Edge effects and canceling measure of transient electromagnetic central loop configuration

摘要

The central loop configuration is one of configurations mostly used in transient electromagnetic survey and its apparent resistivity is usually derived from the field expressions of the loop source center.However,in actual field exploration,the measurement is taken out only in one-third area of the center of the transmitter loop in order to increase the working efficiency.The analysis on field distribution characteristics shows that the difference of induced electromotive force (EMF) between measuring at the central point and at the edge of the measuring area ranges from 15% to 25%,which can not be ignored as compared with anomalies caused by collapse columns and conducting small faults buried at the depth of 800-1000 m widely being in the North China type coal measures.The research indicated that the apparent resistivity based on induced EMF equation of the large rectangular loop source can eliminate the above stated edge effects very well and make the measurement keep in near field zone to enhance the data interpretation precision of coal field hydro-geological exploration.