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US 4,070,612 AGranted
Method and apparatus for measuring terrain resistivity
AssigneeGeonics Limited
InventorsJames Duncan McNeill, Gary George Black, Miroslav Bosnar
Life of this patent
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AssigneeGeonics Limited
InventorsJames Duncan McNeill, Gary George Black, Miroslav Bosnar
Estimating the term…
Apparatus is provided which may be made portable and is calibrated to provide direct readings of terrain conductivity. Transmitter and receiver coils are separated by less than one tenth of the electrical skin depth in terrain having a maximum conductivity to be measured, at the frequency of alternating current signal applied to the transmitter coil, and the signals picked up by the receiver coil are processed to cancel received signal components in phase with the transmitted signal and amplify a received quadrature component so as to provide a direct indication of ground conductivity. The in-phase component received direct from the transmitter coil is cancelled by a preset equal and opposite component, and the in-phase component of the secondary signal received from the terrain is substantially cancelled by a form of negative feedback, in order to avoid swamping of the wanted signal. Optionally, the negative feedback is disabled and the in-phase component is displayed in order to provide an indication of the magnetic susceptibility of the terrain being surveyed.
A direct reading terrain conductivity meter comprising a transmitter coil, a receiving coil horizontally spaced from the transmitter coil above terrain whose conductivity is to be measured, signal generating means connected to the transmitter coil to supply an alternating current thereto, signal processing means connected to receive signals from the receiver coil and the signal generating means and compare said signals so as to isolate from the signal from the receiver any component in quadrature with that from the generating means, and means linearly responsive to the amplitude of said quadrature component and calibrated to provide a direct read out of the conductivity of terrain being surveyed, the spacing D of the coils in meters and the frequency F of the alternating current in Hertz being restricted to values such that D does not substantially exceed about 50/.sqroot.FG where G is the maximum terrain conductivity to be measured in mhos/meter.
A method of measuring terrain conductivity comprising generating an alternating current having a frequency F Hertz, and applying the current to a transmitter coil for transmission to the terrain, receiving signals from the terrain by means of a receiver coil spaced from the transmitter coil above the terrain by a distance D of not more than about 50/.sqroot.FG where G is the highest terrain conductivity to be measured in mhos/meter, isolating the quadrature component of the received signal, and using the amplitude of said signal as a linear indicator of terrain conductivity.
A method of measuring terrain conductivity up to a maximum of about G mhos/meter, comprising generating a signal at a frequency F Hertz and applying it to a transmitter coil mounted on a structure portable over terrain whose resistivity is to be measured, receiving signals from said transmitter coil by means of a receiver coil mounted on said structure at a distance D meters from said transmitting coil, where D is not more than about 50/.sqroot.FG, generating a cancellation signal and combining it with the signal received by the receiver coil, the cancellation signal having a first, preset component equal and opposite to a primary signal received by the receiver coil directly from the transmitter coil, and a second variable component equal and opposite to the in-phase component of a secondary signal received by the receiver coil from the terrain as a result of currents induced therein by the primary signal from the transmitter coil, and synchronously detecting the combined signal on a phase axis in quadrature to the transmitted signal to provide an output whose amplitude is directly proportional to the conductivity of the ground.
Dependencies are inferred from the claim text.