Oblique scanning ground penetrating radar

A ground penetrating radar uses an oblique or grazing angled radiation beam to provide improved coupling of radar energy into the earth reducing forward and back scatter and eliminating the need to traverse the surface of the earth directly over the investigated volume.

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 to provide a relatively larger coverage of 
scanning area 26 above the investigated volume 12. 
The surface traversed by the raster carriage may be a plane. 
It is another object of the invention to provide for a simple raster 
carriage design. A planar raster scan may be readily implemented with 
conventional mechanical elements. 
The received radar signals may indicate time delays in echoes from 
materials beneath the surface of the earth, and the computer may operate 
according to a stored program to shift the radar signals with respect to 
each other to simulate the reception of radar signals at a curved surface 
focused on individual points beneath of the surface of the earth. The 
curved surface may be one where radar signals take equal time to travel 
from a given point of focus beneath the surface of the earth to all points 
on the curved surface. 
Thus, it is another object of the invention to adapt synthetic aperture 
techniques to the unique refractive environment of an oblique ground 
penetrating radar by accommodating for refractive effects in the 
generation of the synthetic aperture. 
The computer may execute a stored program to produce a three dimensional 
representation of material beneath the surface of the earth. 
It is yet a further object of the invention to apply three dimensional 
imaging techniques to the data obtained in an oblique ground penetrating 
radar design. 
The foregoing and other objects and advantages of the invention will appear 
from the following description. In the description, reference is made to 
the accompanying drawings which form a part hereof and in which there is 
shown by way of illustration a preferred embodiment of the invention. Such 
embodiment does not necessarily represent the full scope of the invention, 
however, and reference must be made therefore to the claims herein for 
interpreting the scope of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, an investigation site 10 includes a sub-surface 
volume 12 to be investigated having a buried structure 14 such as a pipe. 
The buried structure could equally be any discontinuity in the dielectric 
properties of the earth, including abrupt moisture and density changes, 
underground pollution plumes and sub-surface air pockets. 
An antenna head 16 is located to the side of the sub-surface volume 12 
above the surface of the earth 18 and includes a transmitting antenna 20 
and receiving antenna 22. Both antennas are directed along an air path 24 
toward a surface point 28 on a scanning area 26 above the sub-surface 
volume 12. 
The scanning area 26, at its edges generally aligned along the air path 24, 
defines a y-axis of a Cartesian coordinate system. Second edges of the 
scanning area 26 crossing the air path 24 defining an x-axis of the 
Cartesian coordinate system. A z-axis of the Cartesian coordinate system 
corresponds generally to a depth within the sub-surface volume 12. 
The air path 24 intersects the surface point 28 at an angle .phi. with 
respect to the surface of the earth 18. Angle .phi. is a grazing or acute 
angle selected to provide a maximum coupling of a radar signal directed 
along air path 24 into the sub-surface volume 12 as will be described. 
The transmitting antenna 20 provides a parallel polarization to the radar 
signal passing along path 24. As mentioned, parallel polarization means 
that the electric vector 34 of the radar signal lies within the plane of 
incidence 36 including the air path 24 and a vertical axis 32 normal to 
the surface of the earth 18. 
Electromagnetic radiation directed along path 24 and striking surface point 
28 is refracted into the sub-surface volume 12 along earth path 30. Earth 
path 30 also lies in the plane of incidence 36 and has an electric vector 
34 lying within the plane of incidence 36. Earth path 30 deviates from the 
vertical axis 32 by an angle .rho.. 
The antenna head 16 is mounted on a horizontal boom 38 defining an x'-axis 
generally parallel to the x-axis. The boom extends approximately twenty 
feet and is initially positioned approximately eighteen feet above the 
surface of the earth 18. The boom 38 may be moved in a y'-axis, between 
its initial altitude and a higher altitude by means of a center 
telescoping tower 40 having its lower end attached to a base such as a 
trailer bed 50 or the like. The telescoping tower 40 is extendible by 
means of motor driven lead screws (not shown) and may be tipped left and 
right, forward and backwards and rotated about a vertical axis by 
secondary lead screws (also not shown) in order to properly align the boom 
38 with the scanning area 26 and to adjust the angle .phi. of the path 24. 
The telescoping tower 40 may be extended approximately twelve feet in the 
y'-direction to cause the antenna head 16 to rise. This in turn moves 
surface point 28 outward in the y-direction. The antenna head 16 may also 
slide back and forth in the x'-direction across the boom 38 drawn by a 
cable and pulley assembly (not shown). These movements combine so as allow 
a scanning of the surface point 28 in the x and y directions. In this way, 
the surface point 28 may be scanned over the scanning area 26 in a raster 
43. 
During a measurement of the sub-surface volume, the boom 38 incremented 
through elevations and the antenna head 16 is moved along the boom 38 at 
each elevation so as to scan a raster pattern 42 in axes x' and y' 
causing, in turn, surface point 28 to scan a similar raster pattern 43 but 
in axes x and y. 
As a result of angle .phi. between the air path 24 and the surface of the 
earth 18 being smaller than forty-five degrees, the separation between 
horizontal rows of the raster pattern 42 in y' will be less than the 
separation horizontal rows of the raster pattern 43 in y in scanning area 
26. Thus, the vertical mechanical motion required of the antenna head 16 
may be limited and still provide substantial coverage over the scanning 
area 26. 
Referring now to FIGS. 1 and 2, the angle .phi..sub.-- is set to a Brewster 
angle. A Brewster angle is an angle at which electromagnetic radiation, 
striking an interface between two materials of difference indices of 
refraction, will reflect only that portion of the electromagnetic 
radiation that is perpendicularly polarized. Electromagnetic radiation has 
perpendicular polarization if the electric vector of the electromagnetic 
radiation is perpendicular to the incident plane, a plane that includes 
the axis of propagation of the electromagnetic radiation and the normal to 
the interface surface. 
If the electromagnetic radiation striking the surface has parallel 
polarization (i.e., the electric vector is parallel to the incident plane) 
none of the electromagnetic energy will be reflected and all will pass 
through the interface. The grazing Brewster angle .phi. is determined by 
the relative index of refraction and is equal to 90 degrees minus the 
arc-tangent of the index of refraction of the entered medium (the earth in 
this case) divided by the index refraction of the incident medium (air in 
this case). The index of refraction, in turn, is the square root of the 
dielectric constant of the medium (the earth in this case). 
In the present situation, the dielectric constant of earth normally varies 
between approximately ten and twenty depending upon soil type, water 
content and radar frequency. At 0.753 GHz representing the frequency of 
average wavelength of the radar signal in the preferred embodiment of this 
invention, the dielectric constant is very close to ten. Accordingly, the 
Brewster angle will be achieved when .phi. equals approximately seventeen 
degrees. 
Referring to FIG. 2, it will be seen under these assumptions that seventeen 
degrees provides the maximum transmission of radar energy through the 
surface of the earth and the minimum reflection. 
When electromagnetic radiation is incident to the surface of the earth at 
the Brewster angle, angle .rho. will also be approximately seventeen 
degrees from vertical. 
Referring now to FIG. 1, the antenna head 16 moves the receiving antenna 22 
and transmitting antenna 20 in tandem to be substantially aligned along 
the air path 24 at the Brewster angle at all times. As will be understood 
in the art, however, the antennas 20 and 22 while having greatest 
sensitivity and transmission efficiency along the air path 24 also 
accepting and transmit radiation at other angles dictated by the shapes 
and size of their primary lobes. Generally the antennas 20 and 22 will 
provide coverage of a substantial portion of the scanning area 26 at each 
position in the raster pattern 42. 
Referring now to FIG. 3, transmitting antenna 20 is connected to 
transmitter circuitry 52 providing a controlled sine wave output between 
620 and 960 MHz according to a command provided by computer 54. 
Accordingly, transmitting antenna 20 produces a frequency modulated radar 
signal 56 beginning at a low frequency and culminating at a high 
frequency. The center frequency is selected to reduce the effect of 
roughness in the surface of the earth 18 at the scanning area 26. 
The signal 56 emitted by transmitting antenna 20 propagates along air path 
24 through the surface of the earth 18 to reflect off buried structure 14 
and to return attenuated and shifted in time as signal 56'. The received 
signal 56' is collected by receiving antenna 22 and provided to receiver 
circuitry 58 and then to one input of a mixer 60. A second input of the 
mixer 60 receives the originally transmitted signal 56. 
As a result of the relative time shift between signals 56 and 56', signal 
56' will be lower in frequency than signal 56 at any given time as a 
function of the time delay caused by the effective path length between the 
antenna head 16 and the buried structure 14. Accordingly, the mixer 60 
outputs a product of these signals 56 and 56' containing sum and 
difference frequencies. The sum frequencies are filtered out by 
conventional techniques and the resulting signal 66 is provided to the 
computer 54 to be captured as a set of sampled and digitized values 
associated with a particular antenna raster position. The computer 
includes an A/D converter sampling at approximately eighty-eight kilohertz 
to obtain 512 samples at each antenna raster position. The A/D converter 
is a sixteen-bit converter. 
As will be understood from this description, the signal captured by the 
computer 54 will contain a mixture of frequency components, one for each 
reflecting point of the buried structure 14. In fact, the mixture of 
frequencies will represent scattering from objects everywhere above and 
below the earth that are within the beam width of the antennas 20 and 24. 
Accordingly, a Fourier transform or spectrum analysis of this signal will 
produce peaks representing reflections at different distances from the 
antenna head 16 in much the same manner as if a short pulse were 
transmitted and echoes were received and measured. 
Referring then to FIG. 6 at a first step of an acquisition of information 
about a sub-surface volume 12 indicated by process block 62, radar signal 
66 is acquired at regular points over the scanning area 26. In acquiring 
this data, the antenna head 16 will move in the raster pattern 42 taking 
thirty-two measurements at spaced points along the x'-axis for thirty-two 
vertical increments along the y'-axis as the boom 38 is raised. The 
acquisition process thus acquires 1,024 separate signals 56' each 512 
points long. 
Referring to FIGS. 1 and 4, each signal 66 is stored as a set of 512 
sampled and digitized points by the computer 54 and may be assembled into 
a data matrix 67 within computer 54 where x and y dimensions of the data 
matrix 67 correspond to particular antenna raster positions at which the 
signal 56' was acquired. The z-axis of the data matrix 67 describes a 
particular sampled point within the signals 56 and more generally a depth 
of a reflection indicated by the signal 66. 
At process block 64, after the matrix of data has been obtained, it is 
preprocessed as indicated by process block 64. First a Fourier transform 
is executed on the signals 66 to produce echo data 66' shown in FIG. 4 in 
which amplitude indicates the strength of echoes and the horizontal axis 
represents the time delay of those echoes. A fixed delay period is 
subtracted from this echo data 66' by truncating the earliest sampled 
points which correspond to intrinsic delays of the system electronics. The 
data 66' is then inverse Fourier transformed to return it to its original 
frequency domain state and the echo data 66 is then normalized to account 
for drift in the sensitivities of the transmitter circuitry 52 and 
receiver circuitry 58. This normalization is based on a monitoring of 
direct coupling between antennas 20 and 22 to determine changes in the 
components' sensitivities that are unrelated to the transmission air path 
24. The antennas 20 and 22 have a minus eighty dB antenna coupling. 
At process block 77, the data 66 is again Fourier transformed and then 
while in the frequency domain, shifted 68 along the time axis, a different 
amount for each z-row of the data matrix 67. The shifting changes the 
apparent delay in receiving the echo signal. The data is then Fourier 
transformed back into the time domain. 
Referring also to FIG. 5, the shifting modifies the data 66 to appear as if 
it had been acquired over a curved raster 76. The desired raster curvature 
is one in which echoes from a single focus point 70 are received 
simultaneously at all points on the curved raster 76 while echoes from 
other points removed from the focus point 70 would be out of phase and 
thus cancel or add destructively. Such synthetic aperture radar (SAR) 
techniques are known generally in the radar art. 
Normally SAR techniques shift the data 66 of the data matrix 67 to simulate 
a raster following a section of the surface of a sphere or an 
approximation thereof. This shape places all points on the receiving 
surface an equal distance and time delay from a focus point in a 
homogeneous medium. In the oblique imaging system of the present invention 
however, this assumption must be modified because of the refractive 
properties of the earth/air interface. 
Referring still to FIG. 5, the amount of shifting 68 of the signals 66 is 
calculated by considering the propagation delay along the air paths 24 and 
24' and earth paths 30 and 30' for different points in the raster pattern 
42. The shifting of data 66 is performed so that the delay over the 
combined air path 24 (or 24') and earth path 30 (or 30') between a focus 
point 70 and all points in the raster pattern 42 will be equalized. For 
example, for a location of the antenna head 16 on the raster pattern 42 
near its top in the y'-axis (designated high point 72) and for a location 
of the antenna head 16 on the raster pattern 42 near its bottom (low point 
74), refractive effects will cause different amounts of bending of the 
combined air path and earth paths. This plus the variation in propagation 
speed between air and earth will require a curvature of the raster 76 to a 
shape that differs substantially from a spherical section surface 78 
(equidistant from point 70) as could be used in an air-only scanning. 
The generation of curved raster 76 and the necessary time shifting 68 may 
be deduced by calculating the air and earth paths for each point on the 
raster 42 using Snell's law, and computing the total delay based on the 
propagation speeds through earth and air, and shifting the data to 
equalize the total delay. 
The shifting of the signals 66 to focus the synthetic aperture on a 
particular focus point 70 is shown by process block 79 of FIG. 6. 
Referring again to FIG. 4, once the shifting is complete, a set of x-y 
plane slices 80 of the data matrix 67 holding the echo data from the focus 
point 70 are transformed by the two-dimensional Fourier transform along 
the x and y axes. The transformation integrates the various data collected 
at different antenna raster positions into an image of focus point 70 to 
provide a synthetic aperture of an antenna comparable to the size of the 
entire raster pattern 42. 
The result of the transformation is a set of slice images 82. A small 
volume 84 about the focus point 70 will also be acceptably in focus. At 
process block 88 of FIG. 6, points within the volume 84 are harvested (at 
different x, y and z values) and placed in an image matrix 86 
corresponding in size to data matrix 67 but providing image data. 
This image matrix 86 may be further transformed geometrically to 
accommodate the fact that the earth path 30 is not vertically through the 
surface of the earth but canted at the refraction angle from vertical. The 
z-axis may also be compressed reflecting the slower propagation velocity 
of electromagnetic waves through the earth. 
At process block 90, a new focus point 70 is established outside the volume 
84 and the processes of process block 77, 79, 88 and 90 are repeated until 
an entire image is developed in image matrix 86. 
The data of the image matrix 86 may be printed on a slice basis or may be 
presented in perspective version to the user for the identification of 
underground materials, objects or pollutants. 
The above description has been that of a preferred embodiment of the 
present invention. It will occur to those that practice the art that many 
modifications may be made without departing from the spirit and scope of 
the invention. In order to apprise the public of the various embodiments 
that may fall within the scope of the invention, the following claims are 
made.