1、SLF Remote Sensing Technique Based Coal MineGas ExplorationQiming QIN、Xia YE、Baishou LI、Bao CAO、Jian LI、Guiting HOU、Peijun LI (1) School of Earth and Space Sciences , Peking University, Beijing, 100871 (2) No. 11 Geological Team, Henan Bureau of Geology and Mineral Resources, Shangqiu, Henan 476000
2、Email: qmqinpku yexa3252 AbstractSuper low frequency remote sensing technique introduced here is, with the use of natural electromagnetic wave, to explore the apparent resistivity of strata on the surface of the earth, and then to educe the degree of enrichment of gas underground and indicate the de
3、pth of gas-baring stratum. The simple theory, exploration method and device are presented in text. The contrasts are made between the results obtained from experiments and those extracted by drilling at the same location for validating this technique. Keywords- SLF electromagnetic wave; apparent res
4、istivity; coal mine gas; remote sensing technique 。 I. INTRODUCTIONWhen coal beds are formed through the compression and heating of organic materials over geologic time spans the generation of coal mine gas entrapped in coal seam and its superstratum, sandstone seam, is an integral part of the proce
5、ss. Coal mine gas is nearly always methane (CH4) and is a useful source of cleanness and high efficiency. As the coal beds are mined, this entrapped, or adsorbed, gas is released into mine and mixed with air. This not only eventuates in the loss of value in use because of decreasing purity of methan
6、e but also make coal mine gas a very serious safety hazard. A new method, which has been adopted by a few countries, is to drill wells into the coal seam in advance of mining in order to extract most of the high quality gas before it is released into the mine, and to reduce the possibilities for exp
7、losions at the same time. The generation of coal mine gas related to coal, however, the reserve of coal mine gas is not in measure as the reserves of coal, which is in certain matter of geology. The location of drilling should be chosen according to the reserves of gas. Unfortunately, in the absence
8、 of means for distinguishing the reserves of gas from mines, the locations of drilling for extracting methane are arranged inexactly and many wells are drilled in vain .To conduct an investigation on exploring the . enrichment of coal mine gas is imperative under the situation. Super low frequency(S
9、LF) remote sensing technique introduced here is, with the use of reflected electromagnetic wave of natural field, to explore the electrical differences of strata, and then to educe the degree of enrichment of gas underground and reserved depth. This technique is superior over others on gas explorati
10、on at plateau and mountainous areas due to its light and portable equipment and less influence from topography. Therefore, the basic theory of the technique used on coal mine gas exploration is explained below and typical experiments are presented with the use of the SLF remote sensing device, which
11、 was developed by the University of Peking. II. BASIC THEORIESThe frequencies of natural electromagnetic field distribute from under 1 Hz to 100 KHz. Ideally, the incident electromagnetic field, in the mode of homogeneous plane electromagnetic wave, transmit perpendicularly into the homogeneous stra
12、ta. The interfaces between the geological strata reflect a certain portion of energy of incident electromagnetic field to the earth surface. During the transmitting and reflecting processes of cascade, the messages of physical appearance of transmission medium are attached onto the incident and refl
13、ected waves .Coal mine gas is .generally entrapped within gas-baring strata, coal bed and sandstone, with a certain pressure before mining. A significant physical quantity of stratum, apparent resistivity, is principal related with the density and the pressure of reserved gas. The higher the pressur
14、e and the density of the reserved gas, the higher the apparent resistivity of the gas-baring stratum appears. Consequently, the higher the apparent resistivity of a certain stratum, the lower the dissipative attenuation appears as the electromagnetic fields propagate within the stratum. The changes
15、of electromagnetic field lead to the received remote sensing waveform changes in respect to amplitude. Shortly, the amplitude changes of remote sensing curve relate to the reverse of coal mine gas in the manner that the amplitude increases as the apparent resistivity decreases, and vice versa. The r
16、eserve of coal mine gas can be concluded according to the degree the amplitude lifts. The depth of interface between gas-baring stratum and other strata is necessary; otherwise the depth of the gas-baringstratum is unavailable. The SLF signal, received by SLF remote sensing device on the ground, is just the synthesis wave, which carry the stepping superposition of the complex amplitudes or the waveforms of
