JP2520042B2 - Underground radar tomography device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention measures the amplitude and propagation time of a transmitted wave, a reflected wave or a diffracted wave by injecting an electromagnetic wave into the ground and performing signal processing on the measured wave. The present invention relates to an underground radar tomography device that obtains information on the distribution of underground strata / soil or buried objects as sectional information, and particularly to detection of a weak signal propagated through the ground and attenuated.

[Prior art] In the investigation of rock mass, it is necessary to evaluate detailed information such as the existence and distribution of faults, shatter zones, cracks, etc., and the geological structure can be analyzed in cross section and displayed graphically. Among the tomography techniques, underground tomography using electromagnetic waves is expected to be effective in terms of resolution and non-destructiveness.

The following proposals have already been made for underground tomography using electromagnetic waves.

(1) Invention according to the application of Japanese Patent Application No. 1-117263 It is an invention proposed by the present applicant, and in order to detect a weak electromagnetic wave attenuated in the ground, the sensitivity is increased by using a pseudo random signal as a transmission wave. As one of the methods, two M-sequence signals with slightly different periods are used.

(2) Invention related to the application of Japanese Patent Application No. 2-194083 This is also an invention proposed by the present applicant, and a pseudo random signal is transmitted to detect weak electromagnetic waves attenuated in the soil. As one of the methods for increasing the sensitivity, two M-sequence signals with slightly different periods are used, and further, the transmission path from the transmission signal generator on the ground to the transmission sonde in the borehole and the reception signal processor on the ground from the borehole. The transmission path to the receiving sonde inside is composed of optical fibers, respectively, to eliminate noise due to the effects of radiated electromagnetic waves, external electromagnetic waves, and induced currents.

[Problems to be Solved by the Invention] In underground tomography, the attenuation of electromagnetic waves greatly changes depending on the electrical characteristics of the soil, and the signal strength of the detected electromagnetic waves greatly changes. The required sensitivity also needs to be changed significantly.

Therefore, in the above-mentioned invention, the amplification factor of the receiving unit is appropriately changed to adjust the sensitivity to an appropriate level. This method is effective for a change in signal intensity above the noise level. However, even if the amplification factor is increased, the noise is also amplified together with the signal for a weak signal below the noise level, and the signal / noise ratio (S / N ratio) is not improved. That is, in soil tomography, S /
While it is necessary to change the N ratio largely, the invention described above could not do so.

[Means for Solving the Problems] The ground-penetrating radar tomography device according to the present invention is
Of the first clock generator (2), which is driven by the output of the first clock generator, and which outputs the first pseudo-random signal, and A second clock generator (3) having a frequency slightly different from the frequency and a circuit configuration identical to that of the first pseudo random signal generator, driven by the output of the second clock generator, A second pseudo random signal generator (4) that outputs a random signal, a first multiplier (7) that multiplies the first pseudo random signal and the second pseudo random signal, and a first multiplier. A low-pass filter (22) for band limiting the output of, and a transmitting means (11) for transmitting a first pseudo-random signal into the soil,
Receiving means (18) for receiving the signal transmitted by the transmitting means through the soil, a second multiplier (13) for multiplying the received signal by a second pseudo random signal, and a second multiplication A low-pass filter (51) for band limiting the output of the receiver and the output of the first low-pass filter and the output of the second low-pass filter, and performing signal processing on these signals. It has a computing device (20, 21, 22) that obtains cross-sectional information of underground strata and soil properties, and the following points are considered in particular. In addition, the reference numerals of the corresponding configurations in the examples described later are added to the above configurations for reference.

B) An oscillator having a variable clock frequency is used for either one or both of the first clock generator and the second clock generator.

(B) The low-pass filter (at least the second low-pass filter of the first low-pass filter and the second low-pass filter) has a cutoff frequency of that of the clock frequency of the first clock generator and that of the second clock generator. It can be varied within a range of about 1 to 20 times the frequency difference from the clock frequency.

C) A pseudo random signal generator in which the code length of the pseudo random signal is variable is used as the first pseudo random signal generator and the second pseudo random signal generator.

D) A signal transmission path between the radar body and the transmitting sonde,
An optical transmission line using an optical fiber cable is used for one or both of the signal transmission lines between the radar main body and the reception sonde.

[Operation] In the present invention, (a) the cycle is slightly different.
In a method of detecting with high sensitivity by using one pseudo-random signal, (b) two clock frequencies are made variable, and a cutoff frequency of a low-pass filter is made variable,
(C) Furthermore, by varying the code length of the pseudo-random signal, the noise removal ratio of the received signal is changed to change the S / N ratio, and (d) the signal transmission line is configured by an optical fiber cable. There is.

The details of (a) are described in the above-mentioned application, but will be described next.

Since the code patterns of the two pseudo random signals are the same, the phases of the two signals match at a certain point in time, but the phases shift as time passes, and when the phase shifts by one code or more, the two pseudo random signals become There is no correlation.

When the two pseudo-random signals are in phase with each other, they are multiplied to generate a positive signal continuously. When this is passed through a low-pass filter, it is integrated and a large value is obtained. On the other hand, when the two pseudo random signals are out of phase with each other, positive and negative values are randomly obtained from these multiplication results, and when these are passed through a low pass filter, they are averaged to zero. Further, when time passes, the phases of the two pseudo random signals again coincide with each other, and a pulsed signal is generated at the output of the low pass filter. Then, these operations are repeated. At this time, even if noise is superimposed on the original signal, this noise is controlled by the low-pass filter, so signal processing with good S / N is achieved.

For example, the first M-sequence signal is directly multiplied by the second M-sequence signal and compared with the maximum value of the signal passed through the low-pass filter, and the first M-sequence signal is transmitted through the transmitting antenna, the ground, and the receiving antenna. From the second M-sequence signal,
The maximum value of the signal passed through the low-pass filter is delayed by the time difference corresponding to the underground propagation time, and its amplitude decreases corresponding to the attenuation due to the underground propagation. By doing so, the information necessary for obtaining a cross-sectional image as radar tomography can be obtained. At this time, as described above, even if the signal attenuation due to propagation in the ground is large, a signal with a good S / N can be obtained, so that a wide area can be searched.

Further, according to the present invention, as will be described below, an output can be obtained as a signal whose speed is slower than the actual propagation time of the electromagnetic wave in the ground, so that the output signal can be handled easily. That is, the propagation time is τ, the frequency of the first clock oscillator is f ₁ , and the frequency of the second clock oscillator is
When f ₂ is set, the time difference T _D in the output signal is greatly expanded as represented by the following equation.

T _D = τ ・ f ₁ / (f ₁ −f ₂ )… (1) Propagation time τ is expanded as time f ₁ / (f ₁ −f ₂ ) times, or measured as T _D slowed down To be done.

Information on the time difference T _D and the signal strength measured in this way is processed to obtain cross-sectional information on the underground strata and soil.

Next, the above (b) will be described. In (a) above, for example, it is easily conceivable to use an amplifier with a good noise figure for the receiver as a method for satisfactorily determining the S / N ratio. However, even if an ideal low noise amplifier is used, thermal noise cannot be avoided. Thermal noise is noise that occurs depending on thermal vibrations of molecules that form a substance, and is always generated as long as there is temperature.

The magnitude Np of this thermal noise is given by the following equation, where T is the absolute temperature, B is the frequency band, and k is the Boltzmann constant.

Np = kTB (2) The frequency band Bt of the transmission signal has two clock frequencies f ₁ and f ₂ (f ₁ > f ₂ ), and when the M-sequence signal driven by f ₁ is transmitted, Bt≈ 5 · f ₁ (3) On the other hand, the frequency band Br of the detection signal obtained by passing the low-pass filter by multiplying the received waveform by the M-sequence signal driven at the clock frequency of f ₂ is determined by the cut-off frequency of the low-pass filter. At this time, cut-off frequency f cu
You need to decide t. In the detected waveform, the frequency band 5 · f ₁ of the correlation function of the M-sequence signal is slowed down by f ₁ / (f ₁ −f ₂ ) as shown in equation (1), so the frequency band Br is Since Br = 5 · (f ₁ −f ₂ ) (4), the cutoff frequency of the low-pass filter is
It is rational to set as the following formula.

(F ₁ −f ₂ ) <f cut <20 (f ₁ −f ₂ ) ... (5) The cutoff frequency of the low-pass filter for simplifying the circuit configuration is fixed, and the subsequent signal processing in which the detection signal is captured is fixed. in the apparatus, a method of reducing noise in a digital signal processing method for a usual f cut = 5 · the cut-off frequency of the low-pass filter _{(f 1 -f 2) ... (} 6) can be considered. Whichever method is used, the noise suppression effect N sup in this case is given by the following equation in consideration of the equations (2) and (3).

N sup = f ₁ / (f ₁ −f ₂ ) ... (7) That is, when Δf = f ₁ −f ₂ is reduced, a large noise suppressing effect can be obtained. However, if Δf is decreased, the speed at which the two M-sequence signals are displaced becomes slower, and the measurement time becomes longer. Therefore, it is effective to make the frequency variable so that Δf can be reduced as necessary to improve the S / N.

Next, the above (c) will be described. When the clock frequency is changed, the measurable distance range also changes. That is, the measurable propagation distance L is given by the following equation, where v is the propagation velocity and N ran is the period of the pseudo-random signal.

L = N ran · v / f ₁ (8) Therefore, when the required measurement distance is determined,
When the frequency is changed, the period of the pseudo random signal needs to be changed according to the equation (8).

Next, the above (d) will be described. One or both of the signal transmission path between the radar main body and the transmission sonde and the signal transmission path between the radar main body and the reception sonde are configured by optical fiber cables, and are not affected by electromagnetic dielectric noise.

[Embodiment] FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, 1 is a clock generator and 2 is an M-sequence signal generator as a first pseudo-random signal generator.
Reference numeral 3 is a clock generator, and 4 is an M-sequence signal generator as a second pseudo random signal generator. Reference numerals 5 and 6 are electric signal / optical signal converters. 7 is a multiplier.

Reference numeral 8 is an optical signal / electrical signal converter, 9 is an amplifier in the transmitting sonde, 10 is a battery for a power source in the transmitting sonde, and 11 is a transmitting antenna.

12 is an amplifier in the reception sonde, 13 is a double balance mixer as a multiplier, 14 is an electric signal / optical signal converter, 15 is a low-pass filter as an integrator, 16 is an optical signal / electrical signal converter, and 17 is a receiving A battery for power supply in the sonde, and 18 is a receiving antenna.

Reference numeral 19 is an optical signal / electrical signal converter, and 51 is a low pasta filter whose cutoff frequency is variable. 20 is an amplitude value measuring device for measuring the amplitude of the maximum part of the detection signal, 21 is a time measuring device for obtaining the propagation time from the time difference between the detection signal and the reference signal, 22 is a low-pass filter as an integrator, and 23 is It is an operation display for displaying an operation for performing a tomographic operation from the information on the intensity of the detection signal or the propagation time and the result of the operation.

Reference numerals 24, 25 and 26 are optical fiber cables. 27 is a transmitting sonde and 28 is a receiving sonde. 29 and 30 are boreholes. 31 is underground. 32 This is the radar body.

 Next, the configuration of the apparatus shown in FIG. 1 will be described.

Clock signal frequency of the generator 1 and 3 1GH from 1 MH _{Z Z}
Using a synthesizer that can vary with high accuracy, the frequency is selected in consideration of radio wave permeability and detection resolution in the soil. In this embodiment, the difference in clock frequencies of the two clock signal generators IH _Z, 100H _Z, can be set with 8KH _Z. In order to keep the frequency of these differences stable, the reference oscillator in the synthesizer uses one of them in common.

Here will be described a case where the clock frequency of about 100 MHz _Z.

For example, the clock signal generator 1 frequency 100.004MH _Z,
Clock generator 3 as a frequency 99.996MH _Z, drives the M-sequence signal generator 2 and the M-sequence signal generator 4, respectively.
The M-sequence signal generator 2 and the M-sequence signal generator 4 are adopted as one of the pseudo-random signal generating means.
For example, a Barker code, a Gold code sequence signal, or a JPL code sequence signal may be used instead of the sequence signal.

In this embodiment, as shown in FIG. 2, a 9-bit shift register 41 is used, and the exclusive OR circuit 42 ...
The code length (cycle) of the M-sequence signal can be changed by switching the feedback circuit constituted by 44 with the switch 45. That is, by switching the number of stages n of the shift register 41 to 9.75,
Set the code length of the sequence signal to 2 ⁹ −1 = 511, 2 ⁷ −1 = 127, 2 ⁵ −1
= 31 can be switched respectively. Therefore, at the above clock frequency, the propagation velocity in soil is v = 0.6.
Assuming × 10 ⁸ m / s, the measurement range in soil from equation (8) is L =
It can be changed to 307m, 76m, 19m.

In this way, the random 2 within the variable code length is
Generate an M-sequence signal that is a cyclic cyclic code of the value signal.

FIG. 3 shows a part of the output waveform of the M-sequence signal generator.
FIG. 10A shows an example of an NRZ (Non Return Zero) waveform in which a logic “1” corresponds to the voltage + E and a logic “0” corresponds to the voltage −E, and FIG. "Is a waveform that changes from + E to -E, and" 0 "is a waveform that changes from -E to + E when PSK (Phaso Shift Keying) is an example of the waveform. Is better, so
In this example, a PSK waveform was used.

The optical signal / optical signal converters 5 and 6, the optical fiber cables 24 and 25, and the optical signal / electrical signal converters 8 and 16 are optical communication devices for high-speed digital signal transmission. The M-sequence signal is transmitted to the transmitting sonde and the receiving sonde. As the amplifier 9, a 3 W wide band power amplifier for power amplifying the M-sequence signal to excite the antenna is used. As the batteries 10 and 17, small nickel cadmium batteries that can be used for a long time were used. The antennas 11 and 18 are both dipole antennas made of a cylindrical aluminum tube and are provided with a paran coil in the center for electrical matching.

The outside of the antenna is made of plastic and is insulated and waterproofed. The electrical signal / optical signal converter 14, the optical fiber cable 25, and the optical signal / electrical signal converter 19 used optical signal transmitters for analog signal transmission for low speed. In the present embodiment, signal processing of an M-sequence signal as described later is performed in the reception sonde, and the signal subjected to band limitation necessary for transmission is transmitted to the radar main body by the optical communication device. There is little decrease in N.

The detection signal transmitted to the radar main body passes through the low-pass filter 51 and undergoes necessary and sufficient band control, noise is suppressed, and a large S / N ratio is obtained. Here, the low-pass filter 51 is composed of a variable inductance 52 and a variable capacitance 53 as shown in FIG. 4, the cutoff frequency is variable, and it is set to about 5 times the frequency of the difference between the two clock frequencies.

The optical fiber cable 24 has a reinforcing member such as a tension member and an outer cover made of non-metal, and also has a role of suspending the transmitting sonde 27 in the borehole. Further, the optical fiber cables 25 and 26 are stored together with a non-metallic tension member in a single non-metallic cable, and also serve to suspend the reception sonde in the borehole.

The amplifier 12 in the reception sonde is a high-sensitivity wide-band amplifier and uses a logarithmic amplifier so that the reception sensitivity is appropriate according to the magnitude of the input signal. Also, this amplifier has a variable attenuator built in at the entrance thereof, so that the sensitivity can be arbitrarily changed. Although not shown in FIG. 1, another optical communication device can be added between the radar main body and the reception sonde to adjust the variable attenuator in this amplifier from the radar main body.

Further, when the transmission power is increased and the power supply for that purpose cannot be sufficiently supplied by the battery in the transmission sonde, a method of supplying power from the radar main body to the transmission sonde with a metal cable is also considered. In this case, it is necessary to attach a toroidal core to the outside of the cable to increase the surface impedance so that the transmitted radio wave does not propagate through the metal cable.

 Next, the operation of this embodiment will be described.

The M-sequence signal generator 2 and the M-sequence signal generator 4 have the same circuit configuration, but the clock signals to be driven are slightly different, and therefore the period time of the two M-sequence signals is 12 each.
Slightly different from 69.9492ns and 1270.0508ns. Thus is generated by circulating the two M-sequence signals M ₁ and M _2, when the patterns of the two M-sequence signal at a certain time t _a match, the deviation of 0.1ns for each time of one period A difference of 10 ns occurs between the two signals after 100 cycles. Here, since the M-sequence signal generates 127 signals in one cycle of 1270 ns, the generation time of one signal is 10 ns.

Therefore, the occurrence of the deviation of 10 ns between the two M-sequence signals M ₁ and M ₂ corresponds to the deviation of one M-sequence signal. The timing relationship between them is shown in FIG.

FIG. 5 is a waveform diagram for explaining the operation of the apparatus shown in FIG. In the figure, (a) shows that the output of one cycle of the reference M-sequence signal generator 4 includes 127 signals and the cycle is 1270 ns, and (b) shows the M-sequence signal generator. 4 shows the state in which the output M ₂ from 4 is cyclically generated from the −100 to the 300th cycle, and (c) shows that the output M ₁ from the M-sequence signal generator 3 is 1 compared with M _2. 0.1 ns in cycle, 100
Period by a short 10 ns, and the two M-sequence signals M ₁ and M ₂ are synchronized at time t _a, the pattern of both signal indicates that a match. Further, when the patterns of the two M-sequence signals M ₁ and M ₂ match at a certain time, the shift gradually increases thereafter, and the patterns of the two signals match again after a lapse of a certain time. In the case of this example, approximately 126.777 cycles of the M-sequence signal M ₁ and 126.767 cycles of M ₂ are approximately every time.
The pattern matching of the _two M-sequence signals M ₁ and M ₂ is repeatedly generated every 16.1 ms.

M output from the M-sequence signal generators 2 and 4, respectively
The series signals M ₁ and M ₂ are branched into two, and one of the signals is input to the multiplier 7. As the multipliers 7 and 13, for example, a wide band double balanced mixer (DBM) is used, and multiplication of two M-sequence signals is performed. The M-sequence signal is a positive or negative voltage signal as described above, the multiplication result with the same sign is a positive voltage, the multiplication result with a different sign is a negative voltage, and the positive or negative voltage signal is output to the multipliers 7 and 13. Is obtained.

Therefore, the output signal of the multiplier 5 in the vicinity of the time t _a which now has two M-sequence signals M ₁ and M ₂ of the pattern was consistent is a pulse train of DC positive voltage or a positive voltage. However, the periods of the _two M-sequence signals M ₁ and M ₂ are slightly different, and a deviation of 0.1 ns occurs between the two signals every time one period elapses. Then, the two after 100 cycles to the time t _a M-sequence signal M ₁ and M
A deviation of 10 ns, that is, a deviation of one signal occurs between the _two .
In this state, there is no correlation between the two signals, and positive and negative pulse train signals are randomly generated at the output of the multiplier 7. The output waveform of the multiplier 7 is shown in FIG.

The output signal of the multiplier 7 is supplied to the low pass filter 22 and converted into a DC voltage. The low-pass filter 22 has a cutoff frequency f _c = 40 KH _Z, and has a function of attenuating an input component having a wave number higher than the cutoff frequency f _c and smoothing the input signal. The cutoff frequency f _c of the low pass filter 22 may also be variable.

The output signal of the lowpass filter 22 becomes a maximum value at time t _a two M-sequence signals M ₁ and M ₂ of the pattern matches, the time of M-sequence signal M ₂ from time t _a is shifted back and forth 100 cycles, That is, it becomes the minimum value at the time of t _an ± 127 μs. Then, the triangular wave voltage signal linearly decreases from this maximum value to the minimum value before and after. The output waveform of the low-pass filter 22 is shown in FIG. The triangular wave voltage signal is output from the low-pass filter 22 every 16.1 ms in which two M-series periodic states occur as described above.

The output signal from the low-pass filter 22 is input to the time measuring device 21, which serves as a reference time for measuring the underground propagation time of the electromagnetic wave by correcting the fixed delay time inside the radar from the triangular wave generation time. M output from the M-sequence signal generator 2
The sequence signal M ₁ is transmitted to the transmitting antenna through the above-mentioned optical communication device or the like.
It is supplied to 11 and is radiated into the ground as an electromagnetic wave. This radiated electromagnetic wave reaches the receiving antenna 18 via the underground 31. At this time, the received M-series signal becomes a signal delayed from the transmitted M-series signal by the propagation time of the electromagnetic wave.

The M-sequence signal M ₂ output from the M-sequence signal generator 4 is
It is supplied to the multiplier 13 in the reception sonde via the above-described optical communication device, is multiplied by the received M-sequence signal, and the result is passed through the low-pass filter 15. Cut-off frequency of the low-pass filter 15 is 100KH _Z, which is intended to limit the bandwidth as needed to transmit the detection signal. The detection signal passed through the low-pass filter 15 is transmitted to the radar main body by the above-mentioned optical communication device. The radar main body passes the transmitted detection signal to the low-pass filter 51.
The operation of this low-pass filter is the same as that of the multiplier 7 and the low-pass filter 22 described above. The difference is that the times at which the patterns of the _two M-sequence signals M ₁ and M ₂ match are different. Strictly speaking, the received M-sequence signal is slightly distorted due to propagation, but this does not affect the operation of this embodiment. Since the cutoff frequency of the low-pass filter 51 can be variably adjusted to be 5 times the difference Δf between the clock frequencies of M ₁ and M ₂ , the optimum noise suppression effect can be obtained.

In this way, the detection signal obtained by passing through the low-pass filter 51 is input to the time measuring device 21 and the amplitude value measuring device 20. The time measuring device 21 can obtain the propagation time of the electromagnetic wave by measuring the time when the maximum value of the detection signal occurs with respect to the reference time described above. Here, a point to be noted when measuring the propagation time in the time measuring device is that the time axis is expanded as compared with the actual propagation time of the electromagnetic wave. For example, when the propagation time is 20 ns in the ground, the time measuring device measures 25 μs, that is, the time axis is expanded by 12,700 times and an extremely low speed signal may be measured.

Further, the amplitude value measuring device 23 can measure the attenuation of the electromagnetic wave by measuring the amplitude of the maximum portion of the detection signal.

As described above, the information about the propagation time and attenuation of the electromagnetic wave in the ground is input to the calculation display 23, and the sectional image of the ground is calculated and displayed based on the information.

The signal processing algorithm itself for obtaining a cross-sectional image is conventionally known, and for example, Alegebraic Recostruction Techniq
ue, Simultaneous lterative ReconstructionTechique
Although so-called and Diffraction Tomography can also be used, the so-called iterative method was applied here.

FIG. 6 is an explanatory diagram of the iterative method. As shown in the figure, the propagation delay time t _k or the amplitude a _k along the broken line Rk from the transmission point to a certain reception point is measured. ), (3)
Expressed as an expression.

In the above equation, V (x, y) is the velocity distribution in the cross section, and a (x, y) is the distribution of damping parameters.

FIG. 7 is a flow chart showing the operation of the iterative method on the operation display 23. After entering the travel time data,
Initial models V ₀ (x, y) and a ₀ (x, y) are set, and t _k or a _k is calculated for the same transmission point-reception point pair as the measurement data according to equation (2) or equation (3). , Modify the model so as to reduce the residual from the actual value, and bring it closer.

[Effects of the Invention] As described above, according to the present invention, two pseudo-random signals having slightly different frequencies are used to inject an electromagnetic wave into the ground, and the amplitude (attenuation) of a transmitted wave, a reflected wave, or a diffracted wave thereof is attenuated. )
And the propagation time can be measured with high accuracy, and the response speed becomes slower when a larger S / N ratio is required, but the S / N ratio can be reduced by reducing the difference between the two clock frequencies. Can be improved. In addition, there is a limit to the measurement time, and if you want a faster measurement time, use S /
N becomes worse, but it has become possible to increase the frequency of the difference between the two clock frequencies.

Also, by using an M-sequence signal generator capable of changing the code length of the M-sequence signal, it is possible to set an appropriate measurement range for the measurement range that is changed by changing the clock frequency. It was

Further, since the optical transmission system using the optical fiber is used, electromagnetic induction noise is prevented from being mixed between the radar main body and the transmitting sonde and between the receiving sonde and the radar main body.

As described above, even in areas such as the Kanto loam layer where the soil has a large amount of water and the electromagnetic waves are greatly attenuated, a good S / N signal can be obtained, and a wide area can be efficiently processed at a time and even when the inspection conditions change. It was possible to respond flexibly and to conduct practical exploration.

[Brief description of drawings]

FIG. 1 is a block diagram showing the structure of an underground radar tomography apparatus according to an embodiment of the present invention, and FIG. 2 is an M of FIG.
Circuit diagram of the sequence signal generator, FIG. 3 is an output waveform diagram of the M sequence signal generator, FIG. 4 is a circuit diagram of a low-pass filter with a variable cutoff frequency, and FIG. 5 shows the operation of the apparatus of FIG. A time chart, FIG. 6 is an explanatory diagram of the iterative method, and FIG. 7 is a flowchart showing the operation of the iterative method.

Claims (3)

(57) [Claims] 1. An electromagnetic wave is injected into the ground to measure the amplitude and propagation time of a transmitted wave, a reflected wave or a diffracted wave at multiple points, and signal processing is applied to this to measure the formation of underground layers and soils. In a ground-penetrating radar tomography device that obtains a distribution as cross-sectional information, a first clock generator and a first clock generator driven by an output of the first clock generator,
A pseudo-random signal generator for outputting the pseudo-random signal, a second clock generator having a frequency slightly different from the frequency of the first clock generator, and the first pseudo-random signal generator A second pseudo-random signal generator having the same circuit configuration as the above, driven by an output of the second clock generator, and outputting a second pseudo-random signal; the first pseudo-random signal and the second pseudo-random signal; A first multiplier for multiplying the pseudo random signal by the first low pass filter for band limiting the output of the first multiplier, and a transmission for transmitting the first pseudo random signal into the soil. Means, receiving means for receiving the signal transmitted by the transmitting means through soil, a second multiplier for multiplying the received signal by the second pseudo-random signal, and the second multiplication A second low-pass filter for band limiting the output of the received signal, the output of the first low-pass filter and the output of the second low-pass filter, and subjecting these signals to signal processing And a computing device that obtains cross-sectional information of the stratum and soil, and the first clock generator and the second clock generator are capable of varying either or both clock frequencies, The second low-pass filter changes a frequency about 1 to 20 times the difference frequency between the clock frequency of the first clock generator and the clock frequency of the second clock generator to be the cutoff frequency. A ground-penetrating radar tomography device characterized by being capable. 2. The first pseudo random signal generator and the second pseudo random signal generator use a pseudo random signal generator capable of varying a code length of a generated pseudo random signal. The underground radar tomography apparatus according to claim 1. 3. The radar main body includes the first clock generator, the first pseudo random signal generator, the second clock generator, the second pseudo random signal generator, and the first multiplication. Unit, the first low-pass filter, the second multiplier, the second low-pass filter, and the arithmetic unit, and a transmitting sonde is equipped with the transmitting means, and a receiving sonde is equipped with the receiving means. And a signal transmission path between the radar main body and the transmission sonde, and an optical transmission path using an optical fiber cable for either or both of the signal transmission path between the radar main body and the reception sonde. The ground-penetrating radar tomography device according to claim 1 or 2, characterized in that.