JPS6236188B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for detecting the position of a crack in a geological formation, and more particularly to a method for detecting the position of the crack on the earth's surface by detecting natural gamma rays emitted from the surface layer. The gamma-ray emitting nuclides contained in the surface layer are potassium-40 (hereinafter referred to as ⁴⁰ K) and thallium-208 (hereinafter referred to as 40 K).
²⁰⁸ Tl), and bismuth-214 (hereinafter referred to as ²¹⁴ Bi
) is typical, and the stratum content rate of the three nuclides has a unique value for each nuclide. However, it has been shown that when there are cracks in the geological formations, the gamma ray dose at the ground surface is higher than under normal conditions where there are no cracks in the geological formations. The reason for this is radon-222 (hereinafter referred to as ^222Rn ), which was produced by the radioactive decay of radium-226, a uranium series member shown in Figure 1, contained in the strata around the crack.
A portion of the 222 Rn is liberated from the stratum particles, rises in large quantities within the cracks, and is stored in the surface layer. When radioactive decay occurs in ^{the 222} Rn stored on the surface, polonium-214 as shown in Figure 1 is generated. The daughter ^nuclides up to This is because the gamma ray dose is higher than the gamma ray dose at the general surface layer. Many attempts have been made in the past to detect the location of geological cracks on the ground surface using this phenomenon. However, this phenomenon cannot be applied unless the surface layer is uniform, the topography is flat, and the gamma ray dose is not disturbed by human conditions, which are very special conditions in Japan. . That is, even if the surface layer is the same geological layer, the dose of gamma rays from the surface layer (hereinafter referred to as detected dose) that enters a detector on the ground surface varies significantly depending on the geometric conditions of the ground surface. In concave terrain, the area on which gamma rays from the surface are incident is larger than in flat conditions, resulting in a higher detected dose; in convex terrain, the area is smaller, resulting in a lower detected dose. In addition, human-made conditions such as road pavement, buildings, concrete, masonry, and fertilizers contain materials with extremely high gamma-ray emitting nuclides, which may result in a higher detected dose than under flat surface conditions. be. At the same time, the artificial conditions may suppress gamma rays from the surface layer and reduce the detected dose. In the case of Japan, the conditions described above generally exist in large numbers, so the detected dose varies significantly from point to point, and the number of points where the detected dose increases is generally several tens of times larger than that caused by cracks in the strata. The number of cracks increases, making it difficult to detect crack points using this method. In order to eliminate changes in the detected dose due to unevenness of the ground surface, the detector was mounted on a helicopter and the height above the ground was set at approximately
There is also a method to reduce the influence of distance by keeping it constant at around 30m. Although this method has been successful in analyzing surface geology in relatively flat areas without anthropogenic conditions,
If there are artificial conditions, the analysis will be insufficient and the location of cracks in the strata cannot be detected. This is because as the distance from the ground increases, gamma rays from the cracks in the surface layer become dispersed, making the rate of increase in the detected dose on the cracks extremely small. In other cases where the earth's surface is covered with vegetation or weathered matter and the covering layer is thick, it is extremely difficult to detect cracks on the earth's surface unless there is a clear drop on the earth's surface due to significant fault movement. The present invention has been made in view of the above points, and provides a method for easily and reliably detecting the location of cracks on the earth's surface by eliminating the phenomenon in which the detected dose fluctuates due to the geometrical conditions of the earth's surface and human-made conditions. The purpose is to In order to achieve such an objective, the present invention provides ⁴⁰ K, which is a gamma ray emitting nuclide contained in the surface layer.
Of the gamma rays emitted from the three nuclides ²⁰⁸ Tl and ²¹⁴ Bi, we measured the peak count rate for the gamma rays that entered the NaI detector at the ground surface and showed the photoelectric effect of each nuclide, and then The method is characterized in that the peak ratio and the fluctuation rate of the peak ratio are determined, and when the peak fluctuation ratio satisfies a predetermined condition, it is determined that there is a crack in the stratum. Hereinafter, details of the method of the present invention will be explained based on FIGS. 2 to 4. First, gamma rays emitted from ⁴⁰ K, ²⁰⁸ Tl, and ²¹⁴ Bi, which are typical gamma ray emitting nuclides contained in the surface layer, are detected by a NaI detector at the surface, and gamma rays emitted by each nuclide are detected. The highest energy of, 1.46MeV at ⁴⁰ K, ²⁰⁸ Tl
For 2.61 MeV and 1.76 MeV for ²¹⁴ Bi, the counting rate (hereinafter referred to as window counting rate) in the energy range showing the photoelectric effect, that is, the respective areas C, A, and B shown in FIG. 2 is measured. In addition, the second
The upper and lower gamma ray energy limits that give the window count rate for each nuclide shown in the figure are determined by the detection efficiency of the NaI detector, and the normal upper and lower energy ranges are approximately ±about It will be about 8%. As shown in Figure 2, the window count rate is
In the case of ²¹⁴ Bi, the count rate D of Compton scattering due to ²⁰⁸ Tl is included, and in the case of ⁴⁰ K, the areas of count rates E and F of Compton scattering due to ²⁰⁸ Tl and ²¹⁴ Bi are included, so these are removed. The following method was used to determine the counting rate due to the photoelectric effect (hereinafter referred to as peak counting rate), that is, the areas of H and G in FIG. G=B-αA H=C+(E+F)=C-(β-αγ)A-γB Here, α, β, and γ are the values of the Na detector, respectively.
It is a constant determined by the shape of NaI crystal, resolution, and geometric conditions of measurement, and is determined by actual measurements using a radiation source of each nuclide and setting the detector used under average geometric conditions. The peak count rates of the three nuclides, that is, the areas A, G, and H in Figure 2, are determined by the above operations, and these values generally include variations due to the geometric conditions of the earth's surface and human conditions, but in principle, This reflects the contents of the three nuclides in the surface layer. Next, in order to largely eliminate the geometrical and anthropogenic conditions on the earth's surface, we decided to use the ratio of peak count rates for each two nuclides (hereinafter referred to as peak ratio) as shown in the formula below as the next index. Peak ratio A = ²¹⁴ Bi peak count rate / ²⁰⁸ Tl peak count rate = Area G / Area A Peak ratio B = ²¹⁴ Bi peak count rate / ⁴⁰ K peak count rate = Area G / Area H Peak ratio C = ⁴⁰ K peak count Rate / ²⁰⁸ Tl peak count rate = Area H / Area A Since the content of each nuclide in a stratum is constant for each stratum, no matter how the peak count rate of each nuclide changes due to geometric conditions, Each peak ratio is constant for each stratum. Therefore, by using the peak ratio, fluctuations in the peak count rate of each nuclide due to the geometrical conditions of each stratum are eliminated, and variations in the peak count rate of each nuclide due to the geometric conditions of each stratum are eliminated, and it is also possible to eliminate variations in the peak count rate of each nuclide due to the geometric conditions of each stratum. The existence of the condition is removed. In other words, under all conditions, the peak count rate of each nuclide varies greatly depending on the measurement position on the earth's surface, but when looking at the ratio of the content of the two nuclides, the difference is small, and by using the peak ratio, The large fluctuations in the peak count rate of each nuclide under these conditions are removed,
Each peak ratio shows a relatively stable value. Therefore, in the surface layer where ²²² Rn has risen through cracks in the stratum, the gamma ray dose of ²¹⁴ Bi due to the radioactive decay of ²²² Rn will be higher than in the general surface layer, and the crack point will be at peak ratio A or peak ratio B. It will be shown as a point where the value suddenly increases. This fact can be explained by the fact that the topography is flat and the surface layer is uniform.
Looking at the results of peak ratios A and B in Yamato Town, Fukushima Prefecture, where a large amount of ^222Rn has risen from underground through cracks due to a series of microearthquakes, as shown in Figure 3, The location is clearly shown as the point of increase in peak ratios A and B. However, in ordinary cracks, a large amount of ²²² Rn does not rise as in this case, and in order to analyze the crack position in ordinary cracks, it is necessary to make the analysis of the peak ratio more detailed and accurate. As a means of this, the fluctuation of the peak ratio value based on the statistical fluctuation of the radioactive decay of the nuclide contained in the surface layer was previously determined as follows (n-
1) A method of comparing the moving average values of the peak ratios was used. Here, (n-1) is an arbitrary number of measurements made immediately before the measurement at the target measurement point, and the number is determined by the interval between the measurement points, and is usually a number in the range of 3 to 10. Each peak fluctuation rate of the measured value at an arbitrary point is
If we classify 100% into positive, zero, and negative, there are 27 combinations, of which the increase in the peak count rate of ²¹⁴ Bi, which includes conditions indicating a crack point, is greater than that of the other two nuclides. The condition that the increase in peak count rate is greater than the increase in the peak count rate is shown in the table below.
Limited to the following three patterns. In other words, the combination conditions for the peak fluctuation rate of the measured values have been reduced to 1/9.

[Table] Therefore, the conditions that indicate the crack position are that the P _A and P _B values are 100 when the pattern ~ in the table is met.
% or more, the accuracy is high. However, since the values of the peak fluctuation rates P _A and P _B include statistical fluctuations of the peak count rate, the threshold value m must be set to be at least (100+m)%, and the value of m If there are statistical errors ±a, ±b for the measured values A and B, for example, the threshold m for the peak fluctuation rate P _A ±pa is expressed as pa/P _A (%).
Then, m is expressed by the following formula. If A _i = B _i = a ² _i = b ² _i = const In addition, the values of peak fluctuation rates P _A and P _B are affected by geological changes and human conditions in the surface layer that could not be fully treated by manipulating the peak ratio, as well as partial potassium from the surface layer due to diagenesis of the stratum. , conditions for leaching and accretion of uranium series elements and thorium series elements are added,
Since the peak count rates of ⁴⁰ K, ²⁰⁸ Tl, and ²¹⁴ Bi are varied, ^{the 214} Bi peak is more important than the rate of increase in the peak count rates of ⁴⁰ K and ²⁰⁸ Tl due to these changes in determining the crack location. It was determined that the rate of increase in the counting rate must be higher than m%. As for this operation method, under the conditions of the pattern shown in the table, P _A −P _C > m ......(B) Under the conditions of the pattern shown in the table, P _B −100 ² /P _C >m ......(B) We decided to add a condition that, when satisfied, is considered a crack location. This is because if we add a bar to the average value of n peak count rates for each nuclide and add Δ to the increase in each nuclide at the target point, the left side of equation (a) becomes As shown in the table, in the pattern ΔK/
is larger than ΔTl/Tl, and the above formula is ΔBi/
This is because the amount corresponding to ΔK/ is determined by the geological conditions, etc., and the ratio of the amount of increase thought to have increased from the remaining cracks to the value of ΔTl/ is calculated. Similarly, the left side of equation (b) is As shown in the table, in the pattern Δ
Tl/ is larger than ΔK/ and the above formula is ΔBi/
This is because the amount corresponding to ΔTl/ is determined by ground surface conditions, etc., and the ratio of the amount of increase thought to have risen from the remaining cracks to the value of ΔK/ is calculated. In addition, in the case of the pattern shown in the table, ²⁰⁸ Tl and
Since the rate of increase at ⁴⁰ K is constant, there is no need to perform these considerations, and cracks in the stratum can be determined by simply considering the following equations (c) and (d). P _A ≧100+m (c) P _B ≧100+m (d) Therefore, putting the above things in order, no matter what conditions the strata are under, if the following formula is satisfied, This means that it can be determined that there is a crack in the stratum. Next, a measurement example of the above-mentioned method of the present invention in Hadano Town, Kanagawa Prefecture, where there is an active fault in the Quaternary period, will be explained, where the topography and strata are complex and there are many human-induced conditions, making analysis difficult. The detector uses 10 4" x 4" NaI crystals, is fixed to the floor of the car at a height of about 25cm above the ground, and is detected every 30 seconds while the car is running at 7km/h. Measurement of natural gamma rays was performed, and the gamma rays measured from the luminescence state within the NaI crystal were converted into counting rates for each energy, and among these, ⁴⁰ K, 208 Tl, ²⁰⁸ Tl,
The window count rate of ²¹⁴ Bi is determined, and the peak count rate of each nuclide is determined using the values of α0.199, β = 0.380, and γ = 0.368, which were previously determined experimentally. Next, calculate the peak ratio A, peak ratio B, and peak ratio C from them, calculate the moving average value of each peak ratio with (n-1) as 5, and calculate the peak fluctuation rate P from both.
Find _A , P _BX , and P _C , and then calculate the P _A − P _C value and P _A
−100 ² /P _C value was determined. They are illustrated in Figure 4. Average peak count rate N _C is 2500 counts, N _A is
If you calculate the value of m from 800 counts and _NB is 700 counts, it will be 5.3%. When this measurement example is considered below using the method of the present invention as a reference, as is clear from FIG. 4, each peak ratio and its peak fluctuation rate fluctuate within a considerable range. Among the points that satisfy the inequalities (1), (2), and (3), it is determined that cracks have occurred at point a, which is approximately 1200 m from point A2, and point b. However, in Figure 4, it is almost
At point c, which is 2200 m away, the peak ratio A of ²¹⁴ Bi and ²⁰⁸ Tl is larger than that at the two points above.
Although cracks appear to be caused by an increase in the detected dose of ²¹⁴ Bi, none of the above-mentioned inequalities (a), (b), (c), and (d) for determining cracks are satisfied. This is due to the low content of ²⁰⁸ Tl in this formation.
As a result, the peak ratio A of ²¹⁴ Bi and ²⁰⁸ Tl showed a larger value than that at the two points a and b, which is thought to be due to a statistical error due to the low content of ²⁰⁸ Tl, and is not due to cracks. It is judged that it is not. Although Fig. 4 shows an example in which a detector is mounted on a car to detect crack positions, a portable detector can also be used to detect crack positions in areas where cars cannot enter. . Furthermore, although the method of the present invention aims to detect the position of cracks in strata, the present invention is not limited to simply detecting the positions of cracks in strata. In addition to examining the distribution state of the values of peak ratios A, B, and C, groundwater under the cracks in the strata, hot spring water,
In addition to analyzing deposits of geothermal steam, uranium, natural gas, oil, etc., it has an extremely wide range of applications, including the ability to predict the distribution of cracks in geological formations and abnormalities on the earth's surface due to diagenesis, etc. As described above, according to the method of the present invention, among the gamma rays emitted from the three gamma ray-emitting nuclides contained in the surface layer, ⁴⁰ K, ²⁰⁸ Tl, and ²¹⁴ Bi, those gamma rays that are incident on the NaI detector at the ground surface are detected. The peak count rate for gamma rays that showed the photoelectric effect for each nuclide was measured, and the peak ratio A of each nuclide,
B, C and peak fluctuation rates P _A , P _B , P _C are determined, and when the peak fluctuation rates P _A , P _B , and P _C satisfy predetermined conditions, it is determined that there is a crack in the stratum. This method has the effect of being able to detect the position of cracks in the strata extremely easily and reliably. At the same time, the cost required for detecting the crack position can be significantly reduced.

[Brief explanation of the drawing]

Figure 1 is a transition diagram showing the radioactive decay process of the uranium series, Figure 2 is an explanatory diagram showing the peak count rates of ⁴⁰ K, ²¹⁴ Bi and ²⁰⁸ Tl, and Figures 3 and 4 are 3
It is a graph showing the relationship between the peak ratio between nuclides, the peak fluctuation rate, etc., and the measurement position.

Claims (1)

[Claims] 1. Potassium-40, thallium-208, and bismuth-214, which are gamma-ray emitting nuclides contained in the surface layer.
Among the gamma rays emitted from the three nuclides, we measured the peak count rate for the gamma rays that entered the NaI detector at the ground surface and showed the photoelectric effect of each nuclide, and further calculated the peak ratio A of each nuclide, B, C and peak fluctuation rates P _A , P _B , P _C are calculated, and when the peak fluctuation rates P _A , P _B , P _C satisfy the following conditions (1) to (3), A method for detecting the position of a stratum crack on the earth's surface, characterized by determining that there is a crack. A = peak count rate of bismuth-214/peak count rate of thallium-208 B = peak count rate of bismuth-214/peak count rate of potassium-40 C=Potassium-40 peak count rate/Thallium-
208 peak count rate pattern (): P _A = P _B > 100, P _C = 100 Pattern (): P _A > P _B > 100, P _C > 100 Pattern (): P _B > P _A > 100, P _C <100 m: Threshold