Electricity: measuring and testing – Of geophysical surface or subsurface in situ – By aerial survey
Patent
1981-07-08
1985-01-08
Strecker, Gerard R.
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
By aerial survey
324334, G01V 3165, G01V 316
Patent
active
044929242
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for determining the electrical conductivity of the ground, particularly in connection with aerial prospecting, with the use of a transmitter unit and a receiver unit located at a distance from said transmitter unit and carried by an aircraft, whereat the transmitter is arranged to generate an electromagnetic field which induces an electric current in the ground, and the receiver is arranged to measure magnetic and electric fields induced by said current at the measuring site, said fields representing a measurement of said electrical conductivity and variations therein. To this end, the method according to the invention is mainly characterized in that the magnetic field from the transmitter is transmitted horizontally and is directed transversally of an imaginary line that connects the transmitter with the receiver; that the electrical field E from the transmitter is measured in the receiver in a radial direction to the transmitter; that the magnetic field H is measured in a direction parallel with the magnetic transmitter-field; and in that of the measured magnitudes the ratio E/H and/or the phase difference .psi. are formed, both of which are dependent on the electrical conductivity of the ground.
The characterizing features of the apparatus and other characterizing features of the invention are set forth in the depending claims.
Different methods using electromagnetic techniques for prospecting ore have been used since 1920. Well known examples include Turam, Slingram, Rotary-field and Input. A common feature of this class of methods is that the mutual magnetic coupling between two coils is measured in one way or another, and utilized as a measurement of the electrical conductivity of the ground.
By utilizing a spectrum of frequencies, or the frequency content of a pulse or a train of pulses, it is possible to expand the scope of measurement with regard to electrical conductivity. In accordance with the invention there has been developed an alternative electromagnetic method, where the mutual coupling between an electric and a magnetic dipole is utilized instead. The invention affords the advantages whereby the whole of the resistivity range of interest can be covered with only one measuring frequency, and that resistivity probes can be made in a simple fashion. The method according to the invention can be applied to particular advantage when measuring the electrical conductivity of the ground from an aircraft. This is mainly due to the absence of primary electrical fields, since the invention pertains to the use of a magnetically induced electromagnetic field for prospecting purposes.
Described in the U.S. Pat. No. 3,936,728 is a method and apparatus for assaying the distribution of the electrical conductivity in the ground. Information is obtained concerning the distribution of the electrical conductivity (resistivity) of the ground between two locations, by generating an electro-magnetic field whose frequency can be varied within a desired range at one location, and by detection and comparison at the other of said locations, at a plurality of transmitted frequencies, of components of the magnetic and/or electric field that are in phase or out of phase with a reference signal generated electromagnetically at the said other location from the transmitted field, whereat no cable or other physical connection between the two locations is required. There is also described in detail the apparatus by which the electric field is produced, the reference signal generated, and the in phase and out of phase components of the magnetic and/or electrical field detected. Among the fields of application, it is mentioned that horizontal or other discontinuities in the distribution of the electrical conductivity are detected, for mapping geothermic and permafrost zones, or for discovering subterranean water fields or anomalous zones formed by bodies of ore.
Described in the U.S. Pat. No. 4,047,098 is a method and apparatus for sea-prospecting, in which an electromagn
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Boliden Aktiebolag
Strecker Gerard R.
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