f Testing of a georadar in tunnel drifting

The radar [1] has been developed by Moscow Institute of Physics and Technology (MIPT) and joint stock company “Geological Prospecting” Rus. Ltd. Nine months, 3 days a week, the measurements of signals were carried out on the face and top of a tunnel, which is being built in Moscow. The results, obtained by the radar, has been comparing with results of borehole drilling. The ground is clay with limestone layers. Sometimes water infiltrated through thin horizontal layers. The drifting included a path below the Yauza river, at a depth 23 m. The radar uses separate transmitting and receiving antennas. Antenna have size 40×40×10 cm, distance between centers of antennas is 1.1 m. The antennas are of the slot type and have large “front to back” ratio. Due to that they are useful in tunnels, where reflections from metallic mechanisms seriously interfere to the operation of a radar. These antennas were developed in MIPT under leadership of Leschansky Yu.I. It appears that the reflections from metallic mechanisms near the face of the tunnel have unacceptable large amplitudes, in spite of large front to back ratio of the antennas. Roughness of the ground surface or spacing between the antenna and the ground diminishes the front to back ratio [2,3]. Except the metallic mechanisms, placed about 2-3 m from the face of the tunnel, cylindrical metallic part of the tunneling machine with diameter 4 m also give reflections. To increase the front to back ratio of the antennas the using of resistive material has been proposed. Several forms of resistive covering have been tested [1]. Resistive covering includes two blocks with size ~60×20×10 cm, placed on sides of the antenna in E plane, a layer with size 60×60×2÷3 cm below the antenna and a layer 60×80×5 cm above the antenna. As has been proposed by Chernokalov A.G., a layer of artificial easily deformable resistive material has been placed in the spacing between the antenna and the ground surface. That lead to diminishing of electric field in the spacing and attenuation of wave, passed through the spacing to the air. The using of this layer also diminishes a number of oscillations in radiated signal. Moreover, the using of this resistive layer enables to diminish the influence of roughness of the ground surface to the results of sounding. The inconstancy of a signal due to the roughness leads to false “objects” near the ground surface after processing of signals. With using of this resistive layer the number and amplitude of these false “objects” diminish.