Gas geochemical exploration methods

The main purposes of gas geochemical exploration are twofold: one is to find solid mineral resources and oil and gas resources, and the other is to evaluate the quality of the atmospheric environment. The current work is mainly focused on ore prospecting geochemistry, looking for ore by studying gas anomalies related to solid minerals and oil and gas resources. Indicator gases used for gas geochemical exploration include: Hg vapor, CO2, SO2, H2S, He, Ne, Ar, Kr, Xe, Rn, F, Cl, Br, I, CH4, etc. The geogas method developed in recent years mainly collects and measures nanometal particles brought by deep underground updrafts to search for geochemical anomalies and achieve the purpose of prospecting.

17.6.1 Mathematical description of gas migration

Gas migration is nothing more than diffusion and air flow. The former can proceed spontaneously faster or slower as long as there is a concentration difference, so this is the most common and basic migration method. Airflow requires a driving force and channel system. The driving force can be a temperature difference, which causes convective motion, or a pressure difference, which causes flow. Of course, in nature, all possible functions are often performed simultaneously. For the convenience of research, different functions are analyzed separately.

17.6.1.1 Diffusion

Diffusion in three-dimensional space is the most complete theoretical description by Fick’s second law. In the Cartesian coordinate system, it is:

Exploration Technology Engineering

In the formula, Dx, Dy, and Dz represent the diffusion coefficients in three directions respectively. For isotropic uniform media, Dx=Dy=Dz=0 . Under the given starting conditions and boundary conditions, by solving equation (17.6-1), the concentration C value at any place at any time can be obtained. In general, this equation has no analytical solution, and the analytical solution can only be obtained under extremely simplified conditions. This is too far from the actual geological situation and has only reference value. For example, the diffusion of a plane gas source into a uniform half space is the simplest case. At this time, equation (17.6-1) becomes a first-degree problem.

Exploration Technology Engineering

The analytical solution of this formula is:

Exploration Technology Engineering

where erf represents the normal distribution The integral function of; C0 is the origin concentration; D is the diffusion coefficient, which is related to the properties of the medium. According to the formula (17.6-3), the test plan for the actual measured D value can be designed. Some measured data show that the diffusion coefficient of gas through dry soil is between 10-1 and 10-6 cm2/s, and the diffusion coefficient through water-saturated sediment is between 10-5 and 10-6 cm2/s. It can be seen that the diffusion of gas through soil is much faster than the diffusion through groundwater.

Since it is difficult to obtain an exact solution to equation (17.6-1), people turn to approximate solutions. Due to the application of computers, this approximate solution method can use numbers to simulate gas anomalies that are as close as possible to real geological conditions, so the results obtained have greater practical significance.

17.6.1.2 Gas flow effect

The forces causing gas flow are diverse, the main ones are atmospheric pressure changes and underground pressure changes. Gas flows from a high-pressure area to a low-pressure area, trying to eliminate the pressure difference. Various underground fissures, faults, unconformity surfaces, and interlayer slips are low-pressure zones that can attract gas to flow toward them and migrate long distances along their directions. Therefore, the formation of gas anomalies is very clearly controlled by tectonics. This is the basis for deep mapping of gas anomalies. On the surface, changes in atmospheric pressure are like a huge piston. When atmospheric pressure decreases, soil gases can escape; when atmospheric pressure increases, air will be driven into the soil, causing abnormal and periodic changes in the air in the soil. Observations show that the range of air pressure affecting the gas content in the soil is limited to within 3 m.

Another important factor that causes gas migration is the movement of groundwater, that is, the gas can first be dissolved in water, flow to another location with the groundwater, and then return to the gas phase under new conditions. There are also some gas anomalies. There was no gas in the first place, but it was completely newly generated through the reaction of local groundwater. Therefore, when conducting gas geochemical prospecting, it is necessary to have an in-depth understanding of the local hydrogeological conditions. In general, the factors that control the movement of groundwater are as important as the movement of underground gases.

17.6.2 Hg vapor measurement

The particularity of the atomic structure of the Hg element makes it a unique metallic element that is liquid at room temperature and has significant vapor pressure. It is now believed that there is a Hg atmosphere surrounding the earth together with other inert gases, and its average concentration is about 1 to 10 ng/m3. It is lower over the ocean than over the continent, indicating that the source of the mercury gas is the continent. If the primary ionization potential of chemical elements is plotted against the atomic number, it can be seen that all inert gases form a series of main peaks, while Hg, Cd, and Zn form secondary peaks, among which the peak of Hg is closest to the inert gases. Therefore, the existence of Hg balloon is not accidental.

17.6.2.1 The geochemical behavior characteristics of Hg and the formation of mercury gas anomalies

The geochemical behavior of Hg has two important characteristics.

First, it is a typical sulfur-loving element.

Therefore, during endogenous mineralization, it mostly enters other sulfides in the form of isomorphs or mechanical mixtures, or exists in the form of thiomercuric anions [HgS2]2- together with other sulfur-loving elements. In the mineral solution, Hg is highly dispersed. Only under low-temperature hydrothermal conditions, Hg crystallizes and precipitates as independent minerals (cinnabar, HgS) to form mineral deposits.

Research shows that [HgS2]2- in hydrothermal fluids reacts with carbon dioxide (CO2) in hydrothermal fluids, either through oxidation-reduction reactions or hydrolysis reactions, which can increase the sulfur concentration in hydrothermal fluids. Decreases and forms HgS (cinnabar) precipitate. Under the influence of acidic (pH<4.46) medium environment, [HgS2]2- forms metallic mercury (Hg0) gas for migration.

Exploration Technology Engineering

When mercury (Hg0) gas rises along faults and surrounding rock gaps into the surface soil or atmosphere, mercury gas anomalies can be formed.

At this time, during the epigenetic process, cinnabar can also be oxidized to produce metallic mercury.

Exploration technology and engineering

As a result, secondary mercury gas anomalies are generated above some metal deposits.

Hg, which is in an ionic state in rocks and exists in the form of isomorphs or mechanical mixtures in sulfides, can be reduced to mercury ions when exposed to the action of Fe2+ or organic matter. Mercury ions are unstable in nature and can form metallic mercury and divalent mercury ions.

Exploration Technology Engineering

Therefore, the mercury gas formed above the oil and gas field may be related to the reduction of organic matter in the oil source rock.

Second, Hg and its compounds have high vapor pressure. This can be illustrated by some data below.

(1) Mercury vapor pressure values ??at different temperatures (Table 17-10)

Table 17-10 Mercury vapor pressure values ??at different temperatures

(2 ) Temperature values ??when the vapor pressure of different metal elements reaches 133.3 Pa (Table 17-11)

Table 17-11 Temperature values ??when the vapor pressure of different metal elements reaches 133.3 Pa

(3 ) Sublimation heat data of different metal elements (Table 17-12)

Table 17-12 Sublimation heat data of different metal elements

The above data show that Hg becomes Volatile. Compared with other metal elements, Hg is the most volatile metal element. Even at normal temperature, the vapor pressure of metallic Hg is significant. Therefore, the metallic Hg formed under epigenetic action can continuously release Hg vapor into the soil and atmosphere to form Hg gas anomalies. Similarly, Hg sulfides (such as cinnabar) can also form Hg gas anomalies. This is mainly because the sulfide of Hg has a high vapor pressure like Hg.

Because of the volatility of Hg and its compounds, which have the important characteristic of high vapor pressure, Hg-Sb deposits and other mercury-containing sulfide deposits are formed in the soil and atmosphere. Hg vapor is abnormal. Generally speaking, Hg vapor rising from deep underground has strong penetrating power. It can rise along structural faults and fracture zones, reaching the surface from hundreds or even thousands of meters below the ground. Even if the loose cover is thick, such as 20-30 m or even hundreds of meters, there will still be abnormal Hg gas in the surface soil. The depth of Hg gas prospecting is shown in Table 17-13.

Table 17-13 Possible detection depths of Hg gas prospecting

The formation of Hg gas anomalies in the soil or atmosphere above the mineral deposits is not only related to the geochemical behavior of Hg, but also to the mineral deposits It is related to the concentration of Hg itself. The statistics of Hg abundance in different types of mineral deposits and other natural systems are shown in Table 17-14.

Table 17-14 The abundance of Hg in rocks, mineral deposits and other formations

It can be seen from Table 17-14 that the Hg abundance in all types of mineral deposits is significantly higher than Rocks and other environmental media. In addition, the degree of Hg richness in ore bodies gradually increases from magmatic deposits to hydrothermal deposits (especially remote hydrothermal ore bodies). Generally speaking, sulfide-rich deposits tend to be Hg-rich. For example, in the ores of Hg-Sb deposits and other low-temperature polymetallic deposits, the Hg content is n×10-4; in the ores of cassiterite sulfide deposits, the Hg content is 0.1×10-4~0.n×10- 4; In the ores of pyrite deposits and other medium-temperature polymetallic deposits, the Hg content is 0.n×10-6~n×10-5. During the oxidation process of the mineral deposit, Hg will be further enriched in the sulfide, so the Hg concentration in the oxidation zone is higher.

Another important reason for abnormal Hg gas above the mineral deposit is the small escape capacity of Hg in the rock. The amount of Hg escaping from the mineral deposits and their dispersion halos is 5 to 1,400 times greater than that of the rocks. Therefore, this factor greatly strengthens the difference in Hg content between the non-mining area and the mining area, causing obvious Hg gas anomalies in the soil or atmosphere above the mining area.

17.6.2.2 Factors affecting the geochemical anomalies of Hg gas

There are many factors that affect the geochemical anomalies of Hg gas in the soil above the mineral deposits and in the atmosphere, which can generally be summarized as geological factors, natural conditions and human factors.

(1) Geological factors

Geological factors include the type, size, occurrence and burial depth of the underlying ore body, as well as the nature and thickness of the overlying strata and surrounding rocks of the ore body. .

a. The type, size, occurrence and depth of the ore body. Different types of mineral deposits have different distributions of Hg, which leads to differences in the intensity of Hg gas anomalies formed in the soil and atmosphere above different types of mineral deposits. The size of the ore body directly controls the scale of the Hg gas anomaly. . Generally, the scale of ore bodies is large, and the scale of Hg anomalies produced in the soil and atmosphere above them is also large. Of course, this must be compared all else being equal. Because the strength of the Hg gas anomaly is also controlled by the occurrence and burial depth of the ore body. Often, the ore body is buried deep and the anomaly strength is weakened; the ore body is steep in appearance and the anomaly width is narrow; the ore body is gentle in appearance and the anomaly develops widely; the hanging wall anomaly of the ore body decreases slowly, and the foot wall anomaly disappears sharply.

b. The nature and thickness of the overlying strata and surrounding rocks of the ore body. The properties and thickness of the overlying strata and surrounding rocks of the ore body have a significant impact on the formation of Hg gas anomalies. Generally, the surrounding rock of an ore body has developed cracks and a large degree of rock fragmentation, which is conducive to the migration and diffusion of Hg gas to the surface. When Hg gas reaches the surface soil, if the pores of the soil develop, especially the development of non-capillary pores, it will be beneficial to the preservation and accumulation of Hg. If the soil layer is too thin, Hg gas will easily escape into the atmosphere, reducing the Hg gas concentration in the soil, so the Hg gas anomalies in the soil will be relatively weakened. The hydrogeological conditions of the soil and the amount of humus in the soil will also affect the formation of abnormal Hg gas in the soil to varying degrees. Generally, soil with more humus and less water content will be conducive to the formation of abnormal Hg gas.

(2) Natural conditions and human factors

Natural conditions mainly include climatic conditions and meteorological conditions. In addition, the occurrence of some other natural phenomena (such as earthquakes, volcanic activities, etc.) will also affect the abnormal changes in Hg gas.

Atmospheric temperature, humidity, atmospheric pressure, atmospheric rainfall, wind direction, wind speed, etc. are important factors affecting changes in Hg gas concentration in the crust (lithosphere), hydrosphere, lower atmosphere and soil.

Temperature. As the temperature increases, the Hg vapor pressure in the atmosphere and soil will be greatly enhanced. For example, when the temperature rises from 0 ℃ to 40 ℃, the Hg vapor pressure above the metal Hg should increase from 0.00019×133 Pa to 0.0063×133 Pa mercury column. Another example is when sampling in a permafrost area with a ground temperature below 3°C, the Hg content in the soil gas is up to 1000 mg/cm3, while in another area with a ground temperature of 14°C, the Hg content in the soil gas is 3800 mg. /cm3. It can be seen that changes in temperature are significant for changes in Hg gas anomalies. Regarding the impact of temperature on Hg anomalies, the diurnal variation of temperature is compared with the seasonal variation of temperature. The latter has a more obvious impact than the former.

Atmospheric pressure. Experiments show that the abnormal gas content decreases with the increase of atmospheric pressure. Therefore, in Hg gas measurement, it is often found that the abnormal Hg gas content measured at the same location at different times in the same weather is different. This has been seen in some areas in the past. For example, in a certain area of ??Arizona, when the air pressure is low at night, the average Hg content in the soil air is twice as high as in the morning. In the Corz gold mining area of ??Nevada, the highest Hg content in soil gas was measured at noon (13:00 to 14:00); at 11:00 and 16:00 to 17:00, the Hg content in the mining area decreased by 1 to 2 times. The Hg content in soil gas is the highest around noon, which is due to the pressure reducing at this time and the largest amount of gas being expelled from the soil.

Humidity and atmospheric rainfall. These two factors have a strong impact on mercury anomalies in the atmosphere and soil, especially heavy rain. This is due to the disruption of the balance of Hg vapor at the interface between the lithosphere and the atmosphere. First, because the porosity of near-surface rocks and overlying soil becomes smaller, the gas in the non-capillary pores in the soil is expelled, and ventilation becomes poor, resulting in a reduction in mercury gas content in the soil after precipitation. The influence of humidity on Hg anomalies in the atmosphere changes with changes in cloud cover, solar action, atmospheric pressure and other conditions. The impact of abnormal Hg gas in the soil increases with the increase in soil medium humidity, which can be confirmed from the evaporation process of Hg in sulfide. For example, at 24°C and 100% relative humidity, the separation of Hg vapor from cinnabar is 50 times greater than at 0% relative humidity, and 75 times greater when Hg is present in the form of HgI2.

Wind direction and speed. Wind direction and wind speed have obvious effects on anomalies of Hg gas in the atmosphere. It also has varying degrees of impact on changes in Hg gas content in the soil. For example, when soil gas is sampled in strong winds with wind speeds up to 10 m/s, it can also be observed that the Hg gas content at a depth of 0.5 m below the surface is also reduced.

According to the measurement results of Hg gas above a mine in the United States, the Hg gas in the atmosphere decreases significantly when the wind speed exceeds 10 km/h; when the wind speed reaches more than 25 km/h, the Hg gas anomalies disappear.

Hg gas anomalies produced by certain natural phenomena that occur in nature, such as earthquakes, volcanic activities, etc., have also been confirmed. It has been found abroad that when seismic activity occurs, the Hg content of gas extracted from boreholes that cut through the fault structure is higher than before the earthquake. The Hg content in geothermal areas is also higher than in areas outside the area.

The influence of human factors mainly refers to manual sampling, sample testing and analysis, waste gas and waste residue discharged from factories and mines, etc. For example, in manual sampling, air samples were taken at different heights. The analysis results of these samples found that the concentration of Hg changes with height, and the Hg content decreases greatly with increasing height (Figure 17-3).

17.6.2.3 Sample collection method

Figure 17-3 Changes in Hg content at different altitudes in the atmosphere

Gas sample collection for mercury gas measurement, Currently, there are three main methods based on different sampling media.

Collection of soil gas samples. The collection of gases in the soil usually involves using a gas extraction tool to extract the gas from the soil, and then using a gold wire sampling tube to capture Hg. The sampling depth is preferably 0.5 m, and the hole spacing is generally not less than 5 m.

Collection of ground air samples. Install an instrument with sufficient sensitivity and stability in the car, and position the air bellows (sampler) directly in front of the car. Surface air samples were collected continuously while driving along a survey line that traversed a known mining area. Tests have shown that the air box tube is most suitable when it is 0.3 m from the ground. Tests on molybdenum-nickel deposits and lead-zinc deposits have shown good ore prospecting results.

Collection of atmospheric samples. Using airplanes to collect atmospheric samples is one of the so-called "aerial geochemical exploration" methods. An air intake pipe is installed on the aircraft to introduce air into the cabin, and gold foil is used to capture Hg, and then measured with a mercury meter. Practice has shown that airborne gas surveying sampling heights below 300 m above the ground can detect Hg gas anomalies above mineral deposits.

17.6.3 Sulfide gas measurement

The pungent smell near the sulfide ore waste rock pile and the smell of rotten preserved eggs in the hot spring area make people never doubt the existence of sulfur gas. Therefore, the idea of ??using SO2 and H2S to prospect is put into trials. Although there have been some reports of SO2 and H2S anomalies detected in known sulfide deposits and geothermal areas, the results were either doubtful or subsequently denied because detailed analysis methods were not stated. The success rate of using a dog's keen sense of smell to find sulfide stones is also unsatisfactory. In addition, large amounts of SO2 and H2S are produced by artificial pollution and surface bacterial activities. Therefore, there is still a question whether these two most common sulfur-containing gases can be used for ore prospecting. Researchers from the United States and the United Kingdom have tried the method of passive enrichment by burying artificial waste rocks at sampling points, but the British gave up because they thought it was worthless. They are now working on ways to extract sulfur-containing gases from soil, a natural adsorbent.

Sulfide gases (SO2, H2S) are typical gaseous indicator components for sulfide deposits, especially sulfur dioxide halo. The mass concentration of SO2 gas formed by the oxidation of sulfides (especially pyrite) in sulfide deposits in the soil above the deposits can reach 25×10-9~50×10-9, while the SO2 background value is 2×10-9～10×10-9. SO2 and H2S haloes may be found in various types of sulfide deposits. They can penetrate through thick layers of loose sediments to reach surface soil or form gas anomalies in the atmosphere. Therefore, SO2 and H2S gas anomalies can be used as signs of ore prospecting.

17.6.4 Carbon dioxide and oxygen measurement

Due to the oxidation of sulfide deposits, the carbon dioxide in the soil above them is high, while the oxygen (O2) is low. Therefore, CO2 or CO2/O2 ratio can be used as a prospecting indicator for certain mineral deposits. Some scholars from the former Soviet Union believe that in arid areas, clear CO2 and O2 anomalies can be generated above hidden ore bodies, and there are rarely false anomalies.

They found a decrease in O2 and an increase in CO2 in the gas in the soil at a depth of 0.4 to 2.5m in the sulfide deposit. They explained this phenomenon as follows: sulfide oxidation consumes oxygen, and the sulfuric acid in the oxidation product reacts with carbonate in the surrounding rock or gangue minerals to generate CO2. These two reactions can be summarized as (represented by pyrite):

Exploration Technology Engineering

Assuming that the effective porosity of the soil is 20%, one ton can be calculated according to the above formula Complete oxidation of pyrite can reduce the oxygen in the range of 350,000 m3 from 21% to 20%; the resulting CO2 can reach a concentration of 0.53% in the same range; this change can be measured with a lightweight instrument out. Since these two gases are constant gases, the background is stable, the measuring instrument is simple, and the accuracy is high. However, there are many reasons for CO2 generation, so abnormal explanation is difficult. As an expanded category of gas measurements that facilitate comprehensive interpretation, measurement work on these two gases is worthwhile. In the former Soviet Union, experiments with CO2 area mapping have been carried out. Field trials have also been carried out in the UK. Positive results were obtained above exotic cover, with CO2 anomalies peaking at 8.0% recorded in glacial deposits of polymetallic deposits.

17.6.5 Ejection measurement

Ejection measurement is an effective method to find radioactive deposits, especially the use of He and Rn to find deep hidden uranium ore bodies, which has a unique effect. According to different measurement media, air emission measurement can be divided into air emission measurement in mineral inclusions, water-soluble air emission measurement and air emission measurement in soil.

17.6.5.1 He gas measurement

He was discovered in 1868 based on the spectral lines in the solar spectrum.

It took 30 years before it was confirmed that this element also exists in the earth's atmosphere. Dick first tried to apply this achievement in basic chemistry to prospecting for minerals. Considering the high cosmic concentration of He, the original material of the Earth may have contained a large amount of He. Since He has a small atomic weight and is easily dissipated, there is probably not much of this original He left. The He currently observed may mainly come from the transformation of radioactive elements. He is actually an α particle that has gained an electric charge. After each 238U, 235U, and 232Th atom transforms into a stable isotope of Pb, it releases 8, 7, and 6 He nuclei respectively. Therefore, all rocks containing U and Th must contain He. . If the average content of uranium in the earth's crust is 4×10-6 and thorium is 12×10-6, it can be estimated that in the past 4.5×109 a, the volume of He per gram of rock should be 9×10-3 cm3. The actual measurement results of crystalline rocks show that the rock only has 10% to 30% of this value, indicating that most of it has been lost, which is the source of the He gas anomaly. According to Banas's observations, the current average flux of He at the ocean floor is 1×106 to 2×106 atoms/cm2·s. Due to the uneven distribution of radioactive elements, the sources of He vary from place to place. During the migration process, He may be captured by some structures (gas capture). Therefore, it is enriched in some natural gases and escapes from the surface along tectonic faults, enters the atmosphere, and finally disperses into space.

According to information from the former Soviet Union, He gas measurement plays a unique role in mapping deep structures because it is inert and highly emissive. Of course, it can also be used to indicate the location of mineral deposits, especially radioactive mineral deposits.

17.6.5.2 Radon gas measurement

Although radon gas measurement has a long history of application, it is limited to uranium ore exploration, so it is not familiar to general geochemical exploration personnel for metal deposits. The method of extracting soil gas in geochemical exploration is transplanted from Rn gas measurement.

Rn is a radioactive gas and an intermediate product of the three major radioactive series in the earth's crust. The half-life of uranium radon (222Rn) is 3.82 d, which makes it possible to migrate out of the ore body 4 to 5 m through diffusion and convection. far away. By pumping the gas and measuring the ionization effect of the particles released by it with an electrometer, its concentration can be determined. This is the earliest gas measurement used in uranium ore surveys. In loose cover, 1×10-6 uranium is generally accompanied by 3×3.7 Bq/L of gas. Above uranium ores, the jet intensity can reach tens to hundreds of Bq/L. Since the half-life of 222Rn is only 3.82 d, its detection depth is limited.

17.6.6 Hydrocarbon gas measurement

The measurement of methane (CH4) and other hydrocarbon gases is currently mainly used in oil and gas survey and exploration work. This method is considered to be one of the most economical and effective direct oil search methods when applied to offshore petroleum geochemical exploration. Its working principle is to detect oil and gas seedlings. Oil and gas seeps are important indicators of hydrocarbon potential. They escape from the lower reaches of the seabed and dissolve in seawater, and are transported and mixed by seawater. Generally speaking, oil and gas seedlings can be detected within a range of 10 to 20 km from the source. The way it works is roughly as follows.

(1) Sampling. Geochemical survey sampling can be carried out continuously on board the ship together with marine seismic or other equipment. Since its towed body and streamer sink almost vertically, it does not affect other geophysical prospecting equipment. The sampling depth is preferably below 75 m, and the effect varies depending on the region, season and climate. Instruments include thermohaline depth probes, high-resolution bottom-view sonar and electromagnetic probes. The temperature-depth profile is used to determine the sampling depth and identify different water bodies. Record flow velocity, flow direction, salinity and temperature at key locations in the vertical profile.

(2) Analysis. There are two analyzers, one is to measure the total amount of hydrocarbons, and the other is to measure the enrichment of methane, ethylene, ethane, propane, isobutane and n-butane to identify the abnormal properties of hydrocarbons . Both spectra and mass spectra were recorded in low-band form. Larger molecular weight hydrocarbons were interpreted as indicators of oil deposits, while ethylene was an indicator of more recent biological processes. Gas proportions can be used to confirm whether anomalies are related to oil deposits. The sensitivity of the analysis must reach 5×10-9 ml of gas per ml of water.

(3) Record. All data are digital tape records and analog low-band records. The recorded data include water parameters that affect the abnormal distribution of hydrocarbons, hydrocarbon enrichment, and measurement lines. The automated records are quickly processed by a computer, and hydrocarbon enrichment contour maps are then automatically drawn.

Finally, use the hydrocarbon enrichment contour map as a basis for further investigation work or compare it with geological and geophysical data to help make decisions about further investigation or drilling.

17.6.7 Geogas measurement

The prospecting mechanism of geogas measurement: There is a vertically migrating updraft inside the earth. When it flows through the ore body or rock mass, Nanoparticles of the elements are carried and migrated to the surface, thus forming geogas anomalies of mineralizing elements and associated elements above the ore body. At present, earth atmosphere measurement is also called earth atmosphere nanometal particle measurement. According to different sampling methods, geogas measurement can be divided into two types: active (instantaneous sampling) and passive (accumulated sampling). Whether active or passive, capture materials must be used, usually polyurethane foam, and must be pretreated.

17.6.7.1 Active geogas measurement and field operation process

Field operation process.

The so-called active method of nano-metal particle measurement refers to the method that causes the nano-metal particles in the underground gas to flow to the collector under the action of external power, and passes through the collection material in the collection device to enrich it in the collection material. A method for measuring nanometal particles.

The active method of nanometer metal particle measurement is to level the topsoil at the area or cross-sectional measurement points in the test area, then use a hammer and a steel drill to drill a hole 0.6 to 0.8 m deep, and use a thread to The sampler is screwed into the hole to a depth of 0.2 to 0.35 m, and the threaded sampler is connected in sequence with a silicone tube. Microporous dust filter, nanometal particle trap and atmospheric sampler or air pump (Figure 17-4), and according to the sampling volume selected for the test, extract 3L/hole gas sample from a single hole. Three holes were drilled within a range of about 5 to 10 m at each sampling point, and 9 L samples were taken.

The following matters should be paid attention to during the sampling process:

(1) The sampling location should be selected in a place with thick soil layer and fine soil particles, avoiding gravel piles and waste rocks. piles and new artificial piles.

(2) When screwing in the threaded sampler, care should be taken to avoid shaking. The threaded sampler must be tightened to ensure the sealing of the mining hole.

(3) The collecting agent should be placed in a dry and clean plastic bag, and non-polluting bamboo clips should be used when taking it in and out. After taking out the collecting agent, place it in a bag that has been washed with aqua regia and dried in advance. Store sealed in a plastic bag.

(4) Each batch of samples should be inserted into 5 to 10 blank capture materials for background and blank checks.

17.6.7.2 Passive method of ground gas measurement

Sampling material is treated polyurethane foam. Dig a pit 40 to 50 cm deep at each measuring point, place a sampler in the pit (Figure 17-5), take out the sampler after 25 days of burial, and use neutron activation to analyze the multi-element content.

Figure 17-4 Schematic diagram of active method ground gas measurement sampling device

Figure 17-5 Passive method sampling device