mining or
environmental work are thus covered. In the time and the frequency domain we find that a system which measures the
horizontal magnetic field has a anomaly index one to two orders of magnitude larger than a system which measures the
vertical magnetic field. Furthermore, conclusions regarding the optimum source-receiver configuration are dependent on
the target orientation. In the time domain, for a vertical thin sheet, the anomaly index is largest when the loop size is of the
order of the depth to the conductor whereas in the frequency domain a much larger loop can be employed.

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