JP2011112368A - Method of monitoring moisture variation of ground - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To readily and quickly monitor moisture the variations in an unsaturated layer, by extracting actual minute variations in he specific resistance by an original technique that uses apparent specific resistance data itself obtained by an electric exploration technique, and by allowing high-speed data processing without having to perform inverse analysis. <P>SOLUTION: A plurality of electrodes are arranged at the ground; the apparent specific resistance data of the ground are temporally obtained by applying the electric exploration technique; the apparent specific resistance, before a rainfall or immediately after the rainfall, is used as a reference; and the variational rate relative to the reference apparent specific resistance is calculated regarding the apparent specific resistance obtained thereafter and variational rate distribution of the apparent specific resistance in the ground is obtained. By temporally observing the variational rate distribution of the apparent specific resistance, the moisture variation of the ground unsaturated layer is monitored. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention uses the apparent resistivity data of the ground obtained by electrical exploration measurement, calculates the change rate of the apparent resistivity with reference to the pre-rainfall or immediately after the rain, and observes the change over time, The present invention relates to a method for monitoring moisture change in an unsaturated layer of the ground. This technique is useful, for example, for grasping the infiltration process of rainwater, entering and exiting the ground, and investigating the state of stagnant water.

  As is well known, a technique for investigating the specific resistance structure of the ground by an electric exploration method is widely used, for example, in resource exploration and civil engineering fields. Typically, electrical resistivity measurement is performed to obtain apparent resistivity data, and the apparent resistivity data is inversely analyzed to create a specific resistivity model (two-dimensional cross section) of the ground. Explore. This inverse analysis of apparent resistivity data is a method in which an initial model is first set and the model is changed until it finally matches the actually obtained resistivity data. The theoretical ratio to the changed model is The residual of the resistance value and the observed specific resistance value is calculated, and the process of correcting the model (inverse analysis) is repeated so that the residual is minimized to obtain the final specific resistance model.

  As an application of such an electric exploration method, estimation of the hydraulic conductivity inside the ground by specific resistance monitoring using a tracer is also performed (see Patent Document 1). In the specific resistance monitoring in the prior art, the specific resistivity model of the ground is created by inverse analysis of the apparent specific resistance data obtained by the electrical exploration measurement, and the change in the specific resistance is compared and observed based on the created resistivity model. (Including a two-dimensional analysis using the rate of change in specific resistance as a parameter).

  That is, in the prior art, it is fundamental to create a specific resistance model by inverse analysis. However, the inverse analysis of the apparent resistivity data for creating the resistivity model has a problem that requires a lot of time. Specifically, it takes tens of minutes to several hours to create one specific resistance model. Therefore, if an attempt is made to create a large number of specific resistance models in order to observe changes at short time intervals, an enormous amount of data processing is required, and it is extremely difficult to implement with the data processing capability of a normal personal computer. Therefore, when trying to grasp the time variation of the resistivity model of the formation, several resistivity models are created at most, and the resistivity change in a short time is the porosity, water saturation, conductivity of the pore water. The current situation is that it must be carried out by complementing it on the assumption that it depends on changes. Therefore, it is not possible to grasp a detailed situation change.

  Inverse analysis of apparent resistivity data is difficult to automatically analyze, such as setting an initial model, and is affected by analysis errors due to differences in analysis methods (analysis parameters), and changes that differ from actual changes may be extracted. There are also problems that lack objectivity. Therefore, it is difficult to create an appropriate specific resistance model unless it is an experienced skilled engineer. In addition, when the apparent resistivity data is inversely analyzed, there is a problem that fine changes are lost due to the filtering process (only large resistivity changes are extracted).

  By the way, there is a soil moisture meter as a device that can directly measure the moisture in the ground over time, but with this, only a measured value at one point in the ground can be obtained, and the moisture distribution in a wide range cannot be observed. In addition, if a meter is inserted after making a hole in the ground, the formation will be disturbed, so even if a large number of soil moisture meters are dispersedly embedded, it is impossible to grasp an accurate change in moisture distribution over time.

JP 2000-121657 A

  The problem to be solved by the present invention is to use the apparent resistivity data itself obtained by the method of electrical exploration measurement, and to enable high-speed data processing without performing complicated operations such as reverse analysis. This is to extract a minute actual change in specific resistance so that the moisture change in the unsaturated layer can be monitored easily and quickly.

  In the present invention, a large number of electrodes are arranged on the ground, and the apparent resistivity data of the ground is obtained over time by electrical exploration measurement, and thereafter obtained based on the apparent resistivity before or after the rain. For apparent resistivity, calculate the rate of change of apparent resistivity in the ground by calculating the rate of change with respect to the standard apparent resistivity, and observe the rate of change of apparent resistivity over time. The soil moisture monitoring method is characterized by monitoring the moisture change of the ground unsaturated layer. In the present invention, it is needless to say that the concept of “ground” includes not only natural ground but also artificial ground (for example, a river bank).

  In the electrical exploration method, after obtaining the apparent resistivity data of the ground, the resistivity data is used to perform reverse analysis to create a soil resistivity model, and the underground structure is interpreted or created based on the created resistivity model. The method of judging is always adopted. The present invention intends to monitor a huge amount of resistivity data and monitor the moisture change of the ground by a completely new method with only a simple calculation without being bound by the fixed concept of requiring such inverse analysis. It is.

  In the present invention, a large number of electrodes are placed on the ground surface and arranged one-dimensionally or two-dimensionally, and the change rate distribution of the apparent resistivity in the vertical cross section in the survey direction or the horizontal cross section at an arbitrary depth is obtained. It is preferable to observe over time. Also, the apparent resistivity data of the ground is acquired at a time interval of several minutes to several tens of minutes, the change rate distribution of the apparent resistivity is obtained, and the time interval is 1 to several tens of minutes to 1 It is also possible to display a moving image of the moisture change of the ground on the display screen.

  Since the ground moisture change monitoring method according to the present invention uses an electric exploration technique, the moisture distribution can be monitored without disturbing the ground. Moreover, the method according to the present invention is based on the apparent resistivity immediately before or just after the rain, without performing time-consuming and cumbersome work such as reverse analysis of the apparent resistivity data. Since it is only necessary to calculate the rate of change of apparent resistivity in the ground over time by calculating the rate of change with respect to the reference apparent resistivity, it is possible to perform high-speed data processing with simple calculations. To extract and emphasize the moisture change.

  By the method of the present invention, it becomes possible to observe the moisture change of the ground in real time, the infiltration process of rainwater into the unsaturated layer of the ground, the change of void due to the penetration of rainwater, the retention and evaporation of moisture in the unsaturated layer It becomes possible to grasp the situation as a change with time.

Explanatory drawing which shows an example of the measurement system for the electric exploration measurement used by this invention. Explanatory drawing which shows an example of the measurement procedure of apparent specific resistance by it. The data processing flow in the method of the present invention. Explanatory drawing which shows an example of the specific resistance model of the ground by reverse analysis. Explanatory drawing which shows an example of specific resistance change rate distribution on the basis of before rainfall. Explanatory drawing which shows an example of specific resistance change rate distribution on the basis of immediately after rainfall.

  The procedure for measuring the apparent resistivity of the ground by electrical exploration measurement may be basically the same as a known method. An example of a measurement system in a two-pole method arrangement is shown in FIG. Far electrodes are installed on the outer sides of both ends of the survey line, and one is a current electrode (C∞) and the other is a potential electrode (P∞). A survey line is set on the ground surface, a large number of electrodes 10 are installed at equal intervals in the survey line, the electrodes are switched by the scanner 12, and the measurement device 14 conducts electricity to the electrodes and measures the potential of the selected electrodes.

  An example of the measurement procedure is shown in FIG. A current is passed between the far electrode (C∞) and the electrode C, the potential of the electrode P and the far electrode (P∞) at that time is measured, and the distance between the electrode C and the electrode P is changed to the exploration depth. Repeat while letting. Here, four potential measurement electrodes (P1, P2, P3, P4) are used, and the measurement position in the ground is indicated by a cross. From the start state, the current electrode C and the potential electrodes P1,..., P4 are switched so as to be shifted one by one in the arrow direction along the measurement line. Note that the ● mark in the ground indicates the measured position. By carrying out such electric exploration measurement along the survey line and continuing to the end state, actual measurement data on the survey line at that time can be obtained.

The apparent specific resistance ρ is expressed by the following equation.
ρ = KV / I (where K is the electrode placement factor)
In actual non-uniform ground, this value does not mean the true resistivity, but it reflects the resistivity distribution in the basement, and is an average value of a kind of resistivity in a fairly wide range around the electrode. Since it can be considered, it is called apparent resistivity. By carrying out electrical exploration measurement as described above, apparent resistivity data (apparent resistivity distribution) in a vertical section in the direction of the measurement line can be obtained.

A data processing flow according to the method of the present invention is shown in FIG. As described above, the apparent resistivity data of the ground is acquired by conducting the electric survey measurement. The actually measured apparent resistivity value is loosely measured in the depth direction, so sensitivity correction is performed as necessary. If it is only necessary to monitor only the state of moisture change on the ground, sensitivity correction is not necessarily required. Next, change the apparent resistivity in the ground by calculating the rate of change of the apparent resistivity before and after the rain as reference data, and the rate of change of the apparent resistivity obtained after that to the reference apparent resistivity. Find the rate distribution. The specific resistance change rate Δρt at an arbitrary time is calculated based on the following equation.
Δρt = {(ρt−ρs) / ρs} × 100 (%)
Here, ρt is an apparent specific resistance at an arbitrary time, and ρs is an apparent specific resistance at a reference time (before or immediately after rainfall). A distribution map of the change rate of the apparent resistivity obtained in this way is created.

  The above steps are repeatedly performed at arbitrary time intervals (for example, intervals of about 5 to 10 minutes), and resistivity change rate data at a number of times are calculated and accumulated. This iterative process is performed on apparent resistivity data obtained at different elapsed times, and is completely different from the iterative calculation in the inverse analysis. The calculation of the rate of change of the specific resistance is obtained instantaneously because only one subtraction and one division are performed for one measurement point and one time point. By monitoring the accumulated data over time, the moisture change in the ground unsaturated layer is monitored. For example, by shortening the elapsed time to one of several hundreds, it is possible to observe a change in the state of moisture change in the ground that has occurred within a few days immediately after rainfall as a moving image of several minutes. Of course, it is also possible to output a distribution map of moisture change in the ground at an arbitrary time point as a hard copy, and based on this, it is possible to monitor the change in the situation of moisture change. It is also possible to monitor the state of moisture change in real time.

  In the method according to the present invention, the ground structure is not changed, and only what is changing is emphasized. Therefore, it is possible to understand the change in water that changes by several percent, and the movement of water in the ground can be seen. Also, what you see depends on where you set the reference point in time. If the reference time is set before the rain, the infiltration process of the rainwater from the surface layer into the unsaturated layer can be understood, and if the reference time is set immediately after the rain, the movement of rainwater in the unsaturated layer, As a result, it is possible to grasp the change of voids and the retention and evaporation of moisture in the unsaturated layer. Specifically, a short-time change, such as where water enters and exits, or a place where it is easy to soak, a place where water easily stagnates, or a place where water drains easily becomes clear.

  By the way, it is necessary to arrange a large number of electrodes in the electric exploration measurement. Although the electrode placement method is arbitrary, in the method of the present invention, it is desirable to place the electrode at a certain position in order to perform measurement over time. It is better to drive the electrode into the ground. Of course, a method of simply placing the electrode on the surface layer or a method of measuring while pulling the electrode on the surface layer is also possible.

  In the above example, a large number of electrodes are arranged one-dimensionally on one survey line, but may be arranged on a plurality of survey lines. Thus, when arranged two-dimensionally, a vertical section of the resistivity change rate at a plurality of survey lines can be obtained, and a horizontal section of the resistivity change rate in the ground at different depths can be obtained.

  Hereinafter, an example of the actual measurement result will be described. The measurement system for electrical exploration measurement shown in FIG. 1 was applied to the ground to be measured, and the apparent specific resistance was measured by a measurement procedure as shown in FIG. FIG. 4 shows a specific resistance model obtained for reference. This is obtained by inverse analysis of apparent resistivity data acquired before the rain, and is a result of normal electric exploration. In FIG. 4, it can be seen that the surface embankment (the loam layer) and the fine sand and tuff clay layer below it are measured.

  5 and 6 are examples of results obtained by the present invention. FIG. 5 is based on the pre-rainfall, and the specific resistance change rate (%) during the rain (a), 6 hours after the rain (b), 1 day after the rain (c), 2 days after the rain (d) ) Distribution map. FIG. 6 is based on the time immediately after the rain, and on the other hand, the distribution of specific resistance change rate (%) after 6 hours of rain (a), 1 day after the rain (b), and 2 days after the rain (c). It is. The apparent resistivity data is processed according to the processing flow shown in FIG.

  Here, due to the limitations of the drawing, only still images can be displayed, so only four distribution maps at different points in time are shown in FIG. 5 and three in FIG. Apparent specific resistance data is acquired at a time interval of about several minutes (for example, 5 to 10 minutes) by high-speed continuous automatic electric exploration measurement, and the specific resistance change rate is calculated. Since this calculation can be obtained by only one subtraction and one division for one point, high-speed data processing is possible, and it can be obtained instantaneously even with a personal computer having a general processing capability. Therefore, it is possible to monitor the moisture distribution of the ground in real time.

  Although it is difficult to understand in FIG. 5 or FIG. 6, a moving image with reduced time can actually be displayed on the personal computer screen, so that the movement of water in the ground (such as entering and exiting water) can be clearly understood.

  Furthermore, if long-term observation is performed, it is possible to calculate the average value for the day and the average value for the month, and by monitoring the rate of change, it is possible to grasp the change with time of the moisture in the unsaturated layer. It could be confirmed.

  When the correlation between the apparent resistivity change rate and the soil moisture meter data is obtained according to the present invention, the soil moisture change can be monitored. In addition, if the correlation between the apparent resistivity change rate and the groundwater level is obtained, the change in the groundwater level can be monitored.

  The method of the present invention can be directly applied to the prediction of landslide because it can directly monitor changes in the moisture distribution of the ground (temporal or local changes in soil moisture, ie, movement). For example, if the method of the present invention is used in advance to know how rain falls and the infiltration of rainwater, landslide warnings can be issued as needed by simply monitoring how rain falls and necessary countermeasures are taken. Construction methods can also be developed. The present invention can also be applied to river bank integrity diagnosis and the like.

  The method of the present invention which can monitor the change in the moisture distribution of the ground can also monitor the groundwater level. Since the flow direction of groundwater can be grasped in a plane, it is possible to accurately investigate the extent of soil contamination. This is because the method of the present invention does not need to form a hole in the ground or embed a meter such as a moisture meter, and therefore does not disturb the ground. By the way, if you dig a hole and embed an instrument, the formation will be disturbed, the effect is inevitable, and the flow direction will change.

10 electrode 12 scanner 14 measuring instrument

Claims (3)

  A large number of electrodes are placed on the ground, and the electrical resistivity method is applied to obtain the apparent resistivity data of the ground over time. For apparent resistivity, calculate the rate of change of the apparent resistivity in the ground by calculating the rate of change relative to the apparent apparent resistivity, and by observing the rate of change of apparent resistivity over time, A method of monitoring moisture changes in the ground, characterized by monitoring moisture changes in the ground unsaturated layer.   Many electrodes are placed on the ground surface and arranged in one or two dimensions, and the change rate distribution of apparent resistivity is observed over time in vertical sections in the direction of measurement or horizontal sections at an arbitrary depth. The ground water change monitoring method according to claim 1.   Acquire the apparent resistivity data of the ground at a time interval of several minutes to several tens of minutes, obtain the change rate distribution of the apparent resistivity, and shorten the time interval to one-hundredth to tens of minutes The moisture change monitoring method for a ground according to claim 1 or 2, wherein the moisture change is displayed as a moving image on a display screen.

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