(Aerogeophysical Remote Sensing Center, Ministry of Geology and Mineral Resources, Beijing 100083)

Due to the shortage of potash fertilizer in China, people have attached great importance to the research of various new methods for finding potassium in recent 30 years. These new methods include: gamma spectrometry logging, airborne gamma spectrometry, remote sensing, gravity and so on. This paper briefly summarizes our main research results in this field. 1963 puts forward and studies gamma-ray spectrometry logging method and underground gamma-ray spectrometer. Before 1980s, the instrument was modified many times, and the gamma-ray spectrum logging instrument and method were successfully used in six provinces. 1995 developed a software system for interactive interpretation of borehole radioactive energy spectrum, including five functional programs: pretreatment, quantitative interpretation, trend analysis, graphic display and comprehensive interpretation. Starting from 1984, high-sensitivity aerial survey was carried out to explore evaporated potash deposits in western China. Usually four parameters are measured: k, Th, u and magnetic field. In order to preprocess, process, stabilize spectrum and process images, an airborne multi-channel gamma energy processing software package (AMGPSP) and an airborne geophysical data image processing software package (AGIP-SP) are developed. Since the mid-1980s, digital image processing technology has been applied to the comprehensive interpretation of geophysical and remote sensing data (MSS and TM) to study the evaporated potassium basin in western China. At the beginning of 1993, a special software package (3DIG) for 3D visualization of gravity field was developed. Firstly, the three-dimensional data array of gravity field is established, and then it is displayed in various forms to reveal the internal characteristics of gravity field.

Gamma spectrometer in potash well; Airborne gamma spectrum; Three-dimensional visualization of remote sensing gravity field

1 gamma spectrum logging method

According to statistics, 50% of the world's total potassium salt reserves are found in boreholes. In order to find potassium salt and distinguish the interference of thorium-bearing and uranium-bearing argillaceous rocks on potassium salt, the gamma-ray spectrometry logging method was studied in 1963, and the borehole gamma-ray spectrometer was developed. Before 1980s, the instrument underwent five modifications.

This method is based on the natural radioactive isotope 40K. The isotope abundance of 40K is 0.0 1 19%, the radiation coefficient of its γ quantum is 1 1.6%, and the medium energy is 1.46MeV, which is suitable for detection and energy spectrum analysis.

The developed gamma spectrometer for four wells uses cylindrical sodium iodide crystal to detect the gamma spectrum. These four channels are (Figure 1): PK channel (differential channel with peak value of 1.46MeV at 40K and energy threshold of1.40 ~1.52 MeV); LI channel (left integral channel with energy threshold ≥1.30 MeV); RI channel (right integration channel, energy threshold ≥1.60 mev); And TC channel (total counting channel with energy threshold ≥0.2MeV). In the process of making in-well gamma spectrometer, the following technical points were studied:

1. 1 sensitivity

Because of the low isotope abundance and gamma radiation coefficient of 40K, it is necessary to improve the sensitivity of gamma spectrometer in well. Therefore, the length of the Nal crystal is increased to 100 ~ 150mm, and its diameter cannot exceed 50mm due to the limitation of the inner diameter of the borehole.

Figure 1 Four-hole gamma spectrometer uses cylindrical sodium iodide crystal to detect gamma-ray energy spectrum.

These four channels are: PK channel (differential channel, the peak value is 1.46MeV at 40K, and its energy threshold is1.40 ~1.52mev); LI channel (left integral channel with energy threshold ≥1.30 MeV); RI channel (right integration channel, energy threshold ≥1.60 mev); And TC channel (total value, energy threshold ≥0.2MeV).

1.2 pulse transmission

In order to maintain the resolution of the instrument, the pulse output by the photomultiplier tube must pass through the preamplifier and be transmitted from the downhole instrument to the ground instrument without distortion in waveform and amplitude. Select the voltage-current transmission scheme. The resolution of 137Cs remains unchanged (10%) after being transmitted through 3600m cable.

1.3 stability

The vertical temperature gradient of the crust is about every 33m 1℃, so the ambient temperature at the depth of 3500m in the borehole may be as high as 100℃. In order to make the spectrometer achieve reliable stability, the following three measures have been taken: selecting high temperature NaI crystal and photomultiplier tube; Compensating the negative temperature coefficient of the photomultiplier gain with the positive temperature coefficient of the high voltage power supply provided to the photomultiplier; 137Cs automatic spectrum stabilization circuit is adopted to keep the peak position stable.

Any photoelectric peak of isotope is superimposed on Compton extension line of higher energy photoelectric peak. The background under the 40K photoelectric peak is proportional to the right integral RI (figure 1), which is the theoretical basis of K dissolution method and principal component analysis. We have studied the methods of principal component analysis and spectral stripping to suppress the interference of uranium and thorium in order to obtain pure potassium anomalies. The system of equations (1) shows the mathematical process of principal component analysis. Its feature vector is obtained by statistics.

Proceedings of the 30th International Geological Congress Volume 20 Geophysics

Where: x is the original variable; E is a new variable and a linear function of the original variable; I'm identity matrix.

1995 borehole radioactive energy spectrum interpretation software system (HRSIS) has been successfully developed, which consists of five functional programs: pretreatment, quantitative interpretation, trend analysis, graphic display and comprehensive interpretation. The sixth group is HELPs (help-seeking function). HRSIS system is written in C language on microcomputer. Fig. 2 shows an image example of logging results of a borehole located in the center of sedimentary activity in Kunteyi Basin, Qinghai Province. In fig. 2, KCl layer shows high left integral (LI), high peak difference (PK) and high total count (TC), but low right integral (RI). In addition to KCl layer, three lacustrine sedimentary stages can be clearly seen.

Fig. 2 Example of logging image

The borehole is located in the center of sedimentary activity in Kunteyi Basin, Qinghai Province.

* The leftmost is the original gamma-ray spectrum logging curve, and the scale is adjusted by using the feature vector.

* The middle part is a composite tricolor image of PK, LI and RI. Rock salt layer is dark because of its low content of radioactive elements, while potassium salt layer or rock salt layer is bright, and argillaceous rock layer is also bright because of its high content of uranium, thorium and potassium.

The trend curve of *K and The trend curve of Th-U show the decrease of th and u and the increase of k, which reflect three stages of basin evolution: semi-salt lake stage; Super-saltwater lake stage; Alternating stage of dry salt lake and super salt lake.

* The K curve obtained by principal component analysis shows the polyhalite-rich layers at the depths of 60m and 130m.

Gamma spectrum logging method has been successfully applied in Yunnan, Hubei, Sichuan, Shandong, Xinjiang and Qinghai provinces. Potassium-bearing layer with thickness ≥0.5m and KCl content ≥2% can be detected in Mengyejing and other mining areas. The Ministry of Petroleum has formulated the technical policy of "developing both oil and salt". Encouraged by this policy, a layer of potassium mirabilite with a thickness of 0.8m was measured in Jianghan Oilfield, and its KCl content was as high as 16.8%. A layer of polyhalite with thickness ≥0.2m was found in Sichuan. 199 1 In a new exploration area in Yunnan Province, all the cores of the first borehole were lost. Only gamma-ray spectrum logging can make up for the borehole data, and several layers containing potassium salt are found, thus avoiding the economic loss of re-drilling.

Airborne gamma-ray spectrometry

Since 1984, a high-sensitivity aerogeophysical survey has been carried out in western China, with the focus on finding evaporated potassium ore. Usually four parameters are recorded: k, Th, u and magnetic field. Multi-channel (5 12) or 4-channel airborne gamma spectrometer can be used. The total volume of the square-column sodium iodide crystal used can reach 50dm3, the flying height is about 90m, the line spacing is 1 ~ 2km, and the measuring scale is1:10000 ~1:200000.

In 1980s, the software packages of airborne multi-channel gamma spectrometry processing (AMGPSP) and airborne geophysical image processing (AGIPSP) were developed, both of which were written in FORIRAN language. Using these two software packages, the gamma-ray spectrum software can be stabilized, preprocessed and processed, as well as the image processing of aviation geophysical data. The area looking for potassium salt in western China has sparse vegetation and flat terrain, which is an ideal area for aerial release. The dynamic range of data is usually as follows: k-0 ~ 8%, th-(0 ~ 40) × 10-6, u-(0 ~ 15) × 10-6.

Digital image processing technology is very helpful to extract the following geological information: lithologic mapping; Looking for potassium directly; Research on potash deposits: extracting structural features: in some cases, it can also judge oil and gas anomalies. Airborne gamma-ray spectrometry is one of the few methods that can directly detect some elements.

For example, the ternary image synthesized by K, Th and U data as the measurement result of airborne gamma-ray spectrum is actually a ternary regional geochemical map, which is synthesized by Landsat image (TM or MSS). Three-element images of k, Th and u can be drawn by supervised or unsupervised classification methods.

Muddy rocks (sandstone, mudstone, etc. ) is rich in potassium oxide, which becomes the main interference in the direct search for potassium salt. By analyzing the images of K and Th elements and their scatter plots, it is found that there is a certain linear correlation between the contents of K and Th in argillaceous rocks. In order to determine KCl directly, a series of image processing techniques are tried, such as principal component analysis, color coordinate system transformation, vegetation extraction and so on.

According to the above phenomena and successful tests, KCl and potassium oxide can be distinguished in Tertiary sandstone in the northeast corner of Qaidam Basin. In this survey area, 10 new KCl anomalies were delineated, one of which was verified on the ground, and the solid KCl reserves increased by 3.245 million tons. KCl brine is of special importance to the survey data. The surface water with K≥0.6% is delineated as KCl brine through the image.

Through supervised classification, two kinds of original 14 lithologic maps have been added. Not only KCl and potassium oxide, but also KCl and argillaceous KCl were successfully distinguished.

3 Remote sensing measurement

The TM data of seven bands have high energy spectrum and spatial resolution, which is more useful for the study of potassium-bearing evaporation basins.

Although it is always hoped to use as many bands as possible in order to extract geological information, it is still inseparable from a ternary image with the best display effect. The best ternary image should contain the most information and the correlation between the three bands is the lowest. The combination correlation factor q is used as the measure and basis of the optimal combination.

Proceedings of the 30th International Geological Congress Volume 20 Geophysics

Where: Si is the variation of I-band; Ri is the correlation coefficient.

The best ternary image should have the largest q value. For example, in the TM data of Kunteyi Basin, the combination effect of 1, 4 and 7 bands is the best. Kunteyi basin is a modern salt lake, which is of certain importance to the exploration of potassium salt. It is located in the northwest of Qaidam Basin in Qinghai Province. Fig. 3a is a black-and-white image, which clearly shows the overall landscape of the Kunteyi Basin. It is a diamond-shaped closed dry salt lake with a length of 80km and a width of 30km, which is crossed by a highway. Dry salt lake is bordered by Altun Mountain in the north, and there are two anticlines in the east, west and south, and some anticlines have oil fields with industrial value.

Fig. 3 Landscape of Kunteyi Basin

The synthetic image composed of 1, 4 and 7 band A-TM data clearly shows the general landscape of Kunteyi Basin in western China, which is a diamond-shaped closed dry salt lake with a length of 80km and a width of 30km. B— A valuable abnormal image is obtained by KL conversion of 1, 4, 5, 6 and 7 band data, and KCl on the image shows white bright spots.

In order to establish the anomaly sign of solid potassium salt, the image was sampled, and 14 geological targets were selected on the image, and their brightness values in 7 TM bands were counted. The result of image sampling is shown in Figure 4. Solid KCl (No.12 and 13) has obvious distinguishable spectral characteristics, which provides a basis for interpretation.

On the basis of spectral analysis, the anomaly of solid potassium salt was extracted by KL transform (principal component analysis) and segmented cutting and stretching (Figure 3b), and the results of these two methods were very close. In addition to detecting KCl, the structural characteristics, salt deposition characteristics and the revelation of the material source of salt basin are also studied.

Fig. 4 image sampling results

The curve is the brightness value of 14 geological target TM7 band. Solid KCl (No.12 and 13) has obvious distinguishable spectral characteristics, which provides a basis for establishing abnormal signs of solid potassium salt.

Fig. 5 Example of interpretation results of 3D ig software package

A-CT-like image of No.20 local negative anomaly in Mengla gravity field, Yunnan Province: the section B-65,438+00 of No.20 negative anomaly along the long axis reveals the internal shape of the anomaly, and describes the deformation of rock strata and its three-dimensional relationship with basin faults in detail.

Fig. 6 Interpretation results of No.20 local negative anomaly manually calculated by geophysical prospecting personnel in Yunnan Province.

A —— the average residual gravity difference plane on the circumference with the radius of r 1, r2, r3, r4 and r5; B- interpretation profile; C—— obtained interpretation result —— rock salt body; Section d

Fig. 7 gravity field profile of Mengla.

The main cutting profile image running through the whole gravity field of Mengla from north to south and then from southeast is marked with isoline. Due to the limitation of exhibition, the whole section is cut into four sections.

Three-dimensional visualization of gravity field

The density difference between salt bed and surrounding medium is usually greater than 0.3g/cm3, while the density difference between NaCl and KCl is about 0. 15g/cm3, so gravity exploration is traditionally used to explore salt deposits.

Geophysical prospectors in Yunnan Province often use the difference of average residual gravity on different radius circles to evaluate the shape and size of salt ore bodies (Formula 3).

Proceedings of the 30th International Geological Congress Volume 20 Geophysics

Where: gr 1 and gr2 are the residual gravity field data on the circumference with the radius of r 1 and r2. N 1, n2 is a point on the circumference with a radius of r 1, and r2 is used for the average calculation.

In principle, this average residual gravity difference is close to the first vertical derivative of gravity. Several examples explained by this method have been confirmed by drilling.

The contribution of our work lies in the three-dimensional visualization of gravity data in order to obtain better visibility and faster interpretation. 1993 developed a software package (3DIG) for 3D visualization of gravity field with FORTRAN language. It uses upward continuation, circumferential average residual gravity difference of different radii to establish a three-dimensional data array of gravity field, and then displays it in different forms.

Using this software package, the gravity field of salt-bearing Mengla County in southwest China was estimated. The dynamic range of gravity field is-3.5×10-5 ~ 2.9×10-5m/S2. There are 20 local negative anomalies. By studying each local anomaly, the results are expressed in the form of profile group and "CT", which can reveal the relationship between the interior of the profile and the surrounding media, and describe the deformation of rock strata and its three-dimensional relationship with basin faults in detail. Fig. 5 is an example of such an image of the No.20 local negative anomaly, and the anomaly is also calculated manually for comparison. Fig. 6 is the result of manual calculation. Both Figure 5 and Figure 6 reveal the internal characteristics of the local negative anomaly No.20.. Fig. 7 shows the main cutting profile of gravity field running through the whole area from north to south and then to southeast, and is marked with isoline. Due to the limitation of exhibition, the whole section was cut into four sections.

The depth scale can be obtained by using correlation analysis (such as least square fitting) of drilling results and profile data. Once the approximate depth scale is obtained, the software can easily calculate the buried depth, thickness and reserves of potassium salt or sodium salt layer.

Thanks to Comrade Mi, Li Shoutian, Chen Xianyao and others for participating in the research and development of in-well gamma spectrometer and method. I also thank the geophysicists in Yunnan Province for their efforts in measuring the gravity of salt mines. Thanks to Zhu Yuee, Shi and Xu for their work in software development, and thanks to Comrade Xu for completing the airborne geophysical survey in Qinghai. I also thank Comrade Gao for his support of S600 image processing system. In addition, I would like to thank Comrade Yang and Comrade Li Yi for typesetting and printing the text and map of this article.

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Zhang Yujun. Using image processing technology to study aeromagnetic and radiation anomalies of an oil and gas field. China Journal of Geophysics, 1994, 37:505~5 15.

Zhang Yujun. Digital image processing of airborne magnetic data in central Qaidam basin. Overview of exploration geophysics in China. Tulsa, American Association of Exploration Geophysicists.1989,517 ~ 535.