Transmission-type radioisotope ground measurement device and method for measuring ground density using radioisotopes

Abstract

To provide a transmission-type radioisotope ground measurement device capable of high-precision measurement and a method for measuring ground density using radioisotopes. [Solution] The transmission-type radioisotope ground measurement device 2 includes a distance measuring means for determining the actual transmission distance between the radiation source 3 and the detection means 6, a relative density calculation means for calculating the relative density based on the dry density calculated by the dry density calculation means, and a corrected dry density calculation means for determining the distance ratio between the actual transmission distance and a preset transmission distance, and calculating the corrected dry density based on the distance ratio and the relative density calculated by the relative density calculation means.

Description

[Technical Field]

[0001] The present invention relates to a transmission-type radioisotope ground measuring device and a method for measuring the density of ground using radioisotopes, which are mainly used to understand the ground properties of embankments and other soils near the ground surface. [Background technology]

[0002] In civil engineering works, it is necessary to understand the properties of the ground before and after the work, and the Radio Isotope (abbreviated as RI) method is known as a technique for measuring such ground properties in situ.

[0003] There are two types of equipment used in the radioisotope method (hereinafter referred to as RI equipment): "transmission type" and "scattering type." The "transmission type" is widely used because it is relatively less affected by the air gap between the ground surface and the bottom of the instrument (see, for example, Non-Patent Document 1).

[0004] A penetrating radioisotope instrument works by inserting a source rod containing a radioisotope into the soil, measuring the amount of gamma rays or neutrons emitted from the source in the soil using a detection device on the ground, calculating the wet density based on the gamma ray radiation dose measured by the detection device, and calculating the water content ratio based on the neutron radiation dose.

[0005] Then, the dry density is determined from the calculated wet density and water content, and based on that, the soil properties, such as the degree of compaction (relative density), are determined.

[0006] However, in conventional general-purpose transmission radioisotopes as described above, the calibration coefficient is set for each instrument on the premise that the distance from the radiation source to the detector is kept constant. If the radiation source rod cannot be inserted to a predetermined depth underground, accurate measurement cannot be guaranteed, and it is necessary to change the measurement location (see, for example, Patent Document 1).

[0007] In conventional RI equipment where the detector is placed on the ground surface, since the distance that radiation (gamma rays and neutron rays) penetrates the ground is limited, the depth from the ground surface to the radiation source, that is, the insertion depth of the radiation source rod (hereinafter referred to as the installation depth) is fixed at a predetermined depth (for example, 20 cm from the ground surface).

Prior Art Documents

Patent Documents

[0008]

Non-Patent Document 1

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] However, in the conventional technology as described above, the installation depth of the radiation source and the installation position of the detector are fixed, and the distance between the radiation source and the detector (hereinafter referred to as the penetration distance) is set to a certain distance in advance. Therefore, when there is a gravel layer in the target ground (hereinafter referred to as hitting gravel) or the insertion length of the radiation source rod deviates from the set value due to insufficient insertion force, etc., the penetration distance fluctuates, and there is a risk that accurate measurement cannot be performed.

[0010] Therefore, in view of such conventional problems, an object of the present invention is to provide a transmission-type radioisotope ground measurement device capable of measuring the density of the ground even when the penetration distance fluctuates, and a method for measuring the density of the ground using radioisotopes.

Means for Solving the Problems

[0011] The feature of the invention according to claim 1 for solving the above-mentioned conventional problems is a transmission-type radioisotope ground measurement device comprising a source rod having a radiation source made of a radioisotope, a detection means having a detector for measuring the radiation dose emitted from the radiation source of the source rod inserted into the ground, and a dry density calculation means for calculating the dry density of the ground based on the radiation dose measured by the detector. The device further comprises a distance measurement means for obtaining the actual transmission distance between the radiation source and the detector, and a corrected dry density calculation means for obtaining the distance ratio between the actual transmission distance and a preset transmission distance and calculating a corrected dry density based on the distance ratio.

[0012] The feature of the invention according to claim 2 is that, in addition to the configuration of claim 1, the device further comprises a detector installation means for bringing the detector into contact with the surface of the ground.

[0013] The feature of the invention according to claim 3 is that, in addition to the configuration of claim 1 or 2, the device further comprises a source rod position measurement means for measuring the insertion length of the source rod into the ground.

[0014] The feature of the invention according to claim 4 is a density measurement method using a radioisotope, in which a source rod having a radiation source made of a radioisotope is inserted into the target ground, the radiation dose emitted from the radiation source is measured by a detector arranged at a position spaced apart from the radiation source by a distance, and the dry density of the ground is calculated based on the measured radiation dose. The method includes a distance measurement step for obtaining the actual transmission distance between the radiation source and the detector, a distance ratio calculation step for obtaining the distance ratio between the actual transmission distance and a preset transmission distance, an added density value calculation step for calculating an added density value based on the distance ratio, and a corrected dry density calculation step for adding the calculated added density value to the calculated dry density to calculate a corrected dry density.

[0015] The feature of the invention according to claim 5 is that, in addition to the configuration of claim 4, the insertion length of the source rod into the ground is measured, and the actual transmission distance is calculated based on the insertion length.

[0016] The feature of the invention described in claim 6 is that, in addition to the configuration of claim 4 or 5, the added density value is multiplied by a device characteristic coefficient that quantifies the characteristics of the device used for the distance ratio.

[0017] The feature of the invention described in claim 7 is that, in addition to the configuration of claim 6, the added density value is obtained by multiplying the distance ratio by a soil property coefficient determined according to the physical properties of the ground. [Effects of the Invention]

[0018] The transmission-type radioisotope ground measurement device according to the present invention, by having the configuration described in claim 1, can calculate the dry density of the ground without being affected by the actual transmission distance between the source rod and the detector.

[0019] Furthermore, by incorporating the configuration described in claim 2, the present invention makes it possible to calculate the dry density of the ground without being affected by the installation depth of the radiation source rod, using a mechanism similar to that of conventional general-purpose RI measuring instruments.

[0020] Furthermore, by providing the configuration of claim 3 in the present invention, the dry density of the ground can be calculated without being affected by the installation depth of the radiation source.

[0021] The method for measuring the density of ground using a radioisotope according to the present invention, by comprising the configuration described in claim 4, can calculate the dry density of the ground without being affected by the installation depth or insertion angle of the source rod.

[0022] Furthermore, by providing the configuration described in claim 5 of the present invention, the dry density of the ground can be calculated without being affected by the installation depth of the radiation source.

[0023] Furthermore, by providing the configuration described in claim 6 of the present invention, the dry density of the ground can be calculated without being affected by the installation depth of the radiation source.

[0024] Furthermore, by providing the configuration described in claim 7, the present invention makes it possible to calculate the dry density according to the physical properties of the target ground, without being affected by the installation depth of the radiation source rod. [Brief explanation of the drawing]

[0025] [Figure 1] This is a longitudinal cross-sectional view showing a schematic of the transmission-type radioisotope ground measurement device according to the present invention. [Figure 2] This is a schematic cross-sectional view showing the actual transmission distance of the same object. [Figure 3] This figure shows the flow chart for calculating the dry density of the ground according to the present invention. [Figure 4] This figure shows the relationship between the rate of change in transmission distance and the change in dry density. [Figure 5] This figure shows the relationship between the rate of change in transmission distance and the change in dry density. [Figure 6] (a) is a diagram showing the relationship between the transmission distance difference and the dry density before correction, and (b) is a diagram showing the relationship between the transmission distance difference and the dry density after correction. [Modes for carrying out the invention]

[0026] Next, an embodiment of the transmission-type radioisotope ground measurement device according to the present invention will be described based on the example shown in Figures 1 and 2. In the figures, reference numeral 1 denotes the target ground, and reference numeral 2 denotes the transmission-type radioisotope ground measurement device (hereinafter referred to as RI measurement device 2).

[0027] As shown in Figure 1, the RI measuring device 2 includes a source rod 5 having sources 3 and 4 made of radioactive isotopes, a detection means having detectors 6 and 7 for measuring the radiation dose (gamma ray dose) emitted from sources 3 and 4, and the wet density ρ of the ground based on the radiation dose measured by the detection means. t A wet density calculation means for calculating the wet density ρ, a water content calculation means for calculating the water content w of the ground based on the radiation dose measured by the detection means, and the calculated wet density ρ d Based on the water content w, the dry density ρ of the ground dIt includes a dry density calculation means for calculating, and can grasp the dry density ρ useful when evaluating the consolidation characteristics of the ground. d It has become possible to do so.

[0028] In addition, this RI measuring device 2 includes a distance measuring means for obtaining the actual transmission distance between the radiation sources 3 and 4 and the detector 6, and based on the distance ratio between the dry density ρ calculated by the dry density calculation means, the actual transmission distance L obtained by the distance measuring means, and a preset transmission distance L0, it includes a corrected dry density calculation means for calculating the corrected dry density ρ, enabling more reliable measurement of the dry density. d d_corr It has become possible to do so.

[0029] The source rod 5 is formed in a thin rod shape, and at the tip, there are radiation sources 3 and 4 made of radioactive isotopes (radioisotopes: RI). Specifically, as the radiation source 3 that emits gamma rays, cobalt 60 ( 60 Co), cobalt 57 (57Co), cesium 137 ( 137 Cs), barium 133 (133Ba), one nuclide or a mixture of two or more nuclides is selected, and as the radiation source 4 that emits neutron rays, americium 241-beryllium ( 241 Am-Be), californium 252 ( 252 Cf), either one is selected and they are each detachably held. Note that the radiation sources 3 and 4 are not limited to the above-described embodiment, and other nuclides are not excluded either.

[0030] Note that the source rod 5 may be directly inserted into the ground 1. Although not specifically described in detail in this embodiment, a drilling drill or a pointed penetration cone for use in inserting into the ground may be provided at the tip of the source rod 5.

[0031] This source rod 5 may be inserted and removed with respect to the ground by a source rod moving means 8 composed of a rack and pinion mechanism or the like, or may be manually penetrated by an operator.

[0032] ​Furthermore, the insertion depth of the radiation source rod 5, i.e., the distance traveled by the radiation source rod 5, is measured by a radiation source rod movement distance measuring device (not shown), and the measurement data output from the radiation source rod movement distance measuring device is input to a distance measuring means.

[0033] The radiation source rod movement distance measuring device is not particularly limited, but it can be a distance sensor such as an encoder or displacement sensor combined with a motor or the like that operates the radiation source rod moving means 8.

[0034] The detection means includes detectors 6 and 7 positioned in contact with the ground 1, and it is preferable that the detectors 6 and 7 are in contact with the ground surface without any gaps being created by the detector installation means.

[0035] The detector installation means are not particularly limited, but for example, if there is a gap between the detectors 6 and 7 and the ground surface due to unevenness, a spacer or backfill may be used to fill the gap, and if the RI measuring device 2 has self-propelled wheels, a wheel storage device or a vehicle height adjustment device may be used.

[0036] For detectors 6 and 7, for example, a GM counter or NaI(Tl) scintillation detector may be used for detector 6 for gamma ray measurement, and a BF3 counter or helium-3 neutron counter may be used for detector 7 for neutron beam measurement.

[0037] The gamma-ray detector 6 then measures the number of gamma rays (count rate) per unit time detected by the detector 6 after passing through the ground 1, and calculates the gamma-ray count rate ratio R shown in the following equation. g and wet density ρ t From the relationship equation, the wet density ρ t The measurement data output from detector 6 is then input to the dry density calculation means. Note that A and B are calibration constants set for each device based on the exponential function of the counting rate ratio and water content obtained with the transmission distance kept constant.

[0038]

number

[0039] On the other hand, the neutron detector 7 indirectly measures the number of fast neutrons (counting rate) by detecting thermal neutrons that are decelerated and forcibly converted from fast neutrons that have penetrated the ground 1 and reached the detector 7, and the neutron counting rate ratio R shown in the following equation n and water content ρ m From the relationship equation, the water content ρ m We find the water content ρ m Based on this, the water content w is calculated, and the calculated water content w is input into the dry density calculation means. C and D are calibration constants set for each device from the exponential function of the counting rate ratio and water content obtained with the transmission distance kept constant.

[0040]

number

[0041] As shown in the figure, the distance measuring means calculates the actual transmission distance L by, for example, the following formula when the insertion depth d of the source rod 5 is input from the source rod movement distance measuring device. Here, h is a constant determined for each device and is the distance in the direction perpendicular to the source rod axis between the sources 3, 4 and the detector 6.

[0042]

number

[0043] The dry density calculation means calculates the wet density ρ of the ground input from the wet density calculation means. t And the dry density ρ is calculated based on the following formula from the water content w input from the water content calculation means. d The system calculates the value, stores it in a storage medium, and outputs the calculated value to the relative density calculation means and the corrected dry density calculation means.

[0044]

number

[0045] The corrected dry density calculation means determines the distance ratio between the actual transmission distance L and a preset transmission distance L0, calculates the added density value based on that distance ratio, and the calculated added density value is used to calculate the dry density ρ calculated by the dry density calculation means. d Add to corrected dry density ρ d_corr The system is designed to calculate the corrected dry density ρ using the following formula. Specifically, the corrected dry density ρ is calculated using the following formula. d_corr The following is calculated. Note that L0 is the standard transmission distance set for each device, and is the same transmission distance used when setting the calibration constant described above.

[0046]

number

[0047] Here, α is a conversion factor determined for each RI measuring device 2 used, and may be determined by preliminary tests if necessary. Also, β is a correction factor for α determined according to the physical properties of the ground being measured, and may be determined by preliminary tests if necessary.

[0048] The dry density calculation means, moisture content calculation means, distance measurement means, relative density calculation means, and corrected dry density calculation means are each composed of computer programs and stored in a computer device (not shown) equipped with computing elements mounted in the housing 9.

[0049] Next, the method for measuring dry density using the RI measuring device 2 described above will be explained with reference to the figure. Components similar to those in the above embodiment will be denoted by the same reference numerals.

[0050] First, the RI measuring device 2, which contains detectors 6 and 7, is brought onto the ground 1 to be measured, and the RI measuring device 2 is installed on the ground surface at the predetermined measurement location (device installation process). At this time, it is desirable to level the ground surface so that there is no gap between the RI measuring device 2 and the ground surface. Alternatively, the detectors 6 and 7 may be brought into contact with the ground surface using a detector installation means.

[0051] Next, with sources 3 and 4 not exposed, the gamma ray dose and neutron dose are measured using detectors 6 and 7 to measure the amount of radiation not caused by radiation from sources 3 and 4, such as natural radioactivity (background measurement process (hereinafter referred to as the BG measurement process)).

[0052] Once the BG measurement is complete, the source rod 5 is lowered to a predetermined depth and inserted into the ground 1 (source rod insertion process). At this time, the insertion depth d, i.e., the distance traveled by the source rod 5, is measured by the source rod distance measuring instrument, and the measurement data is output and input to the distance measuring means.

[0053] Then, once the insertion of the radiation source rod 5 is complete, the amount of radiation emitted from the radiation sources 3 and 4 is measured by the detection means (radiation measurement process).

[0054] Specifically, the number of gamma rays per unit time (count rate) detected by the detector 6 after passing through the ground 1 is measured, and the gamma ray count rate ratio R is calculated based on the formula 1 described above. g and wet density ρ t From the relationship, the wet density ρ t This value is calculated and output to the dry density calculation device.

[0055] Meanwhile, the neutron detector 7 detects thermal neutrons converted from fast neutrons that have penetrated the ground 1 and reached the detector 7, and indirectly measures the fast neutron count rate R n Based on the above equation 2, the water content ρ m We find the water content ρ m Based on this, the water content w is calculated and output to the dry density calculation means.

[0056] Next, in the dry density calculation means, the wet density ρ of the ground 1 output from the wet density calculation means is used. t And based on the water content w output from the water content calculation means, the dry density ρ is calculated using the above formula 4. d The calculation is performed, stored in a storage medium, and the calculated value is output to the relative density calculation means and the corrected dry density calculation means (dry density calculation step).

[0057] However, the dry density ρ calculated above d Since the distance between the radiation source 3 and the detector 6 (hereinafter referred to as the existing transmission distance L) is set to a certain distance in advance, if a gravel layer exists in the target ground 1 (hereinafter referred to as "per gravel") or if the insertion length or insertion angle of the radiation source rod 5 deviates from the set value due to insufficient insertion force, the transmission distance will fluctuate, and there is a risk of errors occurring with the actual dry density, so it is necessary to correct these errors.

[0058] To correct this error, the distance calculation means calculates the actual transmission distance L using, for example, the formula 3 described above, when the insertion depth d of the source rod 5 is input from the source rod movement distance measuring instrument (distance measurement step).

[0059] Next, the distance ratio between the actual transmission distance L and the transmission distance L0, which is set in advance as the existing transmission distance, is determined, and the density value to be added based on that distance ratio is calculated, and the dry density ρ calculated by the dry density calculation means is added to the calculated density value. d Add to corrected dry density ρ d_corr The corrected dry density ρ is calculated using the formula 5 described above. d_corr Calculate.

[0060] Here, the conversion factor α used in the above-mentioned formula 5 is a coefficient determined for each RI measuring device 2 used. Specifically, based on the existing transmission distance determined by the source rod 5 containing the nuclides of sources 3 and 4 equipped in the RI measuring device 2 used, detectors 6 and 7 and their arrangement, the dry density ρ is calculated from the transmission distance using the rate of change (or ratio) of the transmission distance and the existing transmission distance as a fixed value. d It is determined by the relationship between the amounts of change.

[0061] Figures 4 and 5 are graphs showing an example of the relationship between the rate of change in permeability distance and the change in dry density, observed using multiple soil samples.

[0062] Although soil samples differ in physical properties such as density, the rate of change in transmission distance and dry density ρ dThere is a correlation between the change in the value and the value itself, and this correlation coefficient is set as the conversion factor α.

[0063] However, when the difference between the actual transmission distance and the existing transmission distance becomes large, the correction dry density ρ mentioned above cannot be used with the conversion factor α alone. d_corr The accuracy of the calculation may decrease.

[0064] Therefore, by multiplying the conversion factor α, which encompasses the differences in soil properties, by a correction factor β determined for each soil with specific properties, the corrected dry density ρ for that specific soil is obtained. d_corr It is possible to calculate this.

[0065] Figure 6 shows the relationship between the difference in transmission distance relative to the existing transmission distance and the dry density. Of these, Figure 6(a) shows the dry density ρ before correction. d Figure 6(b) shows the corrected dry density ρ d_corr That is the case.

[0066] As shown in Figure 6(a), the greater the difference between the actual transmission distance and the existing transmission distance, the greater the dry density ρ before correction. d The calculation error will increase.

[0067] In contrast, as shown in Figure 6(b), the corrected dry density ρ using the conversion factor α d_corr This reduces the calculation error.

[0068] Furthermore, when the conversion factor α is corrected by the correction factor β, the corrected dry density ρ d_corr Furthermore, this reduces the calculation error.

[0069] The correction of the conversion factor α using the correction factor β is desirable when the difference between the existing transmission distance and the actual transmission distance is large, for example, when there is a large amount of gravel mixed in the assumed target ground.

[0070] Then, once the measurement of the ground conditions at that location is complete, the source rod 5 is lifted, and the measurement is moved to the next measurement location, and the corrected dry density ρ described above is used in the same manner. d_corr A series of tasks are performed to determine the result.

[0071] In the transmission-type RI measuring device 2 and density measurement method using it configured in this way, the actual transmission distance is measured, regardless of the transmission distance predetermined for each device, and the dry density is corrected using that transmission distance. This allows for the calculation of the dry density with high accuracy, unaffected by the insertion depth of the source rod 5. [Explanation of Symbols]

[0072] 1 ground 2 RI measurement device 3 source 4 source 5 Source rod 6 Detectors 7 Detectors 8 Source rod moving means 9 cabinets

Claims

1. A transmission-type radioisotope ground measuring device comprising a source rod having a source made of a radioactive isotope, detection means having a detector for measuring the amount of radiation emitted from the source of the source rod inserted into the ground, and dry density calculation means for calculating the dry density of the ground based on the amount of radiation measured by the detector, A transmission-type radioisotope ground measuring device characterized by comprising: a distance measuring means for determining the actual transmission distance between the radiation source and the detector; and a corrected dry density calculating means for determining the distance ratio between the actual transmission distance and a preset transmission distance, and calculating the corrected dry density based on the distance ratio.

2. The transmission-type radioisotope ground measuring device according to claim 1, further comprising a detector installation means for bringing the detector into contact with the surface of the ground.

3. The transmission-type radioisotope ground measuring device according to claim 1 or 2, further comprising a source rod position measuring means for measuring the insertion length of the source rod into the ground.

4. In a radioisotope-based density measurement method, a source rod having a source made of a radioactive isotope is inserted into the target ground, the amount of radiation emitted from the source is measured by a detector placed at a distance from the source, and the dry density of the ground is calculated based on the measured amount of radiation, A distance measurement step to determine the actual transmission distance between the radiation source and the detector, A distance ratio calculation step to determine the distance ratio between the actual transmission distance and a predetermined transmission distance, A step of calculating an added density value, which calculates an added density value based on the distance ratio, A method for measuring the density of ground using a radioisotope, characterized by a step of calculating a corrected dry density by adding the calculated added density value to the calculated dry density.

5. The method for measuring the density of ground using a radioisotope according to claim 4, wherein the insertion length of the source rod into the ground is measured, and the actual transmission distance is calculated based on the insertion length.

6. The method for measuring the density of ground using a radioisotope according to claim 4 or 5, wherein the added density value is multiplied by a device characteristic coefficient that quantifies the characteristics of the device used for the distance ratio.

7. The method for measuring the density of ground using a radioisotope according to claim 6, wherein the added density value is obtained by multiplying the distance ratio by a ground property coefficient determined according to the physical properties of the ground.

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