Gas measurement includes two main methods, namely soil gas measurement and soil gas measurement. Gas measurement in soil begins with geochemical exploration of oil and gas. In 1930s, Germany (Boemmel, 1929) and Soviet Union (sokolov, 1933) began to look for oil and gas reservoirs by using hydrocarbon anomalies in soil gas. Later, this method was extended to metal ore exploration (Hawkes and Webb, 1962), and the indicators were extended to Hg, CO2, Rn, he, CH4, especially the measurement of mercury gas. However, there are many interference factors and the observation results fluctuate greatly, which limits its application. From 1960s to 1980s, due to the improvement of testing technology, a set of geochemical measurement methods for the determination of bound gases in soil samples was developed, which is called soil gas measurement. By releasing and measuring adsorbed, adsorbed and bound gases in soil by means of decompression, heating and dissolution, relatively stable observation results can be obtained and the prospecting effect can be improved. Widely used in oil and gas and metal mineralization exploration. In the late 1980s, geogas method used the form of gas sampling and improved technology to collect the solid substances carried by underground airflow and analyze the element content, which realized the evolution of gas geochemical method, expanded the application scope and improved the application effect. However, the traditional gas geochemical method is still an important means of deep prospecting. Here are some examples.
Second, the scope of application and application examples
The measurement technology of soil pyrolysis gas in Australia is briefly introduced.
Measurement of trace components of volatile compounds adsorbed on soil clay particles by soil adsorption gas pyrolysis technology. Researchers believe that light hydrocarbon volatiles can easily migrate in thick rocks. Most of these compounds can reach the surface, most of them are dissipated in the atmosphere, and only a small amount of gas is adsorbed on the surface of soil particles. This deep gas can be detected by special methods. As we all know, the continuous degassing of rocks is a common natural phenomenon, which is the result of geological body deposition, metamorphism and interaction with groundwater. The deposit and the fluid forming the deposit are obviously different from their surroundings in chemical properties. When this degassed gas flow passes through different geological bodies, it can carry different components (ore-forming components), which leads to some differences between the gas signal above the deposit and the regional background. Although this difference is very weak, it can be detected by a unique measurement method (SDP).
SDP technology is realized by analyzing the gas composition in the surface soil. The ideal sampling point must meet three conditions: good representativeness; In the past five years, the soil has not been destroyed or polluted. Each soil sample should be collected at multiple points in the range of 1m2, and combined into a combined sample to increase the representativeness of the sample. Samples should be collected and sealed in plastic sample bags, and cloth or paper bags are not allowed. Before sampling, the air in the bag must be exhausted and sealed as much as possible. Compared with the active or passive methods used in the past, SDP technology can not only eliminate the influence of climate change, but also has high sampling efficiency and controllable representativeness, because soil, as a natural adsorbent, captures the deep information in a long geological history period and the results are more stable.
Because gas measurement is influenced by many factors, its measurement accuracy is relatively low, and the effect of directly expressing the results by content is not good. Therefore, SDP measurement results are expressed as ratios. This can not only eliminate the noise of environmental impact, but also filter the abnormal seepage above the fault and strengthen the abnormal mineralization or target body. Therefore, SDP survey adopts special methods to process data, namely, total amount (SDP Sum) and count (SDP Count), so as to better express the comprehensive geochemical information of multiple indicators.
1. Spencer copper mine in Chile
The deposit is a porphyry copper mine, located at 120km northeast of Antofagasta in northern Chile, with an altitude of 1700m and a dry climate. Spence deposit is buried in the transportation gravel with 40 ~ 100 m underground, which is rich in salt groundwater. The deposit has a typical oxidation profile: from bottom to top, sulfide → oxidized sulfide → leaching iron cap.
The soil geochemical gas measurement of Spence porphyry copper mine in Chile can well indicate the deep concealed ore bodies. The total amount of SDP indicates the distribution of shallow oxidized ore bodies, and the abnormal position and range completely correspond to the range of oxidation zone. The unusually strong peak in the east may be caused by the influence of near-surface substances, including a series of gaseous substances such as hydrocarbons, halogens and carbon dioxide. Spence deposit was finally discovered through systematic reverse circulation shallow drilling engineering. Although the new geophysical and geochemical exploration methods can't replace drilling, these technologies are relatively low-cost and fast, which is of great help to determine the drilling position faster and more accurately. In addition, the application of these technologies can greatly reduce the cost of concealed ore exploration.
2. Osborne copper and gold mine, Australia
Located at the southern end of Successive Mountain in the east of Issa Block, Australia, the mine is a copper-gold deposit, surrounded by Mesoproterozoic quartzite and iron stone, and the rock stratum is covered by Mesozoic sediments with a thickness of 30 ~ 40m. In the west and north of the mining area, there are sulfide-bearing thin-layer siliceous ore belts related to magnetite-pyrite. The main ore body in the east is a high-grade siliceous ore body rich in pyrrhotite. The total thickness of the overburden of the eastern ore body is about 300 meters.
The distribution of SDP soil survey points is shown in figure 1. The sampling interval is irregular, and the background area is 100m, which is 50m near mineralization and 25m above mineralization respectively. The gas measurement results are processed by using the standard template of Spencer deposit, and the anomalies of Osborne deposit are also well displayed, which shows that SDP technology has strong exploration ability of copper and gold deposits in arid areas.
Figure 1 Data processing of SDP survey of Osborne deposit by using general model.
SDP technology is not only effective in the above-mentioned arid areas, but also successful in temperate climate conditions. Especially in the Canadian Arctic, SDP technology has been successfully applied to diamond exploration.
According to the information published by BHP Billiton, the success rate of distinguishing kimberlite without ore from kimberlite with diamond is 80% by using SDP technology combined with selective extraction technology.
Three. sources of information
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Tang, Yang, et al. 2004. Research and application of geogas measurement method. Geophysical and geochemical exploration, 03: 193 ~ 198.
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Wang Mingqi. Present situation and development trend of international exploration geochemistry in 2005 ——2 1 Introduction to the International Conference on Exploration Geochemistry. Progress in Earth Science, 04: 477 ~ 478.
Wang Mingqi. The present situation and development trend of international applied geochemistry from the first international conference on applied geochemistry in 2006. Progress in Earth Science, 0 1: 83 ~ 84.
Wu Junhua, Yuan Chengxian, Zhao Gan. Prediction theory, detection method and development status of concealed ore bodies. Geological exploration series, 03: 188 ~ 195.
Zhang Xiangnian, Wang Mingqi, Gao Yuyan, et al. 2007. Research status and main problems of geology and natural gas exploration methods. Guide to scientific and technological consultation, 2 1: 7 1.
Forrest was completely buried in 2006. material world
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SDP Pte Ltd, 2008, What is the endothermic decomposition of soil? A new tool for exploration in overlying strata. [EB/OL]/SDP/tech/technology . html