JPH0361915B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an underground buried object exploration method in which a broadband signal is radiated underground and the buried object is explored using reflected waves from the underground buried object. It is.

[Prior art]

The pulse-echo method is a method in which an antenna or other wave source is scanned over the earth's surface to collect reflected waves, and the obtained reflected waves are drawn and lined up at each coordinate point corresponding to each measurement point, aligning the time origin. Say. Because it is simple, this method has often been used to investigate underground objects and the ground, and the image of the object can be observed at a certain position of its reflected waveform, making it possible to determine whether the object is present or not. However, it is impossible to determine at what depth the object is buried. In order to know this, it is necessary to convert the time scale of the received waveform to a velocity scale, but conventionally, an appropriate exchange factor has been determined based on empirical knowledge or values obtained from other types of measurements as described below. They had no choice but to rely on the less accurate method of setting the value by the observer.

[Problems to be Solved by the Invention] Here, the conventional exchange factor setting method and its drawbacks will be described in more detail. First, to convert the time scale to the depth scale, we must know the wave propagation velocity in the ground. However, since the wave propagation speed in soil varies considerably depending on factors such as water content, it is necessary to estimate the propagation speed through various measurements, but conventional methods cannot solve this problem. There was a problem. In other words, conventional methods can be roughly divided into two types, one of which is that the measurer knows the average speed data of the area and uses this as a representative. However, it is common knowledge that the propagation speed changes even in a fairly narrow area, and even at the same measurement point, if the soil condition changes depending on the time, the speed will also change. Therefore, it is clear that accurate estimation cannot be made using this method.

The second method is to measure physical quantities related to propagation velocity, such as water content in the soil and soil capacitance, and indirectly estimate velocity from these quantities. In this case, quantitatively measuring the soil itself requires quite complicated procedures, so in many cases,
It is inefficient to carry out measurements at the site. In addition, since a soil sample is required, it is not possible to perform non-destructive, non-contact measurement, and the state of the sampled soil may change from its original state. There is no guarantee that the velocity estimates obtained will match the propagation velocity of the soil in the field. Furthermore, when estimating speed by measuring capacitance by placing electrodes on the ground surface or inserting electrodes into the ground, a measurement system different from the pulse echo method is required. In addition, in order to measure non-destructively, electrodes must be placed on the ground surface, but in this case, only the surface layer is measured, and accurate speed measurement is required to reach the depth of the buried object. The drawback was that estimation could not be performed.

The present invention was made to solve these drawbacks, and is based on data measured using the pulse echo method at a deep investigation site, without relying on any other measurement system.
The aim is to estimate the wave propagation speed underground and even the depth of an object with high accuracy. In addition, by combining the pulse compression method and the synthetic aperture method based on the estimated velocity, high-resolution imaging exploration of underground objects is performed.

[Means for solving problems]

In order to solve the above problems, in the present invention,
A transmission antenna connected to a transmission source emits a transmission wave underground where an object to be searched exists, and a reflected wave from the object is sent to the ground at a distance L from the transmission antenna on the ground surface. When searching for the object by receiving it with a reception antenna connected to the reception antenna and detecting it by a receiver connected to the reception antenna, a broadband transmission wave is used, and the reflected wave is detected at a large number of measurement points X _o ( From the received waveform at these measurement points X _o , the propagation time An (n = -M to N) from the transmitting antenna to the receiving antenna is measured, and further, the distance _The propagation time Bn ( _n
= -M~N) and the measured propagation time An, the depth Z _p of the object and the underground propagation velocity V
The method is characterized in that the depth Z _p of the object is determined by applying the method of least squares with and as unknowns.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings. Note that although the following explanation will be given by taking as an example a deep investigation using radar, it is also possible to implement the deep investigation method according to the present invention using waves such as sound waves.

FIG. 1 is a block diagram showing the configuration of one embodiment of the present invention.

In the figure, a transmission/reception control device 1 is a device that mainly controls timing between transmitters and receivers. The transmitter 2 generates an impulse signal for measurement, which is exchanged into radio waves by the transmitting antenna 3 and radiated underground. The waves reflected from the object are captured by the receiving antenna 4 and sent to the receiver 5. The receiver 5 is equipped with a sample and hold circuit, etc., and time-expands the sampled received signal to facilitate subsequent signal processing (for example, expands the time scale by a times) and processes the time-expanded received signal. It is supplied to the A/D exchanger 8a. On the other hand, the antenna moving device 6 carries the transmitting/receiving antennas 3, 4 and the transmitting/receiving devices 2, 5 and moves them. A position measuring device 7 for measuring position information) is loaded. The output signal of this position measuring device 7 is exchanged into a digital signal by an A/D exchanger 8b. In addition, the received signal exchanged into a digital signal by the A/D exchanger 8a is processed by the signal processing device 9, where the S/N ratio etc. are improved, and then the A/D exchanger It is stored in the memory 10 as a pair with the position signal output from 8b. The display device 11 forms a pulse echo image from the position signal in the memory 10 and the received signal, and the depth velocity estimation device 12 estimates the depth of the object and underground radio wave propagation speed from the pulse echo image.

Next, the operation of this embodiment will be explained.
Operations other than the speed estimating device 12 can be easily understood from the drawings and the above description, so a description of these operations will be omitted and only the operation of the depth/velocity estimating device 12 will be described.

First, the measurement principle will be explained. Now, the second
As shown in the figure, when the distance between the transmitting and receiving antennas is L, the coordinates of their midpoints are X, the measurement interval on the ground surface is X _p , and the time point corresponding to the ground surface of the received signal is t=o,
Assume that a hyperbolic response l as shown in the figure is measured from a certain object m (ideally a point or linear object). For simplicity, the X of the vertex of the curve
Let the locus line (corresponding to the X locus line of object m) be x=o, and x
The front M traces (scans) and the rear N traces at =o are selected as one set. Select time points that form a hyperbolic curve from the response part from the object included in each trace, and set each time point to (x _o ), x _o = nx _p (n = M ~ N) ... (1a ).

Object depth z _p and propagation velocity V from these time points
In other words, we estimate. In principle, the above parameters can be estimated using two time points and the intertrain interval x _p based on the principle of triangulation, but since the variation in estimation results due to variations in time points is generally very large, It is dangerous to extrapolate from this alone. In particular, when the target is underground, the above-mentioned principle of triangulation is inappropriate because there are large fluctuations in time points due to factors such as the unevenness of the soil. Therefore, the method described here allows more stable estimation by using many time points.

Now, when x = x _o , the time it takes for a pulse to be reflected by an object and received by the antenna is t(x _o )
Then, the following equation holds from the distance and velocity V between the transmitting and receiving antennas and the object.

t (x _o ) = 1/V [√ ( _o −2) ² + ² ₀ +√ ( _o +2) ² + ² ₀ ] ... (1b) Then, at x = x _o , the time obtained from the measurement is t _p (x _p ) as described above. Therefore,
In order to obtain accurate Z _p and V, equation (1b) is
The conditions imposed on Z _p and V are x=x _o (n=-M~N)
The goal is to minimize the sum of squares of errors with respect to the time point at . That is, E= _N〓nM |t( _xo ) _−tp ⁽ _xo )| ² →min...(2). In order to minimize E in equation (2), the partial differential coefficients with respect to Z _p and V are set to 0.

∂E/∂Z _p =0, ∂E/∂V=0 (3) Then, from equation (3), simultaneous equations regarding Z _p and V can be created.

However, A(x _o , Z _p )=√( _o −2) ² + ² ₀ √( _o +2) ² + ² ₀ From this equation (4), if V is eliminated, an equation regarding Z _p is obtained. That is, f(Z _p )=ΣA ² (x _o , Z _p )/ΣA (x _o , Z _p ) to (x _o ) −ΣA (x _o , Z _p )B(x _o , Z _p )/ΣB (x _o , Z _p )t _p (x _o
)=0...(5) By solving this equation, the object depth Z _p can be found. This equation can be solved quickly using the Newton-Raphson method. Moreover, Z _p
In many cases, the range can be specified by deep investigation conditions, so setting the initial value is relatively easy.

Once Z _p is determined, V can be calculated from equation (4).

That is, becomes.

By the way, in the case of this embodiment, the time scale is expanded by a times in the receiver 5, but even in this case, the above-mentioned equation (5) is as follows: f(Z _p )/a=0...(7) The obtained value of Z _p remains unchanged. Also, from (6), the speed becomes V/a, but
Since time is originally measured as at, the conversion speed from time scale to depth scale (conversion speed x one-way propagation time) is also Z = V / a · at / 2 = Vt / 2 ... (8), and a It remains unchanged. Therefore, it can be seen that there is no problem in measuring each system using its own time scale.

Then, the depth estimating device 12 calculates the object depth Z _p and the propagation velocity V based on the above-mentioned formulas.

FIG. 3 is a block diagram showing the configuration of a modified example of the above-described embodiment. This modification adds a pulse compression filter 13 and a synthetic aperture device 14 to the previously described embodiment. The pulse compression filter 13 is configured to shorten the duration of the reflected waveform from the object, and serves to improve the depth resolution. It serves to compose images and improve horizontal resolution. Therefore, in this modification, high-resolution image reproduction can be performed. Moreover, in order to effectively perform synthetic aperture processing, it is necessary to accurately know the underground wave propagation speed, but in this invention, as described above, Since the average speed of the propagation time can be estimated very accurately, the resolution and reliability of the image reconstruction process can be significantly improved. The operation and function of the pulse compression filter 13 and the synthetic aperture device 14 are already known technologies, and are also described in, for example, Japanese Patent Application No. 136060/1989 filed by the inventors earlier, so details will be given here. Omitted.

〔Effect of the invention〕

As explained above, according to the present invention, the wave propagation velocity and object depth at a desired location are directly calculated based on the data obtained from the reflected signals collected at the location. This provides the advantages described in . No matter how the characteristics of the soil at the measurement location change, it is possible to conduct deep surveys with high precision and determine the location of objects, including their depth. There is no need to use a special measuring device to measure the speed, and a non-destructive and efficient deep investigation can be performed. In addition, by combining the pulse compression method and the synthetic aperture method and creating images based on estimated velocity information, it is possible to obtain high-resolution and highly reliable reconstructed images of underground objects. This will be extremely useful in determining the situation.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing a response from an object in the pulse echo method, and FIG. 3 is a block diagram showing a modification of the same embodiment. 1... Transmission/reception control device, 2... Transmitter, 3...
Transmitting antenna, 4...Receiving antenna, 5...Receiver, 6...Antenna moving device, 7...Position measuring device, 8a, 8b...A/D exchange device, 9...Signal processing device, 10... Memory, 11...display device,
12... Depth velocity estimation device, 13... Pulse compression filter, 14... Synthetic aperture device.

Claims (1)

[Claims] 1. A transmitting antenna connected to a transmitting wave source emits a transmitting wave underground where an object to be searched exists, and a reflected wave from the object is emitted from the transmitting antenna on the ground surface. When searching for the object by receiving it with a receiving antenna arranged at a distance L, and detecting it by a receiver connected to the receiving antenna, using a broadband transmitted wave, and transmitting the reflected wave on the receiving plane. A large number of measurement points X _o (n
= -M to N), and from the received waveforms at these measurement points Xn, the propagation time A _o (n = -M to N) from the transmitting antenna to the receiving antenna is measured, and further, the distance L The propagation time Bn (n=-M to N) obtained based on the position information of each measurement point X _o and the depth Zo of the object, and the measured propagation time An, the depth Z of the object An underground object exploration method characterized in that the depth Z _p of the object is determined by applying a least squares method in which _p and an underground propagation velocity V are unknowns. 2 Numerically determine the object by performing pulse compression processing and synthetic aperture processing from the position information X _o of each measurement point, the received waveform at these measurement points, the propagation velocity V, and the depth Z _p . The underground buried object exploration method according to claim 1, characterized in that the image of the underground object is reconstructed.