JP2022114126A - Underground water level fluctuation detection method - Google Patents

Abstract

To provide an underground water level fluctuation detection method capable of removing noise of short-term diurnal variation close to a ground surface due to a rainfall and an air temperature change and also accurately detecting an underground water level fluctuation in a labor saving manner.SOLUTION: An underground water level fluctuation detection method includes: an electrode interval determination step S1 for determining an electrode interval (a) of four electrodes; an electrode arrangement step S2 for arranging the electrodes at the electrode interval (a) determined in the electrode interval determination step S1; and a measurement step S3 for measuring a time-sequence apparent specific resistance value to be obtained from the electrodes at places arranged in the electrode arrangement step. In the electrode interval determination step S1, the electrode interval (a) is defined as a shallow layer dead electrode interval where a measurement value is not changed even when a surface layer specific resistance is changed, and an underground water level fluctuation is detected from the time-sequence apparent specific resistance value measured in the measurement step S3.SELECTED DRAWING: Figure 1

Description

The present invention relates to a groundwater level fluctuation detection method for detecting groundwater level fluctuations by performing time-series measurements of ground surface electrical prospecting.

In coastal areas, the amplitude ratio or phase difference of groundwater level vibration components propagating inland from the coast is observed at observation holes, and the head diffusivity, permeability coefficient, permeability coefficient, and storage coefficient of the aquifer between observation holes are determined. Hydraulic properties such as are estimated (Non-Patent Document 1). Patent Literature 1 proposes a technique for evaluating the water permeability of a subsurface dam cut-off wall by applying this technique.
For example, in the method of Patent Document 1, it is necessary to observe the groundwater level through observation holes. Even in regional groundwater surveys, observation holes may not be installed near the coast where the existence of seawater is dominant. In the absence of observation holes, there is a demand for a simple method of grasping the vibrational components of the groundwater level near the coast.
On the other hand, in the electric prospecting, which estimates the underground structure by measuring the resistivity of the ground, the depth of the groundwater level and the change in the depth of the groundwater level can be estimated by using the resistivity difference due to the difference in saturation above and below the groundwater table. It's here.
In Non-Patent Document 2, changing the interval of the exploration electrodes placed on the ground surface, repeating one-dimensional exploration to find the resistivity structure in the depth direction assuming a horizontal multilayer structure, and resistivity layer boundaries estimated to be the groundwater table We are looking for the depth change of In Non-Patent Document 3, in a survey line in which a large number of electrodes are arranged at equal intervals on the ground surface, a two-dimensional survey is repeatedly performed to obtain the resistivity structure of the cross section of the underground survey line. is clarified.

Japanese Patent No. 6368014

Katsushi Shirahata, Shuhei Yoshimoto, Takeo Tsuchihara, Satoshi Ishida, "Estimation of Hydraulic Constants of Coastal Aquifers by Tidal Response Method Combined with Frequency Separation Method Optimized for Prevailing Tides", Transactions of the Society for Agricultural and Rural Engineering, June 1991, No. 308, p. I_51-I_60 Shinichi Takakura, "A trial to monitor changes in groundwater level using the resistivity method," Geophysical Exploration, 1991, Vol. 44, p. 227-231 Hiroomi Nakazato, Seiichiro Kuroda, Takashi Inoue, Mutsuo Takeuchi, Shinyo Wang, "Visualization of tide level fluctuations in groundwater by resistivity monitoring", Geophysical Exploration, 2007, Vol. 60, No. 6, p. 501-506

In the method of Patent Literature 1, it is necessary to install an observation hole and then observe the groundwater level.
In addition, in order to estimate groundwater level fluctuations by the methods of Non-Patent Document 2 and Non-Patent Document 3, it is necessary to install electrodes and electric wires, and move the electrodes and electric wires to install survey survey lines. and labor. Since the measurement requires several tens of minutes to several hours, it is difficult to obtain the resistivity change caused by the groundwater level fluctuation that changes with time at high frequency. The phase difference cannot be evaluated accurately.
In order to solve these problems, a minimum of four electrodes were installed at a fixed point, and changes in the response potential or apparent resistivity obtained by high-frequency time-series measurements at intervals of several minutes to ten minutes were used to determine the groundwater level. It is conceivable to evaluate the vibration component.
However, rainfall and temperature changes change the resistivity near the ground surface, which affects the measured values.

The present invention provides a groundwater level fluctuation detection method capable of removing short-term daily fluctuation noise near the ground surface due to rainfall and temperature changes, and capable of detecting groundwater level fluctuations with high precision and labor-saving. With the goal.

The groundwater level fluctuation detection method of the present invention according to claim 1 is a groundwater level fluctuation detection method for detecting groundwater level fluctuations by performing time-series measurement of surface electric prospecting using four electrodes, , an electrode spacing determination step S1 for determining the electrode spacing a of the four electrodes, an electrode installation step S2 for installing the electrodes at the electrode spacing a determined in the electrode spacing determination step S1, and the electrode installation step S2. and a measurement step S3 for measuring the time-series apparent resistivity value obtained from the electrodes at the place installed in step S3, and in the electrode spacing determination step S1, the electrode spacing a is measured even if the surface layer resistivity changes. It is characterized in that the groundwater level fluctuation is detected from the time-series apparent resistivity value measured in the measurement step S3, with the shallow layer insensitive electrode interval having a value that does not change.
According to a second aspect of the present invention, in the method for detecting groundwater level fluctuations according to the first aspect, in the electrode interval determination step S1, the electrode interval a is determined from the thickness of the resistivity changing layer of the surface layer D and the average groundwater level. is determined using
According to a third aspect of the present invention, in the method for detecting groundwater level fluctuations according to the second aspect, in the electrode interval determination step S1, when determining the electrode interval a, the specific resistance above and below the groundwater table X is further determined. It is characterized by using contrast.
The present invention according to claim 4 is the method for detecting a groundwater level fluctuation according to any one of claims 1 to 3, wherein in the electrode installation step S2, a pair of current electrodes 11 constituting the electrodes, 12 is buried under the ground surface.
According to a fifth aspect of the present invention, in the groundwater level fluctuation detection method according to the fourth aspect, a pair of potential electrodes 13 and 14 constituting the electrodes are also buried under the ground surface.

According to the groundwater level fluctuation detection method of the present invention, it is possible to eliminate short-term daily fluctuation noise in the vicinity of the ground surface due to rainfall and temperature changes, and to detect the groundwater level fluctuation with high precision and labor-saving.

FIG. 1 is a conceptual diagram illustrating a method for detecting groundwater level fluctuations according to an embodiment of the present invention; Graph showing the change rate of the measured value when the resistivity changes at a surface layer thickness of 0.1 m Graph showing the change rate of the measured value when the resistivity changes at a surface layer thickness of 0.2 m Graph showing the change rate of the measured value when the resistivity changes at a surface layer thickness of 0.5m Graph showing the change rate of the measured value when the resistivity changes at a surface layer thickness of 1m Graph showing the change rate of the measured value when the resistivity changes at a surface layer thickness of 2m Graph showing the relationship between surface layer thickness and shallow layer dead electrode spacing A graph showing the change rate of the apparent resistivity value due to the electrode spacing when the average groundwater level is 1 m and the upper part of the groundwater level is 100 Ωm and the lower part of the groundwater level is 50 Ωm (assuming fresh groundwater). Graph showing the rate of change in apparent resistivity with the same electrode spacing at an average groundwater level of 2m Graph showing the rate of change in apparent resistivity with the same electrode spacing at an average groundwater level of 5m Graph showing the rate of change in apparent resistivity with the same electrode spacing at an average groundwater level of 10 m A graph showing the change rate of the apparent resistivity value due to the electrode spacing when the average groundwater level is 1 m and the upper part of the groundwater level is 100 Ωm and the lower part of the groundwater level is 1 Ωm (assuming salt water groundwater). Graph showing the rate of change in apparent resistivity with the same electrode spacing at an average groundwater level of 2m Graph showing the rate of change in apparent resistivity with the same electrode spacing at an average groundwater level of 5m Graph showing the rate of change in apparent resistivity with the same electrode spacing at an average groundwater level of 10 m Graphs showing the relationship between the average groundwater level and shallow dead electrode spacing shown in FIGS. 8 to 15 Graph showing the relationship between the average groundwater level and the interval between the shallow layer dead electrodes when the resistivity change layer thickness of the surface layer D is changed, with 100 Ωm above the groundwater level and 1 Ωm below the groundwater level (assuming salt water groundwater). Graph showing change rate of apparent resistivity by buried electrode and surface electrode Graph showing verification results of groundwater level fluctuation detection method according to the present embodiment

The groundwater level fluctuation detection method according to the first embodiment of the present invention includes an electrode spacing determination step of determining the electrode spacing of the four electrodes, and an electrode installation step of installing the electrodes at the electrode spacing determined in the electrode spacing determination step. and a measurement step of measuring time-series apparent resistivity values obtained from the electrodes at the place where they are installed in the electrode installation step. The groundwater level fluctuation is detected from the time-series apparent resistivity value measured in the measurement step, with the shallow layer insensitive electrode interval having no change in value. According to the present embodiment, it is possible to remove short-term daily variation noise near the ground surface due to rainfall and temperature changes, and to detect groundwater level fluctuations with high accuracy while saving labor.

According to a second embodiment of the present invention, in the groundwater level fluctuation detection method according to the first embodiment, in the electrode spacing determination step, the electrode spacing is determined using the thickness of the resistivity change layer of the surface layer and the average groundwater level. It is determined by According to the present embodiment, groundwater level fluctuation can be detected with high accuracy.

According to a third embodiment of the present invention, in the groundwater level fluctuation detection method according to the second embodiment, in the electrode spacing determination step, when determining the electrode spacing, the resistivity contrast above and below the groundwater table is further calculated. It is used. According to this embodiment, it is possible to detect groundwater level fluctuations with higher accuracy.

According to a fourth embodiment of the present invention, in the groundwater level fluctuation detection method according to the first to third embodiments, in the electrode installation step, a pair of current electrodes constituting the electrodes are buried under the ground surface. be. According to the present embodiment, if the electrode spacing must be smaller than the shallow layer dead electrode spacing, short-term diurnal noise near the ground surface due to rainfall and temperature changes will affect the noise. By burying the electrodes under the ground surface, it is possible to eliminate daily noise and detect changes in the groundwater level with high accuracy and labor-saving.

According to a fifth embodiment of the present invention, in the groundwater level fluctuation detection method according to the fourth embodiment, a pair of potential electrodes constituting the electrodes are also buried under the ground surface. According to this embodiment, by also burying the pair of potential electrodes under the ground surface, it is possible to detect groundwater level fluctuations more accurately.

A method for detecting groundwater level fluctuation according to an embodiment of the present invention will be described below.
FIG. 1 is a schematic diagram for explaining the detection method of groundwater level fluctuation according to the present embodiment, FIG. 1(a) shows the installation concept of the electric probe, and FIG. 1(b) shows the processing of the groundwater level fluctuation detection method. showing the flow.
As shown in FIG. 1(a), the electric probe used in the method for detecting groundwater level fluctuations according to the present embodiment has four electrodes 10 consisting of a pair of current electrodes 11 and 12 and a pair of potential electrodes 13 and 14. is used to flow a current I (A) between the current electrodes 11 and 12, and the potential difference V (V) detected between the potential electrodes 13 and 14 is used to calculate the resistance value R (Ω). do.
Assuming that the electrode gap between the potential electrode 13 and the potential electrode 14 is a, the apparent specific resistance when the underground is assumed to be homogeneous is given by the following formula.
Apparent resistivity (Ωm) = 2πaR

As shown in FIG. 1(b), the method for detecting groundwater level fluctuations according to this embodiment includes an electrode interval determination step (S1), an electrode installation step (S2), and a measurement step (S3). Groundwater level fluctuations are detected from the time-series apparent resistivity values measured in the measurement step in .
In the electrode spacing determination step in S1, the electrode spacing a of the four electrodes 10 is determined. In the electrode placement step in S2, the electrodes 10 are placed at the electrode spacing a determined in the electrode spacing determination step (S1). In the measurement step in S3, time-series apparent resistivity values obtained from the electrode 10 are measured at the place where the electrodes are installed in the electrode installation step (S2).
In the electrode spacing determination step in S1, the electrode spacing a is set to a shallow layer insensitive electrode spacing in which the measured value does not change even if the surface layer resistivity changes.

The interval between shallow layer dead electrodes will be described with reference to FIGS. 2 to 7. FIG.
FIG. 2 is a graph showing the rate of change in the measured value when the resistivity changes at a surface layer thickness of 0.1 m, FIG. 3 is a graph showing the rate of change in the measured value when the resistivity changes at a surface layer thickness of 0.2 m, and FIG. Graph showing the change rate of the measured value when the resistivity changes at a thickness of 0.5 m, FIG. 5 is a graph showing the change rate of the measured value when the resistivity changes at a surface layer thickness of 1 m, and FIG. 6 shows the resistivity change at a surface layer thickness of 2 m. FIG. 7 is a graph showing the relationship between the surface layer thickness and the shallow layer dead electrode interval.

Figures 2 to 6 show the rate of change in apparent resistivity depending on the distance between electrodes. changing the layer thickness. In the ground model, when the specific resistance of the surface layer D of the homogeneous ground with a specific resistance of 100 Ωm changes plus or minus 30% (70 Ωm, 90 Ωm, 100 Ωm, 110 Ωm, 120 Ωm, 130 Ωm), the homogeneous ground with a specific resistance of 100 Ωm is used as the initial value. A model calculation is performed on the change rate of the apparent resistivity value for each electrode interval a (apparent exploration depth).
As shown in Figs. 2 to 6, in the case of homogeneous ground, when the surface layer resistivity changes, the measured value changes in the same direction as the surface layer resistivity in the region where the electrode spacing a is small. becomes smaller, and if it is further expanded, it changes in the opposite direction to the surface layer resistivity, and there is a shallow layer dead electrode gap between which the change rate of the measured value is small even if the surface layer resistivity changes. The interval between the shallow layer dead electrodes increases as the layer thickness of the surface layer D where the resistivity changes increases.
In this way, it was found that there is a shallow layer insensitive electrode interval that does not change the measured value even if the surface layer resistivity changes. It is possible to eliminate short-term diurnal noise near the ground surface due to temperature changes and to detect groundwater level fluctuations with high accuracy and labor-saving.

FIG. 7 is a graph showing the relationship between the surface layer thickness shown in FIGS. 2 to 6 and the shallow layer dead electrode interval.
As shown in FIG. 7, there is a relationship that the greater the thickness of the variable resistivity layer of the surface layer D, the greater the interval between shallow layer dead electrodes.
Therefore, the electrode spacing a can be determined using the resistivity change layer thickness of the surface layer D. FIG.
However, if there is a groundwater table X, the specific resistance of the ground changes above and below the groundwater table X due to the difference in saturation. ) and 1 Ωm below the groundwater level X (assuming salt water groundwater), the resistivity change layer thickness of the surface layer D is 0.5m, and the groundwater level X varies by plus or minus 0.5m from the average groundwater level. A model calculation was performed for

8 to 15, the average groundwater level is the initial value, and when the groundwater level changes from the average groundwater level by plus 0.5m and when the groundwater level changes from the average groundwater level by minus 0.5m, the surface layer D When the resistivity changes plus or minus 30% (70 Ωm, 80 Ωm, 90 Ωm, 110 Ωm, 120 Ωm, 130 Ωm), the change rate of the apparent resistivity value for each electrode interval a (apparent exploration depth) is shown.

8 to 11 are graphs showing the change rate of the apparent resistivity value depending on the electrode spacing when 100 Ωm above the groundwater table and 50 Ωm below the groundwater table (assuming fresh groundwater), and FIG. 8 shows the average groundwater. Fig. 9 shows the case of the average groundwater level of 2m, Fig. 10 shows the case of the average groundwater level of 5m, and Fig. 11 shows the case of the average groundwater level of 10m.

As shown in FIG. 8, when the groundwater table X changes by 1 m ± 0.5 m, if the electrode interval a is set to about 3 m, the apparent A change in resistivity can be detected at a rate of change of about 10%. When the groundwater table X falls to 1.5 m, the measured value increases when the electrode spacing a is 3 to 10 m. go.

Further, as shown in FIG. 9, when the groundwater table X changes by 2 m±0.5 m, if the electrode interval a is set to about 4 m, the change in the groundwater table X It is possible to detect the apparent resistivity change due to the change rate of about 10%.
Further, as shown in FIG. 10, when the groundwater table X changes by 5 m±0.5 m, if the electrode interval a is set to about 6 m, the change in the ground water table X is not affected by the change in the resistivity of the surface layer D. It is possible to detect the apparent resistivity change due to the change rate of about 5%.
Further, as shown in FIG. 11, when the groundwater table X changes by 10 m±0.5 m, if the electrode interval a is set to about 10 m, the change in the groundwater table X is not affected by the change in the resistivity of the surface layer D. It is possible to detect the apparent resistivity change due to the change rate of about 2%.

12 to 15 are graphs showing the rate of change in the apparent resistivity value depending on the electrode spacing when 100 Ωm above the groundwater level and 1 Ωm below the groundwater level (assuming salt water groundwater), and FIG. 12 shows the average groundwater. Fig. 13 is for the average groundwater level of 2m, Fig. 14 is for the average groundwater level of 5m, and Fig. 15 is for the average groundwater level of 10m.

As shown in FIG. 12, when the groundwater level X changes by 1 m±0.5 m, by setting the electrode interval a to about 2 m, the ground water level X changes by 2 m±0.5 m as shown in FIG. By setting the electrode interval a to about 2.5 m when the As shown in FIG. 15, when the groundwater level X changes by 10 m ± 0.5 m, by setting the electrode interval a to about 7 m, the change in the ground water level X is suppressed without being affected by the change in the resistivity of the surface layer D. It is possible to detect the apparent resistivity change due to

FIG. 16 is a graph showing the relationship between the average groundwater level shown in FIGS. 8 to 15 and the interval between shallow layer dead electrodes.
As shown in FIG. 16, there is a relationship that the deeper the average groundwater level, the larger the shallow-layer insensitivity electrode interval. There is a relationship that the electrode spacing becomes smaller.
Therefore, the electrode spacing a can be determined using the average groundwater level.
Further, it is preferable to use the resistivity contrast above and below the groundwater table X in determining the electrode spacing a.

FIG. 17 shows the relationship between the average groundwater level and the shallow layer dead electrode interval when the thickness of the resistivity change layer of the surface layer D is changed, with 100 Ωm above the groundwater level and 1 Ωm below the groundwater level (assuming salt water groundwater). It is a graph showing. The resistivity changing layer thicknesses of the surface layer D were set to 0.1 m, 0.5 m, and 1 m, and the resistivity of each surface layer D was changed by plus or minus 30%.
As shown in FIG. 17, the greater the thickness of the variable resistivity layer of the surface layer D, the greater the distance between the shallow layer dead electrodes.

As described above, by setting the electrode interval a to the shallow layer dead electrode interval, it is possible to eliminate short-term daily noise near the ground surface due to rainfall and temperature changes. The resistivity change is determined by the thickness of the layer, the average groundwater level, and the resistivity contrast above and below the groundwater table X, but if there are not enough observation wells or survey materials, shallow layer dead electrodes based on these parameters Determining the interval can be difficult.
However, since the groundwater level is around 0 m above sea level near the coast, for example, when the thickness of the resistivity change layer of the surface layer D is 0.5 m, the shallow layer dead electrode interval can be taken as the elevation at the survey point. . It is also effective to perform one-dimensional exploration as shown in Non-Patent Document 2 to estimate the depth of the groundwater table.
In addition, the depth of the groundwater table X, the thickness of the surface layer D, and the resistivity contrast above and below the groundwater table X are unknown, and there are cases where changes in the resistivity of the surface layer D affect the time series measurement based on the set electrode spacing a. be. In such a case, embedding the electrode 10 under the ground reduces the effect.

FIG. 18 is a graph showing apparent resistivity change rates for buried electrodes and surface electrodes.
In FIG. 18, the groundwater table X is at a position deeper than the depth of 5 m that can be explored with an electrode spacing a of 5 m, which corresponds to the case where the electrode spacing a is smaller than the shallow layer insensitive electrode spacing in FIG. However, implanted electrodes eliminate the effects of such diurnal changes.

FIG. 18 shows the apparent resistivity change rates when all four electrodes are buried and when only the current electrodes are buried, compared with the surface electrodes. Even if there is, the influence of diurnal variation is excluded.
In this way, by making the electrode spacing a smaller than the shallow-layer dead electrode spacing, the pair of current electrodes 11 and 12 is placed below the ground surface, although it is affected by short-term diurnal noise near the ground surface due to rainfall and temperature changes. By burying it in the ground, it is possible to eliminate daily noise and detect groundwater level fluctuations with high accuracy and labor-saving.
In addition, by burying the pair of potential electrodes 13 and 14 together with the pair of current electrodes 11 and 12 under the ground surface, it is possible to detect groundwater level fluctuations more accurately.

FIG. 19 is a graph showing the verification result of the groundwater level fluctuation detection method according to the present embodiment.
In FIG. 19, the results of time-series apparent resistivity measurements by surface electrodes with an electrode interval a of 10 m on the coast of the Ryukyu limestone aquifer were compared with changes in the groundwater level at nearby observation holes.
As shown in FIG. 19, it was found that the apparent resistivity decreased and increased due to the rise and fall of the groundwater level at the nearby observation hole.

As described above, according to the groundwater level fluctuation detection method according to the present embodiment, the groundwater level fluctuation can be detected by performing time-series measurement of the ground surface electrical prospecting using the four electrodes 10 . By performing the detection method at two different locations, or by combining with groundwater level detection using observation holes at other locations, the head diffusivity, permeability coefficient, Hydraulic properties such as hydraulic conductivity and storage coefficient can be estimated.

According to the groundwater level fluctuation detection method according to the present invention, it is possible to easily grasp the vibration component of the groundwater level especially near the coast.

10 Electrodes 11, 12 Current Electrodes 13, 14 Potential Electrodes a Electrode Spacing D Surface Layer E Unsaturated Layer F Saturated Layer X Underground Water Level S1 Electrode Spacing Step S2 Electrode Installation Step S3 Measurement Step

Claims (5)

A groundwater level fluctuation detection method for detecting groundwater level fluctuations by performing time-series measurements of surface electrical prospecting using four electrodes,
an electrode spacing determination step of determining the electrode spacing of the four electrodes;
an electrode placement step of placing the electrodes at the electrode spacing determined in the electrode spacing determination step;
a measurement step of measuring the time-series apparent resistivity value obtained from the electrode at the place where the electrode is installed in the electrode installation step;
In the electrode interval determination step, the electrode interval is set to a shallow layer insensitive electrode interval in which the measured value does not change even if the surface layer resistivity changes,
A method for detecting groundwater level fluctuations, wherein the groundwater level fluctuations are detected from the time-series apparent resistivity values measured in the measuring step. In the electrode spacing determination step,
2. The method for detecting groundwater level fluctuations according to claim 1, wherein the electrode spacing is determined using a resistivity change layer thickness of the surface layer and an average groundwater level. In the electrode spacing determination step,
3. The method for detecting groundwater level fluctuations according to claim 2, wherein a resistivity contrast above and below the groundwater table is further used in determining the electrode spacing. 4. The method for detecting groundwater level fluctuations according to any one of claims 1 to 3, wherein in the electrode installation step, a pair of current electrodes constituting the electrodes are buried under the ground surface. 5. The method for detecting groundwater level fluctuations according to claim 4, wherein a pair of potential electrodes constituting said electrodes are also buried under said ground surface.

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