The minefields are combined as a rule, which means that they include both antitank and anti-personnel mines. The major part of the mines has round cross-section view. The antitank mines have diameter of about 300 mm, and the average diameter of the anti-personnel mine amounts to 75 mm. Some types of the mines contain practically no metal, which substantially impedes their detection. In this case the key give-away factor lies in the blasting charges of the mines, the dielectric properties of which differ from analogous characteristics of the ground that covers them. Thus, relative permittivity er for the typical explosives is between 3.5 and 4.0. At the same time dielectric properties of the ground in the considered frequency range vary within a wide range from 2.6 to 25.0, depending on the composition and moisture content of the ground. The lowest value of the range is related to absolutely dry sand, which is found in the desert. In the overwhelming majority of cases er for the ground exceeds the analogous value for the blasting charge.

MiRascan ground-probing holographic radar that makes it possible to detect and identify slightly deepened (up to 20 cm) objects by their shape has been developed in the Remote Sensing Laboratory. The principle of the multifrequency sounding of the condensed media (construction structures, soils, etc.) was assumed as a basis of the radar design.

In the initial segment of this work, the mock-up of a wide-span mine detector MiRascan, which included, in the capacity of a detecting element, a five frequencies ground penetrating radar receiving signals in two crossed polarities. The detector's sensor was installed on the cart, which was set in motion by an operator manually.

In the course of further research, the mock-up of the mine detector MiRascan underwent modernization. Basic features of it were as follows:

1. On the lower flange of the GPR cylindrical antenna, the head of the metal detector was installed;
2. On the upper flange of the radar antenna, a generator metal detector block was installed;
3. On the axes of chassis front wheels of the mine detector the electrical motors working in the impulse mode were installed;
4. Remote control system of the mine detector movement was assembled. The operator via the remote control box, connected to the cart by the cable of 15 m length, exercises control over the movement of the mine detector.

The radar has five operational frequencies in the range from 1.5 to 2.0 GHz and transmits unmodulated signals at each frequency. Its signals are received in two polarizations. Power emitted by the generator on each frequency is switched in sequence. It amounts up to 10 mW, which provides for the complete safety of staff. As previously mentioned, the induction loop of the metal detector was located on the butt end of the antenna of the ground penetrating radar, which provides spatial coincidence of received images from two channels of the mine detector. Operating frequency of the metal detector is 2 MHz, and the diameter of the induction loop is equal to 120 mm. The successive reception of signals on each frequency and in both polarizations of GPR and from the metal detector is conducted in the process of scanning the ground surface. The frequency switching rate is such that it provides for the spatial matching for all microwave images of the GPR separate frequencies and metal detector image. A general view of the newest model MiRascan mine detector being used during experiments is presented in figure.

Mock-up of MiRascan mine detector

The scanning in the lateral direction is carried out at the expense of electromechanical movement of the SHF device of the radar, and in the longitudinal direction due to the movement of the entire radar. The scanning results are displayed in the form of gray scale images on the monitor screen. Since it is difficult for an operator to perform a simultaneous analysis of all images on different frequencies, one animated image is formed in which sequential frames correspond to different frequencies.

Block diagram of MiRascan radar

The mock-up of the mine detector makes it feasible to survey the lane of movement 112 cm wide and to display the scanning results on the screen of a personal portable computer in real time.

The experiments to detect and identify mock-ups of the plastic-cased anti-personnel and antitank mines were conducted under the full-scale conditions. The experiments were performed on the special proving ground. The proving ground had sites with the key types of soils: sand, chernozem, loamy soil, etc., which ensures wide variation in their dielectric properties.

The mock-ups of the antitank plastic-cased mines of the types of TM-62P3 (manufactured in Russia), TC/6 and TC/2.5 (manufactured in Italy), of the anti-personnel plastic-cased mines of the types PMN-2 and MS-3 (manufactured in Russia), as well as of the metallic antitank mines of the types of TM-62M and PTM-3 (manufactured in Russia) were used in the capacity of the tested objects. All the mines except the PTM-3 mine had a round cross-section view, and the PTM-3 mine had a rectangular cross-section.

Mock-ups of antitank plastic-cased mines TM-62M, TC/6, TC/2.5, TM-62P3, PMN-2, MS-3

The mines were deepened in the ground to the depth of 1 cm for the anti-personnel mines and to 5-8 cm for the antitank mines. The experiments were conducted on the two types of soils: heavily moistened sand and heavily moistened chernozem with turf cover. The soils had the natural moisture content close to the saturation as the experiments were carried out a day after the long heavy showers.

Diagram of mines arrangement in the proving ground	Microwave images of mine mock-ups in the examined lane with sandy soil

The experiments to detect and identify mock-ups of the plastic-case and metallic-case mines were conducted under the full-scale conditions. The experiments were performed on the special proving ground near Moscow. The proving ground has sites with soil key types: sand, chernozem, loamy soil, etc., which ensures wide variation in their dielectric properties. In order to research the impact of humidity upon the quality of images received by different channels, tests were conducted under different weather conditions: both during dry and hot weather standing for a lengthy period of time and after a rain shower.

Experimental tests were conducted on June 29, 1999 while the air temperature was +30° C in bright and sunny weather. Their results are seen in figures for different mines and objects. Microwave images are shown in two polarizations of GPR (the left images are for the cross polarization of received and transmitted signals and the center images are for the parallel polarization) and right images were received by the metal detector. To be short, just one out of five microwave images received for each polarization is chosen - the most distinctive one. It is possible to judge the character of microwave images based on the frequency of the results shown in the previous work (Vasilyev et al., 1998).

The experimental images for MS-3 booby trap are shown in figures below. High contrast in the channel of the metal detector is explained by the presence of a metal ring around the casing of MS-3. In the next experiment two PMN-2 type antipersonnel mines were examined, one of which was fully armed (at the top of the image), and in the second mine (on the bottom) the metal finger of the detonator was missing, e.g., it did not have any metal parts. In GPR channels both mines are seen (the lower one is seen only partially), and the metal detector discovers only the first mine.

The images of MS-3 booby trap in chernozem

The images of two PMN-2 antipersonnel mines in chernozem

The results of scanning a Russian TM-62M antitank mine are shown in figure below. For this type of mine the round form is clearly seen in all channels. Next figure presents the images of glass (on the top) and plastic (on the bottom) bottles filled with water in sand. Since the bottles haven't any metal, there is no signal in channel of metal detector.

The images of Russian antitank mine type TM-62M

The images of glass and plastic bottles filled with water in sand

One more variant of device design is the remote control scout of mine fields. To orientate on terrain and to define coordinates of searching objects, the self-propelled cart of the radar will be equipped with GPS and TV camera. Steering of the cart will be carried out with help of a remote control box. Possible design of the cart with the sensors, which are established on it, is presented in the Figure.