Apparatus for underground radar tomography

Claim 1

An underground radar tomographic apparatus in which an electromagnetic wave is sent into the ground, the amplitude and propagation time of a transmitted wave, a reflected wave or a diffracted wave of the electromagnetic wave are measured at a number of points, and signal processing is performed on the result of measurement to thereby obtain the distribution of strata and soil in the ground as section information, said underground tomographic apparatus comprising: a first clock generator (1); a first pseudo random signal generator (2) driven by said first clock generator (1) for generating a first pseudo random signal; a second clock generator (3) having a frequency (f₂) which is slightly different from a frequency (f₁) of said first clock generator (1); a second pseudo random signal generator (4) driven by said second clock generator (3) for generating a second pseudo random signal, said second pseudo random signal generator (3) having a circuit configuration the same as that of said first pseudo random signal generator (2); a first multiplier (5) for multiplying said first and second pseudo random signals by each other; a transmitter means (6) for transmitting said first pseudo random signal; a receiver means (7) for receiving a transmitted wave, a reflected wave or a diffracted wave of the signal radiated from said transmitter means into the ground (18); a second multiplier (8) for multiplying said first pseudo random signal by the signal received by the receiver means (7); a time difference measuring instrument (49) for measuring a time difference between an output signal of said first multiplier (5) and an output of said second multiplier (8); a signal strength measuring instrument (12, 13) for measuring signal strength of the output signal of said second multiplier (8), and an operator (14) for processing information with respect to said time difference and signal strength obtained by said time difference measuring instrument (11) and said signal strength measuring instrument respectively to thereby obtain the section information of the strata and soil in the ground; characterised in that said underground radar tomographic apparatus further comprises: a first low pass filter (9) for receiving the output signal of said first multiplier (5) and for performing band limitation on the received signal to thereby supply an output to said time measuring instrument (11); a first electro-optical converter (41) for converting said first pseudo random signal into a light signal; a first optical fibre (51) for transmitting said light signal converted by said first electro-optical converter (41); a transmission sonde (54) for converting said light signal transmitted through said first optical fibre (51) into an electromagnetic wave and for radiating said electromagnetic wave into the ground (18); a second electro-optical converter (42) for converting said second pseudo random signal into a light signal; a second optical fibre (52) for transmitting said light signal converted by said second electro-optical converter (42); a reception sonde (55) including a second amplifier (20) for receiving the electromagnetic wave from the ground and for multiplying the reception signal by the signal of said second optical fibre (52), and a second low pass filter (10) for performing band limitation on the result of multiplication of said second multiplier (8), an output of said second low pass filter (10) being outputted as a light signal; a third optical fibre (53) for transmitting said light signal outputted from said reception sonde (55), and a third opto-electrical converter (48) for converting said light signal transmitted through said third optical fibre (53) into an electric signal and for supplying the output signal of said second multiplier (8) to said time difference measuring instrument (11) and said amplitude measuring instrument (12, 13).