Ground survey system and ground survey method

Abstract

A ground survey system (1) includes: at least one ground survey device (10) disposed in the interior of an underground space (41); and an information processing device (20). The ground survey device (10) includes at least one first detector (11) and at least one second detector (12) arranged at a distance from each other to detect cosmic ray muons. The information processing device (20) comprises a control unit (22). The control unit (22) counts the number of muons arriving from the direction connecting the first detector and the second detector by having the first detector (11) and the second detector (12) detect muons substantially simultaneously. The ground survey system (1) detects muons with the ground survey device (10) disposed at a plurality of positions in the underground space (41) simultaneously or in succession, which is achieved by disposing a plurality of the ground survey device (10) and / or by taking measurements while moving the ground survey device (10).

Description

Ground exploration system and ground exploration method

[0001] The present disclosure relates to a ground exploration system and a ground exploration method.

[0002] Cosmic ray muons are a type of secondary cosmic ray generated when cosmic rays collide with the atmosphere, and are high-energy elementary particles. Muons can easily reach the earth's surface and are characterized by attenuation as they pass through the ground. Because their attenuation pattern differs depending on the density of the ground, they can be used for ground exploration (see, for example, Non-Patent Document 1). Ground exploration using cosmic ray muons involves counting muons simultaneously measured by multiple scintillation detectors and measuring any desired position. This measurement widely explores cavities and the like within the ground, allowing for the measurement of muons passing through the detected ground over a wide area.

[0003] Keiichi Suzuki, "Possibilities of civil engineering geophysical exploration using muons", Geophysical Exploration, Vol. 65, No. 4 (2012), pp. 251-259

[0004] However, when exploring a wide area of ​​the ground, the range through which muons pass through the ground becomes wider, which can make it difficult to identify the area where cavities or the like exist. For example, the three-dimensional ground exploration system using muons described in Non-Patent Document 1 is a system that is installed inside an underground structure or the like and measures muons that pass through the ground over a wide area. This device is good at exploring ground over a wide area, but in order to use this device to measure only a specific location in the ground, the number of measurements must be increased. As a result, the time required for measurement also becomes long.

[0005] Therefore, the purpose of the present disclosure, which has been made with these points in mind, is to improve the technology for exploring specific locations within the ground.

[0006] In one embodiment, the ground exploration system includes one or more ground exploration devices arranged inside an underground space and an information processing device, wherein the ground exploration device includes at least one first detector and at least one second detector arranged at a distance from each other to detect cosmic ray muons, and the information processing device is equipped with a control unit, which counts the number of muons arriving from a direction connecting the first detector and the second detector by having the first detector and the second detector detect muons substantially simultaneously, and determines the extent of the existence of cavities in the ground or density change regions having a density different from that of surrounding regions based on the direction and the detection frequency, which is the number of muons counted per unit time.In this ground exploration system, the ground exploration devices are arranged at multiple positions in the underground space simultaneously or sequentially to detect muons by at least one of placing multiple ground exploration devices and taking measurements while moving the ground exploration devices.

[0007] In one embodiment, a ground exploration method includes: acquiring muon detection information from one or more ground exploration devices arranged inside an underground space, the ground exploration devices including at least one first detector and at least one second detector arranged at a distance from each other to detect cosmic ray muons; counting the number of muons arriving from a direction connecting the first detector and the second detector based on the muon detection information, by having the first detector and the second detector detect muons substantially simultaneously; and determining the extent of the existence of a cavity in the ground or a density change region having a density different from that of surrounding regions, based on the direction and the detection frequency, which is the number of muons counted per unit time. In this ground exploration method, the ground exploration devices are arranged simultaneously or sequentially at multiple positions in the underground space to detect muons by at least one of: placing a plurality of the ground exploration devices; and performing measurements while moving the ground exploration devices.

[0008] According to the present disclosure, the ground exploration device is placed simultaneously or sequentially at multiple locations in an underground space to detect muons, thereby improving the technology for exploring specific locations within the ground.

[0009] 1 is a block diagram showing a schematic configuration of a ground exploration system according to an embodiment. FIG. 1 is a schematic configuration diagram of the ground exploration device of FIG. 1. FIG. 2 is a perspective view showing an example of an underground structure in which the ground exploration device is installed. FIG. 3 is a diagram explaining the range of detection directions of detectors of the ground exploration device. FIG. 4 is a diagram explaining an example of a state in which one ground exploration device is installed in an underground space to perform wide-area exploration. FIG. 5 is a diagram explaining a method of performing exploration by arranging multiple ground exploration devices inside an underground space. FIG. 6 is a diagram explaining an example of a method for reducing the range of solid angles of the detection directions of the ground exploration device. FIG. 7 is a diagram explaining an exploration method in which exploration is performed by moving a ground exploration device installed inside an underground space. FIG. 8 is a flowchart explaining the procedure for performing exploration by moving the ground exploration device. FIG. 9 is a diagram explaining an example of a state in which a ground exploration device is installed in an underground space to perform exploration by limiting the range. FIG. 10 is a flowchart explaining the procedure for exploring the ground in two stages using a ground exploration device. FIG. 11 is a diagram explaining a state in which a movable shield is installed on the ground and muons are measured. FIG. 12 is a flowchart explaining the procedure for identifying the position of the ground exploration device using a movable shield. FIG. 13 is a diagram explaining a method of measuring the density of a density change region in the ground by placing an object with a known density on the ground. 10 is a flowchart illustrating a procedure for measuring the density of a density change region using an object whose density is known.

[0010] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The drawings used in the following description are schematic. The dimensional ratios and the like in the drawings do not necessarily correspond to the actual ones.

[0011] A ground exploration system 1 according to one embodiment of the present disclosure is a system that explores the internal structure of the ground by detecting cosmic ray muons, which are generated when primary cosmic rays collide with the Earth's atmosphere, using a ground exploration device disposed in an underground space. Hereinafter, cosmic ray muons will be simply referred to as muons. As shown in FIG. 1 , the ground exploration system 1 includes one or more ground exploration devices 10 and an information processing device 20. In one embodiment, the ground exploration system 1 includes a plurality of ground exploration devices 10. In another embodiment, the ground exploration system 1 includes one or more ground exploration devices 10 that are configured to be mobile.

[0012] (Configuration of the Ground Exploration Device) The ground exploration device 10 is a device that is placed inside an underground space and detects muons that have passed through the ground. The ground exploration device 10 includes one or more first detectors 11 and one or more second detectors 12, which are sensors that detect muons. The ground exploration device 10 may further include a drive mechanism 13 that moves the ground exploration device 10 within the underground space.

[0013] In one embodiment, as shown in Fig. 2, one ground exploration device 10 includes one first detector 11 and one or more second detectors 12a to 12e. Hereinafter, the one or more second detectors 12a to 12e may be collectively referred to as a second detector 12. The first detector 11 and the second detector 12 are, for example, scintillation detectors. A scintillation detector includes a scintillator, which is a material that emits light when charged particles pass through it, and a photodetector that can detect the emission of light from the scintillator. A scintillation detector can detect the passage of muons.

[0014] By detecting muons that pass through the first detector 11 and the second detector 12 at approximately the same time, the direction of arrival of the muons can be detected. The arrows in Figure 2 show an example of the direction of arrival of muons. Since muons travel at a speed close to the speed of light, "approximately simultaneously" means that there is a time difference of approximately the time it takes for light to travel the distance between the first detector 11 and the second detector 12. The direction of arrival of the muons is a direction that crosses both the first detector 11 and the second detector 12. A configuration similar to the ground exploration device 10 having multiple second detectors 12 as shown in Figure 2 is also adopted in Non-Patent Document 1.

[0015] Fig. 2 shows only one example of the configuration of the ground exploration device 10. The ground exploration device 10 may be configured with one first detector 11 and one second detector 12. The ground exploration device 10 may also have multiple first detectors 11. In Fig. 2, the first detector 11 and multiple second detectors 12 are arranged on a single plane along the paper surface. However, the first detector 11 and the second detectors 12 may also be arranged three-dimensionally.

[0016] In one embodiment, the first detector 11 is fixed to an overall frame 14 that forms the framework of the entire ground exploration device 10. The multiple second detectors 12a to 12e are arranged on a rotating frame 15 that is rotatable about a horizontal axis (perpendicular to the plane of FIG. 2 ) that passes through the first detector 11, with each detector being angularly offset from the first detector 11. For example, the second detectors 12a to 12e may be arranged offset from one another by 10°, 15°, or 20°. Furthermore, the rotating frame 15 may be rotatable in increments of an angle that is smaller than the angular difference between adjacent second detectors 12. The above-described configuration for rotating the second detectors 12a to 12e is not essential.

[0017] The drive mechanism 13 can move the entire ground exploration device 10 by any method. The drive mechanism 13 includes, for example, tires and a motor that drives the tires. Alternatively, for example, the drive mechanism 13 may include rails and wheels and be configured to move the ground exploration device 10 along the rails. The drive mechanism 13 can move the ground exploration device 10 in one axial direction, for example, the left and right direction in FIG. 2 . The drive mechanism 13 may also be able to move the ground exploration device 10 in two axial directions.

[0018] The soil exploration device 10 may be placed in an underground space 41 in an existing underground structure 40 as shown in Fig. 3. The underground structure 40 may include, for example, underground tunnels such as subways and roads, deep underground facilities, and mine tunnels.

[0019] (Configuration of Information Processing Device) The information processing device 20 is a PC (Personal Computer), a workstation, or other general-purpose or dedicated computer. As shown in FIG. 1 , the information processing device 20 includes a signal transmission / reception unit 21, a control unit 22, an input / output unit 23, and a storage unit 24. Although FIG. 1 shows the information processing device 20 as a single device, the information processing device 20 is not limited to a single device, and the functions of the information processing device 20 may be shared among multiple devices. For example, the information processing device 20 may include a portion disposed inside the underground space 41 near the ground exploration device 10 and a portion disposed above ground. When the functions of the information processing device 20 are distributed among multiple devices, each device may be capable of communicating with the other devices via any communication means.

[0020] The signal transmitter / receiver 21 includes an interface for transmitting and receiving signals to and from the ground exploration device 10 and / or the movable shield 30 (described later). The signal transmitter / receiver 21 includes a communication module compatible with wired communication and wireless communication. The signal transmitter / receiver 21 may include a detection signal acquirer 21 a, a drive signal transmitter 21 b, and a position information acquirer 21 c.

[0021] The detection signal acquirer 21a is configured to acquire detection information from the ground exploration device 10 indicating that muons have been detected by the first detector 11 and the second detector 12. The drive signal transmitter 21b is configured to transmit to the ground exploration device 10 a drive signal for driving the drive mechanism 13 of the ground exploration device 10. Furthermore, in the case where the ground exploration device 10 has a rotating frame 15, the drive signal transmitter 21b may be configured to transmit to the ground exploration device 10 a drive signal for rotating the rotating frame 15. The position information acquirer 21c is configured to acquire position information of the movable shield 30 when a movable shield 30 described below is used. The position information acquirer 21c is not an essential component.

[0022] The control unit 22 includes one or more processors. In one embodiment, the "processor" may be, but is not limited to, a general-purpose processor or a dedicated processor specialized for a specific process. The processor may be, for example, a central processing unit (CPU), a digital signal processor (DSP), or an application specific integrated circuit (ASIC). The control unit 22 may manage the overall operation of the ground exploration system 1. The control unit 22 may execute processing in accordance with a program stored in the memory unit 24.

[0023] The control unit 22 may include the following components: a counting unit 22a, a determining unit 22b, a drive control unit 22c, and a position identifying unit 22d. Each component of the control unit 22 may be a hardware module or a software module. The processing performed by each component can be executed by the control unit 22. Hereinafter, the processing performed by each component may be described as being executed by the control unit 22.

[0024] The counting unit 22a acquires muon detection information from the first detector 11 and the second detector 12 via the detection signal acquiring unit 21a. The counting unit 22a is configured to count the number of times that the first detector 11 and the second detector 12 detect muons substantially simultaneously for each combination of the first detector 11 and the second detector 12.

[0025] The direction connecting the scintillator of the first detector 11 and the scintillator of the second detector 12 is the arrival direction of the muons detected by the pair of the first detector 11 and the second detector 12. The determination unit 22b is configured to determine the range of the cavity 53 in the ground or the density change region 54 (see FIG. 5 ) whose density differs from that of the surrounding region, based on the arrival direction of the muons and the detection frequency of the muons counted by the counting unit 22a.

[0026] The drive control unit 22c is configured to control the position of the ground exploration device 10 by transmitting a drive signal to the ground exploration device 10 via the drive signal transmission unit 21b to operate the drive mechanism 13. Furthermore, if the ground exploration device 10 has a rotating frame 15, the drive control unit 22c is configured to control the angle of the rotating frame 15.

[0027] The position identifying unit 22d is used to identify the position of the ground exploration device that uses the movable shield 30. When the movable shield 30 is placed on the ground, the position identifying unit 22d acquires position information of the movable shield 30 via the position information acquiring unit 21c. The position identifying unit 22 identifies the position of the ground exploration device 10 based on the position information of the movable shield 30 and relative position information of the movable shield 30 calculated from the detection frequency of muons detected by the ground exploration device 10.

[0028] The input / output unit 23 may include an input device through which a user of the ground exploration system 1 inputs instructions and information, and a display on which the ground exploration system 1 displays the exploration results. The input device may include a keyboard, a mouse, etc. The display may be, for example, a liquid crystal display (LCD), an organic electroluminescence (EL) display, or an inorganic EL display. The input / output unit 23 may include a touch panel that detects input by touching the surface of the display.

[0029] The storage unit 24 may be configured to include, for example, one or more of a semiconductor memory, a magnetic memory, an optical memory, etc. The semiconductor memory may include a volatile memory and a non-volatile memory. The magnetic memory may include, for example, a hard disk. The optical memory may include, for example, a CD (Compact Disc), a DVD (Digital Versatile Disc), and a BD (Blu-ray (registered trademark) Disc). The storage unit 24 may function as, for example, a main storage device, an auxiliary storage device, or a cache memory. The storage unit 24 may store information acquired by the information processing device 20, information processed by the information processing device 20, and information for operating the information processing device 20. The storage unit 24 may store a program for operating the control unit 22.

[0030] The range of directions in which muons arrive that can be detected by a pair of the first detector 11 and the second detector 12 of the ground exploration device 10 will be described with reference to Fig. 4. Fig. 4 shows the scintillator 111 of the first detector 11 and the scintillator 112 of the second detector 12 as having spheroidal shapes of equal size. In Fig. 4, a is the diameter of the scintillators 111 and 112, and d is the distance between the scintillators 111 and 112. Because the detected muons pass through both the first detector 11 and the second detector 12, the solid angle of the direction in which the muons arrive that is detected by the pair of the first detector 11 and the second detector 12 is expressed by the following equation (1):

[0031] Ω=2π[1-cos{tan －１ (a / 2d)}] (1)

[0032] (Basic Ground Exploration Method (Conventional Exploration Method)) As shown in FIG. 5 , one ground exploration device 10 is placed in an underground space 41 inside an underground structure 40 and explores the ground 50 above the underground structure 40. The ground exploration device 10 detects muon electrons arriving from detection ranges 52a to 52e corresponding to the orientation of each of the second detectors 12a to 12e relative to the first detector 11 by combining a first detector 11 with each of the second detectors 12a to 12e. Note that FIG. 5 does not reflect an actual scale. For example, the height of the ground exploration device 10 may be several tens of centimeters to several meters, and the thickness of the ground 50, in other words, the depth from the ground surface of the location where the ground exploration device 10 is placed, may be several meters to several hundred meters. Hereinafter, the detection ranges 52a to 52e may be collectively referred to as detection range 52.

[0033] Muons generated by cosmic rays approximately 10 km above the Earth's surface pass through the atmosphere at a speed close to the speed of light with almost no attenuation, reaching the Earth's surface 51. The number of muons that rain down on the Earth's surface 51 per unit time, unit area, and unit solid angle is known. When muons enter the Earth from the Earth's surface 51, they interact with surrounding atoms and decay into electrons and neutrinos, reducing their number. The attenuation rate of muons depends on the density of the material through which the muons pass and the distance they pass. Therefore, the attenuation rate per length in the direction of muon passage is related to the density of the material along the muon's passage path.

[0034] For example, suppose that there is a cavity 53 inside the ground 50, or a region where the density is different from the surrounding area (referred to as a "density change region 54") and the density is lower than the surrounding area. The region where the density is lower than the surrounding area includes, for example, a region where the ground has become loose due to an earthquake or the infiltration of rainwater. The control unit 22 of the information processing device 20 can detect that the average density of the ground in the detection ranges 52d and 52e is lower based on the detection frequency of muons detected by the combination of the first detector 11 and the second detectors 12d and 12e. Note that the density change region 54 may also be a region where the density is higher than the surrounding material. For example, a high-density structure buried underground is detected as the density change region 54.

[0035] (Method of exploration using multiple ground exploration devices) The above method can explore the presence of cavities 53 or density change regions 54 within a wide area. However, this method alone leaves room for improvement in terms of identifying the precise location of the cavities 53 or density change regions 54. Therefore, in one embodiment of the ground exploration system 1 disclosed herein, as shown in FIG. 6 , ground exploration devices 10 are placed at multiple locations in the underground space 41 to simultaneously detect muons. By using this method, it is possible to simultaneously explore a wider area of ​​the ground.

[0036] For example, multiple ground exploration devices 10a-10c are disposed in the underground space 41. The "multiple" is not limited to three as shown, but may be two, four, or more. The detection range 52 within the ground 50 in which each of the ground exploration devices 10a-10c detects muons may be set narrower than the entire detection range 52a-52e of muons detected by the ground exploration device 10 in FIG. 5. For example, each of the ground exploration devices 10a-10c may perform measurements using only one first detector 11 and one second detector 12 (e.g., second detector 12c). Alternatively, in FIG. 6, the detection range 52 in which muons can be detected by each pair of the first detector 11 and second detector 12 of the multiple ground exploration devices 10a-10c may be set narrower than the example shown in FIG. 5.

[0037] Methods for narrowing the measurable detection range 52 include reducing the size of the scintillator 111 or 112 of both or either one of the first detector 11 and the second detector 12 of the ground exploration device 10. For example, as shown in FIG. 7 , by reducing the size b of the scintillator 112 of the second detector 12, the solid angle Ω of muons that the ground exploration device 10 can detect becomes smaller. The ground exploration device 10 may be configured so that the scintillators 112 of the first detector 11 and / or the second detector 12 can be replaced with scintillators of different sizes. This makes it possible to identify the position of the cavity 53 or density change region 54 detected by the ground exploration device 10 in more detail.

[0038] Alternatively, different from the above, the ground exploration device 10 may be configured to be able to change the distance d between the first detector 11 and the second detector 12. Increasing the distance d between the first detector 11 and the second detector 12 reduces the solid angle Ω of muons that can be detected by the ground exploration device 10. The control unit 22 of the information processing device 20 may be able to control the replacement of the scintillators 111, 112 of the ground exploration device 10 by mechanical means and / or the change of the distance d between the first detector 11 and the second detector 12.

[0039] As described above, the ground exploration system 1 shown in Fig. 6 can specify the location of the cavity 53 or density change area 54 in more detail with a single measurement by simultaneously performing measurements using multiple ground exploration devices 10. Furthermore, as shown in Fig. 6, if the detection range 52 that can be explored by one ground exploration device 10 is set to a narrower range in the vertical direction, the horizontal location of the cavity 53 or density change area 54 can be specified more accurately than when a wide range is explored in the diagonal direction using one ground exploration device 10 as shown in Fig. 5. As described above, according to this embodiment, the technology for exploring specific locations in the ground is improved.

[0040] (Method of exploration while moving a ground exploration device) In another embodiment of the ground exploration device disclosed herein, as shown in FIG. 8 , one or more ground exploration devices 10 detect muons while sequentially moving to multiple positions within the underground space 41. In this case, the detection range of muons of the ground exploration device 10 may be set narrow. This allows the measurement results at different positions to be used to more precisely determine the location of the cavity 53 or the density change region 54, as in the case of performing ground exploration using multiple ground exploration devices 10 of FIG. 6 . Furthermore, the configuration of moving the ground exploration device 10 of FIG. 8 may be combined with the configuration of deploying multiple ground exploration devices 10 shown in FIG. 6 . This allows the ground exploration system 1 to more quickly identify the location of the cavity 53 or the density change region 54.

[0041] The procedure for moving the soil exploration device 10 to explore the ground 50 will be described with reference to FIG. 9 . First, the soil exploration device 10 is placed in the underground space 41 (step S101). The control unit 22 of the information processing device 20 acquires detection information detected by the soil exploration device 10 via the detection signal acquisition unit 21a of the signal transmission / reception unit 21 (step S102). The control unit 22 identifies the range in which the cavity 53 or density change region 54 exists using the counting unit 22a and the determination unit 22b (step S103). In this case, the range in which the cavity 53 or density change region 54 exists is identified as the detection range 52 indicating the direction as seen from the first detector 11. The control unit 22 stores information on the identified range in which the cavity 53 or density change region 54 exists in the memory unit 24. The control unit 22 determines whether to continue measurement according to a predetermined procedure (step S104). For example, the control unit 22 may be programmed to move the soil exploration device 10 a predetermined number of times to perform detection. If measurement is to be continued in step S104 (step S104: Yes), the control unit 22 causes the drive control unit 22c to drive the drive mechanism 13 of the ground exploration device 10 to move to the next exploration position (step S105). After step S105, the control unit 22 returns to step S102. When step S103 is executed for the second or subsequent time, the control unit 22 may be able to more accurately identify the cavity 53 or the density change region 54 from the overlapping region between the region identified previously and the region identified this time. When the predetermined measurement is completed, the control unit 22 determines not to continue measurement (step S104: No) and ends the process.

[0042] (Method of Performing Ground Exploration in Two Stages) In yet another embodiment of the present disclosure, ground exploration performed by the ground exploration system 1 can be performed in two stages. In this case, the first stage is a ground exploration procedure in which the relatively wide detection ranges 52a to 52e shown in FIG. 5 are measured. In the first stage of ground exploration, the range detected by the pair of first detector 11 and second detector 12 is wide, and therefore, the presence of a cavity 53 or a density change region 54 is detected within the wide range. Therefore, in the second stage, as shown in FIG. 10, the first detector 11 and a specific second detector 12 (e.g., second detector 12d or 12e) are used to narrow the measurable detection range 52, and exploration of the cavity 53 or the density change region 54 is performed.

[0043] Furthermore, the ground exploration device 10 may sequentially rotate the rotating frame 15 by a predetermined angle to change the angle of the second detector 12 relative to the first detector 11, thereby obtaining more detailed information on the range in which the cavity 53 or the density change region 54 exists. The predetermined angle may be, for example, 1°, 3°, or 5°.

[0044] In Fig. 10, one soil exploration device 10 is shown, and an example of performing exploration by limiting the exploration range as the second stage is shown. However, this method can be combined with the exploration method of Fig. 6 in which a plurality of soil exploration devices 10 are arranged to perform soil exploration, and / or the exploration method of Fig. 8 in which soil exploration is performed by moving the soil exploration device 10.

[0045] A two-stage procedure for exploring the ground using the ground exploration system 1 will be described with reference to FIG. 11 . First, the ground exploration device 10 is placed in the underground space 41 (step S201). Multiple ground exploration devices 10 may be placed in the underground space 41, as shown in FIG. 6 . Next, muons are detected by the first detector 11 and the second detector 12. The control unit 22 of the information processing device 20 acquires information from a wide detection range 52 within the ground 50 (step S202). Based on the information acquired from the wide detection range 52 within the ground 50 by the ground exploration device 10, the control unit 22 determines whether a cavity 53 or a density change region 54 exists within the ground 50 (step S203). If a cavity 53 or a density change region 54 exists within the ground (step S203: Yes), the control unit 22 controls the ground exploration device 10 to perform a second-stage measurement with a limited exploration range (step S204). That is, the control unit 22 reduces the size of the solid angle Ω detectable by the ground exploration device 10 and measures muons. This allows the control unit 22 of the information processing device 20 to more accurately identify the detection range 52 in which the cavity 53 or the density change region 54 exists (step S205). If the cavity 53 or the density change region 54 does not exist in step S203 (step S203: No), the control unit 22 may end the process.

[0046] (Method for determining the position of the ground exploration device) In the ground exploration system 1 composed of the ground exploration device 10 and the information processing device 20, the relative position of the cavity 53 and / or the density change region 54 can be determined using the position of the ground exploration device 10 as a reference. However, if the exact position and orientation of the ground exploration device 10 underground are unknown, it may be difficult to determine the absolute position of the cavity 53 and / or the density change region 54. For this reason, the ground exploration system 1 may include a movable shield 30 as shown in FIG. 1 .

[0047] The movable shield 30 is an object that is movable and can at least partially block muons coming from the atmosphere. The movable shield 30 may include a material that is highly effective at blocking muons, such as iron or aluminum. The movable shield 30 includes, for example, a large vehicle such as a construction vehicle, as shown in FIG. 12 . Construction vehicles may include dump trucks, bulldozers, excavators, and crane trucks. The movable shield 30 may also be a dedicated vehicle equipped with metal blocks for muon blocking.

[0048] As shown in FIG. 1 , the movable shield 30 may include a position detection unit 31 for detecting its precise position. The position detection unit 31 may include, for example, a receiver compatible with a Global Navigation Satellite System (GNSS). The position detection unit 31 of the movable shield 30 may obtain latitude and longitude information of the current position of the movable shield 30 using a signal from the GNSS receiver. Examples of GNSS include the Global Positioning System (GPS), GLONASS, Galileo, BeiDou, etc. The position detection unit 31 is not limited to one that uses the GNSS. For example, the position detection unit 31 may be configured to identify the current position based on the direction and distance from the ground equipment.

[0049] The position detection unit 31 is configured to transmit position information of the movable shield 30 to the information processing device 20. The control unit 22 of the information processing device 20 can acquire the position information from the position detection unit 31 via the position information acquisition unit 21c of the signal transmission / reception unit 21. The control unit 22 can identify the position of the ground exploration device 10 by using the position information of the movable shield 30. By identifying the position of the ground exploration device 10, the positions of the cavities 53 and / or density change regions 54 inside the ground 50 can also be identified more accurately.

[0050] 13 , a procedure for determining the position of the ground exploration device 10 using the ground exploration system 1 will be described. First, with the ground exploration device 10 placed in the underground space 41, the movable shield 30 is placed on the ground surface 51 (step S301). Next, muon measurements are performed while gradually moving the movable shield 30 (step S302). As the movable shield 30 moves, the number of muon particles arriving from the direction of the movable shield 30 decreases. The control unit 22 of the information processing device 20 determines the relative positional relationship between the ground exploration device 10 and the movable shield 30 from changes in the detection frequency of muon particles counted by the counter 22a (step S303).

[0051] Next, the control unit 22 acquires position information of the movable shield 30 from the position detection unit 31 of the movable shield 30 via the position information acquisition unit 21c (step S304). The control unit 22 identifies the position of the ground exploration device 10 based on the relative position information between the ground exploration device 10 and the movable shield 30 identified in step S303 and the position information (absolute position information) of the movable shield 30 acquired in step S304 (step S305). This makes it possible to identify the position of the ground exploration device 10 placed underground in the ground exploration system 1. Note that although the movable shield 30 is moved in step S302, it is also possible to move the ground exploration device 10 while fixing the movable shield 30.

[0052] 14 , when a density change region 54 such as an underground buried object is present in the ground, the existence and direction of the density change region 54 can be determined by exploration using the ground exploration system 1, but it is not easy to estimate the exact density of the density change region. Therefore, in measurements using the ground exploration system 1 of one embodiment, an object 55 of known material and density is used as a density standard to estimate the density of the density change region 54.

[0053] A method for estimating the density of the density change region 54 using the ground exploration system 1 will be described with reference to FIG.

[0054] First, an object 55 with a known density is placed on the ground surface 51 within the detection range 52 of the ground exploration device 10 (step S401). Preferably, the object 55 is placed directly above the ground exploration device 10 so as to cover the detection range 52 for muon detection by the first detector 11 and the specific second detector 12. Next, the ground exploration system 1 measures the detection frequency of muons using the ground exploration device 10 (step S402). The detection frequency of muons with the object 55 placed is defined as the first detection frequency. The control unit 22 stores information on the density, size, and detection frequency of muons of the object 55 in the memory unit 24 (step S403). The size of the object 55 may include information on the area and thickness parallel to the ground surface 51. Information on the density and size of the object 55 may be input in advance by the user via the input / output unit 23. If the measurement is to be repeated for objects 55 with different densities (step S404: Yes), the object 55 is replaced and steps S401 to S403 are repeated multiple times.

[0055] After completing the measurement of muons with the object 55 placed on the ground surface 51 (step S404: No), the object 55 is removed from the ground surface 51, and the ground exploration system 1 measures the muon detection frequency using the same pair of the first detector 11 and the second detector 12 (step S405). The muon detection frequency measured with the object 55 removed is designated the second detection frequency. Since there is no muon annihilation due to the absence of the object 55 on the ground, the second detection frequency is higher than the first detection frequency. The control unit 22 of the information processing device 20 can calculate the rate of decrease in the muon detection frequency per unit length due to muons passing through the object 55 with a reference density based on the size of the object 55, the first detection frequency, and the second detection frequency. The control unit 22 combines the reference density and the rate of decrease in the muon detection frequency per unit length and stores the combination in the memory unit 24.

[0056] Next, the ground exploration system 1 performs ground exploration using the ground exploration device 10. When a density change region 54 is present inside the ground 50, the ground exploration system 1 measures the detection frequency of muons that have passed through the density change region 54 underground using the control unit 22 of the information processing device 20 (step S406). Preferably, the ground exploration device 10 moves so that the density change region 54 is oriented in the same direction as the direction in which the object 55 was placed in step S401.

[0057] Based on the information acquired up to step S406, the control unit 22 estimates the density of the density change region 54 (step S407). Specifically, the control unit 22 can calculate the rate of decrease in the detection frequency of muons per unit length due to passage through the ground surrounding the density change region, based on the detection frequency of muons detected after passing through regions other than the density change region 54. Next, the control unit 22 calculates the rate of decrease in the detection frequency of muons in the ground 50 from the detection frequency of muons detected after passing through a region in the ground 50 that includes the density change region 54. The control unit 22 calculates the rate of decrease in the detection frequency of muons per unit length in the density change region 54, taking into account the rate of decrease in the detection frequency of muons per unit length in regions other than the density change region 54.

[0058] The control unit 22 can estimate the density of the density change region 54 by comparing this decrease rate with the decrease rate of the detection frequency of muons per unit length by the object 55 of a reference density stored in the memory unit 24. When there is data on a set of reference densities and the decrease rate of the detection frequency of muons, the control unit 22 can estimate the density of the density change region 54 by assuming that the density and the decrease rate of the detection frequency of muons are proportional. When the memory unit 24 stores multiple sets of data on reference densities and the decrease rate of the detection frequency of muons, the control unit 22 can estimate the density of the density change region 54 by interpolating or extrapolating the data.

[0059] The ground exploration system 1 may execute steps S401 to S405 in Fig. 15 using one ground exploration device 10 whose position information is known. The ground exploration system 1 may execute steps S406 and S407 in Fig. 15 using multiple ground exploration devices 10 and / or by sequentially moving the ground exploration devices 10.

[0060] As described above, by using the object 55 whose density is known, the ground exploration system 1 is able to calculate the density of the density change region 54 in the ground.

[0061] Although the above-described embodiments have been described as typical examples, it will be apparent to those skilled in the art that many modifications and substitutions can be made within the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited by the above-described embodiments and examples, and various modifications and / or alterations are possible without departing from the scope of the claims. For example, multiple building blocks described in the embodiments and examples can be combined into one, or one building block can be divided.

[0062] The following additional notes are provided regarding the above-described embodiments.

[0063] (Supplementary Item 1) A ground exploration system including one or more ground exploration devices arranged inside an underground space and an information processing device, wherein the ground exploration devices include at least one first detector and at least one second detector arranged at a distance from each other to detect cosmic ray muons, and the information processing device has a control unit which counts the number of muons arriving from an arrival direction connecting the first detector and the second detector by causing the first detector and the second detector to detect muons substantially simultaneously, and determines the extent to which a cavity in the ground or a density change region having a density different from that of surrounding regions exists based on the arrival direction and the detection frequency, which is the number of muons counted per unit time, and the ground exploration system detects muons by at least one of placing a plurality of the ground exploration devices and moving the ground exploration devices, thereby placing the ground exploration devices simultaneously or sequentially at a plurality of positions in the underground space. (Supplementary Item 2) The ground exploration system according to Supplementary Item 1, wherein the control unit acquires position information of a movable shield that is placed on the ground and blocks muons, and identifies a position of the ground exploration device based on the position information of the movable shield and the detection frequency of the muons. (Supplementary Item 3) The ground exploration system according to Supplementary Item 1 or 2, wherein the control unit estimates the density of the density change region based on a first detection frequency obtained by counting muons that have passed through an object with a known density placed on the ground, and a second detection frequency obtained by counting muons that have passed through the density change region with the object removed.(Supplementary Item 4) A ground exploration method comprising: acquiring muon detection information from one or more ground exploration devices arranged inside an underground space, the ground exploration devices including at least one first detector and at least one second detector arranged at a distance from each other to detect cosmic ray muons; counting the number of muons arriving from an arrival direction connecting the first detector and the second detector based on the muon detection information, by having the first detector and the second detector detect muons substantially simultaneously; and determining the extent of the existence of cavities in the ground or density change regions having a density different from that of surrounding regions based on the arrival direction and the detection frequency, which is the number of muons counted per unit time, wherein the ground exploration method detects muons by at least one of placing a plurality of the ground exploration devices and moving the ground exploration devices, thereby placing the ground exploration devices simultaneously or sequentially at a plurality of positions in the underground space.

[0064] 1 Ground exploration system 10, 10a to 10c Ground exploration device 11 First detector 12, 12a to 12e Second detector 13 Drive mechanism 14 Fixed frame 15 Rotating frame 20 Information processing device 21 Signal transmitting / receiving unit 21a Detection signal acquiring unit 21b Drive signal transmitting unit 21c Position information acquiring unit 22 Control unit 21a Counting unit 21b Determination unit 21c Drive control unit 21d Position identifying unit 23 Input / output unit 24 Memory unit 30 Movable shield 31 Position detecting unit 40 Underground structure 41 Underground space 42 Floor surface 50 Ground 51 Ground surface 52 Detection range 53 Cavity 54 Density change area 55 Object 111, 112 Scintillator

Claims

1. A ground exploration system comprising one or more ground exploration devices arranged inside an underground space and an information processing device, wherein the ground exploration device includes at least one first detector and at least one second detector arranged at a distance from each other to detect cosmic ray muons, and the information processing device is equipped with a control unit which counts the number of muons arriving from a direction connecting the first detector and the second detector by having the first detector and the second detector detect muons substantially simultaneously, and determines the extent to which cavities in the ground or density change regions having a density different from that of surrounding regions exist based on the direction and the detection frequency, which is the number of muons counted per unit time, and the ground exploration system detects muons by at least one of placing a plurality of the ground exploration devices and taking measurements while moving the ground exploration devices, thereby placing the ground exploration devices simultaneously or sequentially at multiple positions in the underground space.

2. The ground exploration system described in claim 1, wherein the control unit acquires position information of a movable shield placed on the ground to block muons, and determines the position of the ground exploration device based on the position information of the movable shield and the detection frequency of the muons.

3. The ground exploration system of claim 1, wherein the control unit estimates the density of the density change region based on a first detection frequency obtained by counting muons that have passed through an object with a known density placed on the ground, and a second detection frequency obtained by counting muons that have passed through the density change region with the object removed.

4. A ground exploration method comprising: acquiring muon detection information from one or more ground exploration devices arranged inside an underground space, the ground exploration devices including at least one first detector and at least one second detector arranged at a distance from each other to detect cosmic ray muons; counting the number of muons arriving from a direction connecting the first detector and the second detector based on the muon detection information, by having the first detector and the second detector detect muons substantially simultaneously; and determining the extent of the existence of cavities in the ground or density change regions having a density different from that of surrounding regions based on the direction and the detection frequency, which is the number of muons counted per unit time, wherein the ground exploration method detects muons by at least one of placing a plurality of the ground exploration devices and taking measurements while moving the ground exploration devices, thereby placing the ground exploration devices simultaneously or sequentially at a plurality of positions in the underground space.

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