Methodological Research Of Surface-to-Underground NMR

Water inrush is one of the major disasters in coal mines.To ensure safe production,it is necessary to identify the hydrogeology of the mine in advance.The existing geophysical methods have played a certain role in coal mine water prevention and control,but there are still many problems that cannot fully meet the needs of mine safety production.Surface nuclear magnetic resonance is a geophysical method that can directly detect the amount of groundwater,but the depth of detection is shallow.We expound the basic principle and working method of surface-to-underground nuclear magnetic resonance disaster water detection,and deduce the underground nuclear magnetic resonance response formula under the ground circular source excitation,and obtain the NMR response characteristics of the surface and surface-to-underground modes.The research results are as follows:First,The response intensity of the underground nuclear magnetic resonance signal is related to the position of the receiving point.Under the same distance from the water body,the ground receiving and the underground receiving signal response amplitude are basically equal.When the local receiving point is close to the water body,the signal response is significantly enhanced.For example,under the same launching device,the water body at 120 m underground generates about 20~30n V/m2 response on the surface,and about 105~120n V/m2 response at 100 m underground.The underground response is 3.6~5 times of the ground response..Second,When the depth of the water-containing anomaly is greater than 135 m,the response of the surface NMR is lower than the minimum amplitude(10n V/m2)that can be measured by the existing instrument under actual noise interference.However,in the underground 100 m observation,the surface-to-underground mode response can still detect the effective value(greater than 50 n V/m2).This indicates that the surface-to-underground nuclear magnetic resonance detection method can detect deeper water bodies than the conventional surface nuclear magnetic resonance method.Third,in the case where the depth of the water-containing anomaly is constant,the larger the volume of the anomalous body and the higher the water content,the stronger the NMR response;the larger the area of the transmitting coil or the receiving coil,the stronger the NMR response;the lower the conductivity of the earth,the greater the interference with the NMR response,and the specific performance is as follows: the amplitude of the nuclear magnetic resonance response is reduced,and the depth of the nuclear magnetic resonance detection is reduced.Our research shows that when the water body is buried deep,the surface-to-underground nuclear magnetic resonance method is more resolving than the surface nuclear magnetic resonance method,and can improve the depth of detecting the water body.