JP2010071834A - Water channel exploration method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water channel exploration method, capable of readily identifying water channels existing in a crystalline rock distribution region, with high accuracy. <P>SOLUTION: The water channel exploration method includes: a fan-shoot seismic exploration step of arranging vibration reception points along a plurality of exploration lines traversing an exploration object region; measuring and analyzing a refracted wave of an elastic wave generated at a vibration generation point distanced not less than a predetermined value in a direction orthogonal to each exploration line by a fan-shoot seismic exploration method; and identifying the peripheral area, including the thickest part T, having a total maximum thickness of a surface deposit 8 and a strongly weathered part 6 of a crystalline rock, and a step of conducting a gravity exploration on the peripheral area and identifying a position of the thickest part T. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a water path search method, and more particularly to a water path search method for searching for a water path existing in a crystalline rock distribution area.

  In mid-mountainous areas where crystalline rocks such as granite are distributed, surface sediments are generally thin, and shallow groundwater systems are mainly developed in weathered areas of crystalline rocks. The permeability of the strong weathered part of crystalline rock such as granite and the weakly weathered part (or fresh rock part) of crystalline rock differ by several orders of magnitude, so the boundary becomes the base of the shallow groundwater system. The boundary between the strongly weathered part of the crystalline rock and the weakly weathered part of the crystalline rock (or fresh rock part) is not uniform, there are local irregularities, and the local concave parts In order to form a structure that collects the shallow groundwater system, a structure in which those structures are continuous becomes a water channel of the shallow groundwater system. In the development of groundwater above a certain scale, excluding extremely small-scale groundwater development, such as household water sources in hilly and mountainous areas, exploration of the shallow groundwater system that develops around the valley bottom plain and creating a pumping well. It has been studied from the past.

  Seismic exploration by refraction and reflection methods is conventionally known as a method to identify the location of water paths in the shallow groundwater system that develops around the valley bottom plain, but the sudden topography change point at the boundary between the valley bottom plain and the mountainous area In addition, the seismic refraction method and the reflection method are used for the development of shallow groundwater systems because the survey line length required at least four times the depth of the survey is not sufficient and the survey accuracy is not sufficient. Exploration was never applied.

Non-Patent Documents 1 and 2 show a method of exploring the flow of groundwater by a two-dimensional resistivity search using a resistivity imaging method, which is a kind of electrical exploration method. Since it is difficult to obtain sufficient exploration accuracy when measuring from the ground surface, there have been many cases of shallow groundwater development failure, and there were fears that cost and time were wasted due to the large number of boring points.
"Applied Geology Vol.31 No.1", Japan Society of Applied Geology, 1990, pp. 12-18; Takeo Tatsuo, Ryoji Nagae, Study on groundwater flow monitoring method by electrical exploration "Applied Geology Vol.33 No.1", Japan Society of Applied Geology, 1992, pp. 1-6; Ryoji Nagae, Ikuo Takeuchi, Study on groundwater flow monitoring method by electrical exploration (Part 2)

  Therefore, an object of the present invention is to provide a water channel search method that can easily and accurately specify a water channel existing in a crystalline rock distribution area.

  The object of the present invention is a method for exploring a water channel existing in a crystalline rock distribution area, wherein a receiving point is arranged along a plurality of exploration survey lines crossing the exploration target area, The total thickness of surface sediments and strong weathered parts of crystalline rocks are measured and analyzed by the seismic exploration method using the seismic reflection method of elastic waves generated at an excitation point with a predetermined separation distance in the orthogonal direction. A water path exploration step comprising: a fan seismic exploration step for identifying a peripheral region including the thickest part having the largest thickness; and a gravitational exploration step for performing a gravitational exploration on the peripheral region to identify a position of the thickest part Achieved by the method.

  In this water path exploration method, it is preferable that the fan method seismic exploration step arranges the plurality of receiving points so that the interval is 1 m or less.

  The gravity exploration step preferably has a gravitational change detection accuracy of about 10 ngal.

  According to the water path exploration method of the present invention, it is possible to easily and accurately identify a water path existing in the crystalline rock distribution area.

Hereinafter, actual forms of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic diagram for explaining a water path exploration method according to an embodiment of the present invention.
The water path exploration method of this embodiment specifies a water path that exists in the valley bottom plain that is the exploration target area in an intermediate and mountainous area where crystalline rocks such as granite are distributed.

  As shown in FIG. 1, the geological section of the crystalline rock is divided into a fresh rock part 2, a weakly weathered part 4 and a strong weathered part 6 according to the degree of weathering, and has a laminated structure. A thin surface layer deposit 8 is deposited on the strong weathering portion 6. In the strong weathering section 6, the permeability coefficient is large and the groundwater is easy to move. On the other hand, in the weak weathering section 4 and the fresh rock section 2, the permeability coefficient is small and the groundwater is difficult to move and is saturated in the layer. The stratum groundwater system develops mainly at the boundary between the weakly weathered part 4 and the strongly weathered part 6. When a fault / joint F exists in such a crystalline rock, weathering proceeds along the fault / joint F, and differential erosion occurs to form a valley bottom plain. In the basement of the valley bottom plain, weathering is progressing along the fault / joint F, so that the surface of the weakly weathered part 4 has a locally concave structure and collects the surrounding shallow groundwater. A water path W is formed by the continuous structure.

  Thus, the formation position of the water channel W is considered to be the thickest portion T where the total thickness of the strong weathering portion 6 and the surface layer deposit 8 is the largest. In order to identify the thickest portion T, the water path exploration method of the present embodiment first performs a fan method seismic exploration step.

  That is, receiving points (such as seismometers) are arranged at regular intervals along a plurality of survey lines crossing the valley bottom plain. The plurality of survey survey lines are preferably arranged in parallel to each other as shown in FIG. 2, for example.

  And an excitation point (seismic source) is installed with a predetermined distance or more in the direction orthogonal to each survey survey line. FIG. 3 shows the positional relationship between a certain surveying line L and the excitation point 10, and the separation distance E between them is such that the direct wave from the excitation point 10 does not reach the receiving point 12 before the refracted wave. It is preferable to set the distance above a certain distance.

  When an elastic wave is generated at the oscillation point 10, the elastic wave goes straight through the surface sediment 8 and the strong weathered portion 6 of the crystalline rock, and the weakly weathered portion 4 (or the fresh rock 2) of the crystalline rock. Refracted at the boundary, then re-refracted in the vicinity of the receiving point 12, goes straight through the surface sediment 8 and the strong weathering portion 6 of crystalline rock, and reaches the receiving point 12. The surface sediment 8 and the strongly weathered portion 6 and the weakly weathered portion 4 (or the fresh rock portion 2) have significantly different elastic wave velocities, and the elastic wave velocities of the surface sediment 8 and the strongly weathered portion 6 are significantly slower. The initial arrival time of the refracted wave varies depending on the total thickness of the surface layer deposit 8 and the strong weathering portion 6.

For example, as shown in FIG. 4A, the thickness of the strong weathering portion 6 from the excitation point 10 to the reception point 12 is 9 m, and the distance from the excitation point 10 to the reception point 12 is 30 m (explanation) For the sake of simplicity, the thin surface layer deposit 8 is not considered), and the initial wave arrival time is assumed when the elastic wave velocities of the weakly weathered portion 4 and the strongly weathered portion 6 are 3.0 km / sec and 1.5 km / sec, respectively. I1 is
I1 = 10.39 / 1.5 + 19.61 / 3.0 + 10.39 / 1.5 = 20.39 (msec).

On the other hand, as shown in FIG. 4B, when the thickness of the strong weathering portion 6 at the oscillation point 10 is 9 m, but the thickness of the strong weathering portion 6 at the receiving point 12 is increased by 1 m and changed to 10 m, the initial movement arrival time. I2 is
I2 = 10.38 / 1.5 + 19.02 / 3.0 + 11.54 / 1.5 = 20.95 (msec).

  Thus, the initial movement arrival time at the reception point 12 is largely dependent on the total layer thickness of the surface layer deposit 8 and the strong weathering portion 6 present in the vicinity of the reception point 12, so the travel time that is the initial movement arrival time is The travel time curve showing the relationship between the distance from the oscillation point 10 and the initial travel arrival time at each receiving point 12 is compared with the standard travel time curve. It can be seen that there is a region (thickest portion T) where the total sum of the layer thicknesses of the surface layer deposit 8 and the strong weathered portion 6 exists in the vicinity of the large receiving point 12.

  FIG. 5 shows an example of the analysis result of the fan seismic survey data in the exploration survey line 1) of FIG. 2 and shows the state where the excitation point 10 is fixed, so that it is far from the excitation point 10. The initial arrival time becomes longer at the receiving point 12, and the graph of the impact sound of the epicenter generated at the starting point 10 shows a hyperbola.

  FIG. 6 shows the result of correcting the difference in distance between the excitation point 10 and the receiving point 12 in the initial movement arrival time of FIG. 5. The corrected receiving point 12 with the latest initial movement arrival time is the survey line position. It can be seen that it exists at 43m and 45m.

  The structure in which the thickest part T is continuous can be grasped by a plurality of survey survey lines. For example, the delay in the initial movement arrival time from the standard travel time curve in each of the three or more survey survey lines crossing the valley bottom plain is the most. When large receiving points are arranged in a straight line, or receiving points with the longest delay in initial movement arrival time from the standard travel time curve in two exploration survey lines crossing the valley bottom plain are arranged parallel to the valley Moreover, it can be considered that the thickest portion T is continuous.

  Unlike the conventional fan-type seismic survey, the water channel search method of this embodiment searches for a narrow water channel structure with a width of about several meters to 5 m that is buried underground in the valley bottom plain. It is preferable to set the interval S (see FIG. 3) densely. Specifically, the reception point interval S is preferably set to 1 m or less, and more preferably 50 cm or less. The data shown in FIG. 5 is obtained by setting the reception point interval S to 50 cm.

  The position accuracy of the thickest part T obtained by such a fan seismic exploration step is low in principle, and in fact, the thickest part is just below the receiving point where the delay in the initial movement arrival time from the standard travel time curve is the largest. It does not necessarily indicate that T exists, but merely indicates that the thickest portion T exists in a range of about several m in the vicinity thereof. That is, it is not the thickest portion T itself but the peripheral region including the thickest portion T and its neighboring region that is specified by the fan method seismic exploration step.

  Therefore, in order to accurately determine the excavation point of the pumping well for the development of shallow groundwater, a gravity exploration step is performed to perform a precise gravity exploration on the peripheral region including the thickest portion T, and the position of the thickest portion T is determined. Is determined with high accuracy.

  Gravity exploration uses the property that the gravity value at each station is proportional to the sum of the product of the density and abundance of the material present immediately below. That is, the density of the surface sediment 8, the strong weathering part 6, the weak weathering part 4, and the fresh rock part 2 are different from each other, and the density of the surface sediment 8 and the strong weathering part 6 is relatively low. Since the density increases to the rock part 2, the gravity value decreases as the total layer thickness of the surface sediment 8 and the strong weathering part 6 immediately below the measurement point increases. Therefore, the position where the gravity value shows the minimum can be obtained as the thickest portion T.

  Even in the gravity exploration step, it is preferable that the station intervals are dense in order to explore the structure that becomes the water channel of the shallow groundwater system with a width of about 5m to 5m under the valley bottom plain. Is preferably set to 1 m or less, more preferably 50 cm or less.

  In the gravity exploration step, it is necessary to detect changes in gravity values due to depth differences of several meters to several tens of meters in weathered structures such as the strong weathered part 6, the weakly weathered part 4 and the fresh rock part 2 that lie beneath the valley bottom plain. . For this reason, high accuracy is required for the detection accuracy of gravity exploration, and specifically, it is preferable to use a method / apparatus that can detect a change in gravity value of about 10 ngal.

  For the gravity value measured in the gravity exploration step, in principle, the gravity value is compared after correcting the drift over time of the gravimeter, tide, station height, topographic effect, etc. By considering the indicated station as the thickest portion T, the station position can be regarded as a passing point of the water path of the shallow groundwater system. As a result, the excavation point of the pumping well for the development of shallow groundwater can be identified easily and with high accuracy.

  Of the survey lines that cross the valley bottom plains, such as rice fields, flat roads that do not require consideration of corrections such as station height and topographic effects, the gravimeter drift has a constant value. On the premise of the following, the measuring point indicating the lowest value of the reading of the gravimeter can be regarded as the thickest portion T.

  FIG. 7 shows an example of precision gravity exploration data in the exploration survey line 1) of FIG. 2 (the same survey survey line as in FIGS. 5 and 6) generated by the gravity exploration step. It can be specified as the position of the thickest portion T. Position A is the 42m point of the exploration survey line 1) in Fig. 2, and the position shift is about 1m from the 43m point of the exploration survey line 1), which was determined to be the latest arrival time by analyzing the seismic survey data. Has occurred. On the other hand, the position B is a position specified as a water path based on a conventional non-resistance method two-dimensional exploration, and a positional shift of about 30 m occurs between the two.

  Table 1 shows the results of forming the wells at positions A and B and measuring the limit yield. In the position A specified by the water path exploration method of the present embodiment, a significantly higher pumping amount is obtained as compared with the position B.

It is a schematic diagram for demonstrating the water path search method of the valley bottom plain which concerns on one Embodiment of this invention. It is a figure which shows the example of a setting of the survey line of the fan method seismic survey in the said water path search method. It is a top view which shows the measurement condition of a fan method seismic exploration. It is explanatory drawing which shows the delay of the initial movement by the difference in the path | route of a refracted wave at the time of a fan method seismic survey. It is a figure which shows an example of the analysis record of fan method seismic exploration data. It is a figure which shows the initial time arrival time of the refracted wave obtained from the fan method seismic exploration data. It is explanatory drawing which shows the position of the water path of the shallow groundwater system estimated from the precise gravity exploration data.

Explanation of symbols

2 Fresh rock part 4 Lightly weathered part 6 Strongly weathered part 8 Surface sediment 10 Excitation point 12 Receiving point L Exploration survey line

Claims (3)

A method for exploring water paths in crystalline rock distribution areas,
Place receiving points along multiple survey lines that cross the survey area, and refract the elastic wave refracted waves generated at the excitation points that have a predetermined distance or more in the direction orthogonal to each of the survey lines. A fan seismic exploration step for measuring and analyzing by the seismic exploration method to identify the peripheral region including the thickest part having the largest total thickness of the surface sediment and the strong weathered part of the crystalline rock,
A water path exploration method comprising: a gravitational exploration step for performing gravitational exploration on the peripheral region and identifying a position of the thickest portion. 2. The water channel exploration method according to claim 1, wherein in the fan method seismic exploration step, the plurality of receiving points are arranged so that an interval is 1 m or less. 3. The water path search method according to claim 1 or 2, wherein the gravity search step has a gravity change detection accuracy of about 10 ngal.

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