JP2905834B2 - Monitoring method for damage to underground pipes - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for monitoring damage to an underground pipe, and more particularly, to a method for monitoring an underground pipe by using a conductive object such as a drilling machine tool. If damage has occurred, it is immediately detected that the damage has occurred.

[Conventional technology]

During excavation work, a conventional method of detecting the occurrence of damage to an underground pipe due to a conductive object such as a cutting blade of a drilling machine tool is to connect an AC power supply to the underground pipe and connect an AC voltage to the ground. Using the generation of an alternating magnetic field due to alternating current leaking from the damaged part of the coating and covering due to the contact of the conductive object to the ground, detecting the point where this alternating magnetic field disappears from the ground,
A method for detecting a damaged part generated in an underground pipe (Japanese Patent Application Laid-Open
-44074), and methods of detecting damaged parts from the ground using various sensors for detection are known. However, these methods only detect damaged parts of underground pipes and cause damage. Could not be monitored at all times.

As a method of constantly monitoring the occurrence of damage to underground pipes,
A method of detecting damage with sound waves or DC bias through an underground pipe or applying an AC signal voltage to an underground pipe and transmitting it, and then this AC signal voltage at a remote receiving point Monitors and detects the decrease in grounding resistance of underground pipes due to contact with conductive objects, that is, changes in the AC signal potential at the receiving point caused by the decrease in grounding resistance due to the occurrence of damaged parts, and damages underground pipes There is known a method for detecting the occurrence of a bleed (Japanese Patent Laid-Open No. 62-81557).

[Problem to be Solved by the Invention] However, the method of monitoring the occurrence of damage to an underground pipe using sound waves or a DC bias is a technique in which a sound wave propagating in an underground pipe is rapidly attenuated over a short distance, and other There is a problem that it is difficult to discriminate the noise, and thus it is not possible to achieve reliable and accurate damage detection.

Further, the AC signal potential applied to the underground pipe at the transmission base attenuates as the distance from the transmission base increases, and becomes a small AC signal potential at the reception base. In the method disclosed in Japanese Patent No. 81557, the receiving potential is very small, so that it is confused with the AC noise potential existing in the underground pipe, and it is difficult to select the used transmitting AC signal.

The amount of change in the AC signal potential at the time of contact with the conductive object at the damaged portion of the underground pipe is attenuated to a small value at the receiving point, and therefore, reliable damage detection cannot be achieved.

The fluctuation of the AC signal potential at the receiving point due to environmental changes is large, and it is difficult to discriminate the change due to damage.

Facility construction of the electrode for current supply is required, and it takes time to implement.

If a low grounded object such as a cathodic protection facility is installed in the underground pipe, the AC signal transmitted to the underground pipe leaks into the ground through this low grounded object, so the receiving base receives the AC signal It becomes impossible to do.

And so on.

Therefore, the present invention was conceived to solve the above-mentioned problems in the prior art, and propagates an AC signal applied to an underground pipe at a transmitting base to a receiving base without attenuating its potential. It is an object of the present invention to achieve accurate and reliable damage monitoring.

[Means for solving the problem]

Means of the present invention for solving the above technical problem is that, when transmitting an AC signal from a transmitting base to a receiving base using an underground pipe as a medium, the frequency f of the AC signal is Here, 1 is the distance from the transmitting base (1) to the receiving base (2), L is the guidance of the underground pipe (3), C is the capacitance of the underground pipe (3), and k is the correction. Value (0.88 to 0.95), and monitoring the occurrence of damage to the underground pipe by the change in the potential of the received AC signal at the receiving base.

In addition to the above means, when a low grounding object such as an anticorrosion facility is connected to the underground pipe, the low grounding is performed through an LC parallel resonance filter that shows the maximum AC resistance value at the frequency f of the AC signal. Goods should be connected to underground pipes.

When the frequency f obtained by the expression (1) cannot be used as the frequency f of the AC signal, a low grounding object is connected to the underground pipe near the receiving base via an adjustment capacitor. It is preferable to change and set the frequency f of the AC signal to a desired value.

Then, in order to reliably discriminate between the change due to the environmental change and the change due to the damage of the reception AC signal potential, the reception AC signal potential at the reception base is sampled every predetermined unit time, and the adjacent sampling potential is sampled. If the difference exceeds a preset value, it is good to judge that the underground pipe has been damaged.

Further, in order to be able to omit the installation work of the power supply electrode at the transmission base, one output terminal is connected to the power supply electrode of the external anticorrosion power supply device attached to the underground pipe near the transmission base. The other output terminal of the signal transmission device connected to the underground pipe is connected via a coupling capacitor, and the connection of the conducting electrode to the external power supply for anticorrosion is
It is preferable to perform the processing through an LC parallel resonance filter that exhibits the maximum AC resistance value at the frequency f of the AC signal.

[Action]

After investigating and analyzing the electrical characteristics of the underground pipe,
When an AC signal potential is applied to the underground pipe, the underground pipe between the transmitting base 1 and the receiving base 2, which is the point to which the AC signal is applied, can be replaced with an equivalent circuit shown in FIG. The result was obtained.

That is, a distributed constant circuit including a series circuit of an inductance L and a resistance R on the pipeline side K and a parallel circuit of a capacitance C and a leakage conductance G between the pipeline side K and the ground side A is provided underground. The equivalent circuit is distributed for each unit distance of the buried pipe.

When an AC signal potential is applied to a 600 mmφ underground pipe, the resistance R is 0.8Ω / km, the inductance L is 1 mH / Km, and the capacitance C is 7.2. μF / Km, leakage conductance G is 1 / 1000S
/ Km.

From this, when an AC signal potential is applied to the underground pipe from the transmission base at one end, at a specific frequency according to the distance between the transmission base and the reception base, the AC signal potential at the reception base increases due to a wave phenomenon. It was predicted that the increase of the AC signal potential at the receiving base could be confirmed by many experiments.

The relationship between the distance between the transmitting base and the receiving base, which increases the AC signal potential at the receiving base, and the frequency of the AC signal can be obtained by a number of experiments as follows.

The AC signal potential distribution when the AC signal is applied to the underground pipe is distributed in a wave shape similar to a sine wave.
The wavelength is Is determined by the general formula.

The AC signal potential in this one wavelength λ changes as the distance from the transmission base increases, rises to the point of / 4λ, drops to the same potential as the transmission base at the point of / 4λ, and
It shows a transition in which it drops to the minimum value at the point λ and rises and returns to the same potential as the transmitting base at the point λ. Therefore, the receiving point of the maximum AC signal potential is 1/4 from the transmitting base.
Since the point is λ, the theoretical optimum reception distance l ₀ from the transmission base to the reception base for obtaining the maximum AC signal potential at the reception base is l ₀ = λ / 4 (3).

Substituting equation (2) into equation (3) gives Is obtained.

When a correction in a realization field is added to this theoretical optimum reception distance l ₀ , the optimum reception distance l becomes Here, k is a correction value that is a value within the range of 0.88 to 0.95,
Determined by actual measurement.

In the situation where the receiving base is determined and the optimum receiving distance 1 is determined, the frequency f of the AC signal represented by the expression (1) is calculated from the expression (5).

As described above, the AC signal potential from the transmission base can be amplified and received at the reception base by utilizing the wave phenomenon caused by the action of the capacitance component C and the induction component L distributed in the underground pipe. However, the wave phenomenon with respect to the AC signal potential in the underground pipe is caused by the conductive object at the damaged part when the conductive object damages the underground pipe between the transmitting base and the receiving base. The same applies to the potential change caused by the decrease in the grounding resistance of all underground pipes, and when the underground pipe is damaged by a conductive object, the potential change at the damaged part is amplified. Since it is observed as a large potential change at the receiving base, it is possible to detect that the underground pipe has been damaged by knowing this large change in the potential.

If a low-grounded object such as an anticorrosion facility is provided in the underground pipe between the transmission base and the reception base, the AC signal voltage transmitted from the transmission base leaks to the ground through the low-grounded object. Due to the leakage, the degree of amplification of the received AC signal potential at the receiving base is reduced. This effect also acts on the degree of amplification of the amount of potential change at the damaged location received at the receiving base when the underground pipe is damaged by a conductive object. Is provided, the receiving base can detect the amount of potential change at the damaged portion as a value at which damage can be detected, but the value is small, and the reliability of damage detection decreases.

Therefore, between the underground pipe and the low grounded object, a frequency-response type LC that shows the maximum resistance at the frequency f of the AC signal voltage and has almost no resistance to the DC voltage (corrosion protection voltage) Insert and connect a parallel resonance filter.

As described above, by inserting and connecting the LC parallel resonance filter between the underground pipe and the low grounded object, the AC signal potential can be reduced via the low grounded object without adversely affecting the anticorrosion effect. Leakage to the ground can be prevented, and abrupt attenuation of the AC signal potential due to a low grounding object is reliably prevented, thereby effectively exhibiting a wave phenomenon with respect to the transmitted AC signal.

The AC signal potential frequency f is determined by the equation (1) according to the distance 1 between the transmission base and the reception base. For example, the optimum frequency f calculated from the equation (1) is determined by the AC frequency used in railways or the like. In the case where interference occurs on the frequency, the frequency of the AC signal potential cannot be set to the optimum frequency f calculated from the equation (1).

Therefore, in the vicinity of the receiving base, an adjusting capacitor is inserted and connected between the underground buried person and a separately provided underground or a low-grounded object such as an anticorrosion facility. By connecting the adjustment capacitor, the capacitance C of the equation (1) changes, so that a sufficient wave phenomenon can be obtained by adjusting and setting the value of the adjustment capacitor to an appropriate value. In the state,
The optimum frequency f can be set to a value that does not interfere with other AC frequencies.

Since the AC signal potential detected at the receiving base is slowly changing due to the influence of the environment, the potential change accompanying the damage is caused by the slow change of the detected potential due to the influence of the environment. Does not reach the preset potential change amount or potential level, and therefore, the occurrence of damage to the underground pipe cannot be detected.

Therefore, the receiving AC signal potential at the receiving base was sampled every preset unit time, and if the difference between the adjacent sampling potentials exceeded a preset value, damage occurred to the underground pipe. Judge. In this way, by sampling the received AC signal potential at the receiving base every unit time and comparing adjacent sampling potentials, the slow change in the detected potential due to the influence of the environment is extremely small. The potential change due to damage is large, so it is detected as a potential change, and the potential change due to damage is reliably detected without being affected by the potential change due to the environment. .

In order to apply an AC signal potential to the underground pipe from the transmission base, it is necessary to connect an energizing electrode provided in the ground to apply a voltage to the underground pipe to the signal transmission device of the transmission base. Providing a dedicated current-carrying electrode only to supply an AC signal to an underground pipe would increase equipment and construction costs. Is installed, the energizing electrode of the external power supply for anticorrosion is preferably shared as an energizing electrode for applying an AC signal voltage to an underground pipe.

That is, the other output terminal of the signal transmission device having one output terminal connected to the underground pipe is connected to the energizing electrode of the external anticorrosion power supply device attached to the underground pipe near the transmission base via a coupling capacitor. DC connection of the external power supply for anticorrosion by connecting the power supply electrode to the external power supply for anticorrosion through an LC parallel resonance filter showing the maximum AC resistance value at the frequency f of the AC signal. The current is cut off by the coupling capacitor and does not affect the signal transmission device at the transmission base, and the AC signal potential of the signal transmission device is cut off by the LC parallel resonance filter, affecting the external power supply for anticorrosion. Do not give.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is an electric circuit configuration diagram of one embodiment of an underground pipe according to the method of the present invention. In FIG. 1, 1 is a transmission base for an AC signal potential having a signal transmission device 5 including a signal source and an amplifier. Reference numeral 2 denotes an AC signal potential receiving base including a measuring device 7 and a digital logic analyzer 8 having a damage detecting device 6 provided with a measuring electrode 14 in the ground, and reference numeral 3 denotes an underground pipe.

In this embodiment, the underground pipe 3 has a diameter of 600 mmφ, and the distance between the transmitting base 1 and the receiving base 2 is 13 km. As described above, the underground pipe 3 can be imitated as the equivalent circuit shown in FIG.
Resistance R is 0.8Ω / Km, inductance L is 1mH / Km, capacitance C is 7.2μF / Km, and leakage conductance G is 1 / 1000S / Km
It was actually measured.

In general, most of the noise on the underground pipe 3 has a frequency of 1000 Hz or less. In the method of monitoring the damage of the buried pipe 3, the frequency of the AC signal potential is set to 3000 Hz.

An actual measurement comparison between the conventional method and the method of the present invention will be described with reference to a received AC signal potential characteristic diagram of the underground pipe 3 shown in FIG.

According to a conventional method, when an AC signal potential was transmitted from the transmitting base 1 to the receiving base 2 (1 V transmission) at a frequency of 3000 Hz via the underground pipe 3, the AC signal potential characteristic of the underground pipe 3 became a curve c, While the received AC signal potential at the receiving base 2 was 0.025 V, according to the method of the present invention, the frequency f was set to 200 Hz and the AC signal potential was transmitted (1 V transmission). 3, the AC signal potential characteristic was curve a, and the received AC signal potential at the receiving base 2 was 1.81 V. That is, by carrying out the method of the present invention under the same conditions, the receiving AC signal potential at the receiving base 2 can be received at a potential 72 times higher than that of the conventional method, thereby greatly improving the receiving accuracy. Improvement has become possible.

In order to confirm the improvement of the receiving accuracy of the method of the present invention,
When the conductive object 4 damages the underground pipe 3 at a distance of 5 km from the transmitting base 1, the AC signal potential characteristic in the conventional method becomes a curve d, and the potential drop continues even after the damage point S. And the receiving AC signal potential at the receiving base 2 is 0.024V
The change amount is 0.001 V, whereas the AC signal potential characteristic in the case of the method of the present invention becomes a curve b, and although the potential decreases to the damage point S, it gradually decreases after passing the damage point S. The received AC signal potential at the receiving base 2 was 1 V, and the amount of change was 0.81 V, which was 810 times the amount of change compared to the conventional method.

From this, it was confirmed that the reception accuracy of the method of the present invention was improved, that is, the high damage detection capability and high accuracy of the method of the present invention were confirmed.

Next, while keeping the conditions for implementing the method of the present invention in FIG.
An example of actual measurement when a low grounded object 10 such as an anticorrosion facility is connected to the underground pipe 3 will be described with reference to FIGS. 1, 4, 5, and 6. FIG.

As shown in FIG. 4, the low grounded object 10 is connected to the underground pipe 3 via an LC parallel resonance filter 9 formed by connecting a capacitance C of 63.3 μF and an inductance L of 10 mH in parallel. The impedance of the LC parallel resonance filter 9 is
As shown in FIG. 6, the frequency characteristic shows the maximum resistance (8850Ω) at a frequency of 200 Hz with respect to the AC signal potential.
The resistance against the DC voltage of the anticorrosion facility is 0.3Ω, which shows almost no resistance.

When an AC signal potential is transmitted while the low grounded object 10 is directly connected to the underground pipe 3, the AC signal potential characteristic becomes a curve f as shown in FIG. Is 1.1 V, whereas the LC parallel resonance filter 9
When the AC signal potential is transmitted with the low grounded object 10 connected to the underground pipe 3 via the
And the received AC signal potential at the receiving base 2 was 1.8 V.

For this reason, the LC parallel resonance filter 9 is
It can be confirmed that the leakage of the AC signal potential from the underground pipe 3 is effectively reduced.

In this state, when a damage point S is generated at a location of the underground pipe 3 at a point 5 km from the transmission base 1, when the LC parallel resonance filter 9 is not provided, the AC signal potential characteristic becomes a curve g,
The received AC signal potential at the receiving base 2 is 1 V, and the change amount is 0.1 V. On the other hand, when the LC parallel resonance filter 9 is provided, the AC signal potential characteristic becomes a curve h, and the receiving base 2 , The received AC signal potential was 1 V, and the amount of change was 0.81 V.

For this reason, when the low grounded object 10 is connected to the underground pipe 3, the low grounded object 10 is connected to the underground pipe 3 via the LC parallel resonance filter 9, whereby the low grounded object 10 is connected to the underground pipe 10. The deterioration of the damage detection accuracy of the method of the present invention can be almost completely prevented, and high damage detection capability and high damage detection accuracy can be maintained.

In FIG. 4, the reason why the lightning arrester 16 is connected in parallel with the LC parallel resonance filter 9 is a countermeasure against surge for the underground pipe 3.

The optimum frequency f of the AC signal potential in the above embodiment is 20
Although it was 0 Hz, assuming a case where this frequency 200 Hz cannot be used because it interferes with the frequency of a signal used in an electric railway or the like, as shown in FIG. Is connected to a low-grounded object 10 via an adjusting capacitor 11, and the adjusting capacitor 11 is adjusted so that the optimum frequency f is 100 Hz.
Was adjusted and set to 80 μF.

As shown in FIG. 7, the frequency f of the AC signal potential is set to 100H
When z is set and the combination of the adjusting capacitor 11 and the low grounding object 10 is not connected to the underground pipe 3 (no adjusting capacitor is provided), the AC signal potential characteristic becomes a curve k and the receiving base 2
Is 1.14 V, whereas in the case where the adjustment capacitor is provided, the AC signal potential characteristic becomes a curve j, and the received AC signal potential at the receiving base 2 is 1.4 V.

Therefore, if the capacitance C of the expression (1) is adjusted with the adjustment capacitor even at a frequency other than the optimum frequency f obtained from the expression (1) of the AC signal potential, a high reception AC is obtained at the receiving base 2. It was confirmed that a signal potential could be obtained.

From this state, when a damage point S is given to the underground pipe 3 at a point 5 km from the transmitting base 1, the AC signal potential characteristic becomes a curve n when no adjusting capacitor is provided, and the receiving AC signal at the receiving base 2 is obtained. The potential is 1.0 V and the change is 0.
In contrast to 14 V, when the adjustment capacitor was provided, the AC signal potential characteristic was represented by a curve m, the received AC signal potential at the receiving base 2 was 1.01 V, and the change amount was 0.39 V.

From this, even if the adjustment capacitor is not provided, damage can be detected, but by installing the adjustment capacitor 11, the damage detection capability can be sufficiently improved, and the damage detection accuracy can be considerably improved. it can.

When the potential fluctuation of the receiving AC potential at the receiving base 2 over a long period of time is actually measured, as shown by a curve o in FIG. 8, it shows a slow change due to the influence of the environment. Damage is monitored by absolute value evaluation in which the value of the slowly changing received AC signal potential is directly compared with a preset evaluation reference value q based on a measurement value of a measuring device 7 such as an AC voltmeter with a filter. Therefore, damage cannot be detected depending on a change in the received AC signal potential due to the influence of the environment, and the accuracy of damage detection decreases.

That is, as shown in FIG. 8, when the received AC signal potential is high due to an environmental change due to a change in the ground resistance of the underground pipe 3 or the like, the potential change due to the occurrence of damage (the potential decreases). ) Even if the pulse p is input, the reduced potential due to the potential change pulse does not drop below the evaluation reference value q, and therefore, the occurrence of damage cannot be detected.

Therefore, as shown in FIG. 1, the damage detection device 6 installed in the receiving base 2 is sampled by the measuring device 7 and the measured value of the measuring device 7 at every set unit time. If the difference does not exceed the set value, there is no output change, and if the difference exceeds the set value, the digital logic analyzer 8 changes the output.

By providing the digital logic analyzer 8 in the damage detection device 6, the received AC signal potential detected by the damage detection device 6 changes slowly even if it is affected by changes in the environment. As shown by the curve o in the figure, the value becomes a constant value having a substantially constant potential difference with respect to the evaluation reference value q. In addition, since the potential change pulse p due to the occurrence of damage to the underground pipe 3 is a potential pulse wave that causes a potential rise above the set value of the digital logic analyzer 8 within a short time, the digital logic analyzer 8 When the potential change pulse p is input, a detection pulse p 'that rises in a pulse shape to a potential higher than the evaluation reference value q is output.

Therefore, even if the received AC signal potential changes slowly due to the influence of the environment, the potential change pulse p accompanying the occurrence of damage is ignored ignoring the slow change of the received AC signal potential due to the environment. Can be reliably detected, and thus the high detection accuracy of the method of the present invention can be reliably maintained.

As shown in FIG. 1, when the external power supply device 12 for corrosion protection is installed near the transmission base 1, the signal transmission device 5
One of the output terminals is connected to the energizing electrode 13 of the anticorrosion external power supply 12 via the coupling capacitor 15, and between the anticorrosion external power supply 12 and the energization electrode 13 as shown in FIG. Work to insert and connect the LC parallel resonance filter 9 so that the current-carrying electrode 13 of the external power supply device 12 for corrosion protection functions as a current-carrying electrode of the signal transmission device 5, and provide a current-carrying electrode dedicated to the signal transmission device 5. Cost and equipment costs have been reduced.

〔The invention's effect〕

The present invention has the above-described configuration, and has the following effects.

The receiving AC signal potential at the receiving base can be made sufficiently high, and the amount of change in the receiving AC signal potential between normal time and damage occurrence is large, so that the detection of the occurrence of damage to the underground pipe can be reliably achieved. At the same time, there is no risk of erroneous detection, and thus detection of the occurrence of damage to the underground pipe can be achieved with extremely high accuracy.

The optimum frequency of the transmission AC signal potential can be easily obtained from the length of the underground pipe between the transmitting base and the receiving base, the inductance and capacitance in an equivalent circuit of the underground pipe, and the correction value. Therefore, the setting is easy and the method of the present invention can be easily carried out.

Even if a low grounded object such as an anticorrosion facility is connected to the underground pipe, leakage of the AC signal potential through the low grounded object is extremely effectively prevented without lowering the original function of the low grounded object Therefore, damage detection capability with high accuracy can be maintained.

Connect an adjustment capacitor as necessary, without limiting the frequency of the AC signal potential to the optimum frequency set from the distance from the transmitting base to the receiving base and the circuit constant of the equivalent circuit of the underground pipe. By this, it is possible to set the desired frequency without lowering the damage detection capability of the underground pipe, and thereby adapt to the laying environmental conditions of the underground pipe and have a high damage detection capability of the underground pipe. Accuracy can be maintained.

Even if the received AC signal potential at the receiving base changes slowly due to the influence of the environment of the underground pipe, the slow change of the received AC signal potential due to the influence of this environment is ignored as a detection signal. Therefore, high accuracy of the ability to detect damage to the underground pipe can be reliably maintained.

When an external power supply for anticorrosion is provided near the transmission base, the current-carrying electrode of the external power supply for anticorrosion can also be used as a transmission electrode for the AC signal potential.
The equipment cost and labor required for implementing the method of the present invention can be greatly reduced.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a connection form of each electric facility to an underground pipe in one embodiment of the method of the present invention. FIG. 2 is an electrical equivalent circuit diagram of an underground pipe between a transmission base and a reception base. FIG. 3 is an AC signal potential characteristic diagram of an underground pipe when the method of the present invention is performed and when the conventional method is performed. FIG. 4 is a diagram showing a circuit configuration of an LC parallel resonance filter used in the method of the present invention. FIG. 5 is an AC signal potential characteristic diagram when a low grounded object is connected to an underground pipe via an LC parallel resonance filter and when it is directly connected to an underground pipe in the method of the present invention. . FIG. 6 is an impedance-frequency characteristic diagram of the LC parallel resonance filter used in the method of the present invention. FIG. 7 shows that when the optimum frequency of the AC signal potential is changed,
FIG. 7 is an AC signal potential characteristic diagram of the underground pipe when the adjusting capacitor suitable for the newly set frequency is connected to the underground pipe near the receiving base and when it is not connected. FIG. 8 shows a relationship between a potential change pulse generated due to damage and a state in which the detected potential changes slowly due to the environment when the AC signal potential received at the receiving base is directly used as the detection output. FIG. FIG. 9 is an explanatory diagram showing the relationship between the detected potential and the damage detection pulse in the case of implementing the method of the present invention in which the influence of the environment is removed by sampling the AC signal potential received at the receiving base. DESCRIPTION OF SYMBOLS 1; transmission base, 2; reception base, 3; underground pipe, 4; conductive object, 5; signal transmission device, 6; damage detection device, 7; measuring instrument, 8; digital logic analysis device, 9; LC parallel resonance filter, 10; low-grounded object, 11; adjustment capacitor, 12; external power supply for anticorrosion, 1
3; conducting electrode, 14; measuring electrode, 15; coupling capacitor, 1
6; lightning arrester, S; damage point, p; potential change pulse, p '; detection pulse, q; evaluation reference value.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Terumi Yaguchi 1819-1 Takagasaka, Machida-shi, Tokyo (72) Inventor Akira Osaka 2-15-40 Misumicho, Higashimurayama-shi, Tokyo (56) References JP Akira 62-81555 (JP, A) JP-A-61-83951 (JP, A) JP-A-53-44074 (JP, A) JP-A-2-203263 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01N 27/00-27/24

Claims (5)

(57) [Claims] When transmitting an AC signal from a transmission base (1) to a reception base (2) using an underground pipe (3) as a medium, the frequency f of the AC signal is determined by: Here, 1 is the distance from the transmitting base (1) to the receiving base (2), L is the guidance of the underground pipe (3), C is the capacitance of the underground pipe (3), and k is the correction. A method of monitoring damage to an underground pipe (3), which is set to a value (0.88 to 0.95) and monitors the occurrence of damage to the underground pipe (3) by a change in the potential of a received AC signal at the receiving base (2). 2. An underground pipe (3) to which a low grounding object (10) such as an anticorrosion facility is connected, an LC parallel resonance filter (9) showing a maximum AC resistance value at the frequency f of the AC signal. The method for monitoring damage to an underground pipe according to claim 1, wherein the low grounded object (10) is connected to the underground pipe (3) via a cable. 3. An underground pipe (3) near a receiving base (2).
3. A method for monitoring damage to an underground pipe according to claim 1, wherein the frequency f of the AC signal is changed and set to a desired value by connecting a low grounding object (10) via an adjusting capacitor (11). . 4. An underground buried pipe when the difference between the adjacent sampling potentials exceeds a preset value, sampling a received AC signal potential at the receiving base (2) every predetermined unit time. The method for monitoring damage to an underground pipe according to claim 1, wherein it is determined that the damage has occurred in (3). 5. An underground pipe (1) having one output terminal connected to a current-carrying electrode (13) of an external anticorrosion power supply (12) attached to an underground pipe (3) near a transmission base (1). The other output terminal of the signal transmission device (5) connected to 3) is connected via a coupling capacitor (15), and the anticorrosion external power supply device (1) is connected.
The connection of the current-carrying electrode (13) to (2) is performed via an LC parallel resonance filter (9) exhibiting a maximum AC resistance value at the frequency f of the AC signal. For monitoring damage to underground pipes.