(1) Test results of Heitouzhai fire area in Rujigou, Ningxia
In order to analyze the measurement effect and abnormal properties of gamma-ray spectrum in coal fire area, two field measurement profiles were made in non-fire area and fire area respectively, and geochemical samples were taken for analysis, as shown in Figure 4? 2? 12。
As can be seen from the figure, for thorium, the content measured by gamma-ray spectrometry in non-ignition area is basically consistent with the content curve of sample chemical analysis, which shows that the measurement results of the two methods are consistent, mainly reflecting the uneven distribution of lithology. In the fire area, there is an abnormal peak in the gamma-ray spectrum measurement results, and there is no abnormality in the indoor analysis results, but it fluctuates up and down with the change of lithology. The curve trends measured by the two methods are different, and the measurement results are inconsistent, indicating that there are other factors that lead to the abnormal gamma spectrum measured in the field besides the influence of near-surface rocks or soil.
The results of potassium content measured by the two methods are basically the same in fire area and non-fire area, which shows that coal seam combustion has no effect on the distribution of potassium in rocks. The distribution of uranium element is complex, and the results measured by the two methods are basically the same in the non-fire area. In the fire area, the field gamma-ray spectrum measurement results show that the weak anomaly is greater than that in the combustion area, while the indoor analysis results are basically normal distribution.
Comparing the difference between fire zone and non-fire zone, it can be preliminarily concluded that the abnormal thorium and uranium content in fire zone is caused by coal seam combustion. Because the gamma-ray spectrum measurement results reflect the radioactivity level in rocks or soil layers near the surface, it is generally not more than 1m thick (Wu Huishan, 1984). Assuming that the distribution of radioactive elements in near-surface rocks or soil is uniform, the field gamma-ray spectrum measurement results at the same measuring point should have good consistency with the analysis results of rocks or soil samples. In the fire area, the results of gamma-ray spectrum measurement are not only contributed by near-surface rocks or soil, but also by deep radioactive sources. That is to say, radon and thorium emanations produced by radioactive elements in deep rocks are separated from the medium along the combustion cracks due to coal seam combustion and migrate to the near surface, and their decay daughters 2 14Bi and 208Tl produce strong gamma rays, thus causing thorium and uranium anomalies in the energy spectrum. When the sample is taken out from the fire area, the indoor analysis results only reflect the contribution of rocks or soil near the surface, because it is separated from radon and thorium sources. In this case, there is a big deviation between the two measurement results. It can be seen that the range of burning zone in coal fire area is related to the abnormal content of radioactive elements uranium and thorium.
(II) Test results of serial number No.8 fire area in Wuda coalfield, Inner Mongolia.
In order to determine whether the correlation between the range of combustion zone and the abnormal content of radioactive elements in the fire area is of universal significance, the area measurement was carried out in the first fire area. In 2003 and 2004, the No.8 fire area of Wuda coalfield in Inner Mongolia was investigated, and reliable first-hand information was obtained. Through the comprehensive analysis of these data, the author thinks that the scope of the fire zone can be delineated according to the abnormal content of radioactive elements in the fire zone.
Uda survey area uses FD produced by Shanghai Instrument Factory No.3.. 3022 multichannel gamma spectrometer. The instrument probe is placed in a flat and open place on the surface to measure the contents of uranium, thorium and potassium and the total counting rate. The measuring time is 120 s, the point spacing is 10m, and the line spacing is 25m.
Figure 4? 2? 13b, c and d are isolines of uranium, thorium and potassium, respectively. Compared with the range of fire zone delineated by geological observation, the abnormal range of thorium content is the best. The abnormal range of uranium content is also in good agreement with it, but the abnormal distribution is scattered; It is difficult to see that the abnormality of potassium is related to the range of the burning zone in the fire area.
In figure 4? 2? The thorium content map of 13c is abnormal at the lower right, and the uranium and potassium contents in the same position are abnormal. Through field geological verification, it is determined that it is caused by different lithology. Therefore, the lithology difference may be the main interference factor when gamma-ray spectrometry is used to measure a large area. In order to define the scope of the burning zone in the fire area correctly, it is necessary to have a certain understanding of the geological conditions in the survey area and try to eliminate the anomalies caused by lithologic differences.
In figure 4? 2? There is an abnormal area in the upper right corner of the thorium content map of 13c. The uranium content map also has abnormal reflection, and the position is not consistent; No sign of fire was seen in the field geological observation, and abnormal reflection was also detected in the same position in the geomagnetic δ T measurement results (as shown in Figure 4? 2? 13a). According to the results of magnetic and gamma-ray spectrum measurement, it is inferred that the underground coal seam here is burning. Because the surface is complete and there are no fire signs such as smoke and fire, the fire in underground coal seam can be detected by radioactive method and ground magnetic method.
(3) Preliminary conclusion
Preliminary experimental research shows that there are differences in radioactivity between fire area and normal area. Underground coal seam combustion can cause radioactive anomalies, mainly thorium anomalies (up to 3× 10-6), uranium also has some reflection, and potassium is not obvious. This anomaly can be detected and distinguished by the detection accuracy of the existing gamma spectrometer, and the range of underground burning coal seam can be delineated by detecting the anomalies of radioactive elements thorium and uranium in the fire area.
Fig. 4-2- 12 Field Determination and Laboratory Analysis of Potassium, Uranium and Thorium Contents in Heitou Village, Rujigou, Ningxia
(a) Analysis results of indoor samples in fire zones K, U and Th; (b) indoor analysis results of K, U and Th in the fire zone; (c) Field measurement results of K, U and Th in the fire zone; (d) The field measurement results of K, U and Th in the fire zone.
Fig. 4-2-13auda Ⅷ magnetic isoline map of fire area
Figure 4-2- 13b Uda VIII Isogram of Uranium Content in Fire Zone
We think that the reason is that the high temperature caused by coal seam combustion, the destruction and transformation of coal seam roof by gravity, and the dynamic action of pressure gradient, temperature gradient and radon and thorium emanation carried by gas have improved the migration conditions of emanation in the combustion area, thus making the emanation move faster and more to the surface and greatly increasing the concentration of emanation near the surface. A large number of emanations migrate to the near-surface decay of the combustion zone, which increases the contents of 2 14Bi and 208Tl near the surface. These foreign elements 2 14Bi and 208Tl lead to the increase of uranium and thorium content measured by energy spectrometer, and the radioactive element content is abnormal above the burning zone in the fire area.
Fig. 4-2- 13c Uda VIII Isogram of Thorium Content in Fire Zone
Fig. 4-2-13duda Ⅷ Isogram of Potassium Content in Fire Area
There is no precedent at home and abroad to define the ignition range of coal seam by γ -ray spectrometry, and our discussion is very preliminary.