Communications: directive radio wave systems and devices (e.g. – Transmission through media other than air or free space
Patent
1999-03-03
2000-07-25
Sotomayor, John B.
Communications: directive radio wave systems and devices (e.g.,
Transmission through media other than air or free space
342129, 342191, G01V 300, G01S 1388
Patent
active
06094157&
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Ground penetrating radar is known for use in detecting underground structure such as pipes or the like. In such systems, microwave frequency radio (radar) signals are transmitted into the earth, and echoes off sub-surface structures are detected and displayed.
In order to increase the signal strength of the returning echo, the microwave transmitting antenna is typically placed close to the surface of the earth to direct energy directly downward. The antenna may be attached to the back of a truck that is driven over the surface of the site to be investigated. Multiple readings or a continuous band of readings may be obtained and a plot produced in which movement of the antenna is plotted in the x-axis and the echo signal is plotted in the y-axis. Stronger echo signals are represented by a darker shading so that a pipe or similar echo producing structure may be identified.
The images produced by these techniques can be of low quality because of the difficulty of coupling adequate radar energy into the earth. The need to move the antenna over the surface of the earth is cumbersome and in many important applications may be difficult or impossible.
SUMMARY OF THE INVENTION
The present invention provides a ground penetrating radar system in which the radar signal is directed at an acute angle with respect to the surface of the earth. In particular, the angle chosen is a Brewster angle at which a parallel polarized radar signal is almost completely coupled into the earth. The acute angle of the radar beam allows scanning of a sub-surface volume eliminating the need to traverse the ground over the volume to be investigated.
More specifically, the present invention provides a ground penetrating radar having a radar transmitter providing a microwave electrical signal that is coupled to a radar antenna transmitting the radar signal along a primary transmission axis at which greatest power is emitted. An antenna head supports the radar antenna with respect to the surface of the earth so that the radar antenna's primary transmission axis intersects the surface of the earth at a Brewster angle, the Brewster angle being a function of the dielectric constant of the earth. A radar receiver receives reflected radar signals from the radar antenna. An electronic computer processes the received radar signals to output an indication of materials beneath the surface of the earth.
The Brewster angle will typically be an angle from ten to thirty-five degrees and the radar antenna will impart a parallel polarization to the transmitted radar signal.
Thus, it is one object of the invention to provide an improved coupling of radar energy into the earth for the purpose of ground penetrating radar measurements. Although intuitively one might expect grazing angles to increase the reflection the radar signal off the earth's surface, for a properly polarized radar signal at the Brewster angle, more energy is coupled into the earth than is coupled into the earth at an angle normal to the earth's surface as is traditionally used.
Better coupling increases the usable energy of the radar signal by: 1) reducing back-scatter from the surface of the earth (such as may mask the echo signals), and 2) reducing forward scatter from uncoupled energy that may reflect from other objects and cause interference with the coupled energy path.
The antenna head may include a raster carriage moving the antenna head to a plurality of points over a surface extending across the primary transmission axis.
Thus, it is yet another object of the invention to allow scanning of a sub-surface volume without the need to move an antenna closely over the surface of the volume being investigated. The grazing angle of orientation of the radar signal allows scanning in an essentially vertical plane from a single position to the side of the investigated volume. This can be important when the surface over the investigated volume is not readily accessible or is hazardous. The low grazing angle of the scanning allows a limited vertical raster scan
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Sotomayor John B.
Underground Imaging, Inc.
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