System for detecting underground objects

A system for detecting underground objects is disclosed wherein image data obtained through a deep range-migration correction and image data obtained through a shallow range-migration correction are combined so that data at the corresponding positions on both the image data at high level may mutually intensify, whereas other data may mutually weaken. The target spot on the thus combined image data is made sharp and provides sufficently high resolution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a system for detecting underground objects 
by a synthetic aperture method, and more particularly, to improvement of 
the resolution in the system. 
2. Description of the Prior Art 
In FIG. 2 is shown a general form of the so-called synthetic aperture 
underground targets detecting system which is structured of a general form 
of the synthetic aperture processing in use for satellite-radar and 
airborne radar and, in addition thereto, means for geological correction 
indispensable to detection of underground targets. As examples of 
techniques to which a system as aforementioned is basically applied, there 
are known such as disclosed in a paper entitled "Electromagnetic Detection 
of Underground Objects", pages 308-311, Proceedings of the Institute of 
Electronics and Communication Engineers of Japan, Vol. 67, No. 3, March 
1984. In the explanatory chart of the general form in FIG. 2, ST1 is a 
step of collecting reflected wave profile data, ST2 is a step of 
performing preprocessing in succession to step ST1, ST3 is a step of 
performing synthetic aperture processing in succession to step ST2, ST4 is 
a step of performing geological correction in succession to step ST3, and 
ST5 is a step of performing output processing in succession to step ST4. 
FIG. 3 is a diagram for explaining the collection of the reflected wave 
profile data at the aforesaid step ST1, referring to which reference 
numeral 1 denotes a target such as a pipe, 2 denotes soil in which the 
target 1 is buried, 3 denotes a transmitter, 4 denotes a transmitting 
antenna for emitting a pulse signal from the transmitter 3 as an 
electromagnetic wave into the aforesaid soil 2, 5 denotes a receiving 
antenna for receiving the wave reflected by the target 1 of the 
electromagnetic wave emitted from the transmitting antenna 4, and 6 
denotes a receiver connected to the receiving antenna 5, where the 
transmitting antenna 4 and the receiving antenna 5 are fixedly held at a 
predetermined mutual distance y and adapted to move in increments of a 
predetermined distance in a direction at right angles with the direction 
in which both the antennas 4, 5 are disposed, as indicated by the arrow X. 
Below will be described the operations. First, at step ST1, collection of 
reflected wave profile data is performed on a plane cutting through the 
soil at right angles with the ground. That is, a monocyclic pulse, for 
example, is emitted from the transmitting antenna 4 at every increment in 
the movement and the reflected wave is received by the receiving antenna 
5. The reflected wave from the target 1 is received in the shortest period 
of time when both the transmitting antenna 4 and the receiving antenna 5 
are directly above the target 1 and the time becomes longer as the 
antennas separate from the position right above the target, and thus 
parabolically spreading reflected wave profile data are obtained for each 
of the targets 1. 
Since the monocyclic pulse propagating through the soil 2 is greatly 
attenuated, largely distorted, and accompanied by noises at high level, 
and further, since there is a direct coupling between the transmitting 
antenna 4 and the receiving antenna 5, the direct coupling and noises, and 
further, the distortions are removed in the preprocessing at step ST2. At 
the following step ST3, such preprocessed reflected profile data are 
subjected to synthetic aperture processing. That is, the reflected wave 
profile data are provided with range migration correction and the 
hyperbolic data corresponding to each target 1 are made to cohere around 
the vertex portion, and thereby, image data thereof are obtained. 
The thus obtained image data are still those expressed with respect to the 
scale of time. Therefore, geological correction is performed in the 
following step ST4 with dielectric constant .epsilon. S of the soil 2 
used, and thereby, image data expressed with respect to the scale of 
length are obtained, and then, the image output of the detected targets is 
displayed on a display device or the like at step ST5. 
Since the prior art underground target detecting systems have been 
constructed as above, there have been such problems with them that the 
target spots on the obtained image data have not been sharp ones and have 
been affected by noises, distortions, or the like that are not completely 
removed in the preprocessing and therefore sufficient resolution has not 
been obtained. 
SUMMARY OF THE INVENTION 
The present invention was made to solve the aforementioned problems, and 
accordingly, a primary object of the present invention is to obtain an 
underground object detecting system providing a sufficiently high 
resolution. 
The underground target detecting system according to the present invention 
is such as to provide a detected image output of targets by combining 
image data obtained by providing reflected wave profile data with a deep 
range-migration correction and image data obtained by providing the same 
with a shallow range-migration correction.

PREFERRED EMBODIMENT OF THE INVENTION 
An embodiment of the present invention will be described below with 
reference to the accompanying drawings. Referring to FIG. 1, diagram 11 
shows reflected wave profile data on a plane cutting through the soil at 
right angles with the ground to be subjected to a range migration 
correction, in which representatively indicated an ideal waveform of the 
reflected wave right above the target 1, while the broken lines are those 
connecting zero-cross points of the reflected wave at respective 
measurement points. Diagram 12 shows processing of a deep range-migration 
correction and diagram 13 shows processing of a shallow range-migration 
correction, in which the minute squares indicate presence of the data to 
be processed in the range migration corrections. 
Diagram 14 shows image data obtained as the result of the deep 
range-migration correction of the aforesaid diagram 12, diagram 15 shows 
image data obtained as the result of the aforesaid shallow range-migration 
correction of diagram 13, and diagram 16 shows image data obtained as the 
result of combination of these image data 14 and 15 by taking the product 
of these data. 
The operations will be described below. The reflected wave profile data 
collected in the same way as in the prior art is subjected to the 
preprocessing, whereby direct coupling between the transmitting antenna 4 
and the receiving antenna 5, noises, and further, distortions, and the 
like are removed. In diagram 11 of FIG. 1, there is shown an ideal 
reflected wave, but, in reality, there are many other vibrational modes 
included therein and various noises superposed thereon. 
Such reflected wave profile data are subjected to a deep range-migration 
correction as shown in diagram 12, whereby the data on the hyperbola 
corresponding to each of the targets 1 are made to cohere around the 
vertex. Here, the number of the range bins to be processed in the deep 
range-migration correction virtually corresponds to the length of the 
synthetic aperture, namely, it corresponds to the length of the synthetic 
aperture sufficient for composing the data, or it is the maximum number of 
the range bins usable for range migration correction. In the example shown 
in diagram 12, those of the maximum number usable for the range migration 
correction (RMC) are subject to the processing. The same reflected wave 
profile data are subjected to shallow range-migration correction as shown 
in diagram 13, whereby, in like manner to the above, the data on the 
hyperbola corresponding to each of the targets 1 are made to cohere around 
the vertex. Here, the number of the range bins to be processed in the 
shallow range-migration correction virtually corresponds to the pulse 
width, namely, it corresponds to the pulse width or to the positive or 
negative half wave. In the example shown in diagram 13, those of the 
number corresponding to the pulse width are subjected to the processing. 
Since processing is made with as great a number of range bins as the 
maximum number usable for the range migration correction in the deep 
range-migration correction, the target spot on the obtained image data 
becomes, as indicated in diagram 14 of FIG. 1, large in height and the 
shape in its plan view becomes an X letter having smaller width in the 
direction of the azimuth. Since, on the other hand, processing is made 
only with as small a number of range bins as the number corresponding to 
the pulse width in the shallow range-migration correction, the target spot 
on the obtained image data becomes, as indicated in diagram 15 of FIG. 1, 
small in height and the shape in its plan view becomes an X letter having 
larger width in the direction of the azimuth, whereas its width in the 
direction of the range becomes smaller than the width in the direction of 
the range of the target spot formed on the image data of diagram 14 of 
FIG. 1. 
Although there appear many other spots on the image data in diagrams 14 and 
15 of FIG. 1 than the indicated target spots due to distortions such as 
vibrational modes of the reflected wave and noises, they are not shown on 
the diagrams. In the present case, the positions of the centers of the 
indicated spots are the same on both the image data, but the spots due to 
aforesaid distortions and noises do not always appear at the same 
positions. 
The image data 14 obtained by the deep range-migration correction and the 
image data 15 obtained by the shallow range-migration correction are 
combined by taking their product. Accordingly, while the data at portions 
at high level in both the image data 14 and 15 mutually strengthen their 
intensity and produce data at still higher level thereat, the data at 
portions at low level mutually weaken their intensity and produce data at 
still lower level thereat, on the combined image data. And, the portion at 
zero level in one of the image data produces data at zero level on the 
combined image data even if the corresponding portion in the other image 
data is at high level. Since, as described above, one of the target spots 
on the image data 14 and 15 is narrower in the direction of the azimuth 
and the other is narrower in the direction of the range, the leg portions 
of the X letter cancel each other, although their central portions 
intensify each other, and therefore, a very sharp target spot with 
virtually no leg portions of the X letter remaining is obtained on the 
combined image data. Diagram 16 of FIG. 1 indicates such combined image 
data. 
The thus obtained image data is subjected to geological correction whereby 
the time scale is converted into the length scale and then to output 
processing so as to be displayed as a detected image output of the 
targets. 
Although, in the case described in the foregoing example the geological 
correction was made after the synthetic aperture processing has been 
performed, the reflected wave profile data may first be subjected to the 
geological correction whereby the time scale is converted into the length 
scale and then the data may be subjected to the synthetic aperture 
processing to obtain the same effects as in the above described 
embodiment. 
In the present invention the reflected wave profile data are subjected to 
both deep range-migration correction and shallow range-migration 
correction and the thus obtained image data are combined to provide a 
detected image data output, and therefore, such effects are obtained that 
the image spot on the combined image data becomes sharp and provides 
sufficiently high resolution.