WO1999010731A1

WO1999010731A1 - Microwave enhanced infrared thermography

Info

Publication number: WO1999010731A1
Application number: PCT/US1998/017762
Authority: WO
Inventors: Charles A. Dimarzio; Carey M. Rappaport
Original assignee: Northeastern University
Priority date: 1997-08-29
Filing date: 1998-08-27
Publication date: 1999-03-04
Also published as: WO1999010731A8

Abstract

A detection apparatus (10) incorporating microwave-enhanced infrared thermography for providing the detection, location and identification of underground devices. Microwave energy at a wavelength determined by the depth of the object (60) and the soil conditions is utilized to heat the underground target and the surrounding ground (80). The contrast in dielectric properties of the object with respect to the ground produces a contrast in absorbed energy of the underground device with respect to the surrounding soil. This contrast in absorbed energy as well as the contrast in thermal conductivity and specific heat of the underground object (60) with respect to the surrounding soil result in a concomitant contrast in the temperature at the surface of the ground (70). This contrast in temperature at the surface of the ground is detected and recorded by a device (40) such as an infrared camera from which the underground device can be precisely located and identified.

Description

TITLE OF THE INVENTION MICROWAVE ENHANCED INFRARED THERMOGRAPHY

CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority under 35 U.S.C. §119 (e) to provisional patent application serial number 60/057,253 filed August 29, 1997; the disclosure of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT Partial funding under U.S. Department of Defense contract

No. DAAG55-97-1-0013 was provided for the development of the invention disclosed herein.

BACKGROUND OF THE INVENTION There is a need to detect, locate and possibly identify underground or buried objects. These underground objects may be mines, buried waste or utilities. One prior attempt to detect, locate and identify underground objects utilized ground-penetrating radar. While the use of ground-penetrating radar has proven to be useful in detecting underground objects, the ground-penetrating radar suffers from limited resolution which makes the identification of the underground device difficult. The limited resolution additionally makes it difficult to pinpoint the exact location of the underground device.

Another attempt to detect, locate and identify underground objects utilizes thermal images from solar heating of the ground. This technique is severely limited in that sunlight does not penetrate the ground at any appreciable distance, and that the technique can only be used during certain hours when sunlight levels are changing. Differential heating by sunlight can be measured in a time scale of minutes. The nominal 1000 watts per square meter of sunlight are concentrated in the optical region of the spectrum and are absorbed at the surface of the ground. The image is weak because it is produced entirely by heat transfer and is also noisy due to the uneven absorption of sunlight at the surface.

Other known mine detection systems such as metal detectors are subject to high rates of false alarms since any metal object will trigger the detector. It would be desirable to have an apparatus and method of detecting and locating underground devices which also provides sufficient resolution such that the detected underground devices can be identified.

BRIEF SUMMARY OF THE INVENTION

The present invention comprises a detection apparatus incorporating microwave-enhanced infrared thermography for providing the detection, location and identification of underground devices. The present invention utilizes microwave energy at a wavelength determined by the depth of the object and the soil conditions to heat the underground target and the surrounding ground. The contrast in the dielectric properties of the object with respect to the ground produces a contrast in absorbed energy of the underground device with respect to the surrounding soil. This contrast in absorbed energy as well as the contrast in thermal conductivity and specific heat of the underground object with respect to the surrounding soil result in a concomitant contrast in the temperature at the surface of the ground. This contrast in temperature at the surface of the ground is detected and recorded by a device such as an infrared camera from which the underground device can be precisely located and identified.

The use of microwave energy which penetrates the ground to the target region results in the infrared thermography being more sensitive than current detection devices. This increased sensitivity allows for faster detection which would allow for mine detection from a moving vehicle. The resolution provided by the present invention is much improved over traditional ground penetrating radar, which in turn results in an increased ability to determine the shape of the underground device and decreases false alarms. The additional contrast parameters due to the dielectric properties of the underground device and the soil increases the sensitivity of the present invention over conventional infrared thermography.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING The invention will be more fully understood from the following detailed description taken in conjunction wit the accompanying drawings in which:

Fig. 1 is a diagram of the apparatus for detecting and identifying underground objects utilizing microwave enhanced infrared thermography; and

Fig. 2 is a flow chart of the method for detecting and identifying underground objects utilizing microwave enhanced infrared thermography.

DETAILED DESCRIPTION OF THE INVENTION An apparatus and method which are able to detect, locate and identify underground objects such as mines, buried waste or utilities is presented. The present invention utilizes microwave energy to heat the target and surrounding ground at a predetermined depth. Contrasts in the dielectric properties of the object and soil create a contrast in the absorbed energy. Additionally, the contrast in thermal conductivity and in specific heat between the surrounding ground and the underground object, when combined with the contrast in absorbed energy result in contrast in temperature at the surface of the ground. This contrast in the surface temperature of the ground is recorded by a device such as an infrared camera, from which the precise location and identification of the underground device is determined.

Referring to Fig. 1 an apparatus 10 for detecting underground devices utilizing microwave-enhanced infrared thermography is shown. The apparatus 10 includes a microwave source 20 in communication with an antenna 30 which couples microwaves generated by the microwave source 20 to the ground 50. The microwaves penetrate an area 80 of the ground 50 and heat the ground area 80 and any objects 60 in the ground area 80. The frequency of the microwaves provided by the microwave source 20 are selected according to the depth of the target and/or the soil conditions, such that the microwaves heat the ground area 80 and any other objects 60 in the ground at a preselected depth. Accordingly, due to the object 60 having different dielectric properties, as well as different thermal conductivity and specific heat as compared to the ground, a difference in temperature of the ground surface 70 above the object 60 is produced. The antenna 30 in a preferred embodiment comprises an elliptical reflector antenna oriented with respect to the ground at a pseudo-Brewster angle using a microwave beam is transverse-magnetically (TM) polarized. The pseudo-Brewster angle is the angle at which the maximum amount of power is coupled from air into soil, and almost all of the radiated microwave energy is used in heating the soil, with a minimal amount reflected back towards the air. The pseudo-Brewster angle varies with soil type, density and moisture content. For common variations in soil characteristics, the pseudo-Brewster angle tends to be greater than sixty degrees and less than eighty degrees .

The elliptical reflector antenna 30 is mounted above the ground, with a feed 32 from the microwave source 20 at one focal point and the soil volume target at another focal point, and with the major axis (the line intersecting the two focal points) of the antenna 30 aligned at the pseudo-Brewster angle with respect to the ground 50. For a given antenna height above the ground h, the target will be 2h to 5.6h forward of the antenna. The converging rays from all parts of the elliptical reflector antenna 30 will arrive at the target focal point with a range of angles. As long as the major axis of the antenna is close to the pseudo-Brewster angle corresponding to the average soil characteristics of the area being examined, many of the rays will be incident on the ground surface with nearly optimal orientation, despite small local variations in soil type. The utilization of the elliptical reflector antenna 30 thus provides power concentration and ensures that most of the power enters the soil, despite soil conditions which change with position, and does so for targets 60 well in front of the antenna 30. The coupling of the microwaves into the ground causes the ground and any objects in the ground to be heated.

Once the object has become heated with respect to the surrounding ground a recording device such as an infrared camera 40 is used to measure the temperature of the ground surface 70 located above the underground object 60. Infrared cameras normally work in either the 3-5 or 8-12 micrometer band. With filters, narrower bands can be used (at the expense off sensitivity because a smaller portion of the spectrum is used) . A camera with filters for different wavelengths around 3 micrometers can detect spectral changes is surface reflectivity as a result of water content in the soil. Thus, the ratio of reflectivities at two closely spaced wavelengths can be used to measure water content. The water content will be altered by objects buried slightly below the surface of the ground. This contrast, used separately or combined with the thermal contrast can be used for underground object detection.

In a particular example, if an object absorbs as little as

1 Watt per square centimeter and is located 1 centimeter below the surface, assuming the worst case specific heat of water, the object will heat up by approximately 0.25 Kelvin per second. The resulting radiance change caused by a temperature change of 0.25 Kelvin at 300 Kelvin is about 0.02W/m²/sr in the 3-5 micrometer (MIR) band and 0.25W/m²/sr in the 8-12 micrometer (FIR) band. A good infrared camera has a sensitivity of 2.5E-4 W/m/sr, thus offering two to three orders of magnitude margin. The preference would be for the FIR band, since the signal is stronger and because scattered sunlight would not contaminate the measurement, however the MIR band will also provide acceptable results.

Referring now to Fig. 2 a flow chart representative of the method 100 for detecting underground objects using microwave- enhanced infrared thermography is shown. At a first step 110, a plurality of microwaves are generated. The wavelength of the microwaves is preselected according to the desired depth of the search for the underground objects and also according to the soil conditions.

The next step 120 is to couple the microwaves into the ground. The most efficient manner to couple the microwaves into the ground is by way of an antenna, and in particular by use of an elliptical reflector antenna oriented at the pseudo- Brewster angle with respect to the ground.

As shown in step 130, the coupling of the microwaves to the ground results in an area of the ground being heated by the microwaves. Additionally, any objects in the area will also be heated, which results in a contrast in surface temperature above the heated area.

Finally, as shown in step 140, the resulting contrast in surface temperature is recorded. The recording of the contrast in surface temperature is preferably done with an infrared camera, the resulting image from which is useful in determine what the underground object comprises, as well as determine the exact location of the underground device. The present invention thus provides a method and apparatus for detecting, locating and identifying underground objects such as mines, buried waste or utilities. The present invention results in less false alarms than prior detection devices since the use of microwaves provides additional thermal contrast between the underground object and the surrounding ground which results in increased resolution so that, for example, when sweeping a mine field, the mines can be distinguished from shell casings. Having described preferred embodiments of the invention, it will now become apparent to those of ordinary skill in the art that other embodiments incorporating these concepts may be used. Accordingly, it is submitted that the invention should not be limited to the described embodiments but rather should be limited only by the spirit and scope of the appended claims.

Claims

CLAIMS We claim:

1. An apparatus for detecting underground objects utilizing microwave-enhanced infrared thermography comprising: a microwave source operative to provide a plurality of microwaves at a predetermined frequency; an antenna in communication with said microwave source, said antenna coupling said plurality of microwaves into a ground area, wherein said plurality of microwaves heat the ground area and other objects in the ground area at a desired depth; and a thermal recording device operative to record a resulting contrast in temperature at the surface of the ground area, said resulting contrast in temperature indicative of the detection of an underground object.

2. The apparatus of claim 1 wherein said antenna comprises an elliptical reflector antenna.

3. The apparatus of claim 1 wherein said antenna is disposed at a pseudo-Brewster angle with respect to the ground area.

4. The apparatus of claim 1 wherein said antenna is disposed at an angle of between approximately 60 degrees and approximately 80 degrees with respect to the ground area.

5. The apparatus of claim 1 wherein said antenna is mounted above the ground area, said antenna having a feed at a first focal point, said antenna having the ground area at the desired depth as a second focal point, said antenna having a major axis intersecting said first focal point and said second focal point, said major axis aligned at a pseudo-Brewster angle with respect to the ground area.

6. The apparatus of claim 1 wherein said resulting contrast in temperature is caused by a difference in dielectric coefficients between the ground area and the underground object .

7. The apparatus of claim 1 wherein said resulting contrast in temperature is caused by a difference in thermal conductivity between the ground area and the underground object .

8. The apparatus of claim 1 wherein said resulting contrast in temperature is caused by a difference in specific heat between the ground area and the underground object.

9. The apparatus of claim 1 wherein said predetermined frequency of said microwaves is dependent upon the depth within the ground that the detecting is to take place at.

10. The apparatus of claim 1 wherein said predetermined frequency of said microwaves is dependent upon the conditions of the ground area.

11. The apparatus of claim 1 wherein said antenna is mounted above the ground at a height h, and wherein said area of the ground which is heated is between approximately 2h and approximately 5.6h forward of the antenna.

12. The apparatus of claim 1 wherein said thermal recording device comprises an infrared camera.

13. The apparatus of claim 12 wherein said infrared camera operates in the 3-5 micrometer (MIR) band.

14. The apparatus of claim 13 wherein said infrared camera operates in the 8-12 micrometer (FIR) band.

15. A method for detecting underground objects utilizing microwave-enhanced infrared thermography comprising the steps of: generating a plurality of microwaves at a predetermined frequency; coupling said plurality of microwaves into a ground area with an antenna wherein said plurality of microwaves heat the ground and other objects in the ground at a desired depth; and recording a resulting contrast in temperature at the surface of the ground area, said resulting contrast in temperature indicative of detection of an underground object.

16. The method of claim 15 wherein said step of coupling comprises coupling with an elliptical reflector antenna.

17. The method of claim 15 wherein said step of coupling further comprises coupling with an antenna disposed at a pseudo-Brewster angle with respect to the ground.

18. The method of claim 15 wherein said step of coupling further comprises coupling with an antenna disposed at a an angle between approximately 60 degrees and approximately 80 degrees with respect to the ground.

19. The method of claim 15 wherein said step of coupling comprises coupling with an antenna mounted above the ground wherein said antenna has a feed at a first focal point, said antenna having the ground volume target at a second focal point, said antenna having a major axis intersecting said first focal point and said second focal point, and aligning said major axis at a pseudo-Brewster angle with respect to the ground .

20. The method of claim 15 wherein said step of recording comprises recording with an infrared camera.