Ground structure estimation method and receiving unit used therein

Abstract

To provide a ground structure estimation method capable of two-dimensionally or three-dimensionally estimating a ground structure efficiently regardless of a ground state such as underground water existence.SOLUTION: A ground structure estimation method comprises: sequentially generating vibrations with a vibration generator 10 at a plurality of underground vibration generating points 62 different in depth to each other in an underground vibration generating hole 60; receiving S wave of the vibrations generated at each of the plurality of the underground vibration generating points 62 at either of a plurality of vibration receiving points 70 on the ground surface different in horizontal positions to each other and the vibration generating hole 60 and a plurality of underground vibration receiving points 82 different in horizontal position from the vibration generating hole 60 in an underground vibration receiving hole 80 and different in depths to each other; and estimating a S wave velocity structure of the ground G using elastic wave tomography based on a received S wave. In this way, using the S wave without relying on the P wave of the vibration enables the structure of the ground G to be two-dimensionally or three-dimensionally estimated efficiently regardless of a state of the ground such as existence of underground water.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a ground structure estimation method for estimating a ground structure, and a receiving unit used in the ground structure estimation method. [Background technology]

[0002] Methods for estimating ground structure include the ground elastic wave velocity logging method (JGS 1122 2012) and elastic wave tomography using artificially generated elastic waves. For example, Patent Documents 1 and 2 disclose methods for estimating ground structure using elastic wave tomography. Patent Document 1 discloses a method for generating inter-well tomography measurement data using only a surface source without using a source in the borehole. Patent Document 2 also discloses a ground structure estimation method that reduces processing load by estimating the S-wave velocity structure of the ground using a surface wave exploration method, setting initial parameters based on the estimated S-wave velocity structure, and estimating the P-wave velocity structure of the ground using elastic wave tomography based on P waves received at multiple underground receiving points. Furthermore, ground strength, i.e., the hardness and compaction of the ground, is generally investigated based on the Standard Penetration Test Method (JIS A 1219), the Mechanical Cone Penetration Test Method (JIS A 1220), and the Dynamic Cone Penetration Test Method (JGS 1437 2014). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-298369 [Patent Document 2] Japanese Patent Application Publication No. 2020-167726 Summary of the Invention [Problem to be solved by the invention]

[0004] The estimation method using elastic wave tomography described above uses P-waves. While P-waves correlate with the hardness of bedrock, in saturated ground below groundwater, they are the same as the underwater velocity, making it difficult to estimate the hardness of the ground. In contrast, the elastic wave velocity logging method for ground can obtain not only the P-wave velocity of each layer but also the S-wave velocity, which correlates with the hardness of the ground, but can only obtain S-wave data from a single test site. Furthermore, the standard penetration test method, mechanical cone penetration test method, and dynamic cone penetration test method can only obtain hardness data from a single test site. To obtain planar or three-dimensional hardness data, it is necessary to conduct the test at multiple sites. The present invention has been made in view of the above-mentioned problems, and its object is to efficiently estimate the ground structure in two dimensions or three dimensions, regardless of the ground condition such as the presence or absence of groundwater. [Means for solving the problem]

[0005] (Aspects of the invention) The following embodiments of the present invention are examples of the configuration of the present invention, and are described in terms to facilitate understanding of the various configurations of the present invention. Each term does not limit the technical scope of the present invention, and while taking into consideration the best mode for carrying out the invention, some of the components of each term may be replaced or deleted, or other components may be added, and these may also be included in the technical scope of the present invention.

[0006] (1) A method for estimating ground structure, which involves generating vibrations sequentially at a plurality of underground excitation points at different depths within an underground excitation hole using an excitation device, receiving the S-waves of the vibrations generated at each of the plurality of underground excitation points at at least one of a plurality of surface receiving points whose horizontal positions are different from each other and from the excitation hole, and a plurality of underground receiving points whose horizontal positions are different from each other and from the excitation hole, within an underground receiving hole whose horizontal positions are different from the excitation hole, and estimating the S-wave velocity structure of the ground using elastic wave tomography based on the received S-waves.

[0007] The ground structure estimation method described in this section estimates ground structure using elastic wave tomography, in which S waves of vibrations generated at a vibration generating point are received at a vibration receiving point, and estimation is performed using the S waves of the received vibrations. The vibrations to be received are generated by setting multiple underground vibration generating points at different depths in a vibration generating hole drilled underground in the target ground, and using a vibration generating device at these multiple underground vibration generating points in sequence. In this case, vibration generating holes may be drilled at gradually increasing depths, and vibrations may be generated at the tip of the vibration generating hole each time the depth of the vibration generating hole reaches a set underground vibration generating point.

[0008] The S-waves of the vibrations generated at each of the multiple underground vibration generating points as described above are received at at least one of multiple surface receiving points and multiple underground receiving points set within the target ground. The multiple surface receiving points are set at different horizontal positions, and at different horizontal positions from the vibration generating holes. The multiple underground receiving points are set at different depths within the receiving holes, with receiving holes set at different horizontal positions from the vibration generating holes. The S-wave velocity structure of the target ground is then estimated using elastic wave tomography from the S-waves of the vibrations received at at least one of the multiple surface receiving points and multiple underground receiving points set in this way.

[0009] As a result, when vibrations are received only at multiple surface receiving points, the structure of the ground located between the multiple underground excitation points and the multiple surface receiving points is estimated, and when vibrations are received only at multiple underground receiving points, the structure of the ground located between the multiple underground excitation points and the multiple underground receiving points is estimated. Furthermore, when vibrations are received at both multiple surface receiving points and multiple underground receiving points, the structure of the ground located between the multiple underground excitation points and the multiple surface receiving points and the structure of the ground located between the multiple underground excitation points and the multiple underground receiving points are estimated. In either case, the ground structure is efficiently estimated in two dimensions or three dimensions, and particularly when vibrations are received at both surface receiving points and underground receiving points, missing data when vibrations propagate by bypassing soft ground is filled in, thereby further improving the estimation accuracy. Furthermore, because the elastic wave tomography method uses the S-waves of vibrations to estimate the S-wave velocity structure of the ground, rather than the P-waves of vibrations, the ground structure can be estimated without any problems even in saturated ground such as ground below groundwater or the seabed. Therefore, the hardness and softness of the ground structure can be efficiently estimated regardless of the state of the ground.

[0010] (2) In the above paragraph (1), a method for estimating ground structure is provided in which a plurality of three-way receivers, each of which is a combination of three vibration sensors to measure vibrations in three mutually perpendicular directions, are attached to the outer periphery of a tubular member at intervals along the longitudinal direction of the tubular member, and the spacing between the three-way receivers is adjusted to match the spacing between the underground receiving points, and the tubular member is expanded within the receiving hole to bring the three-way receivers into close contact with the hole wall, thereby simultaneously receiving vibrations at the underground receiving points where the three-way receivers are located.

[0011] The ground structure estimation method described in this section uses a device called a receiving unit, which includes a three-way receiver and a tubular member, to receive vibrations at multiple underground receiving points. Specifically, a three-way receiver is fabricated by combining three vibration sensors to measure vibrations in three mutually perpendicular directions, and multiple three-way receivers are prepared. These multiple three-way receivers are then attached to the outer periphery of a tubular member, spaced apart from one another in the longitudinal direction of the tubular member, with the spacing between the multiple three-way receivers adjusted to match the spacing between the multiple underground receiving points.

[0012] Furthermore, a tubular member equipped with multiple three-directional geophones is placed inside the receiving borehole, and the tubular member is expanded inside the receiving borehole to bring the multiple three-directional geophones into close contact with the borehole wall. At this time, the positions of the multiple three-directional geophones are aligned with one of the multiple underground receiving points set in the receiving borehole and brought into close contact. The multiple three-directional geophones installed in this manner then receive S-waves of vibrations generated at multiple underground vibration generating points. This allows simultaneous reception at the multiple underground receiving points where multiple three-directional geophones are located, which is expected to reduce labor and time required for the work and enable more efficient estimation of ground structure. Furthermore, because each three-directional geophone is configured to measure vibrations in three mutually perpendicular directions, S-waves of vibrations can be measured without any problems at each underground receiving point.

[0013] (3) In the above item (2), when receiving S-waves of vibrations at the plurality of ground surface receiving points, at least two of the vibration excitation holes are provided at positions different from each other in horizontal position, and vibrations are generated at the plurality of underground vibration excitation points in each of the at least two vibration excitation holes. In the ground structure estimation method described in this section, when receiving S-waves of vibrations at multiple surface receiving points, at least two vibration excitation holes are provided at mutually different horizontal positions. Then, multiple underground vibration excitation points are set in each of these at least two vibration excitation holes, and vibrations are generated at each of the multiple underground vibration excitation points. That is, for example, when two vibration excitation holes are provided, vibrations are generated sequentially at the multiple underground vibration excitation points of one vibration excitation hole using a vibration excitation device, and the S-waves of the vibrations are received at the multiple surface vibration excitation points. Furthermore, at a different timing, vibrations are generated sequentially at the multiple underground vibration excitation points of the other vibration excitation hole using a vibration excitation device, and the S-waves of the vibrations are received at the multiple surface vibration excitation points.

[0014] Among multiple surface receiving points, a surface receiving point located near one of the vibration generating holes may dominate the received vibrations, making it difficult to distinguish between P waves and S waves due to its close distance from the vibration generating point. However, since such a surface receiving point is located far from the other vibration generating hole, it is easy to distinguish between P waves and S waves when receiving vibrations generated at the underground vibration generating point of the other vibration generating hole. In this way, by generating vibrations in at least two vibration generating holes, it becomes easy to distinguish between P waves and S waves generated by vibrations from at least one vibration generating hole at all multiple surface receiving points. In other words, ground information, such as that directly above the vibration generating hole, is complemented. This enables more accurate estimation of ground structure using elastic wave tomography using S waves, improving the reliability of the estimation results. Note that when vibrations are also received at multiple underground receiving points, it is of course possible to receive the vibrations generated in each vibration generating hole at multiple underground receiving points.

[0015] (4) The above (2) In (3), the method for estimating ground structure comprises setting the plurality of underground receiving points at depths of 1 m each within the receiving hole, and attaching five or less of the three-directional receivers to the tubular member at intervals of 1 m. In the ground structure estimation method described in this section, multiple underground receiving points are set in the receiving hole at depths of 1m each. In accordance with this, the installation intervals of the multiple three-directional receivers attached to the outer periphery of the tubular member are set to 1m, and the number of three-directional receivers attached to the tubular member is set to five or less.

[0016] This allows the tubular member to be as long as about 5 m, suppressing the water pressure difference between the top and bottom of the tubular member, even when the tubular member is expanded in a receiving borehole where groundwater or seawater has flowed in. Therefore, the internal pressure imbalance caused by the water pressure difference, which can be a concern when the tubular member is too long, is suppressed, and the expanded tubular member can easily fit five or fewer three-way receivers to the borehole wall, regardless of their longitudinal installation position. Furthermore, by setting multiple underground receiving points every 1 m in depth, vibrations can be received every 1 m in depth, and the data can be used to estimate the ground structure, allowing the ground structure to be estimated at appropriate intervals.

[0017] (5) The above ( 2 ) to (4), a ground structure estimation method in which an instrument or device used in any one of a dynamic cone penetration test, a static cone penetration test, and a standard penetration test is used as the excitation device, and the one test is performed in parallel. The ground structure estimation method described in this section involves generating vibrations at multiple underground excitation points in a vibration excitation hole by using an instrument or device used in any one of the dynamic cone penetration tests, static cone penetration tests, and standard penetration tests as a vibration excitation device, and conducting the test in parallel. In other words, one of the three penetration tests is conducted in the vibration excitation hole, and the vibrations generated in that test are received at multiple surface receiving points and multiple underground receiving points.

[0018] Specifically, in the case of a dynamic cone penetration test or a static cone penetration test, a rod with a cone attached to the tip and a hammer striking the rear end of the rod are used as vibration generators. In the case of a standard penetration test, a boring rod with a sampler attached to the tip and a drive hammer striking the rear end of the boring rod are used as vibration generators. This reduces costs by using the same equipment or devices used in each penetration test as vibration generators to generate vibrations without using a dedicated vibration generator. Furthermore, by conducting one of the penetration tests in parallel and using the test results to estimate the ground structure, the accuracy of the ground structure estimation can be improved.

[0019] (6) A receiving unit that receives S waves of vibrations generated at a vibration source at a plurality of underground receiving points at different depths within an underground receiving hole in order to estimate the ground structure, the receiving unit including a three-way receiver that combines three vibration sensors to measure vibrations in three mutually perpendicular directions, and a tubular member that can expand in the radial direction, and the plurality of three-way receivers are attached to the outer periphery of the tubular member along the longitudinal direction of the tubular member at intervals that match the spacing of the plurality of underground receiving points.

[0020] The receiving unit described in this section receives S-waves of vibrations generated at a vibration source at multiple underground receiving points at different depths within an underground receiving hole provided in the target ground in order to obtain data used for estimating the ground structure. Specifically, this receiving unit includes a three-way receiver and a tubular member, and the three-way receiver is formed by combining three vibration sensors so as to measure vibrations in three mutually perpendicular directions. The tubular member is expandable in the radial direction, and multiple three-way receivers are attached to its outer periphery. In this case, the multiple three-way receivers are attached at intervals along the longitudinal direction of the tubular member, matching the spacing of the multiple underground receiving points set in the receiving hole.

[0021] With this configuration, when acquiring data, the receiving unit is placed in the receiving borehole with the positions of the multiple three-directional receivers aligned with the depths of the multiple underground receiving points. Then, by pumping air or the like into the tubular member in this state, the tubular member expands in the radial direction, causing the multiple three-directional receivers to adhere tightly to the borehole wall. This allows the S-waves of vibrations generated at the excitation point to be measured simultaneously at the multiple underground receiving points where the multiple three-directional receivers are located, thereby reducing labor and time required for the work. Furthermore, because each of the multiple three-directional receivers is configured to measure vibrations in three mutually orthogonal directions, S-waves of vibrations can be measured without any problems at each underground receiving point. The number of multiple three-directional receivers does not need to be the same as the number of multiple underground receiving points. Measurements may be performed multiple times, with the receiving unit positioned at different depths, to measure all of the multiple underground receiving points.

[0022] (7) In the above item (6), a receiving unit in which five or less three-way receivers are attached to the tubular member at intervals of 1 m. The receiving unit described in this section has five or fewer three-way receivers attached at 1m intervals around the outer periphery of a tubular member. In other words, multiple underground receiving points are set at depths of 1m each inside the receiving hole, and three-way receivers are attached at these set intervals. Furthermore, by limiting the number of three-way receivers to five or less, the length of the tubular member can be kept to around 5m, which prevents pressure imbalances between both ends of the tubular member due to water pressure differences.

[0023] In other words, assuming that the device is installed in saturated ground, such as ground deeper than groundwater or the seabed, the tubular member will expand in the groundwater or seawater inside the receiving borehole. The influence of external water pressure is suppressed by the tubular member's length of approximately 5 m, so that there is not much difference between the two ends of the tubular member (between the upper and lower ends), allowing the entire tubular member to expand without any problems. This allows all five or fewer three-directional receivers to be in close contact with the borehole wall, enabling accurate reception of vibration S-waves. Furthermore, because S-waves are measured at multiple underground receiving points set every 1 m deep, the ground structure can be estimated at appropriate intervals. [Effects of the Invention]

[0024] Since the present invention has the above-described configuration, it is possible to efficiently estimate the ground structure in two dimensions or three dimensions regardless of the ground conditions such as the presence or absence of groundwater. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is an image diagram showing a schematic arrangement of members used in a ground structure estimation method according to an embodiment of the present invention; [Figure 2] FIG. 2 is an image diagram showing a schematic configuration of a receiving unit. [Figure 3] 1 is an image diagram showing a schematic representation of the positional relationship between a plurality of underground vibration generating points, a plurality of ground surface vibration receiving points, and a plurality of underground vibration receiving points in a ground structure estimation method according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Here, detailed descriptions of parts that are the same as or corresponding to those in the prior art will be omitted, and the same reference numerals will be used throughout the drawings to indicate the same or corresponding parts. 1 shows an example of the configuration and arrangement of components used in a ground structure estimation method according to an embodiment of the present invention. As shown in the figure, the ground structure estimation method according to an embodiment of the present invention estimates the structure of ground G using a vibration generator 10, multiple surface geophones 20, a measuring instrument 24, an analyzer 26, a receiving unit 30, and a compressor 56.

[0027] The vibration exciter 10 is used to generate vibrations in an underground vibration generating hole 60 formed in the ground G, and in this embodiment is composed of striking means 12 such as a hammer and a long, struck object 14 such as a rod. That is, the rear end of the struck object 14 placed in the vibration generating hole 60 is struck by the striking means 12, thereby generating vibrations at the tip of the vibration generating hole 60 where the tip of the struck object 14 is located. As will be described later, a plurality of underground vibration generating points 62 (see FIG. 3 ) at different depths are set in the vibration generating hole 60, and the position of the tip of the struck object 14, i.e., the depth of the vibration generating hole 60, is adjusted so as to generate vibrations at each of the underground vibration generating points 62.

[0028] Furthermore, tools and devices used in dynamic cone penetration tests, static cone penetration tests, standard penetration tests, and the like may be used as the vibration generator 10. That is, when tools and devices for dynamic cone penetration tests or static cone penetration tests are used, a wooden hammer, a large hydraulic hammer, or the like is used as the striking means 12, and a rod with a cone attached to the tip is used as the struck object 14. When tools and devices for standard penetration tests are used, a drive hammer is used as the striking means 12, and a boring rod with a sampler attached to the tip is used as the struck object 14.

[0029] The multiple surface geophones 20 are intended to receive the S-waves of vibrations generated by the vibration generator 10 as described above on the surface of the ground G to be measured, and are installed to receive vibrations at multiple surface receiving points 70 (see Figure 3) set as described below. Any vibration sensor capable of receiving S-waves of vibrations on the ground surface is used for the surface geophones 20. The multiple surface geophones 20 are connected to a measuring instrument 24 via a take-out cable 22. The measuring instrument 24 converts the vibrations received by the multiple surface geophones 20 into data and transmits it to an analyzing device 26.

[0030] The receiving unit 30 receives S-waves of vibrations generated by the vibration exciter 10 as described above in an underground receiving hole 80 formed in the ground G to be measured, and includes a plurality of three-directional receivers 32 and a tubular member 48. FIG. 2 illustrates a detailed structure of the receiving unit 30 according to an embodiment of the present invention. First, as shown in FIG. 2(a), each of the three-directional receivers 32 includes three vibration sensors 34, a cylindrical case 36, two stop valves 40, and a cable 42. The three vibration sensors 34 are fixed in position and orientation within the cylindrical case 36 so as to measure vibrations in three mutually orthogonal directions (height, side, and depth).

[0031] The three vibration sensors 34 are fixed in place by silicone 38 filled inside the cylindrical case 36, and both the top and bottom ends of the cylindrical case 36 are closed by stopcocks 40 with the three vibration sensors 34 and silicone 38 inside. Cables 42 house and protect data communication lines extending from each of the three vibration sensors 34. Any vibration sensor capable of measuring vibrations in each direction may be used for the three vibration sensors 34, and the three vibration sensors 34 may be fixed in place by any method other than silicone 38. Furthermore, although not limited to these, for example, the cylindrical case 36 may be made of aluminum with an inner diameter of approximately 40 mm, and the two stopcocks 40 may be made of a material with excellent water-tight properties, such as silicone resin.

[0032] Next, referring to FIG. 2(b), the vibration receiving unit 30 according to the embodiment of the present invention has five three-way receivers 32, as shown in FIG. 2(a), attached to the outer periphery of a tubular member 48 along the longitudinal direction of the tubular member 48. The installation intervals between the five three-way receivers 32 are the same as the spacing between multiple underground receiving points 82 (see FIG. 3) set in the vibration receiving hole 80, as described below. In this embodiment, this spacing is 1 m. Therefore, the length of the tubular member 48 in this embodiment is approximately 5 m so that the five three-way receivers 32 can be attached at 1-m intervals. The tubular member 48 is expandable in the radial direction and expands when air is pumped into it. The tubular member 48 may be made of, but is not limited to, a multi-purpose hose containing polyvinyl chloride or synthetic fiber. The three-way receivers 32 can be attached to the tubular member 48 using any method, such as an adhesive, depending on the material of the tubular member 48.

[0033] Both ends of the tubular member 48 are closed by caps 50, such as screw-on caps, with a weight 52 connected to the lower cap 50 and a supply pipe 54 connected to the upper cap 50, which passes through the cap 50 and reaches the inside of the tubular member 48. The weight 52 is a weight made of stainless steel, for example, that has enough weight to prevent the vibration receiving unit 30, with air pumped into the tubular member 48, from floating up in the vibration receiving hole 80 when the vibration receiving unit 30 is installed in a vibration receiving hole 80 formed in saturated ground or the like and containing groundwater. The supply pipe 54 is used to pump air into the inside of the tubular member 48 and extends from the compressor 56 shown in FIG. 1.

[0034] Returning to FIG. 1 , the compressor 56 sends air to the tubular member 48 via the supply pipe 54. In addition to the supply pipe 54, FIG. 1 also shows a data line 58 extending from the receiving unit 30. This data line 58 is an image of an integrated line, such as the cable 42 of the three-way receiver 32 shown in FIG. 2, and is connected to the measuring instrument 24 and the analysis device 26. Vibration reception results from the five three-way receivers 32 are transmitted to the measuring instrument 24 and the analysis device 26 via this data line 58. The analysis device 26 analyzes and estimates the structure of the ground G from the vibration reception results from the multiple surface receivers 20 and the receiving unit 30. Most of the processing and calculations required for the ground structure estimation method according to the embodiment of the present invention are performed by the analysis device 26. The analysis device 26 may be any hardware, such as a laptop PC or tablet PC, equipped with any software. Note that FIG. 1 does not show power supplies or other components required for the operation of each device.

[0035] Next, the specific contents of the ground structure estimation method according to the embodiment of the present invention will be described with reference to Fig. 3. Note that for the configuration of the members used in the ground structure estimation method, please refer to Figs. 1 and 2 as appropriate. In Figure 3, multiple underground vibration generating points 62, multiple surface vibration receiving points 70, and multiple underground vibration receiving points 82 set around the ground G to be measured are shown with circles, with the underground vibration generating points 62 shown as black circles, the surface vibration receiving points 70 shown as white circles, and the underground vibration receiving points 82 shown as gray circles. In addition, improved ground IG, which is the part of the ground G that has been improved, is shown colored gray, and for convenience, auxiliary lines extending vertically and horizontally at 1m intervals are also shown on the ground G.

[0036] First, we will explain an example in which vibrations are generated in two vibration generating holes 60 and received at multiple surface vibration receiving points 70 for analysis in order to estimate the ground structure, mainly focusing on the improved ground IG. In this case, multiple surface vibration receiving points 70 are set at different horizontal positions within the area of ​​the ground surface covering directly above the improved ground IG. In the example of Figure 3, these are the multiple surface vibration receiving points 70 set at 1-m intervals between 70A to 70C. Then, multiple surface geophones 20 are installed so that vibrations are received at each of the multiple surface vibration receiving points 70. Furthermore, a first vibration generating hole 60A is set at a different horizontal position from the multiple surface vibration receiving points 70, and multiple underground vibration generating points 62 are set in the vibration generating hole 60A. In the example of Figure 3, nine underground vibration generating points 62 are set at 1-m intervals in the depth direction in the vibration generating hole 60A. Then, vibrations are generated at each of these underground vibration generating points 62 using the vibration generating device 10 described above.

[0037] More specifically, the vibration excitation hole 60A is formed to the depth of the underground vibration excitation point 62 that is set closest to the ground surface (the top) among the multiple underground vibration excitation points 62 set in the vibration excitation hole 60A, and in this state, vibrations are generated at the tip of the vibration excitation hole 60A using the vibration excitation device 10. The vibrations generated at the uppermost underground vibration excitation point 62 of the vibration excitation hole 60A are then received at multiple surface vibration receiving points 70 (in the range of 70A to 70C), and the results are transmitted to the analysis device 26. Next, the vibration excitation hole 60A is formed to the depth of the second underground vibration excitation point 62 from the top of the vibration excitation hole 60A, and in this state, vibrations are generated at the tip of the vibration excitation hole 60A, and the results of receiving the vibrations at the multiple surface vibration receiving points 70 are transmitted to the analysis device 26. Thereafter, the above procedure is repeated in order until vibrations are generated at the lowest underground vibration excitation point 62 set in the vibration excitation hole 60A. In this case, if the vibration generating device 10 used in a dynamic cone penetration test, a static cone penetration test, or a standard penetration test is used as the vibration generating device 10, the penetration test will also be conducted in parallel in the vibration generating hole 60A, and the results will be input into the analysis device 26.

[0038] Next, a second vibration generating hole 60B is provided at a horizontal position different from the vibration generating hole 60A and the plurality of surface vibration receiving points 70, and nine underground vibration generating points 62 are set in the vibration generating hole 60B at 1 m intervals, and vibrations are generated at each of these underground vibration generating points 62. The generated vibrations are then received by the plurality of surface vibration receiving points 70, and the received vibration results are transmitted to the analysis device 26. At this time, the vibration generating hole 60B is formed while gradually decreasing in depth until vibrations are generated at all of the underground vibration generating points 62 set in the vibration generating hole 60B, and vibrations are generated at the tip of the vibration generating hole 60B, and a penetration test is performed in parallel in the vibration generating hole 60B, in the same manner as in the case of the vibration generating hole 60A. In addition, Figure 3 shows, with dashed lines, a wavy image of vibrations being transmitted in a straight line from the top underground vibration excitation point 62 of the vibration excitation hole 60A and the top underground vibration excitation point 62 of the vibration excitation hole 60B to multiple surface vibration receiving points 70 and multiple underground vibration receiving points 82.

[0039] Next, using the analysis device 26, received data on S waves is extracted from the results of receiving the vibrations generated in the two vibration bores 60A and 60B as described above using the surface geophones 20 installed at the multiple surface receiving points 70. Then, based on the received S wave data, the S wave velocity structure of the ground G is estimated using elastic wave tomography. At this time, if a penetration test has been conducted in the vibration bores 60, the test results are also taken into account to estimate the structure of the ground G. Estimation of the structure of the ground G using elastic wave tomography is similar to conventionally known methods except for the fact that only the S waves of the vibrations are used, so a detailed explanation will be omitted here.

[0040] Here, as in the example of Figure 3, when improved ground IG is present near the surface, the area of ​​the improved ground IG is harder than other areas of the ground G, so the vibrations generated at the underground vibration generating point 62 do not propagate linearly, but instead bypass the soft areas of the ground G and propagate through the improved ground IG. Specifically, the vibrations generated at each underground vibration generating point 62 first propagate approximately vertically upward, then refract at the boundary between the other areas and the improved ground IG, and from there propagate approximately linearly through the improved ground IG to each surface vibration receiving point 70. For this reason, when vibrations are received only at multiple surface vibration receiving points 70, there are no vibration wavy lines that pass through the lower left or lower right ends of the improved ground IG in Figure 3, and there is a risk that the ground structure at such ends cannot be estimated.

[0041] Therefore, to estimate the structure of the end of the improved ground IG, the following measurements were performed as a method of receiving vibrations through the end of the improved ground IG. Here, a method for supplementing the structure of the lower right end of the improved ground IG in Figure 3 is described as an example. However, to supplement the structure of the lower left end of the improved ground IG, the following method can be performed by reinterpreting the left and right terms. First, multiple surface receiving points 70 were set in a wide area to the right of the improved ground IG in the figure, extending to the area indicated by reference numeral 70D in the example of Figure 3. Furthermore, a receiving hole 80 was set near the right end of the improved ground IG in Figure 3, approximately 5 m to the right of the improved ground IG in the example of Figure 3, and multiple underground receiving points 82 were set within the receiving hole 80. In this embodiment, nine underground receiving points 82 were set at depths that differed by 1 m from each other.

[0042] The receiving unit 30 is then placed in the receiving borehole 80 so that the positions of the three-directional receivers 32 coincide with the depths of the underground receiving points 82. At this time, the receiving unit 30, which is equipped with five three-directional receivers 32, is placed with the intention of measuring all nine underground receiving points 82 in the receiving borehole 80 in two separate runs. Air is then sent from the compressor 56 to the tubular member 48 via the supply pipe 54, causing the tubular member 48 to expand in the radial direction, and the five three-directional receivers 32 are brought into close contact with the borehole wall at positions corresponding to the underground receiving points 82. Once this preparation is complete, vibrations are sequentially generated at the multiple underground excitation points 62 in the right-hand excitation borehole 60B in Figure 3 out of the two excitation bores 60.

[0043] The vibrations generated in the vibration excitation hole 60B as described above are received by multiple surface receiving points 70 (in the range of symbols 70B-70D) located to the right of the vibration excitation hole 60B in FIG. 3 , and also by multiple underground receiving points 82 within the vibration receiving hole 80. As a result, at least a portion of the vibration wave lines from the multiple underground vibration excitation points 62 to the multiple surface receiving points 70 and the vibration wave lines from the multiple underground vibration excitation points 62 to the multiple underground receiving points 82 propagate through the lower right edge of the improved ground IG in FIG. 3 . Therefore, the S-waves of the vibrations received in this manner are used to estimate the structure using elastic wave tomography, and the structure of the lower right edge of the improved ground IG is complemented. Note that in the example of FIG. 3 , the positional relationships between the vibration excitation hole 60, the surface receiving points 70, and the vibration receiving hole 80 are shown only in the left-right direction in the figure. However, they may also exist in the depth direction perpendicular to the plane of the paper in FIG. 3 .

[0044] The configuration of the ground structure estimation method and the components used therein according to the embodiment of the present invention is not limited to the configurations shown in FIGS. 1 to 3 and may be other configurations. For example, unlike the example shown in FIG. 3, the structure of ground G may be estimated in the absence of improved ground IG. Furthermore, the number of excitation holes 60 and the number of receiving holes 80 may be more or less than those shown in FIG. 3. The number of underground excitation points 62 in each excitation hole 60, the number of surface receiving points 70, and the number of underground receiving points 82 in each receiving hole 80 may also be more or less than those shown in FIG. 3. Furthermore, the number of three-directional receivers 32 included in the receiving unit 30 is not limited to five, but may be four or less, or six or more. Furthermore, the installation intervals of the three-directional receivers 32, the intervals between the underground receiving points 82, and the intervals between the surface receiving points 70 are not limited to 1 meter and may be other distances. Furthermore, the test conducted in parallel in the vibration hole 60 may be a test other than the dynamic cone penetration test, static cone penetration test, or standard penetration test, as long as it is related to the estimation of the ground structure. Furthermore, the vibration generator 10 is not limited to a combination of a striking means 12 such as a hammer and a long struck object 14 such as a rod, and may be one that generates vibrations by any means, as long as it applies an impact to the ground G at the relevant depth to generate vibrations.

[0045] According to the embodiment of the present invention configured as described above, the following effects can be obtained. That is, the ground structure estimation method according to the embodiment of the present invention estimates the ground structure using elastic wave tomography, in which S waves of vibrations generated at a vibration generating point are received at a vibration receiving point, and estimation is performed using the S waves of the received vibrations. As shown in Figures 1 and 3, the vibrations to be received are generated by setting multiple underground vibration generating points 62 at different depths in a vibration generating hole 60 provided underground in the ground G, and sequentially using the vibration generating device 10 at these multiple underground vibration generating points 62.

[0046] The S-waves of the vibrations generated at each of the plurality of underground vibration generating points 62 as described above are received at at least one of a plurality of surface vibration receiving points 70 and a plurality of underground vibration receiving points 82, which are set within the range of the ground G to be measured. The plurality of surface vibration receiving points 70 are set at horizontal positions that are different from each other and also different from the vibration generating holes 60. The plurality of underground vibration receiving points 82 are set at different depths within the vibration receiving holes 80, which are set at horizontal positions that are different from the vibration generating holes 60. The S-waves of the vibrations received at at least one of the plurality of surface vibration receiving points 70 and the plurality of underground vibration receiving points 82 set in this way are then used by an analysis device 26 or the like to estimate the S-wave velocity structure of the ground G using elastic wave tomography.

[0047] As a result, when vibrations are received by the surface geophone 20 only at the multiple surface receiving points 70, the structure of the ground G located between the multiple underground vibration generating points 62 and the multiple surface receiving points 70 is estimated, and when vibrations are received only at the multiple underground receiving points 82, the structure of the ground G located between the multiple underground vibration generating points 62 and the multiple underground receiving points 82 is estimated. Furthermore, when vibrations are received at both the multiple surface receiving points 70 and the multiple underground receiving points 82, the structure of the ground G located between the multiple underground vibration generating points 62 and the multiple surface receiving points 70 and the structure of the ground G located between the multiple underground vibration generating points 62 and the multiple surface receiving points 70 and the multiple underground vibration receiving points 82 are estimated.

[0048] In either case, the structure of the ground G can be efficiently estimated in two dimensions or three dimensions. In particular, when receiving vibrations at both the surface receiving point 70 and the underground receiving point 82, missing data (such as the data at the lower right end of the improved ground IG in Figure 3) can be supplemented when vibrations propagate around the soft ground G, further improving estimation accuracy. Furthermore, since the elastic wave tomography method estimates the S-wave velocity structure of the ground G using the S-waves of vibrations rather than the P-waves of vibrations, the structure of the ground G can be estimated without any problems even in saturated ground such as ground G deep below groundwater or ground G on the seabed. Therefore, it is possible to efficiently estimate the structure of the ground G, such as its hardness or softness, regardless of the condition of the ground G.

[0049] Furthermore, in the ground structure estimation method according to the embodiment of the present invention, when S-waves of vibrations are received at a plurality of surface vibration receiving points 70, at least two vibration excitation holes 60 (60A, 60B) are provided at positions different from each other in horizontal position. A plurality of underground vibration excitation points 62 are set in each of the at least two vibration excitation holes 60, and vibrations are generated at each of the plurality of underground vibration excitation points 62. That is, for example, when two vibration excitation holes 60A, 60B are provided as shown in FIG. 3 , vibrations are generated sequentially at the plurality of underground vibration excitation points 62 in one vibration excitation hole 60A by the vibration excitation device 10, and the S-waves of the vibrations are received at the plurality of surface vibration receiving points 70. Furthermore, at a separate timing, vibrations are generated sequentially at the plurality of underground vibration excitation points 62 in the other vibration excitation hole 60B by the vibration excitation device 10, and the S-waves of the vibrations are received at the plurality of surface vibration receiving points 70.

[0050] Here, among the multiple surface receiving points 70, for example, at the surface receiving point 70B located near the vibration generating hole 60B, due to the close distance from the underground vibration generating point 62 of the vibration generating hole 60B, P waves of the received vibrations may be dominant, making it difficult to distinguish them from S waves. However, because such surface receiving point 70B is located far from the other vibration generating hole 60A, when vibrations generated at the underground vibration generating point 62 of the vibration generating hole 60A are received, it is easy to distinguish between P waves and S waves. In this way, by generating vibrations in at least two vibration generating holes 60, it is easy to distinguish between P waves and S waves associated with vibrations from at least one vibration generating hole 60 at all of the multiple surface receiving points 70. In other words, it is possible to complement information on the ground G, for example, directly above the vibration generating hole 60. This allows for more accurate estimation of ground structure using elastic wave tomography using S waves, thereby improving the reliability of the estimation results.

[0051] Furthermore, the ground structure estimation method according to the embodiment of the present invention receives vibrations at a plurality of underground receiving points 82 using a receiving unit 30 including a three-directional receiver 32 and a tubular member 48 as shown in Fig. 2. That is, as shown in Fig. 2(a), a three-directional receiver 32 is fabricated by combining three vibration sensors 34 so as to measure vibrations in three mutually orthogonal directions, and a plurality of such three-directional receivers 32 are prepared. Then, as shown in Fig. 2(b), these plurality of three-directional receivers 32 are attached to the outer periphery of a tubular member 48 at intervals in the longitudinal direction of the tubular member 48, and the spacing between the plurality of three-directional receivers 32 is set to match the spacing between the plurality of underground receiving points 82.

[0052] Furthermore, as shown in FIG. 1, the above-described receiving unit 30 is placed in a receiving borehole 80, and the tubular member 48 is expanded within the receiving borehole 80, thereby bringing the multiple three-directional receivers 32 into close contact with the borehole wall. At this time, the multiple three-directional receivers 32 are positioned to fit closely to one of multiple underground receiving points 82 set in the receiving borehole 80, as shown in FIG. 3. The multiple three-directional receivers 32 installed in this manner receive S-waves of vibrations generated at multiple underground vibration generating points 62. This allows simultaneous reception at multiple underground receiving points 82 where multiple three-directional receivers 32 are located, thereby reducing the labor and time required for the work and enabling more efficient estimation of the ground structure. Furthermore, because each three-directional receiver 32 is configured to measure vibrations in three mutually orthogonal directions, S-waves of vibrations can be measured without any problems at each underground receiving point 82.

[0053] In addition, in the ground structure estimation method according to the embodiment of the present invention, as shown in FIG. 3, multiple underground receiving points 82 are set in a receiving borehole 80 at depths of 1 meter each within the receiving borehole 80. Accordingly, the installation intervals of the multiple three-directional receivers 32 attached to the outer periphery of the tubular member 48 are set to 1 meter, and the number of three-directional receivers 32 attached to the tubular member 48 is set to five or less (five in the example of FIG. 2). This allows the length of the tubular member 48 to be approximately 5 meters, so that even when the tubular member 48 is expanded within the receiving borehole 80 into which groundwater or seawater has flowed, the water pressure difference between the top and bottom of the tubular member 48 can be suppressed. In other words, the pressure imbalance within the tubular member 48 due to the water pressure difference, which is a concern when the tubular member 48 is too long, can be suppressed. Therefore, the expanded tubular member 48 can easily fit five or fewer three-directional receivers 32 to the borehole wall, regardless of the installation position of the tubular member 48 in the longitudinal direction. Furthermore, by setting multiple underground receiving points 82 at depths of 1 m each, vibrations are received at depths of 1 m each and the data is used to estimate the ground structure, making it possible to estimate the structure of the ground G at appropriate intervals.

[0054] Furthermore, in the ground structure estimation method according to the embodiment of the present invention, when generating vibrations at a plurality of underground vibration excitation points 62 in the vibration excitation hole 60, an instrument or device used in any one of the dynamic cone penetration test, static cone penetration test, and standard penetration test is used as the vibration excitation device 10 to generate vibrations, and the test is further performed in parallel. In other words, any one of the three penetration tests is performed in the vibration excitation hole 60, and the vibrations generated in that test are received at a plurality of surface vibration receiving points 70 and a plurality of underground vibration receiving points 82.

[0055] That is, in the case of a dynamic cone penetration test or a static cone penetration test, a rod 14 with a cone attached to its tip and a hammer 12 that strikes the rear end of the rod 14 are used as vibration exciters 10 to generate vibrations. In the case of a standard penetration test, a boring rod 14 with a sampler attached to its tip and a drive hammer 12 that strikes the rear end of the boring rod 14 are used as vibration exciters 10 to generate vibrations. In this way, costs can be reduced by using the vibration exciters 10 for each penetration test to generate vibrations without using a dedicated vibration exciter 10. Furthermore, by conducting one of the penetration tests in parallel and using the test results to estimate the structure of the ground G, the accuracy of the estimation of the ground structure can be improved.

[0056] On the other hand, the receiving unit 30 according to the embodiment of the present invention receives S-waves of vibrations generated at a vibration source at a plurality of underground receiving points 82 at different depths within an underground receiving hole 80 provided in the ground G in order to obtain data used for estimating the ground structure. Specifically, as shown in FIG. 2 , the receiving unit 30 includes a three-directional receiver 32 and a tubular member 48. The three-directional receiver 32 is formed by combining three vibration sensors 34 so as to measure vibrations in three mutually orthogonal directions. The tubular member 48 is expandable in the radial direction, and the plurality of three-directional receivers 32 are attached to its outer periphery. In this case, the plurality of three-directional receivers 32 are attached at intervals along the longitudinal direction of the tubular member 48, matching the arrangement intervals of the plurality of underground receiving points 82 set in the receiving hole 80.

[0057] With this configuration, when acquiring data, the receiving unit 30 is placed in the receiving hole 80 with the positions of the multiple three-directional receivers 32 aligned with the depths of the multiple underground receiving points 82. Then, when air is pumped into the tubular member 48 in this state, the tubular member 48 expands in the radial direction, causing the multiple three-directional receivers 32 to adhere tightly to the wall of the receiving hole 80. This allows the multiple underground receiving points 82 where the multiple three-directional receivers 32 are located to simultaneously measure S-waves of vibrations generated at the excitation point, thereby reducing the labor and time required for the work. Moreover, because each of the multiple three-directional receivers 32 is configured to measure vibrations in three mutually orthogonal directions, S-waves of vibrations can be measured without any problems at each of the underground receiving points 82.

[0058] Furthermore, the receiving unit 30 according to the embodiment of the present invention has five or less (five in the example of FIG. 2) three-way receivers 32 attached at intervals of 1 m around the outer periphery of a tubular member 48. That is, multiple underground receiving points 82 are set at depths of 1 m each within the receiving hole 80, and the three-way receivers 32 are attached at these set intervals. Furthermore, by limiting the number of three-way receivers 32 to five or less, the length of the tubular member 48 can be kept to around 5 m, thereby suppressing pressure imbalance between both ends within the tubular member 48 due to water pressure differences.

[0059] In other words, assuming that the receiving unit 30 is installed in saturated ground, such as ground G deeper than groundwater or the seabed, the tubular member 48 will expand in the groundwater or seawater in the receiving borehole 80. The influence of external water pressure can be suppressed so that there is not much difference between both ends (between the upper and lower ends) of the tubular member 48 because the length of the tubular member 48 is approximately 5 m, allowing the entire tubular member 48 to expand without any problems. This allows all five or fewer three-directional receivers 32 to be in close contact with the borehole wall, enabling accurate reception of vibration S-waves. Furthermore, because S-waves are measured at multiple underground receiving points 82 set every 1 m deep, it is possible to estimate the structure of the ground G at appropriate intervals. [Explanation of symbols]

[0060] 10: vibration generator, 30: receiving unit, 32: three-way receiver, 34: vibration sensor, 48: tubular member, 60 (60A, 60B): vibration generating hole, 62: underground vibration generating point, 70 (70A, 70B, 70C, 70D): ground surface receiving point, 80: receiving hole, 82: underground receiving point, G: ground

Claims

1. A method for estimating a ground structure, comprising: Vibrations are sequentially generated by a vibration generator at a plurality of underground vibration points at different depths within an underground vibration generating hole; The S-waves of the vibrations generated at each of the plurality of underground vibration generating points are received at at least one of a plurality of ground surface vibration receiving points whose horizontal positions are different from each other and from the vibration generating hole, and a plurality of underground vibration receiving points whose horizontal positions are different from each other and whose depths are different from each other within the underground vibration receiving hole whose horizontal positions are different from the vibration generating hole, Based on the received S-waves, the S-wave velocity structure of the ground is estimated using elastic wave tomography. A method for estimating ground structure, characterized in that multiple three-way geophones, each consisting of three vibration sensors that measure vibrations in three mutually perpendicular directions, are attached to the outer periphery of a tubular member at intervals along the longitudinal direction of the tubular member, and the spacing between the multiple three-way geophones is adjusted to match the spacing between the multiple underground receiving points, and the tubular member is expanded inside the receiving hole to tightly press the multiple three-way geophones against the hole wall, thereby simultaneously receiving vibrations at the multiple underground receiving points where multiple three-way geophones are located.

2. A method for estimating ground structure according to claim 1, characterized in that when S-waves of vibrations are received at the plurality of ground surface receiving points, at least two of the vibration excitation holes are provided at positions different from each other in horizontal position, and vibrations are generated at the plurality of underground vibration excitation points of each of the at least two vibration excitation holes.

3. The plurality of underground receiving points are set at depths of 1 m each in the receiving hole, 3. The method for estimating ground structure according to claim 1, wherein five or less three-directional receivers are attached to the tubular member at intervals of 1 m.

4. A ground structure estimation method according to any one of claims 1 to 3, characterized in that the excitation device is an instrument or device used in any one of a dynamic cone penetration test, a static cone penetration test, and a standard penetration test, and the one test is conducted in parallel.

5. A receiving unit that receives S waves of vibrations generated at a vibration source at a plurality of underground receiving points at different depths in an underground receiving hole in order to estimate a ground structure, a three-way receiver in which three vibration sensors are combined to measure vibrations in three mutually orthogonal directions; a tubular member that is expandable in a radial direction, A receiving unit characterized in that multiple three-way receivers are attached to the outer periphery of the tubular member along the longitudinal direction of the tubular member, spaced apart to match the spacing of the multiple underground receiving points.

6. 6. The receiving unit according to claim 5, wherein five or less three-directional receivers are attached to the tubular member at intervals of 1 m.

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