Gamma measurements can be made on the ground, in the air and in wells. According to the different physical quantities measured, it can be divided into two types: γ total measurement and γ energy spectrum measurement. Total gamma measurement, referred to as gamma measurement for short, is an integral gamma measurement, which records the total irradiation rate of gamma rays emitted by uranium, thorium and potassium, but it cannot be distinguished. Gamma-ray spectrometry is a differential gamma measurement, which records the gamma-ray irradiation rate on the characteristic energy spectrum profile, and then determines the contents of uranium, thorium and potassium in rocks, thus solving a wider range of geological problems.
12. 1. 1 ground gamma measurement
Calculation of γ -ray irradiation rate 12. 1. 1.
The surface γ -ray radiation rate measured by γ -ray radiometer is related to the shape, scale, radionuclide content, γ -ray energy spectrum composition, caprock characteristics and measurement conditions of geological bodies. Only some simple models are discussed below to understand the basic characteristics of γ -ray irradiation rate distribution around geological bodies.
γ -ray irradiation rate of (1) point source
If the point gamma source is in a uniform medium, the gamma irradiation rate at the distance R(cm) from the point source inside the medium is
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Where m is the mass (g) of radioactive material in the point source; μ is the absorption coefficient (cm-1) of the medium for gamma rays; K is a gamma constant, which is numerically equal to the irradiation rate of gamma rays from a point source with a mass of 1 g 1 cm without gamma ray absorption. K values of uranium, radium, thorium and potassium are respectively
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When measured with different types of instruments, the value of K changes slightly.
When the gamma rays produced by point source pass through several different media, the gamma ray irradiation rate at the distance from point source R is
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In the formula, μi is the absorption coefficient (cm- 1) of the I-th medium for gamma rays, and Ri is the distance (cm) that gamma rays pass through the I-th medium.
(2) γ -ray irradiation rate of frustum rock mass
As shown in figure 12- 1, there is a frustum-shaped rock exposed with a height of l and a radius of upper and lower bottoms of r, with a density of ρ, a mass fraction of radionuclides of w, a self-absorption coefficient of the rock for γ-rays of μ, and an absorption coefficient of air for γ-rays of μ0. Therefore, the gamma-ray irradiation rate generated by the volume element dV with the mass of radioactive material dm in the frustum at a point with a height of h is 0.
Figure 12- 1 Calculation Parameters of γ -ray Irradiance of Truncated Rock Mass
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Take P as the origin of spherical coordinates, and substitute dm=wρdV, dV=r2sinφdrdφdθ into the above formula to integrate the whole volume, then
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Since r 1-r0=lsecφ and r0=Hsecφ, the above formula becomes
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For the integral in (12. 1-4), Jiang function can be introduced.
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Where t=xsecφ. Ginger function is a list function that decays faster than exponential function e-x (see table 12- 1). When x→0, φ (x )→1; When x→∞, φ (x )→ 0. Can prove
Table 12- 1 ginger menu
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Substituting the formula (12. 1-5) into the formula (12. 1-4) (x=μ0H or x=μl+μ0H), the γ -ray irradiation rate generated by frustum at any point P in the air is
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Where φ0 is the opening angle of point P to the radius of the top and bottom of frustum of a cone, and there are
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If the thickness of the truncated cone is infinite (l→∞), the formula (12.5438+0-6) becomes
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When measuring on the ground, the instrument probe moves close to the ground, which can be considered as H→0. The above formula is simplified as follows.
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It is easy to prove that the solid angle of observation point P to the frustum of a cone is
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Therefore, the formula (12. 1-8) can be written as follows.
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The formula (12. 1-9) shows that for the same radioactive rock with uniform radionuclide content, different solid angles of observation points on the rock mass will have a great influence on the ground gamma measurement results. As shown in figure 12-2, the γ -ray irradiation rate measured in the slit is higher than that in the plane, while the γ -ray irradiation rate measured at the top of the micromorphological protrusion is lower. Therefore, we should pay attention to the influence of micro-topography on the measurement results in ground gamma measurement, and generally record the measurement data on the flat surface.
Figure 12-2 Influence of different solid angles on γ measurement
(3) γ -ray irradiation rate of semi-infinite strata
For semi-infinite volume strata, l→∞, R→∞, φ 0→π/2. So in the formula (12. 1-6), cosφ0→0, φ (μ l+μ 0h )→ 0. At this time, the γ -ray irradiation rate of point P at the height h from the ground is
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It can be seen that the γ -ray irradiation rate at point P will decrease according to the law of ginger function with the increase of height.
In the ground survey, at any point on the rock surface, H→0, φ (μ 0h )→ 1, at this time, the γ -ray irradiation rate reaches the maximum.
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(4) γ -ray irradiation rate when there is a covering layer on the semi-infinite rock.
Assuming that the thickness of the non-radioactive coating is H and the absorption coefficient of the coating is μ 1, the γ -ray irradiation rate at any point on the coating surface can be obtained by a derivation method similar to (12. 1- 10).
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The above formula shows that the γ -ray irradiation rate on the overburden of infinite rock mass decreases according to the law of Gingert function with the increase of the thickness of the overburden. The attenuation degree of γ -ray irradiation rate by different density covering materials is different. The greater the coverage density, the more gamma rays absorbed and the faster the irradiance attenuation.
12. 1. 1.2 ground gamma radiometer
The radiometer used for ground gamma measurement consists of a gamma detector and a recording device. The most commonly used γ detector is scintillation counter, which consists of scintillator (phosphor) and photomultiplier tube, and its function is to convert light energy into electric energy (Figure 12-3). When the ray enters the scintillator, its atoms are excited, and when the excited atoms return to the ground state, they will emit photons and flicker. These photons hit the photocathode of the photomultiplier tube, producing photoelectric effect, releasing photoelectrons from the photocathode, and then accelerating diffusion under the action of each multiplier electrode in the photomultiplier tube, finally forming an electron beam, and outputting a voltage pulse on the anode that amplifies the initial optical signal by 105 ~ 108 times. The radiation ray is strong, and the number of pulses generated per unit time is more; The energy of the radiation particles is large, and the amplitude of the pulse is also large. Therefore, the scintillation counter can measure the intensity and energy spectrum of radiation.
Figure 12-3 Working Principle Diagram of Scintillation Counter
Scintillators can be divided into inorganic scintillators (NaI, CsI, ZnS, etc. ) and organic scintillators (anthracene, terphenyl, etc. ). The commonly used NaI(Tl) crystal is that thallium is infiltrated into the sodium iodide crystal as an activator to make the crystal emit visible light and prevent the light from being absorbed by the crystal itself. Since the luminescence time of the crystal is only 10-7s, the highest counting rate can reach 105 cps. Large volume crystal is used to measure gamma rays, and thin crystal (thickness 1 ~ 2 mm) is used to measure x rays.
The recording device of radiometer consists of a set of electronic circuits. The voltage pulse output by the flicker counter is displayed by the reading part of the circuit after amplification, discrimination (selecting pulses with a certain amplitude), shaping (changing irregular pulses into rectangular pulses) and counting.
12. 1. 1.3 working method of ground gamma measurement
Generally, ground gamma measurement should be arranged in areas where geological conditions and geophysical and geochemical exploration conditions are favorable for mineralization. It is most beneficial to measure γ in areas with topographic cutting, well-developed water system, good outcrop, thin overburden and developed mechanical halo and salt halo.
Ground gamma measurement can be divided into three stages: general survey, general survey and detailed investigation. See table 12-2 for the working scale and the distance between points and lines in each stage. The census is conducted in areas where gamma measurement has never been carried out or where the exploration level is low. The work scale of the general survey is 1 ∶ 1 10,000 ~1∶ 50,000, which circles the distant scenic spots for the next work. Generally, the remote scenic spots selected in the investigation stage are surveyed, and the working scale is1:25000 ~1:1:0000. Its task is to study the geological structure characteristics of the work area, find abnormal points and abnormal zones, study their distribution laws, explain the causes of anomalies, and delineate the prospective areas in detail. Carry out detailed investigation in the selected remote scenic area or the periphery of the mining area, with the working scale of1:5000 ~1:1000. Its task is to find out the shape, scale, intensity, geological conditions and mineralization characteristics of the discovered anomalies, so as to evaluate the anomalies and provide basis for deep exposure.
Table 12-2 γ measurement accuracy and point-to-point distance requirements
General survey and general survey adopt route survey method, and the γ survey route should be consistent with the geological survey route. The observation adopts continuous measurement, mainly through strata and structural strike. When geological phenomena such as lithologic changes, structural zones and fracture zones are found, they can be tracked appropriately along the strike. In order to ensure that no anomalies are missed in the range on both sides of the survey line, the survey route can be tortuous. The detailed survey adopts the area survey method, and the survey network is laid in advance according to the selected scale, and the survey line should pass through the geological body to be detected vertically as much as possible.
When working, the γ detector should be placed in a flat place to avoid the influence of micro-topography. Geological conditions near the measuring point shall be recorded. When encountering favorable horizons or obvious changes in lithology, structure and base number, the measuring points should be properly densified.
When measured by gamma-ray radiometer, the recorded gamma-ray exposure rate is caused by many factors, which can be expressed as
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Where: γ -ray irradiation rate produced by radionuclides in rocks or soil near the measuring point; Is the gamma-ray irradiation rate produced by cosmic rays; Is the instrument base; Is the natural base of the instrument.
Because the irradiation rate of cosmic rays varies with latitude, altitude and time of day and night, the cardinal number of the instrument is also affected by the radionuclide content in the detector, the pollution degree of the instrument, the noise intensity and the number of false pulses of the instrument, and the service time of the instrument. Therefore, the natural basis of radiometer is not constant. However, this change is generally not big, and its share in the bedrock is small, which can be regarded as a constant. Different musical instruments may have different natural foundations. When gamma measurement is carried out by multiple instruments, especially in environmental gamma background investigation, radionuclide quantitative measurement and measurement of low gamma field below background, in order to unify and compare the measurement results, it is necessary to determine the natural background of each instrument.
There are many methods to determine natural basement, such as lead screen method, underwater method and water surface method, among which underwater method is the simplest. Select the water area with water depth above 1.5 m, water surface diameter above 2 m and no radioactive pollution, seal the γ -ray radiometer with plastic sheets and place it 50 cm underwater. The reading obtained at this time is the natural base.
The instrument readings produced by the normal content of radionuclides in rocks are called rock reference values or background values. Various rocks have different bases, which can be obtained as normal field values by statistical methods. In field work, the γ -ray irradiation rate is more than 3 times higher than that of the surrounding rock basement, which is controlled by certain lithology or structure and is uranium or uranium-thorium mixture, which is called anomaly. If the gamma-ray irradiation rate is high (higher than the surrounding rock basement plus three times the mean square deviation), but it does not meet the standard of abnormal irradiation rate, and the geological quality ore-controlling factors are obvious and have a certain scale, it is also called abnormal point. It should be pointed out that the above standards are not suitable for solving non-uranium geological problems. For example, when looking for water storage structures, the anomaly is only 10% ~ 80% higher than the cardinal number. Therefore, when solving the geological problems of non-uranium deposits, all the above cardinal numbers are abnormal points. Abnormal distribution is controlled by the same rock stratum or structure, and the length is continuously greater than 20 m, which is called abnormal zone. Major anomalies should be exposed by Guangshan Project. On the basis of geological and geophysical logging and sampling analysis, suggestions for further work are put forward.
When the balance of radium and uranium in the survey area is destroyed and the balance is obviously biased towards uranium, because the γ -ray irradiation rate of uranium is very small, β+γ measurement should be adopted, that is, the total irradiation rate of β -rays and γ -rays should be measured with the instrument recording β -rays. When it is necessary to find out the uranium prospect in the residual soil covered area, in-hole gamma measurement can be used.
In order to evaluate the quality of ground gamma measurement, inspection routes should be arranged. Inspection routes should be arranged in areas with favorable geological conditions or areas with questionable work quality. The inspection workload shall not be less than 10% of the survey workload. The standard of high working quality is that there are no major anomalies and omissions, and there is no obvious difference between the measured curve and the original measured curve.
The main factor affecting the measurement accuracy is the statistical fluctuation of nuclear decay. According to the formula (1 1.2- 16), the way to improve the accuracy is to have enough pulse counting. In practical work, we can solve this problem by extending the measurement time and increasing the number of measurements.
In order to ensure the quality of work, the performance of the instrument must be checked with the working standard source before and after work every day. When the reading difference between standard source and non-standard source at a fixed point is within the allowable range of statistical fluctuation, the instrument can be considered to work normally; Otherwise, the instrument should be recalibrated. At the same time, we should regularly check the stability and accuracy of the instruments and the consistency of comparison among multiple instruments.
12. 1. 1.4 collation and illustration of ground gamma measurement data
(1) ground gamma measurement data collation
The arrangement of ground gamma measurement data includes converting reading (counting rate) into gamma ray exposure rate, determining rock basement and calculating the mean square error of statistical fluctuation of rock gamma ray exposure rate.
In order to obtain the rock basement, the frequency histogram (or probability distribution curve) should be drawn according to the measured γ -ray irradiation rate. If the gamma ray irradiation rate of rock obeys the arithmetic normal distribution, the (arithmetic) average value of the rock irradiation rate is
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Mean square deviation is
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Where n is the number of statistical groups; Is the frequency of group I; Is the group median of group I.
If the gamma irradiation rate of rock obeys lognormal distribution, the geometric mean and mean square error of rock irradiation rate are
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Take it as the rock foundation and +3σ as the lower limit of anomaly (except for non-uranium geological work).
The rock cardinal number and abnormal lower limit can also be directly read out on the cumulative percentage distribution map or cumulative percentage distribution curve.
(2) Map of ground investigation results
Ground gamma measurement results mainly include: gamma exposure profile, gamma exposure profile, gamma exposure contour map and relative gamma exposure contour map.
Draw the isogram of γ irradiation rate according to 3 σ, 2 σ and σ. Different rocks have different bases, and the variation range (mean square deviation) of γ -ray irradiation rate of different rocks is also different, which will affect the accuracy of γ -ray irradiation rate isoline map. Therefore, in each lithologic range, according to their respective+σ, +2σ and +3σ, the gamma field can be divided into three levels: high field, high field and abnormal field, and then the points with the same gamma irradiation rate of various lithology are connected respectively (regardless of whether their lithology is the same or not), thus forming an isogram of relative gamma irradiation rate (figure 12-4). This map avoids the interference caused by different rock background values, comprehensively reflects the gamma field characteristics of different lithology, and can clearly reflect the relationship between gamma halo and mineralization and structure, which is conducive to studying the metallogenic law and speculating favorable metallogenic areas.
Figure 12-4 Isogram of Relative γ -irradiation Rate in a Region
12. 1. 1.5 ground gamma measurement data interpretation and examples
The data interpretation of surface gamma measurement is qualitative, because the detection depth of gamma measurement is shallow, 1 ~ 2 m, and generally only the area with increased surface radionuclides can be circled, so it is difficult to find deep-buried ore bodies. In addition, the γ -ray irradiation rate does not always reflect the enrichment degree of uranium. Because the main γ radiator in uranium series is a nuclide belonging to radium family, the source of γ anomaly is mainly radium rather than uranium.
Radionuclides are widely distributed in nature. It is not difficult to find anomalies in gamma measurement, but it is not easy to evaluate anomalies. When the deposit is exposed to the surface or oxidation zone and there are signs of fracture nearby, uranium is easily taken away by acid dissolution due to weathering and leaching. Results With the increase of radium, the balance is biased towards radium, resulting in high γ -ray irradiation rate and low uranium content. If the transported uranium is reduced and deposited in an appropriate environment, or radium is taken away, and uranium dissolves little in the reducing environment, the balance will be biased towards uranium. At this time, the γ -ray irradiation rate is not high, but uranium is rich. Therefore, special attention should be paid to judging whether uranium and radium are in a long-term equilibrium state by using the uranium and radium equilibrium coefficient, and it is not possible to evaluate the anomaly only by using the γ -ray irradiation rate. At the same time, geophysical data such as geology and geochemistry of abnormal points (zones) should be comprehensively applied for analysis to make correct abnormal judgment.
Figure 12-5 Comprehensive plan of geology and relative gamma irradiation rate of a certain area
Ground gamma measurement has the characteristics of light instrument, simple method, flexible work, low cost and high efficiency. In addition to directly searching for uranium and thorium deposits and determining metallogenic prospects, it is also used for geological mapping, searching for other minerals related to radionuclides, detecting groundwater and solving other geological problems.
Figure 12-5 is an example of ground gamma survey for uranium deposits. Early Yanshanian granite was found in this area, and the main lithology is medium-fine grained granite. The floating soil area in the area is large, magmatic activity is frequent, and the structure is complex, showing east-west distribution. Gamma survey delineated two anomalies and two high fields, both of which have a certain scale and still exist after being exposed to the surface. In the high area, we have carried out emanation measurement, uranium quantity measurement and associated element prospecting, and all of them have shown results. After exploration, uranium deposits were found in 1, No.2 anomaly and No.3 high area, and uranium mineralization was found in No.4 high area.
12.10.2 ground gamma-ray spectrum measurement
As mentioned earlier, both uranium and thorium systems have several main gamma radiation sources. Therefore, in the uranium-thorium mixed area, it is difficult to determine the nature of anomalies by ground gamma measurement, while ground gamma spectrometry can often obtain better geological results.
12. 1.2. 1 ground gamma spectrometer and instrument energy spectrum
The scintillation counter of the ground gamma spectrometer can convert the energy of gamma rays into electric pulses, and the amplitude of the output pulse is proportional to the energy of gamma rays, so the energy spectrum measurement is actually to analyze the pulse amplitude. The circuit that completes this function is called pulse amplitude analyzer. Its principle is shown in figure 12-6(b), which consists of an upper discriminator, a lower discriminator and an anti-coincidence circuit. Discriminator is a device that only allows pulses whose amplitude is higher than a certain value (called discrimination threshold) to pass through. The threshold voltage of the upper frequency discriminator is high, and only the pulse with large amplitude (such as pulse No.9) can pass. The threshold voltage of the lower discriminator is low, so pulses that can pass through the upper discriminator (such as pulse No.9) can pass, and pulses with amplitudes between the upper discriminator and the lower discriminator (such as pulses No.3, No.5 and No.8) can also pass. The signals output by the two discriminators are sent to the anti-coincidence circuit. The characteristic of anti-coincidence circuit is that when the upper discriminator and the lower discriminator have the same signal output at the same time, these signals cancel each other in anti-coincidence circuit. Therefore, the anti-coincidence circuit only outputs pulses (No.3, No.5 and No.8 pulses) between the upper and lower discrimination threshold voltages, and then counts and records them.
The difference between the upper and lower discrimination threshold voltages is called the track width. After the track width is fixed, by adjusting the lower discrimination threshold voltage (the upper discrimination threshold voltage changes accordingly), pulses with different amplitudes can be gradually picked out. This method of pulse amplitude analysis is called differential measurement. The measured spectral lines are called differential spectra.
If the pulse amplitude analyzer uses only one lower discriminator, record all pulses (No.3, No.5, No.8 and No.9 in Figure 12-6(a)) whose amplitudes exceed the threshold voltage of the lower discriminator. This method of pulse amplitude analysis is called integral measurement. The measured spectral lines are called integral spectra.
In practical work, the γ energy spectrum measured by γ energy spectrometer is not a line spectrum, but an instrument spectrum complicated by various factors (Figure 12-7). It is a continuous spectrum formed by photoelectric effect, Compton scattering and electron pair effect when gamma rays pass through substances (rocks, soil, detector elements of spectrometer, etc.). ). Compared with the line spectrum, the characteristic peaks of U, th and K mentioned above are not prominent enough, but they can still be distinguished.
Figure 12-6 Principle of Pulse Amplitude Analyzer
Fig. 12-7 NAI(TL) differential instrument spectrum and selection of u, Th and k channels.
12. 1.2.2 Calculation of uranium, thorium and potassium contents
The gamma spectrometer records the total gamma ray counting rate above a certain energy threshold with an integral channel (> > 50 kiloelectron volts), and also measures the counting rate generated by three energy spectrum segments of gamma rays with three differential channels. Among them, the width of the potassium channel is 0.2 MeV, and the center of the identified γ spectrum segment can be selected at the 40K characteristic peak of 1.46 MeV; The width of uranium channel is 0.2 MeV, and the spectrum center can be selected at the characteristic peak of uranium series 2 14Bi 1.76 MeV; The width of the thorium channel is 0.4 MeV, and the center of the spectrum can be selected at 2.62 MeV of the characteristic peak of the thorium system 208Tl. All three spectral bands are selected in the high energy region, which can reduce the influence of scattered gamma rays. The three spectral bands are independent of each other, and the target nuclide spectral line is the main component in each spectral band, which is beneficial to improve the stability of the solution of the calculation equation (Figure 12-7).
Let the counting rates (minus radix) of potassium, uranium and thorium channels be I 1, I2 and I3 (calculated by cpm) respectively, then their mass fractions with U, th and K are w(U), w(Th) and w(K) (calculated by 10-6 and 66 respectively.
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In the formula, the coefficients ai, bi and CI (I = 1, 2,3) are called conversion coefficients, which respectively represent the counting rates of uranium, thorium and potassium per unit content in different measuring channels (CPM/ 10-6, CPM/ 10-6, CPM/respectively).
The mass fractions of uranium, thorium and potassium can be obtained by solving the above equations.
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formula
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12. 1.2.3 working method and result diagram of ground gamma-ray spectrometry.
The working method of ground gamma-ray spectrometry is similar to ground gamma-ray spectrometry, but ground gamma-ray spectrometry requires fixed-point timing reading according to the pre-arranged measurement network, and the reading time is generally 1min. Microcomputer-based γ -ray spectrometer has realized automatic data collection, preliminary data arrangement and drawing profile.
Indoor, the data collected in the field can be directly input into the computer, and various maps can be quickly formed on the screen, and explained through human-computer interaction.
Ground gamma-ray spectrum measurement results include: profiles, profiles and isolines of uranium, thorium and potassium contents, and sometimes profiles or isolines of thorium-uranium ratio [w (th)/w (u), thorium-potassium ratio w (th)/w (k) and uranium-potassium ratio w (u)/w (k)] are drawn.
12. 1.2.4 application of ground gamma-ray spectrum measurement
Ground gamma-ray spectrometry can directly search for uranium and thorium deposits, as well as metal and nonmetal deposits related to radionuclides. Using the distribution data of uranium, thorium and potassium contents and their ratios, we can also infer the formation conditions and evolution process of magmatic rocks and sedimentary rocks, and detect the metallogenic characteristics and genesis of the deposit.
Figure 12-8 is an example of searching for gold-bearing structural zones by using gamma energy spectrum. Near the gold-bearing vein, the γ total curve and K content curve are low, while the U and Th content curves are high, while the W (u)/W (th), W (u)/W (k) and W (th)/W (k) values are obviously abnormal. Combined with these curves, the location of gold-bearing veins can be determined. According to the positions of two high values in the potassium content curve on both sides of the vein, the width of the potassium ore belt can be roughly estimated.
Figure 12-8 Measurement curve of ground gamma-ray spectrum in Shandong Province