JPH0558152B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a method for exploring underground buried objects using wave motion, and more specifically, in collecting information on reflection of wave motion from underground. This invention relates to an underground exploration method that allows effective use of broadband signals.

[Prior art]

In the conventional underground buried object exploration method, the first
The configuration shown in the figure is used.

This is based on the principle of pulse radar, which is used in the air for various purposes, but because the distance to buried objects is short, it requires higher ranging resolution than airborne radar. Therefore, a baseband impulse signal with a pulse width narrower than that of airborne radar, a few nanoseconds at most, is used. As is well known, impulse signals have an extremely wide frequency band. It is well known that this wide frequency band contributes to increasing the resolution of ranging. From this point of view, attempts have been made to use chirp pulses with a swept frequency in addition to impulse signals, but there is no essential difference and they are included in the conventional example shown in Figure 1, so the case of impulse signals will be explained here. do.

An impulse-like electrical signal having an extremely wide frequency band generated by a pulse transmitter 1 passes through a T/R switch 2 which has the function of switching between a transmission state and a reception state, and is fed to a single wideband antenna 3. The broadband antenna 3 emits an impulse-like radio signal 4 toward the ground. The radio signal 4 is reflected by an underground object 5, detected by a broadband antenna 3, converted to a potential signal again, and guided to a receiver 7 through a T/R switch 2 switched to a receiving state. , is subjected to amplification, detection, etc., and is recorded in the data recorder 8 as waveform data. Furthermore, it is displayed on the graphic recorder 9 as necessary. Exploration is performed by moving the antenna and repeating measurements while determining the position of the antenna with a position generator 6 built into the moving mechanism. Although there is a method of separating a wideband antenna dedicated to transmission and a wideband antenna dedicated to reception without using the T/R switch 2, they are essentially the same type.

As mentioned above, in the conventional method. In order to ensure the resolution of distance measurement, an extremely wide band electric signal was used, and this was applied to a single antenna for transmission, and the reflected wave was received by the single antenna. The drawback was that antennas required for this purpose had extremely wide frequency characteristics, making it difficult to realize antennas that were essential for collecting information from underground. In addition, the implemented method is one in which the radiated impulse-like electrical signal 4 has a significant distortion and lowers the resolution of exploration compared to the applied impulse-like electrical signal because the band is insufficient. However, if emphasis was placed on ensuring broadband performance, the efficiency of the antenna would drop significantly and there would be a problem in that sufficient wave energy could not be obtained for lossy underground exploration.

Furthermore, in conventional systems, a single antenna handles a wideband signal, making it difficult to reduce negative effects such as fluctuations in antenna characteristics due to the presence of the ground and strong reflections from the ground surface. There is. That is, in order to suppress these effects, it is necessary to match the antenna with the ground over a wide band, but this has been difficult to achieve. Additionally, in order to reduce reflections from the ground surface, a method using a cross dipole antenna has been proposed, which takes advantage of the fact that reflections from near-flat reflectors such as the ground surface do not change the plane of polarization. However, it has the disadvantage that it may become unresponsive to reflections from underground exploration objects and that energy use efficiency is low. Also,
In order to reduce the effects of antenna mismatch, a method has been proposed in which a dummy antenna is connected in parallel to the transmitting and receiving antennas to generate opposite-phase reflections and cancel out unnecessary reflections, but this method eliminates the energy that does not contribute to exploration. The loss was always as high as 3 dB, and it could not be an advantageous solution for a lossy underground exploration method.

On the other hand, a method has been proposed that uses one frequency and does not require the use of a wideband antenna, but this method has the disadvantage that the ranging resolution is extremely low because it uses only a single frequency component. Furthermore, there was a problem that alignment with the ground for efficient underground transmission was not taken into consideration.

[Object of the Invention] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide an underground exploration method that reduces excessive burden on antennas and allows effective use of wideband signals. With the goal. Furthermore, another object of the present invention is to provide an underground buried object exploration method that can reduce harmful effects such as ground surface reflection over a wide band by dividing frequencies.

[Structure of the invention]

The present invention decomposes a wideband signal required for exploration into a plurality of narrow frequency bands, and divides each band into a plurality of relatively narrowband antenna elements to share the signal aggregation process. The method is characterized in that, after reception, while effectively utilizing the movement information of the transmitting and receiving antennas, a pulse signal with an equivalent short duration is synthesized using the divided frequency components to perform underground exploration.

〔Example〕

Hereinafter, the present invention will be explained in detail based on FIGS. 2 to 9.

The following description will be made with a case in mind where electromagnetic waves are used to collect reflection information from underground and to search for buried objects. For this reason, the term antenna refers to a transducer for electrical signals and radio wave signals, but it goes without saying that the method of the present invention can also be applied to other wave media such as sound waves.

FIG. 2 shows an embodiment of the present invention. First, the characteristic points of the exploration method of the present invention are shown in the figure. In other words, the broadband signal necessary for exploration is divided into multiple bands, and This article explains how to reduce the requirement for wideband characteristics of the antenna by allocating it to a separate antenna, and to synthesize an equivalent wideband pulse after reception by using the position signal associated with the movement of the antenna. be.

In this method, a wideband signal required for exploration is divided in advance into N bands and handled. That is, the transmitter 11 sequentially switches and generates signals belonging to N divided frequency bands under the control of the transmission control unit 10. The specific shape of the signal is, for example, a sine wave, and N frequencies f ₁ , f ₂ , f ₃ , ... f _N are assigned to each of N bands, and these are sequentially switched from f ₁ . generate it. At this time, since the frequency is switched within a finite time, the generated signal has a band broadening somewhat depending on the duration, but it is approximately f ₁ to _{f N}
It becomes a narrowband signal with a frequency of . If one frequency is generated for a duration that includes a sufficiently large number of cycles relative to the frequency, each signal can be considered to be substantially a continuous wave and can be considered to have a line spectrum.

First, the entire system will be explained assuming that a signal of frequency f ₁ belonging to the first band is generated at a certain point in time. The generated signal is sent to a T/R switch that switches between a transmission state and a reception state, and an antenna 13 (described later).
The antenna element having the first band in the antenna element group 14 passes through the functional block 12 having the function of establishing a path for applying an electric signal to a predetermined antenna element in the antenna element group 14 constituting the antenna element group 14. is applied to The functional block 12 establishes the transmission/reception state and the application path to the antenna element in synchronization with the applied signal using the control signal from the transmission control section 10. Antenna 13, antenna element group 14
Each antenna element of the antenna element group 14 has a function of converting an electric signal into a radio wave signal and radiating it by sharing a different relatively narrow frequency band.

Now, the signal f ₁ is radiated from the antenna element sharing the first band in the antenna element group 14, and when it enters the ground and is reflected by the buried object 15, the signal f ₁ belongs again. The receiver 18 is detected by the antenna element in charge of the frequency band and passes through the functional block 12 which is switched to the receiving state.
be guided. A reference signal 17 that is coherent with the electrical signal applied to the antenna 13 is sent from the transmitter 11 to the receiver 18, and the information in the reflected signal necessary for performing the subsequent pulse synthesis processing is determined from the reflected signal. is taken out. For example, by comparing the reflected signal and the reference signal 17, phase and amplitude information of the reflected signal at frequency f ₁ can be obtained.
The process of extracting such information in the frequency domain is well known as the detection of amplitude and phase information in network analyzers or the technique of electronic reference waves in long wavelength holography, and will not be described in detail.

The complex amplitude information of signal f ₁ is A/D converted and stored in memory 19 in an appropriate format. At this time,
As the antenna 13 moves by the antenna moving mechanism, the position coordinate signal generated from the position coordinate signal generator 16 indicating the position of the antenna 13 is
It is essential to store it in the memory 19 in association with the reflected signal.

Upon completion of the data collection using the signal _f1 , the transmission control unit 10 controls the functional block 12 to transmit data to a predetermined antenna element in the antenna element group 14 that handles the second frequency band. A signal path is established, and in synchronization with this, the transmitter 11 is brought into a state of transmitting the signal _f2 . The signal f ₂ is
Through the same process as for the signal f ₁ , the necessary information is extracted by the receiver 18 and stored in the memory 19 together with the signal indicating the measurement position of the signal f ₂ .

Similarly, similar operations are performed for the quasi-order signals f ₃ , f ₄ , . . . f _N .

In this way, the information extracted in the frequency domain by the signals f ₁ to f _N belonging to each of the divided frequency bands is then synthesized into a pulse signal in the pulse synthesis processing section 20 . The synthesized pulse waveforms are arranged, for example, in the order of position coordinates and displayed in a cross-sectional diagram on the graphics recorder 21, or more advanced signal processing is stored in the data recorder 22 for analysis and display. .

Knowing the frequency components of a signal in the frequency domain and synthesizing them into an Ars waveform in the time domain is possible based on the Fourier transform relationship, and can be performed numerically using an FFT algorithm. However, if each phase component of information extracted from signals belonging to different frequency bands is simply extracted using the reference signal 17, arbitrary phase shifts may occur between the signals. Therefore, the reference signal 17 supplied from the transmitter 11 to the receiver 18 must be phase-locked and provide a common phase reference to the signals belonging to each frequency band. .

Furthermore, in the above description of the pulse synthesis processing section 20, it is implicitly assumed that the signals f ₁ to f _N are collected at the same observation point, but as is clear from FIG. , each antenna element forming the antenna element group 14 is located at a spatially different location and does not satisfy this assumption. It will be explained next that despite such an arrangement of the antenna element group 14, the signals f ₁ to f _N at the same observation point can be stored in the memory 29 and read out for pulse synthesis processing.

FIG. 3 is an enlarged view of a portion of the antenna 13 in the embodiment shown in FIG. The antenna 13 has internal antenna element groups 14 that share signals belonging to divided frequency bands, and these are respectively named A ₁ , A ₂ , A ₃ , . . . _AK , . . . _AN . The antenna 13 also includes a mechanism for movement and a position coordinate signal generator 16 that generates a position coordinate signal indicating the position of the antenna 13. Each antenna element A ₁ , A ₂ , A ₃ , ...A _K , ...A _N of the antenna element group 14 receives a signal.
Paths for transmitting and receiving f ₁ ·f ₂ , ... f _K , ..., f _N are provided independently. Antenna elements A ₁ , A ₂ , A ₃ ,
_{. _} In this respect, it is essentially different from a log-periodic antenna, which has multiple elements but is configured to function as a single broadband antenna as a whole.

Each antenna element A ₁ , A ₂ , A ₃ ,..., A _K ,...A _N
are at spatially different positions, the signals f ₁ , f ₂ ,
…f _K and …f _N are transmitted and received at different observation points. Compared to the moving speed of the antenna 13, the time required for transmitting _and receiving signals can be made sufficiently small, so the deviation of the points for transmitting and receiving signals _can be ignored _. ,… _AK ,…
Since the deviation of A _N causes each signal to have a separate underground propagation path, the pulse waveforms must be synthesized using the information collected in one measurement cycle from signal f ₁ to f _N. is inconvenient. However, if we use the fact that the antenna 13 moves along the survey line of underground exploration, the signals f ₁ ~
Synthesis using _N is possible. Now, suppose that at a certain point, antenna element _A1 is at a point on the ground where X=Xo,
Suppose that we perform observation using signal f ₁ . At this time, the other antenna elements A ₂ , A ₃ , ..., A _K , ... A _N are arranged by the sum of the arrangement intervals of the antenna elements A ₁ , A ₂ , A ₃ , ... A _K , ... A _N , respectively. While located behind X=Xo, they conduct observations using their respective signals. Therefore, the signal f ₁ is associated with a position coordinate that clearly indicates that it is the result of observation at X = Xo, and the information obtained by the signal f ₁ is set as R(f ₁ ),
(R(f ₁ ), Xo) is stored in the memory 19. Antenna elements A ₁ , A ₂ , A ₃ ,..., A _K ,... _AN
Since the array spacing is known, for example, the information from antenna element A ₂ can be expressed as a label such as (R(f ₂ ), Xo−d ₁ ) (where d ₁ is the array spacing between A ₁ and A ₂ ). be attached. While moving the antenna 13,
As the measurements from signal f ₁ to f _N are repeated, a time will come when antenna element A ₂ reaches the point where X=Xo, and at this time (R(f ₂ ), Xo) is accumulated. Similarly, any Kth antenna element
For A _K , (R(f _K , We can extract f _N from ₁ .

FIG. 4 shows the antenna 1 shown in the embodiment of FIG.
3 is a plan view of a configuration example of No. 3, which is an example in which a die-ball antenna with characteristics suitable for shared signals is used as a relatively narrow band antenna element group 14. FIG. Of course, it is also possible to use antennas other than die-ball antennas, and the essence is to use antenna element groups 14 that divide and share the frequency band.

Furthermore, in the embodiment shown in FIG. 2, a case has been described in which the antenna 13 is switched between the transmitting state and the receiving state, but as shown in FIG. 5, the transmitting antenna 23 and the receiving antenna 23' are separated, Each antenna element group 24, 24' may be provided. At this time, the function block 12 does not need to have the function of a T/R switch. Furthermore, in the functional block 12, a directional coupler may be used instead of the T/R switch to separate the transmitted signal and the received signal.

Next, a description will be given of the configuration of an antenna to be applied to this method, which aims to improve matching with the ground by dividing the frequency band and assigning the frequency band to each antenna element.

In FIG. 6, each antenna element A ₁ , A ₂ , A ₃ ,
..., A _K , ...A _N is _the wavelength min (λ _K ) ( ₁
The antenna elements A ₁ , A ₂ , A ₃ , ..., A _K , ... A _N are placed close enough to the ground at a distance h that is sufficiently small compared to the ground. If the antenna 25 is matched to the impedance of the ground when the influence of n _s becomes strong,
This enables efficient transmission and reception. With this configuration, the reflection from the ground during transmission almost overlaps with the radiated wave, which prevents the exploration signal from becoming complicated due to multiple reflected waves. It also reduces the effects of corners, etc. Each antenna element A ₁ , A ₂ , A ₃ ,...
Since the signals shared by the antenna elements A ₁ , A ₂ , A ₃ , ... _{, A K , ...A N are narrowband, the impedance matching of A K , ...A N with the ground} is far greater than that in the case of wide band. This makes it easier to design roses when using baluns. 27 in the figure is antenna 2
5 shows an antenna moving mechanism that moves the antenna and is configured to generate a position coordinate signal.

FIG. 7 shows another embodiment in which an antenna element group 29 constituting an antenna 28 is arranged in a dielectric medium 30. The dielectric medium 30 has a refractive index n that is equal to or approximately equal to the refractive index n _s of the earth, and therefore, the boundary between the antenna 28 and the earth, where the refractive index differs, can be ignored. Reflection from the surface can also be ignored, and if the antenna elements A ₁ to A _N are impedance matched to the signals f ₁ to f _N in the induction band medium 30, then
Efficient transmission and reception of radio waves becomes possible. In the figure, 31 is an antenna moving mechanism that moves the antenna 28, and is configured to generate a position coordinate signal. Since the refractive index n _s of the earth is always greater than 1,
An antenna element A ₁ placed in a dielectric medium 30,
A ₂ , A ₃ , . . . , _{A K} , . Therefore, this configuration has the advantage that a smaller antenna 28 can be used. In addition, since the earth can be considered to be non-magnetic in most cases, the refractive index mentioned above is actually equivalent to the dielectric constant.

FIG. 8 shows still another embodiment, in which the antenna 3
The antenna element group 33 constituting 2 has a relative refractive index n
In the dielectric medium 34, the height from the ground is such that the wavelengths of the signals handled by the antenna elements A ₁ , A ₂ , A ₃ , ..., A _K , ... _AN are reflected in the dielectric medium 34 . They are placed at positions h ₁ ...h _K ..., etc. that satisfy the phase condition for prevention. Generally, reflection from the ground surface occurs, but since the antenna element group 33 is located at a position that satisfies the phase condition for preventing reflection from the ground surface, the ground surface reflection returns to the antenna 32 in the opposite phase and eliminates unnecessary reflected signals. reduce In the figure, 35 is an antenna moving mechanism that moves the antenna 32, and is configured to generate a position coordinate signal.

FIG. 9 shows that an antenna element group 37 constituting an antenna 36 is arranged in a dielectric medium 38 whose refractive index satisfies the amplitude condition for anti-reflection with respect to the refractive index n _S of the earth, and its arrangement height is as follows. The distance from the ground surface is determined to satisfy the anti-reflection phase condition, and each antenna element A ₁ , A ₂ , A ₃ , ..., A _K , ...
A _N shows an embodiment in which the antennas are arranged at different heights from the ground surface, such as distances h' ₁ . . . h' _K . . . from those shown in FIG. This satisfies the requirements for a so-called anti-reflection layer in optics, and can be expected to more effectively suppress ground surface reflection. Reference numeral 39 denotes an antenna moving mechanism that moves the antenna 36, and is configured to generate a position coordinate signal.

In the embodiment shown in FIGS. 8 and 9, the heights from the ground where the antenna element groups 33 and 37 are arranged are antenna elements A ₁ , A ₂ , A ₃ , ..., A _K , ...
A is different for each _N. Therefore, in order to synthesize data from the same observation point, it is necessary to correct the phase of the collected data by a known amount in advance, but this correction amount can be stored in the memory 19 in advance. can.

The configurations shown in FIGS. 6 to 9 are made possible by the method of the present invention in which a signal is divided into a plurality of relatively narrowband antenna elements and received by a plurality of antenna elements. This cannot be achieved using a system that uses a wideband antenna.

In the embodiments described above, a wideband signal component is divided into a plurality of relatively narrowband signals, and a single frequency component signal representing each band is divided into
We have explained an example of assigning the frequency to each antenna element, but by dividing the band into bands that can be reasonably achieved with a single antenna and assigning one or more frequency components to one antenna element, the number of antenna elements can be reduced. The present invention can be applied even if an antenna having a similar configuration is used.

〔Effect of the invention〕

As explained above, in the present invention, the wideband signal components required for exploration are divided into multiple relatively narrow frequency bands, and each frequency band is assigned to a different antenna element to collect data. Since this method performs exploration by synthesizing short-duration, wide-band pulse waveforms through signal processing after data collection, the requirements for frequency characteristics are strict, and due to the wide band, there are drawbacks such as reduced efficiency. This method has the advantage that a wideband signal can be used without using a wideband antenna, and it is possible to effectively search for buried objects with good resolution.

Furthermore, since each antenna element only has to share a relatively narrow signal band, it is possible to use an antenna configuration suitable for suppressing strong unnecessary reflections from the ground surface, which can be used underground more effectively. It has the advantage of being able to search for buried objects.

Therefore, if the present invention is used for underground buried object exploration or underground structure exploration, broadband underground information can be obtained and exploration with high resolution can be performed.

Furthermore, the method of the present invention is useful not only underground but also when it is desired to non-destructively inspect the inside of a medium other than soil using a broadband signal.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of a conventional underground buried object exploration method using a broadband antenna, Figure 2 is a schematic diagram of an embodiment of the underground buried object exploration method of the present invention, and Figure 3 is an explanatory diagram of an underground buried object exploration method of the present invention. FIG. 4 is an enlarged side view of the configuration of the antenna in the method of searching for objects buried underground, and FIG. 5 is an enlarged plan view of the configuration of the antenna in the method of searching for underground objects of the present invention. Schematic diagram, Figure 6 is a schematic diagram of the configuration of the antenna in the underground buried object exploration method of the present invention, Figure 7
8 and 9 are schematic diagrams of other embodiments of the structure of an antenna designed to reduce the influence of reflection from the ground surface, which is used in the exploration of underground objects according to the present invention. 10... Transmission control unit, 11... Transmitter, 12...
... Functional block, 13 ... Antenna, 14 ... Antenna element group, 15 ... Buried object, 16 ... Position coordinate signal generator, 17 ... Reference signal, 18 ... Receiver, 19 ... Memory, 20 ... ...Pulse synthesis processing unit, 21...Graphics recorder, 22...Data recorder, 23...Antenna, 23'...Antenna, 24...Antenna element group, 24'...Antenna element group, 25...Antenna, 26... Antenna element group, 27... Antenna moving mechanism, 28...
...Antenna, 29...Antenna element group, 30...
Dielectric medium, 31... Antenna movement mechanism, 32...
...Antenna, 33...Antenna element group, 34...
...Dielectric medium, 35...Antenna movement mechanism, 36
...Antenna, 37...Antenna element group, 38...
...Dielectric medium, 39...Antenna movement mechanism.

Claims (1)

[Claims] 1. In a method of collecting underground information using wave motion and exploring underground objects, in order to obtain a broadband signal required for exploration, , using a transmitting and receiving antenna consisting of a set of a plurality of independent antenna elements arranged separately from each other in space or in a dielectric medium having a specific refractive index, and dividing the frequency band to separate the plurality of antenna elements. While sequentially supplying electric signals with different frequency components to each of the transmitting and receiving antennas, the transmitting and receiving antenna is moved, and each of the plurality of antenna elements corresponds the reflected wave of each frequency component with a position coordinate signal indicating the position of the transmitting and receiving antenna. From these received waveforms, a group of received waveforms with different frequency components belonging to the same observation point is selected, and these are used to synthesize short pulse waveforms for each observation point. This is an underground buried object exploration method using frequency division and synthesis, which is characterized by conducting underground exploration using the broadband pulse waveform thus obtained. 2. A plurality of antenna elements constituting the transmitting/receiving antenna are located close to the ground surface and move at a distance sufficiently shorter than the minimum wavelength among the center wavelengths of waves radiated by each antenna element. Frequency division according to claim 1
Exploration method for underground buried objects by synthesis. 3. A plurality of antenna elements constituting the transmitting/receiving antenna are located and moved in a dielectric medium having a refractive index equal to or almost equal to the refractive index of the earth. Exploration method for underground objects using frequency division and synthesis. 4 The plurality of antenna elements constituting the transmitting/receiving antenna are configured such that the height of each antenna element from the ground surface is set to a phase condition for preventing reflection of waves radiated by each antenna element into the surrounding medium of the antenna element. 2. A method for exploring underground objects by frequency division and synthesis according to claim 1, characterized in that the underground object is moved at a height that satisfies the following. 5. The plurality of antenna elements constituting the transmitting/receiving antenna are located in a dielectric medium having a refractive index that satisfies anti-reflection conditions for the waves radiated by each antenna element, and the ground surface satisfies the anti-reflection conditions. 2. A method for exploring underground objects by frequency division and synthesis according to claim 1, characterized in that the method locates and moves at a height of .