Transient electromagnetic method

Transient electromagnetic method (TEM) is to establish a stable primary magnetic field in the underground by using the power supply of ungrounded return line or ground wire source. At the moment when the power supply is turned off, the magnetic pulse excites the downward and outward secondary eddy current in the ground, and receives the induced magnetic field strength or induced electromotive force generated by the eddy current on the ground. This is the secondary field which gradually decays with the delay time and reflects the electrical structure in different depths, It is the rule of the secondary field changing with time to explain the underground geological structure and structure. Its detection effect and detection depth [13] are affected by various factors, such as emission magnetic dipole moment, spectrum of electromagnetic pulse, size and electrical property of target layer or target body, separation degree of signal and various noises, etc.

In order to adapt to the complex terrain and geological conditions, the selection of the best observation mode of transient electromagnetic method has always been a hot issue. Its principle is to take into account the theoretical feasibility and the convenience of field application. In the 20th century, people began to pay more attention to the "small central wire frame" (i.e. the smaller central wire frame is used in this paper). This kind of observation method has been more controversial. After the correlation analysis, the author thinks that this observation method can meet the requirements of conventional exploration, and has certain advantages in some aspects.