JP7250873B1 - Ground resistivity monitoring device and slope failure warning system - Google Patents

Abstract

[Problem] To easily acquire the specific resistance of the ground at a desired depth in a short period of time while saving power, evaluate the risk of slope failure based on the acquired results, and provide a stable and highly reliable alarm. To provide a ground resistivity monitoring device and a slope failure warning system that can perform A resistivity monitoring device includes a pair of current electrodes installed on the ground and a plurality of sets of potential electrodes arranged outside the pair of current electrodes. The potential difference between the two potential electrodes of each set is measured, and the resistivity at a given depth of the ground is calculated based on the given current and the measured potential difference, respectively. In addition, the slope failure warning system calculates the ratio of the thickness of the aquifer in the soil layer as a risk parameter representing the risk of slope failure, based on the ground resistivity obtained by the resistivity monitoring device. Trigger an alert when the criticality parameter reaches a threshold. [Selection drawing] Fig. 1

Description

The present invention uses a resistivity monitoring device that monitors the resistivity of the ground over time through a large number of electrodes installed on the ground, and the monitoring results, and the risk of sloping ground is high. The present invention relates to a slope failure warning system that issues a warning when a slope failure occurs.

In general, the ground consists of layers of various types of soil layered on top of an impermeable basement. Slope failure may occur. As a disaster prevention system for evaluating the risk of such a slope failure and detecting it in advance, conventionally, there are systems based on the soil rainfall index or sediment disaster warning and evacuation standard rainfall based on rainfall, and systems installed on the ground surface. There are also those that observe the deformation of the ground surface by extensometers, slope failure sensors, or inclinometers. The former disaster prevention system does not take topography and geology into account, so it is not possible to carry out detailed risk assessments of slope failures according to them. On the other hand, the latter disaster prevention system cannot properly assess the possibility of slope failure until deformation occurs on the ground surface, even if the possibility of slope failure has increased inside the ground.

In addition, the behavior of groundwater due to rainwater permeating into the ground due to rainfall and moisture in the soil layer is one of the major factors that cause slope failures. Can not do it. Of course, as a method of measuring changes in soil moisture, a borehole is drilled to the aquifer of the soil layer, and a sensor is installed in the borehole to observe the water level. It is known to measure the moisture content of the ground by inserting the probe into the ground. However, with these methods, rainwater permeates into the soil layer along the boreholes and probes, making it impossible to properly measure changes in moisture content.

On the other hand, the state of groundwater, specifically, the porosity of the ground, the water saturation of the pores, and the resistivity of the ground are highly correlated. Is possible. Moreover, resistivity measurements are generally a non-destructive technique by placing relatively small electrodes on the ground surface and do not allow rainwater to seep through the boreholes and probes mentioned above. Conventionally, a device described in Patent Document 1, for example, is known as a device that acquires the specific resistance of the ground over time and monitors changes in the moisture content of the ground.

In the monitoring method of Patent Document 1, in the ground where the resistivity is to be measured, a current electrode (distant current electrode) is placed on one of the far outer ends of the survey line, and a potential electrode (distant potential electrode) is placed on the other. do. In addition, a large number of electrodes are arranged at regular intervals along the survey line. Then, a predetermined current value I is applied between the far-current electrode and the current electrode on the survey line, and the potential difference V between the four potential electrodes near the current electrode and the far-potential electrode is measured. Potential difference measurements are made across the line, shifting the electrodes one by one from one end of the line to the other. Then, the apparent resistivity ρa of the ground vertically below the survey line is calculated by the following formula (1).
ρa=KV/I (1)
Note that K in Equation (1) is an electrode placement coefficient that is determined by the distance between the electrodes. Also, the apparent specific resistance ρa is the specific resistance when the ground is assumed to be homogeneous, and reflects the average specific resistance around the electrodes.

As a technique for measuring the resistivity of the ground, for example, the Schlumberger method is known. FIG. 9 shows a ground resistivity measuring device by electrical survey using the Schlumberger method. As shown in the figure, in the resistivity measuring device 21, a measuring unit 22 and a large number of measuring units electrically connected to the measuring unit 22 and installed along the survey line S on the ground G (FIG. 9A) 18 electrodes 23 are provided. The measurement unit 22 has a power supply 22a, a measurement control section 22b for controlling measurement, and an electrode switching section 22c for switching current electrodes to be energized. On the other hand, the large number of electrodes 23 are composed of a plurality of sets of electrodes (9 sets in FIG. 9(a)) each consisting of two electrodes. It is arranged in the central part of the survey line S, and outside the potential electrodes 23P, 23P, each pair of current electrodes 23C, 23C is arranged so as to gradually increase the distance from each other.

In the resistivity measuring device 21 configured as described above, the electrode switching section 22c of the measuring unit 22 switches the pair of current electrodes 23C, 23C to be energized so as to be connected to the power source 22a during measurement. Specifically, first, as shown in FIG. 9B, a predetermined current Electric current is applied at a value I, and the potential difference V at that time is measured at the two potential electrodes 23P, 23P. Next, as shown in FIG. 4(c), a pair of current electrodes 23C (C12) and 23C (C22) arranged immediately outside the pair of current electrodes C11 and C21 to which the current is applied are charged in the same manner as above. Electricity is applied and the potential difference is measured at the two potential electrodes 23P, 23P. Furthermore, as shown in FIG. 4(d), a pair of current electrodes 23C (C13) and 23C (C23) arranged immediately outside the pair of current electrodes C12 and C22 to which the current is applied are charged in the same manner as above. Electricity is applied and the potential difference is measured at the two potential electrodes 23P, 23P. After that, in the same manner as described above, energization is repeated in order up to the pair of current electrodes 23C, 23C on both ends of the survey line S, and each time, the potential difference is measured at the two potential electrodes 23P, 23P.

As described above, the potential difference between the potential electrodes 23P and 23P is repeatedly measured while the intervals between the current electrodes 23C and 23C of each pair to be energized are sequentially increased. Using the formula (1), it is possible to obtain the apparent resistivity ρa of the ground G vertically below the survey line S, and to know the moisture state of the ground G and the like.

Japanese Unexamined Patent Application Publication No. 2011-112368

When the apparent resistivity is obtained by the monitoring method and the Schlumberger method of Patent Document 1 described above, the former shifts the current electrode and the plurality of potential electrodes in order along the survey line, while the latter sets each pair of currents to be energized. While switching the electrodes in order, the current electrodes are energized and the potential difference between the potential electrodes is measured. For this reason, when trying to measure the apparent resistivity over the entire survey line of the ground, in any of the above-described cases, the power consumption increases and the measurement time increases.

Moreover, the above-mentioned Patent Document 1 describes that it is possible to predict landslides and issue warnings by monitoring changes in ground moisture content. However, Patent Literature 1 does not describe any specific criteria for issuing the warning, and cannot issue a stable and highly reliable warning.

The present invention has been made to solve the above problems, and can easily obtain the specific resistance at a desired depth in the ground with low power consumption and in a short time. An object of the present invention is to provide a ground resistivity monitoring device and a slope failure warning system capable of evaluating the risk of slope failure and issuing a stable and highly reliable warning.

In order to achieve the above object, the invention according to claim 1 is a ground resistivity monitoring device for monitoring the resistivity of a predetermined ground while measuring it over time, wherein A pair of current electrodes installed on the ground in a state in which the pair of current electrodes are placed in between, and two electrodes installed on the ground with a predetermined distance in a predetermined direction from each other. A plurality of sets of potential electrodes, in the set closest to the current electrode and in the set farthest from the current electrode, set such that the distance between the two electrodes in each set gradually increases, and electrically connected to the pair of current electrodes, A potential difference measuring means electrically connected to a plurality of sets of potential electrodes and measuring a potential difference between the two potential electrodes of each set. and an apparent resistivity calculating means for calculating the apparent resistivity at a predetermined depth of the ground corresponding to the distance between the two potential electrodes of each set based on the predetermined current and the measured potential difference; Resistivity structure analysis means for estimating the resistivity structure in the ground based on the apparent resistivity calculated by the reverse analysis method, and the aquifer in the soil layer of the ground based on the estimated resistivity structure. and an aquifer thickness ratio calculating means for detecting the aquifer as a low resistivity layer and calculating the ratio of the thickness of the aquifer in the soil layer of the ground.

According to this configuration, a pair of current electrodes are installed on the ground with a predetermined distance in a predetermined direction from each other, and a plurality of sets of potential electrodes each having two electrodes between the current electrodes. is installed on the ground. Also, the plurality of sets of potential electrodes are set such that the distance between the two potential electrodes in each set gradually increases from the set closest to the pair of current electrodes to the set farthest. In other words, in the resistivity monitoring device of the present invention, the positional relationship between the current electrodes and the potential electrodes is reversed with respect to the aforementioned Schlumberger method. In the following description, the technique of measuring the resistivity of the ground by arranging the pair of current electrodes and the plurality of sets of potential electrodes described above will be referred to as the "reverse Schlumberger method".

In a pair of current electrodes and a plurality of sets of potential electrodes installed on the ground as described above, during measurement, a predetermined current is passed through the pair of current electrodes from an energizing power source, thereby causing a current to flow through the ground and measuring the potential difference. Means measure the potential difference between the two electrodes of each set. In this case, since the pair of current electrodes is common to each set of potential electrodes, the potential difference between the two potential electrodes of the entire set can be measured by energizing the current electrodes once for a relatively short period of time. can be done. Then, the apparent resistivity calculating means calculates the apparent resistivity at a predetermined depth of the ground based on the predetermined current applied to the pair of current electrodes and the potential difference between the two potential electrodes of each pair.

In the resistivity monitoring device that employs the electrode arrangement based on the reverse Schlumberger method described above, the greater the distance between the two potential electrodes, the more the state of the deeper part of the ground is reflected in the potential difference. can be obtained from the ground surface to the deep part. As described above, according to the present invention, it is possible to easily obtain the apparent resistivity at a desired depth in the ground in a short period of time while saving power.
Also, based on the above apparent resistivity, the resistivity structure in the ground is estimated by a predetermined inverse analysis method. Then, based on the estimated resistivity structure, the aquifer in the soil layer of the ground is detected as a low resistivity layer, and the ratio of the thickness of the aquifer in the soil layer of the ground is calculated.

The invention according to claim 2 is the ground resistivity monitoring device according to claim 1, further comprising a power control means for controlling the power supply, and the power control means controls a pair of The energizing power source is controlled by a first energizing cycle in which the current electrodes are energized for a second predetermined time each shorter than the first predetermined time.

According to this configuration, the power source control means performs the first energization cycle in which the pair of current electrodes are energized for the second predetermined time every first predetermined time. By setting the shortest possible time for which the potential difference between the potential electrodes can be measured, the apparent resistivity of the ground to be monitored can be obtained every first predetermined time with low power consumption.

The invention according to claim 3 is the ground resistivity monitoring device according to claim 2, further comprising rain detection means for detecting the presence or absence of rain on the predetermined ground, wherein the power supply control means comprises the rain detection means. When rain is detected by the first energization cycle, and when rain is not detected, the pair of current electrodes is energized every third predetermined time longer than the first predetermined time. The power source is controlled by a second energization cycle that is performed for two predetermined time intervals.

According to this configuration, the energizing power supply is controlled so as to energize the pair of current electrodes in the above-described first energizing cycle when rainfall on the predetermined ground is detected by the rainfall detecting means. . On the other hand, when rainfall is not detected, the energization is performed in a second energization cycle in which the pair of current electrodes are energized for the second predetermined time every third predetermined time longer than the first predetermined time.

Normally, when the amount of water in the ground changes due to rainfall, current tends to flow in areas with a large amount of water in the ground, and the specific resistance changes so as to decrease. In this way, when it rains (especially when the amount of rainfall is large), the change in the specific resistance inside the ground is relatively fast, so the first predetermined time is set to a relatively short time, and the pair of By energizing the current electrodes and measuring the potential difference between the entire set of two potential electrodes, the ever-changing apparent resistivity of the ground can be obtained.

On the other hand, when the ground is not raining, such as when the weather is fine or cloudy, there is almost no change in the moisture content inside the ground, and therefore there is almost no change in the resistivity inside the ground. Therefore, when rainfall is not detected, the third predetermined time is set to a relatively long time, and the energization and potential difference measurement are performed in the second energization cycle, thereby obtaining the apparent resistivity of the ground with low power consumption. can be done.

The invention according to claim 4 is the resistivity monitoring device according to claim 2 or 3, wherein the resistivity monitoring device is composed of a plurality of resistivity monitoring devices, and each of the plurality of resistivity monitoring devices The power source control means of (1) controls the corresponding energized power sources so that the energized timings do not overlap with each other.

According to this configuration, when a plurality of resistivity monitoring devices are installed on the ground, the power supply control means of each resistivity monitoring device controls the timing of energizing the corresponding pair of current electrodes so that they do not overlap each other. , controls the corresponding energized power supply. As a result, when measuring the potential difference between all sets of two potential electrodes in each resistivity monitoring device, the apparent ratio of the ground by each resistivity monitoring device is not affected by the energization of other resistivity monitoring devices. Acquisition of resistance can be performed well.

The invention according to claim 5 monitors the risk of slope failure of the sloped ground by the resistivity monitoring device according to any one of claims 1 to 4, and issues an alarm when the risk becomes high. In a slope failure warning system, risk parameter setting for setting the ratio of the thickness of the aquifer in the soil layer of the ground obtained by the resistivity monitoring device as the risk parameter representing the risk of slope failure. threshold setting means for setting a threshold; and alarm means for issuing an alarm when the set risk parameter reaches the set threshold. Characterized by

According to this configuration, the ratio of the thickness of the aquifer in the soil layer of the ground obtained by the resistivity monitoring device described above is used to monitor the risk of slope failure of the inclined ground, and the risk Triggers an alarm when is high. Specifically, first, based on the apparent resistivity of the ground at a predetermined depth obtained by the resistivity monitoring device, the thickness and resistivity of each layer of the multilayer structure in the ground are estimated using a known inverse analysis method. do. Generally, in the ground, the higher the saturation of pore water, the lower the resistivity. Therefore, an aquifer with a high saturation can be detected as a low resistivity layer. Next, the ratio of the thickness of the aquifer in the soil layer of the ground is set as a risk parameter representing the risk of slope failure. Then, when the set risk parameter reaches the threshold set by the threshold setting means, an alarm is issued. As described above, by adopting the thickness ratio of the aquifer as the risk parameter, the risk of slope failure can be evaluated and a stable and reliable warning can be issued.

The invention according to claim 6 is the slope failure warning system according to claim 5, wherein the set threshold value is 40%.

According to this configuration, by setting the threshold value to be compared with the risk parameter to 40% in order to issue an alarm, an appropriate alarm is issued before a slope failure of the ground occurs, as will be described later. can be executed.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram schematically showing the overall configuration of a slope failure warning system to which a ground resistivity monitoring device according to an embodiment of the present invention is applied; FIG. 4 is a diagram schematically showing a resistivity measuring device of a resistivity monitoring device according to an embodiment of the present invention; 1 is an explanatory diagram for explaining an unsaturated zone, a wet zone, and a saturated zone (aquifer) in a soil layer laminated on a base in a sloped ground, and (a) is an unsaturated zone only in the soil layer; One state, (b) shows a state in which the soil layer is in the wet zone and the saturated zone. FIG. 4 is a diagram sequentially showing moisture changes in the ground (soil layer) due to rainfall in the sloped ground shown in FIG. 3 ; Soil layer No. shown in FIG. 1 is a graph showing the relationship between the 1/2 distance r between potential electrodes and the apparent resistivity ρa by simulation calculation, corresponding to models 1 to 11, respectively. Soil layer No. shown in FIG. No. 6 to No. 9 models of the apparent resistivity curve shown in FIG. 6 is a graph showing the relationship between ground depth and resistivity obtained by reverse analysis of 6 to 9. FIG. It is the same figure as FIG.3(b), and is a figure for demonstrating the safety factor which makes the potential collapse surface between the base of a sloped ground, and a soil layer into a slip surface. 4 is a graph showing a plurality of iso-factor-of-safety curves with parameters of the inclination angle of the ground and the thickness of the soil layer. FIG. 1 is a diagram for explaining a method for detecting the resistivity of the ground by electrical survey using the Schlumberger method, (a) is an overall view of the electrical survey device, (b) to (d) are a pair of potential electrodes and a plurality of A set of current electrodes is enlarged to show a state in which a current is applied to each set of current electrodes.

Preferred embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 schematically shows the overall configuration of a slope failure warning system to which a ground resistivity monitoring device according to an embodiment of the present invention is applied. As shown in the figure, the slope failure warning system 1 includes a plurality of (only two are shown in FIG. 1) resistivity measuring devices 2 for measuring the resistivity of the sloped ground G, and each resistivity measuring device 2 A server 4 ( apparent resistivity calculation means, resistivity structure analysis means, aquifer thickness ratio calculation means, risk parameter setting means, threshold value setting means) and an alarm device 5 (alarm means) that issues an alarm according to a command from the server 4 .

In FIG. 1, the resistivity measuring device 2 on the lower side is installed on the horizontal ground G for convenience of illustration, but in reality, like the resistivity measuring device 2 on the upper side, it is installed on the inclined ground G. is set up. Moreover, these resistivity measuring devices 2 are installed at a predetermined distance from each other.

As shown in FIG. 2(a), the resistivity measuring device 2 includes an electrode array 11 having a large number of electrodes arranged on a survey line S set to extend in a predetermined direction on the ground G; and a measurement unit 12 electrically connected to each of the electrodes 11 via a multicore cable. The electrode array 11 is composed of a pair of current electrodes C1 and C2 installed in a state of being inserted into the ground G with a predetermined distance from each other along the survey line S, and the current electrodes C1 and C2 interposed therebetween. A set of two electrodes installed in a state of being inserted into the ground G at a predetermined distance along the survey line S is defined as a set, and the set closest to the pair of current electrodes C1 and C2 is the farthest. Each set includes a plurality of sets of potential electrodes P1n, P2n (n: number of sets of potential electrodes) set such that the distance between two electrodes in each set gradually increases. As described above, the electrode array 11 of the resistivity measuring device 2 is configured to be able to measure the resistivity of the ground G by the reverse Schlumberger method in which the positional relationship between the current electrodes and the potential electrodes according to the conventional Schlumberger method is reversed. ing.

On the other hand, the measurement unit 12 includes a power supply 12a (energization power supply), a measurement control section 12b (power supply control means), and a communication control section 12c. The power source 12a has, for example, a battery body and a solar cell (neither of which is shown). Electricity generated by the solar cell is stored in the battery body, and current flows through each part of the measurement unit 12 and the current electrodes C1 and C2. supplied. The measurement control section 12b controls energization of the current electrodes C1 and C2 by the power source 12a, and controls measurement of the potential difference between each set of potential electrodes P1n and P2n. The communication control unit 12c receives a command from the server 4, and transmits to the server 4 data such as the measured potential difference between each pair of potential electrodes P1n and P2n.

The resistivity measuring device 2 configured as described above is installed as follows on the slope of the ground G recognized as requiring monitoring due to the possibility of slope failure in the future. . That is, first, the ground G is preliminarily surveyed by a generally known two-dimensional resistivity survey, boring survey, or the like, and a place where the resistivity measuring device 2 is to be installed is selected. Next, specifications such as the length of the survey line S and the electrode spacing are determined according to the depth of the potential collapse surface confirmed by the preliminary investigation. Then, a pair of current electrodes C1, C2 and a plurality of sets of potential electrodes P1n, P2n of the resistivity measuring device 2 are installed on the ground G along the survey line S according to the determined specifications. Thereby, the electrode array 11 is installed on the ground G, and the measurement unit 12 is installed near the electrode array 11 .

A rainfall sensor 13 (rainfall detection means) is installed at an appropriate position on the ground G described above. This rainfall sensor 13 detects the presence or absence of rainfall on the ground G, and the detection signal is transmitted to the server 4 .

In the slope failure warning system 1 configured as described above, the resistivity monitoring device of the present invention is composed of the resistivity measuring device 2, the server 4, and the rainfall sensor 13 described above.

Here, with reference to FIGS. 3 and 4, the structure of the sloping ground G and the moisture change of the ground (soil layer) due to rainfall will be described. FIG. 3 schematically shows the structure of the sloping ground G, where (a) shows the state before rainfall and (b) shows the state during or after rainfall. As shown in the figure, in this ground G, soil layers of various types of soil are laminated on a water-impermeable base, and the ground as a whole shows an inclined state. The boundary between the base and the soil layer is a potential collapse surface where the soil layer is highly likely to collapse.

FIG. 4 shows changes in soil layer moisture due to rainfall in order on the sloped ground G shown in FIG. In FIG. 4, the soil layers with different moisture conditions are divided into 1st to 11th layers according to the infiltration process of rainfall, and indicated by circled numbers. Each of the 11th soil layers is referred to as soil layer No. shall be described as 1-11.

Soil layer No. in FIG. 1 shows the condition before rainfall, similar to FIG. In the ground G having such a soil layer, when it starts to rain, soil layer No. 2 to 5, moisture due to rainfall penetrates into the soil layer from the upper surface of the soil layer, that is, the ground surface, and the soil layer No. As the wetting front indicated by the dashed-dotted lines of 2 to 5 descends, the thickness of the wet zone containing moisture gradually increases. In the wet zone, although the degree of saturation is almost constant, it is not saturated. As the rain continued, soil layer No. As shown in 6, the wet front reaches the basement and the entire soil layer becomes a wet zone.

After that, as the rain continued, soil layer No. As shown in order from 7 to 10, as the water level, which is the upper surface of the saturation zone, rises from the bottom of the soil layer toward the ground surface, the thickness of the saturation zone gradually increases. This saturation zone is a state in which the water content is very high and the gaps between the soil particles are filled with water. As the rain continued, soil layer No. As indicated by 11, the water level in the saturation zone reaches the ground surface and the entire soil layer becomes the saturation zone. Since this saturated zone is a layer containing a very large amount of water, it will be referred to as an "aquifer" in the following description.

Next, the procedure for measuring the resistivity by the resistivity measuring device 2 in the slope failure warning system 1 of FIG. 1 described above will be described. When the measurement interval of the measurement unit 12 of the resistivity measuring device 2 is set by the measurement command from the server 4, the measurement control unit 12b controls the current of the predetermined current value I from the power supply 12a based on the measurement command for a predetermined time ( A pair of current electrodes C1 and C2 are energized for a second predetermined time (for example, several seconds), and during this energization, the potential difference V between all pairs of potential electrodes P1n and P2n is simultaneously measured. Then, the communication control unit 12 c transmits the current value I and the potential difference V of all the measured sets to the server 4 . As described above, at the time of measurement by the resistivity measuring device 2, only by passing a current through the pair of current electrodes C1 and C2 for the above-described predetermined time, which is a relatively short time, the voltage between all pairs of potential electrodes P1n and P2n is reduced. The potential difference V can be measured in a short time.

Moreover, the above measurement interval is different between when it rains and when it is not raining, such as fine weather or cloudy weather. Specifically, when the rainfall sensor 13 detects that it is raining on the ground G, it is remotely operated to measure at relatively short predetermined time intervals (first predetermined time period, for example, 5 to 10 minutes). Configure the measuring unit 12 by: Further, instead of using the rainfall sensor 13, it is also possible to set the measurement interval of the measurement unit 12 by transmitting the above measurement command to the measurement unit 12 based on the rainfall information and weather forecast by AMeDAS of the Japan Meteorological Agency. . As described above, when it rains, the energization is performed at short intervals (first energization cycle).

On the other hand, when it is not raining, the server 4 sets the measurement unit 12 so as to perform measurement at relatively long predetermined time intervals (third predetermined time period, for example, one hour). In addition, as in the case of rain described above, it is also possible to set the measurement unit 12 to measure at long predetermined time intervals based on AMeDAS or weather forecasts. As described above, when it is not raining, the energization is performed at long intervals (second energization cycle).

As described above, when a plurality of resistivity measuring devices 2 are installed on the ground G, the timing of the measurement command to each measurement unit 12 and the timing of energization of the pair of current electrodes C1 and C2 are controlled so that they do not overlap each other. As a result, when measuring the potential difference V between all sets of potential electrodes P1n and P2n in each resistivity measuring device 2, each resistivity measuring device 2 is not affected by the energization of other resistivity measuring devices 2. Be able to take measurements properly.

Based on the received current value I and potential difference V of all pairs, the server 4 calculates the apparent resistivity ρa corresponding to each pair of potential electrodes P1n and P2n using the above-mentioned formula (1).
ρa=KV/I (1)

Then, based on the calculated apparent resistivity ρa and the ½ distance r between each pair of potential electrodes P1n and P2n, an apparent resistivity curve representing the relationship between the two is calculated. Then, by analyzing this apparent resistivity curve using a known inverse analysis method, the resistivity structure of each layer in the depth direction of the ground G is estimated. Specifically, the thickness and resistivity of each layer in the ground G are estimated by the nonlinear least-squares method so that an apparent resistivity curve that can be approximated to the observed apparent resistivity curve is calculated. Next, based on the estimated resistivity structure, the ratio of the thickness of the aquifer in the soil layer is calculated as a risk parameter representing the risk of slope failure.

Then, the calculated risk parameter is compared with a preset threshold (for example, 40%), and when the risk parameter reaches the threshold, that is, the thickness of the aquifer in the soil layer When the slope ratio reaches 40%, the server 4 sends an alarm command to the alarm device 5, indicating that there is a high possibility of slope failure, and the alarm device 5 issues an alarm. The alarm by the alarm device 5 is issued to the installer (local government, etc.) and residents in the vicinity of the ground G, for example, by means of a broadcast or an alarm e-mail that urges them to evacuate or prepare for evacuation.

Next, referring to FIGS. 5 to 8 in addition to FIGS. 3 and 4 described above, a simulation analysis was performed to confirm the criteria for issuing a warning in the slope failure warning system 1 of this embodiment. and the results. The conditions for this simulation analysis are as follows. That is, the soil layer No. shown in FIG. 1 to 11 as models (hereinafter, these soil layer models are referred to as "soil layer models No. 1 to 11"), and each layer of the soil layer, specifically, the unsaturated zone, the wet zone and the aquifer zone. , and the saturation and resistivity of the substrate were set as in Table 1 below.

In addition, in this simulation analysis, the above soil layer model No. The thickness of 1-11 was set to 2.5 m. Furthermore, in this simulation analysis, a pair of current electrodes and a plurality of sets of potential electrodes are formed by a predetermined length (for example, 20m).

Based on the above conditions, FIG. 1 to 11 respectively show apparent resistivity curves obtained when a current is applied to a pair of current electrodes and the potential difference between all sets of potential electrodes is measured. These apparent resistivity curves are displayed using a double logarithmic graph, with the horizontal axis representing the half distance r between the potential electrodes and the vertical axis representing the apparent resistivity ρa. 5, the apparent resistivity curves with circled numbers 1 to 11 correspond to the soil layer model No. 1 shown in FIG. It corresponds to 1 to 11. In the following explanation, soil layer model No. The apparent resistivity curves corresponding to 1 to 11 are appropriately designated as curve No. shall be described as 1-11.

As shown in FIG. 1 to 11 are represented by curves having the following tendency as the distance r increases.
・Curve No. 1 : Up to a distance r of 0.4 to 2.0 m, the apparent resistivity ρa is maintained at a high resistivity (2000 Ωm) corresponding to the unsaturated band, and when the distance r is 2.0 m or more, the apparent resistivity The resistance ρa changes so as to decrease toward the high resistivity (1000 Ωm) corresponding to the substrate.
・Curve No. 2 : When the distance r is around 0.4 m, the apparent resistivity ρa is a medium resistivity (500 Ωm) corresponding to the wet zone, and when the distance r is 0.4 to 2.0 m, the apparent resistivity ρa is The apparent resistivity ρa rises sharply, exceeds 1000 Ωm around 2.0 m, rises gently until the distance r is around 2.0 to 6.0 m, and when the distance r is 6.0 m or more, the apparent resistivity ρa The multiplication resistivity ρa changes so as to decrease toward a high resistivity (1000 Ωm) corresponding to the substrate. That is, curve no. 2 is convex upward as a whole.
・Curve No. 3 : Until the distance r is around 0.4 to 0.7 m, the apparent resistivity ρa is maintained at a middle resistivity (500 Ωm) corresponding to the wet zone, and until the distance r is around 0.7 to 4.0 m , the apparent resistivity ρa rises sharply and exceeds 1000 Ωm near 4.0 m. Above 8.0 m, the apparent resistivity ρa changes so as to decrease toward the high resistivity (1000 Ωm) corresponding to the substrate. That is, curve no. 3 corresponds to curve No. 3 above. As with 2, the whole is convex upward.
・Curve No. 4 : Until the distance r is around 0.4 to 1.0 m, the apparent resistivity ρa is maintained at a medium resistivity (500 Ωm) corresponding to the wet zone, and until the distance r is around 1.0 to 10.0 m , the apparent resistivity ρa increases and changes to exceed 1000 Ωm near 10.0 m.
・Curve No. 5 : Until the distance r is around 0.4 to 1.0 m, the apparent resistivity ρa is maintained at a medium resistivity (500 Ωm) corresponding to the wet zone, and until the distance r is around 1.0 to 10.0 m , the apparent resistivity ρa is determined by curve No. Compared with that of 4, it changes so that it rises moderately.
・Curve No. 6 : Until the distance r is around 0.4 to 2.0 m, the apparent resistivity ρa is maintained at a medium resistivity (500 Ωm) corresponding to the wet zone, and until the distance r is around 2.0 to 10.0 m , the apparent resistivity ρa is determined by curve No. Compared to that of 5, it changes so as to rise gently and linearly.
・Curve No. 7 : Until the distance r is around 0.4 to 3.0 m, the apparent resistivity ρa is maintained at a medium resistivity (500 Ωm) corresponding to the wet zone, and until the distance r is around 3.0 to 10.0 m , the apparent resistivity ρa increases and changes to exceed 700 Ωm near 10.0 m. That is, curve no. 7 is convex downward as a whole.
・Curve No. 8 : Until the distance r is around 0.4 to 1.0 m, the apparent resistivity ρa is maintained at a medium resistivity (500 Ωm) corresponding to the wet zone, and until the distance r is around 1.0 to 4.0 m , the apparent resistivity ρa gradually decreases, increases when the distance r is 4.0 m or more, and changes to exceed 600 Ωm near 10.0 m. That is, curve no. 8 is the above curve No. As with 7, it is convex downward as a whole.
・Curve No. 9 : Until the distance r is around 0.4 to 1.0 m, the apparent resistivity ρa is maintained at a medium resistivity (500 Ωm) corresponding to the wet zone, and until the distance r is around 1.0 to 3.0 m , the apparent specific resistance ρa decreases, and at 3.0 or more, the apparent specific resistance ρa increases and changes to exceed 600 Ωm near 10.0 m. That is, curve no. 9 corresponds to curve No. 9 above. Like 7 and 8, it is convex downward as a whole.
・Curve No. 10 : When the distance r is around 0.4 m, the apparent resistivity ρa is a medium resistivity (500 Ωm) corresponding to the wet zone, and the apparent resistivity ρa drops sharply, and when the distance r is around 2.0 to 3.0 m, the apparent resistivity ρa is maintained slightly below 300 Ωm, and when the distance r is 3.0 m or more, the apparent resistivity ρa is It rises sharply and changes to exceed 500 Ωm near 10.0 m. That is, curve no. 10 is the above curve No. Like 7 to 9, it is convex downward as a whole.
・Curve No. 11 : Up to a distance r of 0.4 to 2.0 m, the apparent resistivity ρa is maintained at a low resistivity (222 Ωm) corresponding to the aquifer, and when the distance r is 2.0 m or more, the apparent resistivity The resistance ρa rises sharply and changes to exceed 500 Ωm near 10.0 m.

Curve No. above. From the change of 1 to 11, the shorter the 1/2 distance r between the potential electrodes, that is, the closer the set of potential electrodes to the current electrode arranged in the center of the survey line, the more the apparent resistivity ρa based on the potential difference is Soil layer model no. 1 to 11 represent the resistivity on the upper surface (ground surface) side. On the other hand, the longer the distance r and the farther the set of potential electrodes from the current electrodes, the higher the apparent resistivity ρa based on the potential difference. It can be seen that the specific resistances of 1 to 11 on the deep side are represented.

6 shows the soil layer model No. shown in FIG. Curve Nos. 6 to 9 shown in FIG. The relationship between the depth of the ground and the specific resistance obtained by reverse analysis of 6 to 9 is shown, the solid line shows the specific resistance obtained by the analysis result, and the broken line shows the specific resistance of each soil layer model. Further, in the following description, in FIG. 6, the resistivity of the solid line and the dashed line marked with circled numbers 6 to 9 are the analyzed resistivity Nos. 6 to 9 and model resistivity no. shall be described as 6-9. Although a detailed description of the above inverse analysis is omitted, in this inverse analysis, the curve No. In order to obtain a resistivity structure with an apparent resistivity approximating 6 to 9, the nonlinear least-squares method was used to iteratively modify the thickness and resistivity of each layer to obtain an analytical resistivity No. 6 to 9. 6 to 9 are calculated respectively.

As shown in FIG. 6 to 9 are model resistivity Nos. indicated by dashed lines. 6 to 9, the following results were obtained.
・Analysis resistivity No. 6 : Model resistivity No. 6 is the soil layer model No. of FIG. At a depth of 2.5 m from the ground surface (depth 0 m), the resistivity is medium (500 Ωm) corresponding to the wet zone, and the lower layer of the wet zone is high corresponding to the bedrock. It is a specific resistance (1000 Ωm).
The above model resistivity no. 6, the analyzed resistivity No. 6 is consistent throughout the soil layer and bedrock.
・Analysis resistivity No. 7 : Model resistivity No. 7 is the soil layer model No. of FIG. 7, at depths up to 2.0 m from the ground surface, there is a medium resistivity (500 Ωm) corresponding to the wet zone, and in the lower layer of the wet zone, at depths up to 2.5 m, the zone A low resistivity (222 Ωm) corresponding to the water layer, and a high resistivity (1000 Ωm) corresponding to the substrate beneath the aquifer. Therefore, in the above case, the thickness of the aquifer is 0.5 m.
The above model resistivity no. 7, the analysis resistivity No. 7 agrees up to about 2.0 m from the ground surface and at depths below 4.0 m, but disagrees at depths between 2.0 and 4.0 m. This is a problem called "equivalent layer" in electrical prospecting, and is due to the theoretical existence of multiple resistivity structures with almost the same apparent resistivity curve.
・Analysis resistivity No. 8 : Model resistivity No. 8 is the soil layer model No. of FIG. 8, at depths up to 1.5 m from the ground surface, there is a medium resistivity (500 Ωm) corresponding to the wet zone, and in the lower layer of the wet zone, at depths up to 2.5 m, the zone A low resistivity (222 Ωm) corresponding to the water layer, and a high resistivity (1000 Ωm) corresponding to the substrate beneath the aquifer. Therefore, in the above case, the thickness of the aquifer is 1.0 m.
The above model resistivity no. 8, the analysis resistivity No. 8 is almost the same as the ground as a whole, although there is a slight deviation at a depth of about 1.5 m and a depth of about 2.5 m from the ground surface.
・Analysis resistivity No. 9 : Model resistivity No. 9 is the soil layer model No. of FIG. 9, at depths up to 1.0 m from the ground surface, there is a medium resistivity (500 Ωm) corresponding to the wet zone, and in the lower layer of the wet zone, at depths up to 2.5 m, the zone A low resistivity (222 Ωm) corresponding to the water layer, and a high resistivity (1000 Ωm) corresponding to the substrate beneath the aquifer. Therefore, in the above case, the thickness of the aquifer is 1.5m.
The above model resistivity no. 9, the analyzed resistivity No. 9 is consistent throughout the soil layer and bedrock.

Based on the above analysis results, for example, the soil layer model No. 1 in FIG. Analytical resistivity no. When the thickness of the aquifer (0.5m) is thinner than the thickness of the entire soil layer (2.5m) and the ratio of the aquifer to the soil layer (20%) is low, as in case 7. may not be able to adequately detect low-resistivity aquifers. On the other hand, the soil layer model No. of FIG. Analytical resistivity no. 8, the thickness of the aquifer (1.0 m) is relatively thicker than the thickness of the entire soil layer (2.5 m), and the ratio of the aquifer to the soil layer is a predetermined ratio (40 %) or more, it is possible to detect an aquifer with a low specific resistance almost properly. In addition, since the law of similarity generally holds in electric prospecting, the ratio of the aquifer to the soil layer that can properly detect the low resistivity layer (predetermined ratio (40%)) does not change even if the thickness of the soil layer changes. is.

Next, referring to FIG. 7, which is similar to FIG. 3(b), the safety factor Fs, which assumes the potential collapse surface between the base of the sloped ground G and the soil layer as the slip surface, will be described. In the inclined ground G as an infinitely long slope shown in FIG. hereinafter referred to as "slip resistance force"), the safety factor Fs is represented by the following equation (2).
Fs=T/Wt (2)

In addition, D is the thickness of the soil layer shown in FIG. Assuming that the unit volume weight of the aquifer is γsat, the unit volume weight of water is γw, and the adhesion force is c, the slip resistance force T and the slip force Wt in the above equation (2) are expressed by the following equations (3) and ( 4).
T={(D−H)γwet+H(γsat−γw)}cos ² β·tanφ+c (3)
Wt={(DH)γwet+H・γsat}cosβ・sinβ (4)

For the internal friction angle φ and the adhesive force c, it is possible to adopt, for example, an average value of massed soil in various places in Japan. In addition, the unit volume weight γwet of the wet zone and the unit volume weight γsat of the aquifer are calculated using the porosity, dry density and soil particle density of masado as described above and the saturation in Table 1 above. Is possible.

Then, by substituting the equations (3) and (4) into the equation (2), the safety factor Fs is obtained from the following equation (5).

Regarding the safety factor Fs, when Fs>1.0, the slope of the sloped ground G is in a stable state. Slope failure occurs at As will be described later, it has been confirmed that under general slope conditions, the safety factor Fs>1.1 when the ratio of the thickness of the aquifer in the soil layer reaches 40%. . Therefore, as described above, 40% is adopted as a threshold value to be compared with the ratio of the thickness of the aquifer as a risk parameter representing the risk of slope failure. By issuing an alarm when the thickness ratio reaches 40%, an appropriate alarm can be realized before a slope failure occurs.

FIG. 8 shows a plurality of (four in FIG. 8) with the inclination angle β of the sloped ground G and the soil layer thickness D as parameters when the thickness of the aquifer in FIG. 7 is 40% of the entire soil layer. , the isosafety factor curves L1 to L4 are shown. A region R surrounded by a dashed line represents the range of inclination angles (25 to 45 degrees) and soil layer thicknesses (2 m or less) in which general surface collapse occurs. In the following description, the equal safety factor curves L1 to L4 are simply referred to as curves L1 to L4, respectively.

As shown in FIG. 8, the curve L1 with the safety factor Fs=1.0 has a soil layer thickness D of about 5 m when the inclination angle β of the ground G is about 35 degrees, and when the inclination angle β is 60 degrees. Sometimes the soil layer thickness D is about 1 m, and it curves convexly to the lower left.

On the other hand, the curve L2 with a safety factor of Fs=1.1 curves downward to the left so as to extend downward from the left side of the curve L1. A curve L3 with a safety factor of Fs=1.2 extends downward from the left side of the curve L2, and a curve L4 with a safety factor of Fs=1.5 extends downward from the left side of the curve L3. Both of them are convexly curved to the lower left. That is, the smaller the inclination angle β or the smaller the soil layer thickness D, the greater the safety factor when the thickness of the aquifer is 40% of the entire soil layer.

As is clear from FIG. 8, if the inclination angle β of the ground G and the soil layer thickness D are in the range from the left to the lower side of the curve L2 where the safety factor Fs = 1.1, the zone with low specific resistance At the timing when the water layer can be detected, the safety factor Fs>1.1. Moreover, as described above, general surface failures often occur within the range of the slope angle β and the soil layer thickness D of the region R in FIG. In .1, by appropriately detecting the aquifer with low resistivity and by issuing an alarm when the ratio of the thickness of the aquifer in the soil layer reaches 40%, a slope failure is detected. Appropriate warning can be performed before it occurs.

As described in detail above, according to the present embodiment, the resistivity measuring device 2 equipped with the electrode array 11 according to the reverse Schlumberger method enables the apparent ratio of the desired depth in the ground G to be obtained in a short time with low power consumption. The resistance ρa can be easily obtained. In addition, the thickness and specific resistance of each layer in the basement and soil layers of the ground G are estimated by reverse analysis of the obtained apparent specific resistance ρa. Then, by adopting the ratio of the thickness of the aquifer in the soil layer as a risk parameter representing the risk of slope failure of the ground G, the risk of slope failure can be evaluated, and stable and reliable Alerts can be performed.

It should be noted that the present invention is not limited to the above described embodiments and can be implemented in various ways. For example, in the embodiment, 40% is used as the threshold value to be compared with the risk parameter, but the present invention is not limited to this. It is also possible to set Further, the detailed configurations of the slope failure warning system 1 and the resistivity measuring device 2 shown in the embodiment are merely examples, and can be appropriately changed within the scope of the present invention.

1 Slope failure warning system 2 Resistivity measuring device 4 Server ( Apparent resistivity calculation means, resistivity structure analysis means, aquifer thickness ratio calculation means, risk parameter setting means, threshold value setting means)
5 alarm device (alarm means)
11 electrode array 12 measurement unit 12a power supply (energization power supply)
12b measurement control unit (power supply control means)
12c Communication control unit 13 Rain sensor G Ground C1 Current electrode C2 Current electrode P1n Potential electrode P2n Potential electrode r Half distance between potential electrodes ρa Apparent resistivity Fs Safety factor

Claims (6)

A ground resistivity monitoring device for monitoring while measuring the resistivity of a given ground over time,
a pair of current electrodes installed on the ground with a predetermined distance from each other in a predetermined direction;
With the pair of current electrodes interposed therebetween, two electrodes placed on the ground at a predetermined distance in the predetermined direction are set as one set, and the set closest to the pair of current electrodes and the set farthest from , a plurality of sets of potential electrodes arranged such that the distance between the two electrodes of each set becomes progressively larger;
an energizing power supply that is electrically connected to the pair of current electrodes and applies a predetermined current to the pair of current electrodes during measurement to cause current to flow through the ground;
potential difference measuring means electrically connected to the plurality of sets of potential electrodes for measuring a potential difference between two potential electrodes of each set;
Apparent resistivity calculating means for calculating the apparent resistivity at a predetermined depth of the ground corresponding to the distance between the two potential electrodes of each set based on the predetermined current and the measured potential difference. ,
Resistivity structure analysis means for estimating a resistivity structure in the ground based on the calculated apparent resistivity by a predetermined reverse analysis method;
Based on the estimated resistivity structure, the aquifer in the soil layer of the ground is detected as a low resistivity layer, and the aquifer thickness ratio of the aquifer in the soil layer of the ground is calculated. thickness ratio calculation means;
A soil resistivity monitoring device comprising: further comprising power control means for controlling the energized power supply,
The power source control means controls the energizing power source by a first energizing cycle in which the pair of current electrodes are energized for a second predetermined time shorter than the first predetermined time every first predetermined time. The ground resistivity monitoring device according to claim 1. Further comprising rain detection means for detecting the presence or absence of rain on the predetermined ground,
The power supply control means performs the energization in the first energization cycle when rain is detected by the rain detection means, and the first energization cycle longer than the first predetermined time when the rain is not detected. 3. The soil specific resistance monitoring device according to claim 2, wherein the power source is controlled by a second energization cycle in which the pair of current electrodes are energized for the second predetermined time every three predetermined times. . The resistivity monitoring device is composed of a plurality of resistivity monitoring devices,
4. The ground according to claim 2 or 3, wherein the power supply control means of each of the plurality of resistivity monitoring devices controls the corresponding energizing power supply so that the energization timings do not overlap each other. Resistivity monitoring device. A slope failure warning system that monitors the risk of slope failure on sloping ground using the resistivity monitoring device according to any one of claims 1 to 4 and issues a warning when the risk increases. ,
risk parameter setting means for setting the ratio of the thickness of the aquifer in the soil layer of the ground obtained by the resistivity monitoring device as a risk parameter representing the risk of slope failure;
threshold setting means for setting a threshold;
alarm means for executing the alarm when the set risk parameter reaches the set threshold;
A slope failure warning system comprising: 6. A slope failure warning system according to claim 5, wherein said set threshold is 40%.

Images