Forward Numerical Simulation Of Surface-To-Underground NMR

Water disaster is the main accident of coal mine water hazard.It is very necessary to identify the hydrological condition in the mine.The existing geophysical prospecting methods have a certain role in the coal mine water hazard geological detection work,but each has advantages and disadvantages,and the precision of exploration can not meet the fine requirements.Based on the characteristics and current situation of mine water disaster in China,In this paper,the method of surface-to-underground NMR detection is proposed,One dimensional numerical simulation is used to study the characteristics of NMR response signals.First,aiming at the problem of double Bessel function oscillation,this paper calculate the expression of excitation magnetic field,and do the Gauss integral for every part of the expression,and calculate the magnetic field at any point of the large line source.Through these,this paper get a high accuracy magnetic field value,and analyze the distribution of magnetic field components Hr and Hz.The results show that the real part of magnetic field varies with depth and horizontal distance,while the imaginary part of magnetic field is much smaller than the real part value.Second,Based on the existing nuclear magnetic resonance theory and the ground separation coil formula,this paper get the formula for surface-to-underground forward modeling.The initial amplitude curve of the receiving coil is obtained through numerical integration.The research shows that the depth of surface-to-underground NMR method is increased compared with the surface NMR method,and the initial amplitude response of NMR is obvious.Third,Based on the forward calculation formula and forward modeling of surface-to-ground,the influence factors such as launch coil,receiving coil and water bearing structure are studied in this paper.The following rules are as follows: the greater the thickness of aquifer,the greater the detection depth;the higher the water content,the greater the detection depth;the larger the radius of the receiving coil is,the larger the depth of NMR will be.;The closer the receiving coil is from the aquifer,the greater the signal response.the depth of exploration increases with the increase of excitation current,but the radius of the transmitting coil has no significant influence on the detection depth.