JP5464548B2 - Measurement method of fresh salt water interface - Google Patents

Description

  The present invention relates to a method for measuring a boundary surface between fresh water and salt water.

In coastal areas with little surface water that can be used without large rivers and dams, groundwater is often used as water resources for areas such as agriculture and industrial water. In coastal aquifers, generally high-density seawater penetrates into the lower groundwater in a wedge shape (Fig. 1 (a)). When groundwater is used in such areas, seawater intrusion into freshwater bodies can be an obstacle to securing good quality water resources. In order to continuously use fresh water with low salinity, it is necessary to grasp the thickness of the fresh water layer that changes due to pumping, rainfall, tide, etc. It is important to monitor the “fresh salt water interface”.
An electric exploration method can be cited as a method for measuring the fresh saltwater interface. In this method, a fresh saltwater boundary surface is detected nondestructively from a specific resistance distribution in the ground measured by a current applied from the ground surface. However, the measurement depth is limited by the output of the equipment and the electrode spacing, and the measurement accuracy is lower than that of direct observation in a well (for example, the error may be 10 m or more in the electromagnetic method). In addition, the direct method of manually dropping the sensor into the well and observing the state of salt water intrusion based on the sudden change point of the vertical conductivity profile of groundwater also requires a great deal of effort to investigate the time variation of the interface due to tides and rainfall, Cost and time are required.
On the other hand, in recent years, for example, Patent Document 1 introduces the measurement of soil water content, electrical conductivity, water level, and the like by time domain reflection (TDR).
However, TDR has a problem that measurement may be impossible due to signal attenuation in a high salinity environment.

JP 2009-204601 A

  In view of the above, an object of the present invention is to provide a method for measuring a fresh salt water boundary surface by reversely utilizing the attenuation characteristics of microwave signals in a high salinity environment.

(数1中、ε_fwは前記淡水の比誘電率、Lは前記TDRプローブのロッド長、cは光速とする。)
とから構成されることを特徴とする。
また、請求項２記載の本発明は、請求項１記載の発明において、前記TDRプローブの根端側をケーブルに接続し、地表面から前記淡塩水境界面までの距離を、前記地表面以下のケーブル長とロッド長の和から前記h_iを差し引いた距離として求めることを特徴とする。 In order to solve the above-mentioned problems, the solution means has been found as follows.
That is, according to the method for measuring a fresh salt water interface of the present invention, as described in claim 1, a) a TDR probe is inserted into a multilayer system of fresh water and salt water from the fresh water side, and the root end of the TDR probe and Placing the TDR probe so that the interface between fresh water and salt water is located between the tips,
b) measuring the time t _fw until the TDR waveform sudden decrease point; and
c) A step of obtaining a distance h _i from the tip of the TDR probe to the boundary surface from the following formula 1.

(In _Equation 1, ε _fw is the relative permittivity of the fresh water, L is the rod length of the TDR probe, and c is the speed of light.)
It is comprised from these.
Further, the present invention according to claim 2 is the invention according to claim 1, wherein the root end side of the TDR probe is connected to a cable, and the distance from the ground surface to the fresh salt water interface is less than the ground surface. It is obtained as a distance obtained by subtracting the h _i from the sum of the cable length and the rod length.

  According to the present invention, the fresh salt water boundary surface can be detected with high accuracy.

Schematic diagram of (a) observation image and (b) measured TDR waveform of fresh saltwater interface Schematic of the experiment for carrying out the measurement of the fresh salt water interface of the present invention TDR waveform obtained by measuring the boundary surface between distilled water (σ _fw = 0.0005S · m ^-1 ) and salt water (σ _sw = 5.2S · m ^-1 ) A graph showing the relationship between the propagation time t _fw and the measured value of the fresh saltwater interface position h _i or the value calculated by Equation 1 TDR waveforms measured in distilled water and NaCl solutions with different conductivity TDR waveform obtained by interface measurement between fresh water (σ _fw = 0.1 to 0.4 S · m ^-1 ) and salt water (σ _sw = 5.2 S · m ^-1 ) with different conductivity Comparison of measured values of interface position h _i between fresh water and salt water with different electrical conductivity and values calculated by Equation 1 TDR waveform when the fresh salt water interface changes Comparison of measured and calculated values of h _i when the fresh brine boundary changes

In TDR (Time Domain Reflectometry), electromagnetic energy such as microwaves is absorbed by the medium in the process of propagating through the conductive medium along the electrode rod. The amount of energy absorption increases as the propagation distance of the pulse increases and the conductivity of the surrounding medium increases. Therefore, when the difference in energy absorption between fresh water and salt water is compared under the same pulse propagation distance, the absorption amount of salt water is much larger.
In the present invention, based on the above findings, the fresh salt water boundary surface is detected from the energy absorption characteristics of the microwave step pulse measured by TDR.

[Measurement model of fresh salt water interface]
When the time required for the microwave step pulse applied to the TDR probe to reciprocate the probe rod is t _t (s), the relative permittivity ε _t of the medium around the probe is t _t , the speed of light c (3.0 × 10 ⁸ m · s ⁻¹ ) and rod length L (m) are expressed by the following formula 2.

Pulses propagating from the rod root 2a toward the tip 2b when the probe 1 is inserted vertically into a layered system of fresh water (upper) and salt water (lower) as shown in FIGS. 1 (a) and 1 (b) When the water reaches the salt water area, the electric nelkey is absorbed into the salt water. The amount of absorption has a positive correlation with the electrical conductivity, and appears on the TDR waveform as a sudden decrease in the reflection coefficient ρ (electric energy index) (FIG. 1 (b)). Considering the sudden decrease point (t = t ₁ position in Fig. 1 (b)) as the position of the fresh salt water boundary surface 3, the boundary surface from the rod root 2a position (t = t ₀ position in Fig. 1 (b)). Assuming that the propagation time of pulses up to 3, that is, the propagation time of pulses in fresh water is t _fw (s) and the distance from the rod tip 2b to the boundary surface 3 is h _i (m), the relative permittivity ε _fw of fresh water is Based on Equation 2, it is given by

From the above equation 3, h _i is expressed by the following equation 1.

When ε _fw is already known, h _i can be obtained by substituting t _fw measured by TDR into equation (1).

[Outline of experiment and analysis]
The situation of fluctuation of fresh water quality (electrical conductivity) or boundary position was reproduced by laboratory experiments, and the feasibility and accuracy of measurement of fresh salt water boundary using microwave energy absorption characteristics in TDR were investigated.
Figure 2 shows a schematic diagram of this experiment. In the experiment, a 2-wire TDR probe 1 with a rod length of 48 cm was connected to a 1502C type cable tester 4 (Tektronix), and software (WinTDR18) was used for measurement control and TDR waveform recording. A NaCl solution with σ _sw = 5.2 S ・ m ⁻¹ corresponding to the electrical conductivity of seawater is considered as an alternative solution for salt water, and the electrical conductivity σ _fw = 0.0005,0.013,0.1,0.2,0.3,0.4 S ・ m ^-1 The NaCl solution was used as an alternative solution for fresh water. In this embodiment, the NaCl solution having the above electric conductivity is referred to as salt water or fresh water for convenience. Each solution temperature was about 22 ° C.

In order to obtain ε _fw given by Equation (3), the longitudinal direction of the probe 1 is installed in the center of an acrylic cylindrical column 5 having an inner diameter of 7 cm and a height of 52 cm so that the rod 2 of the probe 1 The TDR waveform was measured with fresh water filled to the root tip 2a. The acquired waveform was analyzed with WinTDR to obtain ε _fw (= 75.11). This is given as an actual measurement value of ε _fw of the model formula number 1. The measured ε _fw underestimated the theoretical value (= 79.63) at a water temperature of 22 ° C. This is the measurement of the relative permittivity of the wall of the acrylic cylindrical column 5 (the relative permittivity of acrylic is about 3) used in the experiment. It was because of the influence. Thereafter, salt water was slowly supplied from the lower end of the column 5 to the root end 2 a of the rod 2. In the experiment for ascending the fresh salt water interface reproduced in this way, the time when the salt water reached the rod tip 2b was set to h _i = 0 cm, and thereafter, a TDR waveform with a h _i interval of 5 cm was measured.

In addition, in order to confirm the effectiveness of this method when the saltwater wedge moves forward or backward, we decided to reproduce the vertical fluctuation of the fresh saltwater interface by laboratory experiments. That is, the rising process of the boundary surface was reproduced by slowly supplying salt water from the lower end of the column to h _i = 30 cm by the method described above. After that, salt water is drained from the lower end of the column 5 and at the same time, fresh water of the same amount as the drainage amount is gently supplied from the upper end of the column 5 to maintain the full state, and the descending process of the interface up to h _i = 10 cm is reproduced. did. A series of ascent and descent was repeated three times, and TDR waveforms were recorded at intervals of 5 cm. In this experiment, the boundary surface fluctuation range was set to h _i = 10 to 30 cm in order to avoid disturbance of the fresh salt water boundary surface due to water supply and drainage at the upper and lower ends.
The position of the boundary surface in the above experiment was determined by measuring the column capacity corresponding to the fluctuation range in advance and supplying and draining an equal amount of salt water.

The waveform data acquired in each experiment was developed into a spreadsheet and analyzed manually. That is, in this analysis, the branch point that occurs when the TDR waveforms measured in advance in the column in air and distilled water are superimposed on t ₀ (see Fig. 1 (b)), and the waveform line before and after the sudden decrease of the ρ value. the point of intersection of the approximate straight line drawn was t _1. By calculating t _fw from the difference between t ₀ and t ₁ and substituting the determined t _fw and the predetermined ε _fw into Equation 1, h _i at each stage was calculated.

[Results and discussion]
(1) When non-conductive distilled water is located in the upper layer
As an example of the TDR waveform measured when h _i changes, the upper layer distilled water (σ _fw = 0.0005S ・ m ^-1 ) was measured in the process of pushing up the salt water (σ _sw = 5.2S ・ m ^-1 ) The waveform is shown in FIG. Since distilled water is a non-conductive medium, the amount of electric energy absorbed by the step pulse applied to the rod 2 is small. Therefore, when h _i = 0 m, a TDR waveform with a sharp rise at t _{2 of the} rod tip 2b was obtained. In the other h _i conditions, instead of the rising point being recognized, a sudden decrease point t ₁ (upward arrow in the figure) is recognized, and the position of t ₁ is to the left of the figure as h _i increases. Shifted to. T _fw obtained from t ₁ decreases linearly as h _i increases (Fig. 4), and the square root RMSE (Root Root Mean Square Error) of the measured value of h _{i and} the measured TDR value based on Equation 1 mean square error) was 0.8cm.
In reality the coastal aquifer show fresh water conductivity Located at the top, in the ideal condition is a medium, such as a small distilled water energy absorbed in the upper layer is present, can be evaluated in h _i with high accuracy by TDR It is believed that there is.

(2) When conductive fresh water is located in the upper layer When the propagation path is the same, the amount of electrical energy absorbed by the conductive medium increases as the propagation distance increases and the conductivity of the medium increases. I mentioned earlier. Therefore, in a homogeneous medium that does not show a local change in electrical conductivity, such as a fresh salt water interface, the longer the rod and the higher the conductivity of the medium, the more uneven the TDR waveform becomes, and the t ₂ It became difficult to specify the position (Fig. 5).

A similar trend occurred in the measurement of the fresh salt water interface (Fig. 6). The ρ value at each t ₁ position when NaCl solution of σ _fw = 0.1,0.2,0.3,0.4S ・ m ^-1 was used for the fresh water in the upper layer is the distilled water (σ _fw = 0.0005S ・m ^-1 ) was smaller than that used. In any of the σ _fw conditions, the amount of energy lost until the pulse reaches the boundary surface is larger than in the case of distilled water. Due to this, the tendency of the t ₁ position to become unclear was recognized, and the detection of the t _l position became more difficult as h _i was smaller and σ _fw was larger. In particular, it was not possible to detect the t ₁ for sigma _fw = the _{^{0.3S · m -1 h i = 0.05~0.2m}} , σ fw = 0.4S · m -1 in h _i = 0.05~0.3m. It was compared with the measured values and the TDR measurement value based on the number 1 of the h _i in each sigma _fw condition t ₁ is obtained, for each h _i in σ _fw = 0.1,0.2,0.3,0.4S · m ^-1 RMSE was 0.5, 0.8, 0.7, and 0.7 cm (Fig. 7), and it was found that the measurement accuracy was almost equivalent to that of distilled water (Fig. 4) considered to be ideal conditions.

Since it was observed that t ₁ could not be detected under the high σ _fw condition, in order to investigate the application limit of the method of the present invention, the maximum thickness of the fresh water layer that can detect t ₁ , that is, the maximum fresh water layer thickness D _max (m ) Was investigated. As a result, D _max for sigma _fw = 0.1 and 0.2 S · m ^-1 is at least 0.45 m, the ^{_{σ fw = 0.3S · m -1 D}} max = 0.23m, the σ _fw = 0.4S · m ^-1 It is considered that D _max = 0.13 m. That is, the range where h _i can be measured is, sigma _fw = 0.1 and 0.2 S · m ^-1 in h _i = 0～0.48M or a range wider than, σ _fw = 0.3S · m ^-1 in h _i = 0.25 It was h _i = 0.35 to 0.48 m at ˜0.48 m and σ _fw = 0.4 S · m ^−l .
From the above results, it is considered that the fresh brine boundary surface can be evaluated with sufficient accuracy through Equation 1 even when σ _fw is high as long as it does not exceed D _max .

(3) When 8 the boundary surface fluctuates up and down, the light from the TDR waveform in a case where the position of the saltwater boundary fluctuates up and down, h _i = 025m in h _i = 0.15 m and descending processes at elevated process The waveform for is shown. When the boundary surface fluctuated, it was thought that the vertical gradient of electrical conductivity was reduced by promoting mixing of fresh water and salt water, and the boundary between the two became unclear, but either h _i = 0.15 or 0.25 m In some cases, no significant difference was observed in the shape of the TDR waveform even when the rise and fall were repeated 1 to 3 times. In addition, a clear sharp decrease in ρ value was observed in any waveform. Similar trend was also common in other h _i conditions (h _i = 0.1,0.2,0.3m). In the TDR waveforms under all h _i conditions, the t ₁ position was almost equal, and the calculated value of h _i and the measured value were in good agreement (FIG. 9).

  From the results of this experiment, it was confirmed that the detection of the boundary surface of fresh saltwater by TDR maintained the electrical conductivity difference between freshwater and saltwater, and that there must be a sudden change point of electrical characteristics. In an actual observation well, the behavior of the fresh salt water boundary surface due to rainfall and tide is characterized by the surrounding hydrogeology. For example, in this example, a conductivity difference is formed in this example in places where the boundary surface variation and groundwater flow are intense. There is a possibility that it is smaller than the calculated difference. However, unless the hydrologic geology is such that the boundary surface is constantly disturbed by groundwater flow, etc., it is highly possible that the location can be detected by TDR even if the position fluctuates due to tides or rainfall.

In this study, a double layer of fresh water and salt water was reproduced in a cylindrical column, and an attempt was made to detect the fresh salt water interface using time domain reflection TDR.
When a TDR probe was inserted perpendicularly to the boundary surface between distilled water and salt water, and a microwave step pulse was applied thereto, a sharp decrease point t ₁ of the reflection coefficient ρ (electrical energy index) was recorded on the TDR waveform. It was. The relationship between the pulse propagation time t _fw from the probe root to the boundary surface obtained from t ₁ and the distance h _i from the rod tip to the boundary surface is expressed by a linear expression (Equation 1). ability to measure h _i in the range of RMSE <1 cm through the revealed.

On the other hand, when the electrical conductivity σ _fw of fresh water is increased, energy lost in the process of propagation of the pulse in fresh water is increased, and it may be difficult to detect t ₁ (FIG. 6 or FIG. 7). However, when the fresh water layer thickness is thin, the net energy loss is reduced, so t ₁ can be detected, and the RMSE under such conditions is less than 1 cm. It was also confirmed that even if the position of the boundary surface fluctuates up and down, h _i can be determined with the same accuracy in a flow field without disturbance.
However, there are some problems in practical use at the field level. Considering the fluctuation range of the boundary surface due to rainfall and tide, it is desirable that the measurement range is relatively wide, but the application range of the method of the present invention depends on the maximum freshwater layer thickness D _max from the rod root that can detect t ₁ I discussed what to do. As is clear from the experimental results, D _max decreases as σ _fw increases and the energy absorption of fresh water increases. Therefore, it should be noted that the measurement of h _i is limited to a range that does not exceed D _max for σ _fw of the target water area.

DESCRIPTION OF SYMBOLS 1 Probe 2 Rod 3 Fresh salt water interface 4 Cable tester 5 Acrylic cylindrical column

Claims (2)

a) Insert the TDR probe into the layered system of fresh water and salt water from the fresh water side, and place the TDR probe so that the boundary surface between fresh water and salt water is located between the root and tip of the TDR probe Step to do,
b) measuring the time t _fw until the TDR waveform sudden decrease point; and
c) A step of obtaining a distance h _i from the tip of the TDR probe to the boundary surface from the following formula 1.

(In _Equation 1, ε _fw is the relative permittivity of the fresh water, L is the rod length of the TDR probe, and c is the speed of light.)
A method for measuring a boundary surface of a fresh salt water characterized by comprising: The root end side of the TDR probe is connected to a cable, and the distance from the ground surface to the fresh salt water boundary surface is obtained as a distance obtained by subtracting the h _i from the sum of the cable length and the rod length below the ground surface. The method of measuring a fresh salt water boundary surface according to claim 1, wherein:

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