JPH0619473B2 - Groundwater dynamics survey method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for investigating groundwater dynamics, and more specifically, measuring the three-dimensional flow velocity of water in a vertical borehole having a small diameter to measure groundwater dynamics. Concerning the method of investigating precisely.

[Conventional technology]

Since groundwater dynamics surveys are indispensable for the use of water resources, elucidation of water permeability, landslides and other geological formations, design of structures and disaster prevention facilities, construction plans for civil engineering works, etc. Was being investigated.

As a conventional method for investigating the dynamics of groundwater, a two-dimensional velocity meter method for measuring resistance change between electrodes due to movement of electrolyte, isotope tracer method, ordinary tracer method, propeller method and the like are known.

In the two-dimensional anemometer method, a large number of electrode rods are inserted into a container filled with a liquid having a specific electric resistance different from that of water. The state in which the liquid is being replaced is detected from the change in the specific resistance between the electrode rods. Such an investigation method has a drawback that it requires drilling a large bore hole having a diameter of 100 mmφ or more.

The isotope tracer method uses radioactive isotopes,
Ordinary tracer method uses a substance that is different from water, such as electrolyte and dye, into the input hole and observes the appearance status in the separately drilled observation hole. It is an essential requirement to drill the borehole for observation, and only one-dimensional or two-dimensional measurement can be performed. In this method, the three-dimensional movement of groundwater is estimated from the groundwater surface gradient or the surface gradient. Not only is this method expensive to survey because it digs a large number of boreholes, but it is also inaccurate because it involves estimation as described above.

The propeller method is a method of inserting a propeller driven by a water flow into a boring hole. It is possible to measure the water flow in a boring hole with a relatively small diameter, but it is limited to the measurement of only one direction, for example, the vertical direction component, and it is not suitable for high-precision investigation.

[Problems to be solved by the invention]

An object of the present invention is to provide a method for investigating the dynamics of groundwater, which has improved the drawbacks of the above-mentioned conventional methods.

According to the present invention, a boring hole having a large diameter is not required to be drilled, a 66 mmφ boring hole which is conventionally most easily performed is used, and a separate charging hole and observation hole are not required, It is possible to measure the three-dimensional flow velocity of water in a borehole, and a high-performance and highly accurate groundwater dynamics survey method can be realized by drilling a minimum number of small-diameter boreholes.

[Means for solving problems]

In order to achieve the above-mentioned object, the present invention inserts a sensor having an ultrasonic wave transmitting / receiving device and a direction detecting device in which a small diameter boring hole is arranged in a three-dimensional space in the boring hole,
A groundwater dynamics survey characterized by calculating a three-dimensional flow velocity of water at a water level of groundwater in the borehole from the ultrasonic signal and the direction signal of the sensor, and analyzing the dynamics of the groundwater from the calculated value. The method is the gist.

Hereinafter, the method of the present invention will be described in detail with reference to the ground. FIG. 1 conceptually shows a method of investigating groundwater on a sloping land, which schematically shows the method of the present invention.

When investigating the water resources in the target area shown in FIG. 1, for example, the presence of infiltrated water 1 and the presence of the impermeable layer 2 are confirmed by surface survey, and the water level 5 of groundwater is confirmed by the observation well 4.

In the present invention, the actual condition of the target area to be surveyed is ground-surface surveyed as described above, and then the small-diameter boring hole 6a,
6b, 6c. Dig ...

The number of boring holes is determined depending on the actual condition of the object to be investigated and the purpose of the investigation, but the method of the present invention requires drilling a smaller number of boring holes than any of the conventional methods. This is because the three-dimensional flow velocity can be measured. The boring hole is usually a vertical boring hole, but it is also possible to use an inclined boring depending on the actual condition. The diameter of the boring hole may be 66 mmφ, which is the most widely used small diameter. Since the boring hole has a special machine to be used depending on the size of the bore and causes a huge difference in required cost, the present invention has an extremely excellent economical and technical advantage in this respect.

Next, a sensor is inserted in the borehole to measure the three-dimensional flow velocity of water at each water level of the water table 5.
For this measurement, it is most preferable to use a sensor utilizing ultrasonic waves developed by the present inventor.

FIG. 2 illustrates the appearance of such a sensor. The ultrasonic wave transmitting elements 11 face each other and are arranged at both ends of the same diameter on the circumference of a circle having a diameter of about 50 mmφ. The number is four or more. One receiving element 12 is installed in the central portion with a certain distance from the transmitting element 11. The ultrasonic wave emitting / receiving elements 11 and 12 are coupled to a measuring unit 13 having a built-in ultrasonic wave generator and a receiving device.
Is suspended in the boring hole by the flexible cable 14.

The outer diameter of the measuring portion 13 is 6 to 10 mm smaller than the bore diameter.
The size is small, the diameter is larger than the diameter of the array circle of the ultrasonic wave transmitting elements 11, and the length is about several tens of centimeters, which can be easily moved up and down along the bore hole, and The sound wave transmitting element 11 is guided so as not to collide with the wall of the boring hole. A space is provided between the ultrasonic wave transmitting / receiving elements 11 and 12 and the measuring unit 13 so as not to disturb the three-dimensional flow velocity of water.

Furthermore, a small azimuth detecting device, for example, a compass using a small magnet and a Hall element is built in the measuring unit 13.

The flexible cable 14 has sufficient strength for suspending the measuring unit 13 and flexibility that allows the measuring unit 13 to freely rotate within the boring hole, and has a built-in power line and communication line. ing. This cable 14 is wound around an appropriate cable drum on the ground, and is held in the boring hole so that it can be freely wound and rolled up, and the depth position in the hole of the ultrasonic transmitter / receiver can be detected.

The ultrasonic waves sequentially time-divided transmitted from each element of the ultrasonic wave transmitting element 11 are sequentially received by the ultrasonic wave receiving element 12, and the arrival time difference is measured on the order of 10 ⁻¹² seconds.

Here, ΔT: time difference between a pair of ultrasonic wave transmitting / receiving elements L: distance between ultrasonic wave transmitting / receiving elements C: speed of sound V: flow velocity of water between ultrasonic wave transmitting / receiving elements, ΔT = 2VL / C ² There is a relationship, and the flow velocity V can be obtained by measuring ΔT. In this case, changes in the sound velocity C due to water temperature, density, etc. are automatically corrected by direct sound velocity measurement between the transmitting and receiving elements. When such a method is used, a flow velocity of about 5 mm / sec or more can be accurately detected by setting the spacing between the transmitting and receiving elements to about several cm.

The measurement between the pair of transmitting and receiving elements is performed for a plurality of receiving elements, and if the combination of these measured values and the positions of the transmitting and receiving elements is analyzed, the three-dimensional flow velocity of water in the borehole can be calculated. It is possible to measure easily. The analytical calculation of this measurement can be immediately calculated from the data input to the analyzer on the ground by the communication cable built in the flexible cable 14. The output of this arithmetic unit can be recorded in a chart recorder or input to a digital data logger, and this can be connected to a computer for various kinds of analysis.

As described above, it is possible to measure upflow, downflow, and horizontal flow even within a single hole, and it is possible to obtain a three-dimensional flow velocity vector. In addition, the velocity vectors at each water level are measured for multiple boreholes, and the results show the dynamics of groundwater in the complete measurement area.

For example, in Figure 1, there is an unconfined aquifer 7 above the impermeable layer 2, its dynamics can be analyzed, and the piezometer head surface 9 is confirmed to obtain rational development data for water resources in this area. You can

According to the method of the present invention, it is possible to analyze groundwater dynamics with extremely high accuracy, and it is possible to easily and inexpensively provide a survey that meets a high-level survey objective.

〔Example〕

The infiltration flow on the river embankment was analyzed by the method of the present invention. There are several phenomena on river levees that threaten the safety of the levees due to floods, and one of them is the phenomenon called piping. Ono biping is a phenomenon in which the material of the levee is washed away when the seepage flow in the levee exceeds a certain velocity (critical velocity) depending on the soil quality, which leads to the destruction of the levee.

As shown in FIG. 3, 66 mmφ boring holes 6a, 6b, ... Are dug in the bank 1, a floodwater level reservoir 22 is provided on the river side of the bank, and the dynamics of the seepage flow 23 are analyzed by the present invention. did.

As a result, it is possible to compare the three-dimensional flow velocity in the levee during a flood with the critical flow velocity, to accurately judge the risk of piping, and to ensure the safety during a flood. Could be made clear.

In addition, the levee at the time of flood was modeled, the seepage flow was calculated by the finite element method, the flow velocity vector at the tip of the levee body method where the flow velocity was the maximum was obtained, and the relationship between the hydraulic conductivity of the levee body and the flow velocity However, it was possible to find a relatively good agreement between the calculated flow velocity and the measured flow velocity.

〔The invention's effect〕

Since the present invention employs a method of obtaining and analyzing the three-dimensional flow velocity of water in a small-diameter boring hole, it is possible to inexpensively and highly accurately investigate the dynamics of groundwater and to make a great contribution.

[Brief description of drawings]

1 and 3 are explanatory views of the method of the present invention, and FIG. 2 is a perspective view of a sensor which can be suitably used for carrying out the method of the present invention. 1 ... Seepage water, 3 ... Impermeable layer, 4 ... Observation well, 5 ...
Water table, 6a, 6b, 6c …… Boring hole, 7 ……
Unconfined aquifer, 9 ... Piezometer head surface, 11 ... Ultrasonic wave transmitting element, 12 ... Ultrasonic wave receiving element, 13 ... Measuring section, 14 ... Flexible cable, 21 ... Bridge body, 2
2 ... Reservoir, 23 ... Seepage flow

Claims (1)

[Claims] 1. A plurality of small-bore boreholes are drilled at required positions in an area where groundwater dynamics should be investigated, and four or more transmitter elements arranged in a three-dimensional space in the boreholes and the transmitter elements. And a sensor equipped with an ultrasonic wave transmitting / receiving device and a direction detecting device each consisting of one receiving element separated by a certain distance are inserted, and the sensor is maintained at each water level of groundwater in the boring hole. Ultrasonic waves are sequentially transmitted from each element in a time-division manner, the arrival time of the transmission at the receiving element is measured, and the three-dimensional flow direction and flow velocity of water at the water level are obtained from the measured values. The azimuth is obtained from the azimuth detecting device, the flow velocity vector of water for each water level in the groundwater in the borehole is obtained, and the dynamics of groundwater in the area is calculated from the flow velocity vector group for a plurality of boreholes. Dynamics research methods of underground water, characterized in that the analysis.