Development and experiment of transient electromagnetic common centerzero-flux coil

Transient electromagnetic method(TEM) based on the common-center arrangement with small-scale coil is widely used in engineering geological exploration in mining, transportation, water conservancy and other fields because of its advantages of convenient construction, fast construction and sensitivity to low resistance bodies. However, the theoretical and practical results show that the early-stage signal measured by this method is obviously distorted, and the calculated apparent resistivity value is seriously deviated from the actual value. As a result, the detection ability of this method to shallow geoelectric information is missing, which means there are exploration blind area. In order to overcome this defects, the TEM field signal recorded by the instrument under the common-center observation method is reconstructed. Based on the theoretical analysis, the influence characteristics of the transient process of the receiving system and the primary field on the TEM field observation signal are studied. The results show that the transient process of the receiving system and the primary field are the two causes of the early-stage data distortion. And compared with the former, the latter has a greater influence on the effective resolution time lag of TEM field, which is the key factor. It is also pointed out that further eliminating the primary field interference is an effective way to reduce the exploration blind range under the condition of modulating the damping matching of the receiving system. Based on this, considering the opposite polarity of the inner and outer magnetic fields of the transmitting coil, the winding method of the common-center zero-flux coil is proposed. It is clear that the total magnetic flux of the primary field in the receiving coil tends to zero(namely, zero flux) by using the inner and outer receiving coils in series. And the theoretical calculation method of the turns-ratio of inner and outer receiving coils is given by formula deduction. The field test results show that:(1) the theoretical value of the turns-ratio of inner and outer receiving coils is relatively reliable. But in practice, due to the error of coil winding, it is necessary to correct it in combination with the characteristics of the measured signal of the coils.(2) Compared with the center coil, the primary field interference on the zero-flux coil is significantly reduced. The effective resolution time of the TEM field is significantly earlier, and the early-stage signal has high fidelity. On this basis, the signal-to-noise ratio of late-stage data can be effectively improved by increasing the number of turns of inner and outer receiving coils.(3) Compared with the center coil, the data measured by the zero-flux coil significantly enhance the resolution of the shallow geoelectric information in the test area. It better solves the technical problem that there are shallow exploration blind range in the TEM method. The above results reflect the reliability and superiority of the zero-flux coil, which can provide equipment support for the actual detection.