Experimental Study On The Characteristics Of Small Coil System Of Transient Electromagnetic Method

In recent years, water damage accident in China’s coal mines often happened, itseriously threat to the safety of coal mine production and workers, thus becoming thefirst task of prevention work of each mine. The mine transient electromagneticmethod—with its flexible, convenient, sensitive response to low resistance object andsmall size effect, etc—becomes the preferred method of water exception explorationof coal mine. However, the domestic and international instruments all use advancedformulas of central coil transient electromagnetic method to calculate the apparentresistivity, making the apparent resistivity values of small coil observing system weregrossly low, affecting the reliability of data interpretation.Firstly, field experiment carried out on the ground to study the factors(transmitter coil size, transmitter coil turn and the transmitter current) which may leadto low apparent resistivity value using a single variable method. Through comparisonand analysis of the experimental results, it is concluded that the influence law for theresistivity computation of the three factors, and probes into the applicability of theapparent resistivity formula of instrument for small coil device.The paper studied how to optimize the observing system for the phenomenonthat the apparent resistivity value of small coil device is low and does not complywith the real situation. Using small coil device of frequency domain sounding andfigure-8Coil device to highlight secondary field by changing the geometricalrelationship between transmitter coil and receiver coil. Through the comparison andanalysis of experimental results, we found out that when transmitter coil isperpendicular to receiver coil, receiver coil doesn’t cut the magnetic field lines oftransmitter coil, and the abnormal body must be on the direction the midperpendicularof receiver coil pointed, thus the secondary field signal could be highlighted; thesuperposition of electromagnetic response signals of figure-8Coil device also hasstrong suppression of noise, and when the abnormal body is equidistant from twotransmitter coil ring, the suppressive effect is best. Then the paper studied the affectsof the distance of transmitter coil and receiver coil, the size and turn of coil on theobservation results, and it provides the referential basises for selecting the bestobservation system parameters and optimizing observing system.