Research On Zero Mutual Inductance Overlapping Loop Transient Electromagnetic Method

In recent years,shallow underground space has become a concern of more and more geophysicists.The application of transient electromagnetic method in shallow exploration has just begun.Transient electromagnetic method was mostly used in deep underground exploration before,but seldom used in shallow exploration.The main reason is that there are blind areas in shallow exploration.There are many reasons for blind areas in shallow exploration,the most important of which are two aspects:one is the influence of off-time,the other is the mutual inductance between transmitting coil and receiving coil,which is difficult to eliminate.In fact,the two aspects are interrelated.At present,scholars have done a lot of research on the former aspect,but little research has been done on the mutual inductance between two coils.Mutual inductance between the transmitting coil and receiving coil leads to the superposition of the primary and secondary fields in the early received signals of the receiving coil,which can not be separated,thus affecting the interpretation of the early signals.In this paper,a new transient electromagnetic coil device with zero mutual inductance between transmitting coil and receiving coil is proposed.The device is formed by two overlapping coils,that is,the two coils overlap only partially.According to this feature,we named it zero mutual inductance Overlapping loop TEM.The zero mutual inductance overlapping loop transient electromagnetic method can effectively eliminate the mutual inductance between transmitting coil and receiving coil,thus achieving the purpose of detecting underground pure secondary field.This paper completes the derivation of one-dimensional forward theoretical formulas for general overlapping loops.Compared with coincident loop,a zero-order Bessel function is added to the theoretical formula.The theoretical formula can be regarded as a general formula for various loop transient electromagnetic methods.Through theoretical formula calculation and physical experiment,we find that the distance between the two coils is about 1.5 times of the coil radius when the mutual inductance is zero under the condition of the same radius of transmitting coil and receiving coil.The results of one-dimensional forward modeling show that the late response curves of zero mutual inductance overlapping loop and coincident loop coincide,and the early response signal is weaker than overlapping loop,which indicates that there is a relationship between them.This paper also shows an example of the application of this method in the inversion of a section of the river dyke of the Xixiaojiang Channel of the Hangyong Canal.Application examples show that the zero mutual inductance overlapping loop method can be effectively applied to shallow geological prospecting,and satisfactory results are obtained.