JP7560868B2 - Method for detecting groundwater level fluctuations - Google Patents

Description

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

In coastal areas, the amplitude ratio or phase difference of the vibration component of the groundwater level propagating inland from the coast is observed in observation holes, and the hydraulic properties of the aquifer between the observation holes, such as the head diffusivity, permeability coefficient, hydraulic conductivity, and storage coefficient, are estimated (Non-Patent Document 1). Patent Document 1 proposes a method for applying this method to evaluate the permeability of a cut-off wall of an underground dam.
For example, the method of Patent Document 1 requires observation of the groundwater level through an observation hole, but in particular in the evaluation of the permeability of an underground dam's water cutoff wall, there are cases where there is no suitable observation hole downstream of the water cutoff wall, and even in coastal area groundwater surveys, there are cases where observation holes are not installed near the coast where the presence of seawater is dominant. In cases where no observation hole is installed, a method is required to easily grasp the vibration component of the groundwater level near the coast.
On the other hand, electrical exploration, which estimates underground structures by measuring the resistivity of the ground, has been used to estimate the depth of the groundwater level and changes in depth of the groundwater level by using the difference in resistivity due to differences in the degree of saturation above and below the groundwater table.
In Non-Patent Document 2, the spacing between exploration electrodes placed on the ground surface is changed, and one-dimensional exploration is repeatedly performed to obtain the resistivity structure in the depth direction, assuming a horizontal multi-layer structure, and the depth change of the resistivity layer boundary estimated to be the groundwater surface is obtained. In Non-Patent Document 3, a two-dimensional exploration is repeatedly performed to obtain the resistivity structure of the cross section underground of the survey line on a survey line where many electrodes are placed at equal intervals on the ground surface, and the resistivity change near the groundwater surface corresponding to the tidal fluctuation is clarified.

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," Journal of the Japanese Society of Irrigation, Drainage and Forestry Engineering, June 1991, No. 308, pp. I_51-I_60 Shinichi Takakura, "Attempt to monitor groundwater level changes using resistivity method", Geophysical Exploration, 1991, Vol. 44, pp. 227-231 Hiroomi Nakazato, Seiichiro Kuroda, Takasuke Inoue, Mutsuo Takeuchi, and Zhenyang Wang, "Visualization of tide level fluctuations in groundwater by resistivity monitoring," Geophysical Exploration, Vol. 60, No. 6, 2007, pp. 501-506

The method of Patent Document 1 requires the installation of an observation hole and then observation of the groundwater level.
Furthermore, in order to estimate groundwater level fluctuations using the methods of Non-Patent Documents 2 and 3, it is necessary to install electrodes and electric wires, and then move the electrodes and electric wires to install survey lines, which requires a lot of materials and labor. Since measurements take from several tens of minutes to several hours, it is difficult to frequently obtain resistivity changes caused by groundwater level fluctuations that change over time, and it is not possible to accurately evaluate the period of the vibration component of the resistivity change or the phase difference with tidal fluctuations.
To address these issues, one possible approach would be to install the minimum number of electrodes (four) at fixed points and evaluate the vibration components of the groundwater level from changes in response potential or apparent resistivity obtained by high-frequency time-series measurements at intervals of several minutes to 10 minutes.
However, this has never been done before because rainfall and temperature changes change the resistivity near the ground surface, which affects the measured values.

The present invention aims to provide a method for detecting groundwater level fluctuations that can remove noise from short-term daily changes near the ground surface caused by rainfall and temperature changes, and can detect groundwater level fluctuations accurately and in a labor-saving manner.

The method for detecting groundwater level fluctuations of the present invention described in claim 1 is a method for detecting groundwater level fluctuations by performing time-series measurements of surface electrical exploration using four electrodes, and includes 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 a measurement step S3 for measuring time-series apparent resistivity values obtained from the electrodes at the locations installed in the electrode installation step S2, wherein in the electrode spacing determination step S1, the electrode spacing a is set to a shallow layer insensitive electrode spacing whose measured value does not change even if the surface layer resistivity changes, and the groundwater level fluctuations are detected from the time-series apparent resistivity values measured in the measurement step S3.
The present invention described in claim 2 is characterized in that, in the method for detecting groundwater level fluctuations described in claim 1, in the electrode spacing determination step S1, the electrode spacing a is determined using the resistivity change layer thickness of the surface layer D and the average groundwater level.
The present invention as set forth in claim 3 is the method for detecting groundwater level fluctuations as set forth in claim 2, characterized in that in the electrode spacing determination step S1, the electrode spacing a is determined by further using resistivity contrast above and below the groundwater table X.
The present invention described in claim 4 is characterized in that, in the method for detecting groundwater level fluctuations described in any one of claims 1 to 3, in the electrode installation step S2, a pair of current electrodes 11, 12 constituting the electrodes are buried below the ground surface.
According to a fifth aspect of the present invention, in the method for detecting groundwater level fluctuations according to the fourth aspect, a pair of potential electrodes 13, 14 constituting the electrodes are also buried beneath the ground surface.

The groundwater level change detection method of the present invention can eliminate noise from short-term daily changes near the ground surface caused by rainfall and temperature changes, making it possible to detect groundwater level changes with high accuracy and in a labor-saving manner.

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 rate of change in measured value when resistivity changes at a surface layer thickness of 0.1 m Graph showing the rate of change in measured values when resistivity changes at a surface layer thickness of 0.2 m Graph showing the rate of change in measured values when resistivity changes at a surface layer thickness of 0.5 m Graph showing the rate of change in measured values when resistivity changes at a surface layer thickness of 1 m Graph showing the rate of change in measured values when resistivity changes at a surface layer thickness of 2 m Graph showing the relationship between surface layer thickness and shallow layer insensitive electrode spacing A graph showing the rate of change in apparent resistivity due to electrode spacing when the average groundwater level is 1 m and the area above the groundwater surface is 100 Ωm and the area below the groundwater surface is 50 Ωm (assuming fresh groundwater) A graph showing the rate of change in apparent resistivity due to the electrode spacing when the average groundwater level is 2 m Graph showing the rate of change in apparent resistivity due to electrode spacing when the average groundwater level is 5 m A graph showing the rate of change in apparent resistivity due to the electrode spacing when the average groundwater level is 10 m A graph showing the rate of change in apparent resistivity due to electrode spacing when the average groundwater level is 1 m and the area above the groundwater surface is 100 Ωm and the area below the groundwater surface is 1 Ωm (assuming saline groundwater) A graph showing the rate of change in apparent resistivity due to the electrode spacing when the average groundwater level is 2 m Graph showing the rate of change in apparent resistivity due to electrode spacing when the average groundwater level is 5 m A graph showing the rate of change in apparent resistivity due to the electrode spacing when the average groundwater level is 10 m Graph showing the relationship between the average groundwater level and the shallow layer insensitive electrode interval shown in FIG. 8 to FIG. 15 . A graph showing the relationship between the average groundwater level and the shallow layer dead electrode spacing when the resistivity change layer thickness of the surface layer D is changed, with the groundwater level above the groundwater level set at 100 Ωm and the groundwater level below the groundwater level set at 1 Ωm (assuming saline groundwater) Graph showing the apparent resistivity change rate between buried electrodes and surface electrodes Graph showing verification results of the method for detecting groundwater level fluctuations according to this embodiment

The method for detecting groundwater level fluctuations according to the first embodiment of the present invention includes an electrode spacing determination step for determining the electrode spacing of four electrodes, an electrode installation step for installing electrodes at the electrode spacing determined in the electrode spacing determination step, and a measurement step for measuring time-series apparent resistivity values obtained from the electrodes at the locations installed in the electrode installation step. In the electrode spacing determination step, the electrode spacing is set to a shallow-layer insensitive electrode spacing where the measured value does not change even if the surface resistivity changes, and groundwater level fluctuations are detected from the time-series apparent resistivity values measured in the measurement step. According to this embodiment, it is possible to remove noise from short-term daily fluctuations near the ground surface due to rainfall and temperature changes, and it is possible to detect groundwater level fluctuations with high accuracy and in a labor-saving manner.

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

The third embodiment of the present invention is a method for detecting groundwater level fluctuations according to the second embodiment, in which the electrode spacing determination step further uses resistivity contrast above and below the groundwater surface when determining the electrode spacing. According to this embodiment, groundwater level fluctuations can be detected with even greater accuracy.

In the fourth embodiment of the present invention, in the method for detecting groundwater level fluctuation 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. According to this embodiment, when the electrode spacing must be made smaller than the shallow layer insensitive electrode spacing, the electrode is affected by noise from short-term daily fluctuations near the ground surface due to rainfall and temperature changes. Therefore, by burying a pair of current electrodes under the ground surface, the noise from daily fluctuations can be removed, and groundwater level fluctuations can be detected with low labor and high accuracy.

The fifth embodiment of the present invention is a method for detecting groundwater level fluctuations according to the fourth embodiment, in which a pair of potential electrodes constituting the electrodes are also buried below the ground surface. According to this embodiment, by burying the pair of potential electrodes below the ground surface, it is possible to detect groundwater level fluctuations with even greater accuracy.

A method for detecting groundwater level fluctuation according to one embodiment of the present invention will be described below.
FIG. 1 is a schematic diagram for explaining the method for detecting groundwater level fluctuations according to this embodiment, in which FIG. 1(a) shows the concept of installing an electrical probe, and FIG. 1(b) shows the processing flow of the method for detecting groundwater level fluctuations.
As shown in Figure 1 (a), the electrical prospecting instrument used in the method for detecting groundwater level fluctuations according to this embodiment uses four electrodes 10 consisting of a pair of current electrodes 11, 12 and a pair of potential electrodes 13, 14. A current I (A) is passed between the current electrodes 11 and 12, and a resistance value R (Ω) is calculated using the potential difference V (V) detected between the potential electrodes 13 and 14.
If the electrode distance between the potential electrodes 13 and 14 is a, the apparent resistivity 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 spacing determination step (S1), an electrode installation step (S2), and a measurement step (S3), and detects groundwater level fluctuations from the time-series apparent resistivity values measured in the measurement step in S3.
In an electrode spacing determination step S1, an electrode spacing a is determined for the four electrodes 10. In an electrode installation step S2, the electrodes 10 are installed at the electrode spacing a determined in the electrode spacing determination step (S1). In a measurement step S3, time-series apparent resistivity values obtained from the electrodes 10 are measured at the locations where the electrodes were installed in the electrode installation step (S2).
In the electrode spacing determination step S1, the electrode spacing a is set to a shallow layer insensitive electrode spacing at which the measured value does not change even if the surface layer resistivity changes.

The shallow layer insensitive electrode interval will be described with reference to Figs.
Figure 2 is a graph showing the rate of change of the measured value when the resistivity changes when the surface layer thickness is 0.1 m, Figure 3 is a graph showing the rate of change of the measured value when the resistivity changes when the surface layer thickness is 0.2 m, Figure 4 is a graph showing the rate of change of the measured value when the resistivity changes when the surface layer thickness is 0.5 m, Figure 5 is a graph showing the rate of change of the measured value when the resistivity changes when the surface layer thickness is 1 m, Figure 6 is a graph showing the rate of change of the measured value when the resistivity changes when the surface layer thickness is 2 m, and Figure 7 is a graph showing the relationship between surface layer thickness and shallow layer insensitive electrode spacing.

2 to 6 show the rate of change in apparent resistivity due to the electrode spacing, and change the thickness of the surface layer D (resistivity change layer) where resistivity increases or decreases due to temperature changes and rainfall in a homogeneous ground with a resistivity of 100 Ωm. Note that the ground model calculates the rate of change in apparent resistivity for each electrode spacing a (apparent exploration depth) using a homogeneous ground with a resistivity of 100 Ωm as the initial value when the resistivity of the surface layer D of the homogeneous ground with a resistivity of 100 Ωm changes by ±30% (70 Ωm, 90 Ωm, 100 Ωm, 110 Ωm, 120 Ωm, 130 Ωm).
As shown in Figures 2 to 6, in the case of homogeneous ground, when the surface layer resistivity changes, in areas where the electrode spacing a is small, the measured value changes in the same direction as the surface layer resistivity, but when the electrode spacing a is increased, the rate of change becomes smaller, and when it is further increased, the measured value changes in the opposite direction to the surface layer resistivity, and it can be seen that there is a shallow layer dead electrode spacing in between where the rate of change of the measured value is small even if the surface layer resistivity changes. The shallow layer dead electrode spacing becomes larger as the layer thickness of the surface layer D where the resistivity changes is greater.
In this way, it has been discovered that there exists a shallow layer insensitive electrode spacing such that the measured value does not change even if the surface layer resistivity changes.By installing the electrode spacing a at the shallow layer insensitive electrode spacing, it is possible to eliminate noise from short-term daily changes near the surface caused by rainfall and temperature changes, and groundwater level fluctuations can be detected accurately and with little effort.

FIG. 7 is a graph showing the relationship between the surface layer thickness and the shallow layer insensitive electrode spacing shown in FIGS.
As shown in FIG. 7, there is a relationship in which the greater the resistivity change layer thickness of the surface layer D, the greater the shallow layer insensitive electrode gap.
Therefore, the electrode distance a can be determined using the thickness of the resistivity changing layer of the surface layer D.
However, when there is a groundwater level X, the resistivity of the ground changes above and below the groundwater level X due to differences in the degree of saturation. Therefore, model calculations were performed for cases where the area above the groundwater level X is set to 100 Ωm, the area below the groundwater level X is set to 50 Ωm (assuming fresh groundwater), and the area below the groundwater level X is set to 1 Ωm (assuming saltwater groundwater), with the resistivity change layer thickness of surface layer D set to 0.5 m, and the groundwater level X changes by plus or minus 0.5 m from the average groundwater level.

Figures 8 to 15 show the rate of change in apparent resistivity for each electrode spacing a (apparent exploration depth) when the average groundwater level is set as the initial value, the groundwater level changes by 0.5 m from the average groundwater level, and when the groundwater level changes by 0.5 m from the average groundwater level, and when the groundwater level changes by 30% (70 Ωm, 80 Ωm, 90 Ωm, 110 Ωm, 120 Ωm, 130 Ωm).

Figures 8 to 11 are graphs showing the rate of change in apparent resistivity due to electrode spacing when the resistance is 100 Ωm above the groundwater level and 50 Ωm below the groundwater level (assuming fresh groundwater). Figure 8 is for the case where the average groundwater level is 1 m, Figure 9 is for the case where the average groundwater level is 2 m, Figure 10 is for the case where the average groundwater level is 5 m, and Figure 11 is for the case where the average groundwater level is 10 m.

As shown in Figure 8, when the groundwater level X changes by 1m ±0.5m, if the electrode spacing a is about 3m, the apparent resistivity change due to the change in the groundwater level X can be detected with a change rate of about 10% without being affected by the change in resistivity of the surface layer D. When the groundwater level X drops to 1.5m, the measured value increases when the electrode spacing a is 3 to 10m, but when the electrode spacing a becomes narrower than 3m, the influence of the change in resistivity of the surface layer D becomes greater.

Furthermore, as shown in FIG. 9, when the groundwater level X changes by 2 m±0.5 m, if the electrode spacing a is about 4 m, the apparent resistivity change due to the change in the groundwater level X can be detected with a change rate of about 10% without being affected by the change in resistivity of the surface layer D.
Furthermore, as shown in FIG. 10, when the groundwater level X changes by 5 m±0.5 m, if the electrode spacing a is about 6 m, the apparent resistivity change due to the change in the groundwater level X can be detected with a change rate of about 5% without being affected by the change in resistivity of the surface layer D.
Furthermore, as shown in FIG. 11, when the groundwater level X changes by 10 m±0.5 m, if the electrode spacing a is about 10 m, the apparent resistivity change due to the change in the groundwater level X can be detected with a change rate of about 2% without being affected by the change in resistivity of the surface layer D.

Figures 12 to 15 are graphs showing the rate of change in apparent resistivity due to electrode spacing when the resistance above the groundwater table is 100 Ωm and below the groundwater table is 1 Ωm (assuming saline groundwater). Figure 12 is for the case where the average groundwater level is 1m, Figure 13 is for the case where the average groundwater level is 2m, Figure 14 is for the case where the average groundwater level is 5m, and Figure 15 is for the case where the average groundwater level is 10m.

As shown in Figure 12, when the groundwater level X varies by 1m ± 0.5m, the electrode spacing a can be set to about 2m. As shown in Figure 13, when the groundwater level X varies by 2m ± 0.5m, the electrode spacing a can be set to about 2.5m. As shown in Figure 14, when the groundwater level X varies by 5m ± 0.5m, the electrode spacing a can be set to about 4.5m. As shown in Figure 15, when the groundwater level X varies by 10m ± 0.5m, the electrode spacing a can be set to about 7m. This makes it possible to detect apparent resistivity changes due to changes in the groundwater level X without being affected by resistivity changes in the surface layer D.

FIG. 16 is a graph showing the relationship between the average groundwater level and the shallow layer insensitive electrode interval shown in FIGS.
As shown in Figure 16, there is a relationship in which the deeper the average groundwater level is, the larger the shallow dead electrode spacing is, and also in which the larger the resistivity contrast above and below the groundwater level X is, the smaller the shallow dead electrode spacing is for the same groundwater level.
Therefore, the electrode spacing a can be determined using the average groundwater level.
In addition, when determining the electrode spacing a, it is preferable to further use the resistivity contrast above and below the groundwater table X.

17 is a graph showing the relationship between the average groundwater level and the shallow layer dead electrode spacing when the resistivity change layer thickness of the surface layer D is changed, with the resistivity change layer thickness being set to 100 Ωm above the groundwater table and 1 Ωm below the groundwater table (assuming saline groundwater). The resistivity change layer thickness of the surface layer D was set to 0.1 m, 0.5 m, and 1 m, and the resistivity of each surface layer D was changed by ±30%.
As shown in FIG. 17, the greater the resistivity change layer thickness of the surface layer D, the greater the shallow layer insensitive electrode gap.

As described above, by setting the electrode spacing a to the shallow layer dead electrode spacing, it is possible to eliminate noise caused by short-term daily changes near the surface due to rainfall and temperature changes. The shallow layer dead electrode spacing is determined by the resistivity change layer thickness of the surface layer D, the average groundwater level, and the resistivity contrast above and below the groundwater level X. However, if there are insufficient observation wells and survey materials, it may be difficult to determine the shallow layer dead electrode spacing based on these parameters.
However, since the groundwater level near the coast is near 0 m above sea level, if the resistivity change layer thickness of the surface layer D is 0.5 m, for example, the shallow layer dead electrode interval can be taken as the elevation at the survey point. It is also effective to estimate the groundwater table depth by performing a one-dimensional survey as shown in Non-Patent Document 2.
In addition, if 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, the time-series measurement based on the set electrode interval a may be affected by changes in resistivity of the surface layer D. In such a case, the effect can be reduced by burying the electrodes 10 below the ground surface.

FIG. 18 is a graph showing the apparent resistivity change rate due to buried electrodes and surface electrodes.
In Figure 18, the electrode spacing a is 5 m, and the groundwater level X is located deeper than the 5 m depth that can be explored. This corresponds to the case where the electrode spacing a is smaller than the shallow layer insensitive electrode spacing in Figure 7. Surface electrodes are affected by daily variations due to temperature fluctuations, but buried electrodes eliminate the effects of such daily variations.

FIG. 18 shows the rate of apparent resistivity change when all four electrodes are buried and when only the current electrodes are buried, in comparison with the surface electrodes. Even when only the current electrodes 11 and 12 are buried, the effects of diurnal variation are eliminated.
In this way, by making the electrode spacing a smaller than the shallow layer insensitive electrode spacing, the electrode is subject to the noise of short-term daily variations near the surface due to rainfall and temperature changes. However, by burying a pair of current electrodes 11, 12 below the surface, the noise of daily variations can be eliminated, making it possible to detect groundwater level fluctuations accurately and in a labor-saving manner.
Furthermore, by burying the pair of potential electrodes 13, 14 below the ground surface together with the pair of current electrodes 11, 12, fluctuations in the groundwater level can be detected with even greater accuracy.

FIG. 19 is a graph showing the verification results of the method for detecting groundwater level fluctuation according to this embodiment.
In Figure 19, the time-series apparent resistivity measurements using surface electrodes with an electrode spacing of 10 m on the coast of a Ryukyu limestone aquifer were compared with the groundwater level changes in a nearby observation well.
As shown in Figure 19, a decrease and increase in apparent resistivity value was observed in the nearby observation well in association with a rise and fall of the groundwater level.

As described above, according to the method for detecting groundwater level fluctuations in this embodiment, groundwater level fluctuations can be detected by performing time-series measurements of surface electrical exploration using four electrodes 10. Therefore, by performing this detection method at two different locations, or by combining it with groundwater level detection using an observation hole at another location, it is possible to estimate hydraulic properties of the aquifer between two different points, such as the head diffusivity, permeability coefficient, hydraulic conductivity, and storage coefficient.

The method for detecting groundwater level fluctuations according to the present invention makes it easy to grasp the vibration components of groundwater levels, particularly near the coast.

10 Electrode 11, 12 Current electrodes 13, 14 Potential electrodes a Electrode spacing D Surface layer E Unsaturated layer F Saturated layer X Groundwater table S1 Electrode spacing determination step S2 Electrode installation step S3 Measurement step

Claims (5)

A method for detecting groundwater level fluctuations by performing time-series measurements of surface electrical prospecting using four electrodes, comprising:
an electrode spacing determination step of determining an electrode spacing between the four electrodes;
an electrode installation step of installing the electrodes at the electrode interval determined in the electrode interval determination step;
a measuring step of measuring a time-series apparent resistivity value obtained from the electrode at a location 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 at which the measured value does not change even if the surface layer resistivity changes,
A method for detecting groundwater level fluctuations, comprising detecting the groundwater level fluctuations from the time-series apparent resistivity values measured in the measuring step. In the electrode interval determination step,
2. The method for detecting groundwater level fluctuation according to claim 1, wherein the electrode interval is determined using a resistivity change layer thickness of the surface layer and an average groundwater level. In the electrode interval determination step,
3. The method for detecting groundwater level fluctuation according to claim 2, further comprising the step of determining the electrode interval using resistivity contrast above and below the groundwater surface. 4. The method for detecting groundwater level fluctuation according to claim 1, wherein the electrode installation step comprises burying a pair of current electrodes constituting the electrodes below the ground surface. 5. The method for detecting groundwater level fluctuation according to claim 4, wherein a pair of potential electrodes constituting said electrodes are also buried beneath the ground surface.