System and method for earth probing with deep subsurface penetration using low frequency electromagnetic signals
First Claim
1. A system for earth exploration by generating and radiating low frequency electromagnetic energy deeply into the earth and analyzing return information, the system comprising:
- a) means for generating a series of pulsed electromagnetic signals in a low frequency range less than about 3 Megahertz, so as to form a series of electromagnetic pulses having respective spectra which collectively cover a portion of the low frequency range that is substantially larger than any individual pulse'"'"'s spectrum;
b) means for radiating the electromagnetic pulses into the earth, so as to penetrate the earth to substantial depths;
c) means for detecting electromagnetic reflections from subsurface layers or objects; and
d) means for extracting deep subsurface return information from the electromagnetic reflections.
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Accused Products
Abstract
An earth probing system uses deep penetration of electromagnetic waves into soil and other media. Advantage is taken of the lower attenuation of radar waves in soil by frequencies of about three megahertz or less. Bursts of electromagnetic energy of various frequencies in this range are consecutively transmitted. The transmitting antenna is continuously tuned, so as to maintain resonance during each burst, allowing large circulating currents and high power output. In a receiving antenna system, a dual antenna arrangement is providing for obtaining improved reception. A corresponding dual antenna circuit employs "spatial notch filtering", automatic adjustment of antenna gain-frequency variations, as well as compensation for transmitter gain variation. The system may be implemented in a totally analog, totally digital, or hybrid manner. Preferably, a signal processing method detects and digitally samples signals reflected from subsurface layers and buried objects. The invention provides means for removing system distortions and interfering signals, for compensating for aliasing errors and frequency-dependent antenna gain and phase variations, and for avoiding masking errors introduced by strong reflections.
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Citations
34 Claims
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1. A system for earth exploration by generating and radiating low frequency electromagnetic energy deeply into the earth and analyzing return information, the system comprising:
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a) means for generating a series of pulsed electromagnetic signals in a low frequency range less than about 3 Megahertz, so as to form a series of electromagnetic pulses having respective spectra which collectively cover a portion of the low frequency range that is substantially larger than any individual pulse'"'"'s spectrum; b) means for radiating the electromagnetic pulses into the earth, so as to penetrate the earth to substantial depths; c) means for detecting electromagnetic reflections from subsurface layers or objects; and d) means for extracting deep subsurface return information from the electromagnetic reflections. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
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14. A deep probing radar generating and radiating system for use in obtaining reflected signal data to permit mapping of layers or objects beneath the surface of the earth or other media, the system comprising:
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a) means for generating pulses of high power electromagnetic waves of various respective frequencies, each frequency being in a low frequency range of less than about 3 MHz; b) a transmitting antenna which is small in comparison to the electromagnetic waves'"'"' wavelengths, the transmitting antenna responsive to the generating means to radiate the pulses of electromagnetic waves deeply into the earth or other media; c) timing means for controlling the frequency of the waves generated by the generating means so as to cause respective frequency spectra of the pulses to collectively cover a substantial percentage of the low frequency range; and d) means for tuning the transmitting antenna to resonance in response to a control signal from the timing means so as to generate the high power pulses, the control signal changing between at least some of the pulses. - View Dependent Claims (15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28)
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29. A receiver subsystem for use in a system for earth probing having a transmitting antenna, the receiver comprising:
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a) first and second receiving antennas for producing respective first and second antenna output signals, the first receiving antenna positioned much closer to the transmitting antenna than the second receiving antenna; b) means for attenuating the first antenna output signal to produce an attenuated signal; and c) means for combining signals derived from the attenuated signal and the second antenna output signal, so as to substantially remove signal components from the second antenna output signal that are caused by direct signal transmission from the transmitting antenna. - View Dependent Claims (30)
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31. A receiver subsystem for use in a system for earth probing having a transmitter driving a transmitting antenna at a band of frequencies, the receiver comprising:
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a) first and second receiving antennas for producing respective first and second antenna output signals, the first receiving antenna positioned much closer to the transmitting antenna than the second antenna; b) first and second amplitude and phase detectors, connected to receive the respective first and second antenna output signals and provide respective first and second detector outputs expressed as respective first and second complex numbers; and c) means for dividing the second complex number by the first complex number to provide a quotient complex number which contains accurate information regarding subsurface reflections of transmitted energy but is compensated for frequency-dependent amplitude and phase variations of the transmitter, variations in earth loading and changes in antenna operating height above the ground.
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32. A receiver subsystem for use in a system for earth probing having a transmitter and an associated transmitting antenna, the receiver comprising:
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a) first and second receiving antennas for producing respective first and second antenna output signals, the first receiving antenna positioned much closer to the transmitting antenna than the second receiving antenna; b) means for attenuating the first antenna output signal to produce an attenuated signal; c) means for combining signals derived from the attenuated signal and the second antenna output signal, so as to substantially remove signal components from the second antenna output signal that are caused by direct signal transmission from the transmitting antenna, the combining means providing a combiner output; d) first and second amplitude and phase detectors, connected to receive the combiner output and the attenuator output, respectively, and provide respective first and second detector outputs expressed as respective first and second complex numbers; and e) means for dividing the second complex number by the first complex number to provide a quotient complex number which contains accurate information regarding subsurface reflections of transmitted energy but is compensated for frequency-dependent amplitude and phase variations of the transmitter, variations in earth loading and changes in antenna operating height above the ground. - View Dependent Claims (33, 34)
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Specification