Theoretical Study On Response Characteristics Of Cone-shaped Source In Transient Electromagnetic Method

Large-size transmitter sources,such as large source loops and long line sources,are usually used for deep transient electromagnetic(TEM)detection in the fields of hydrogeological exploration and metal ore prospecting,etc.In recent years,TEM is increasingly used in underground environments,for instance,coal mine roadway,engineering tunnel,and so on,as well as in the environmental and engineering fields,where the big size sources are limited because of the narrow environments and the shallow targets.The traditional transient electromagnetic devices mainly consist of coaxial and dipole devices,in order to enhance the initial electromagnetic energy,the method of multi-turn winding is used to increase the transmitting magnetic moment,so as to achieve the goal of reducing the length and area of the transmitting coil.These kinds of equipments have strong mutual inductance coupling,which makes turn-off time longer,and eventually makes the "blind zone" deeper.In this paper,a cone-shaped source device which is based on the field source was proposed,and the theoretical study on its response characteristics was carried out.For the proposed source,the radius formula of each coil of the device were derived.According to the idea of superposition principle,the forward modeling method of cone-shaped source transient electromagnetic was discussed,and numerical calculation was carried out with the application of Fast Hankel transform,G-S transform,sine and cosine transform numerical filtering algorithm.The characteristics of the primary and second fields of the cone-shaped source were analyzed,and the mutual inductance coefficient and the turn-off time of the cone-shaped source and the multi-turn coils under different parameters were calculated and compared.The results show that the inductance of multi-turn coils are 9 times more than that of the cone-shaped source with the same equivalent magnetic moment of 926.1A·m2,and the turn-off time is about 8 times greater than that of the latter,which demonstrates that the new source leads to a much shallower "blind zone".Increasing the height of the cone-shaped device can reduce the mutual inductance coefficient,but the further variation of its bottom radius has limited influence on mutual inductance.The magnetic behavior of the cone is the same as that of the multi-turn coils,but the transient responses of the secondary field and the total field are more stronger than that of the multi-turn coils.The all-time apparent resistivity of cone-shaped source transient electromagnetic field was calculated by using the translation method,a “smoke ring” inversion method was used for the transient electromagnetic data of cone-shaped source according to the "smoke ring" effect of transient electromagnetic field.The equivalence transformation of the cone-shaped source transient electromagnetic sounding data to the MT plane wave field data was carried out by using the corresponding time-frequency conversion relation.The results of the "smoke ring" inversion are taken as the initial model,by using the least square regularization inversion technique,the quasi MT optimization inversion of the apparent resistivity data of the cone-shaped source was carried out.Both the smoke ring inversion and the least square regularization inversion were verified by a homogeneous half space model with a resistivity of 100?·m,the results show that the relative error between the improved inversion resistivity and the model resistivity is about 0.2%,and the relative error of the "smoke ring" inversion is more than 0.8%.Then they were applied in the forward inversion calculation of two layered,three layered and four layered typical geoelectric models,respectively.Finally,a field experiment was launched and the data derived was processed by the two methods.The results show that the inversion resistivity improved by the least squares regularization method is closer to the model resistivity,and the inversion depth is approximate with the layer interface.Moreover,a certain stratification effect was also found.The reconstruction error between the fitting curves of apparent resistivity and that of the model is in the range of 10-4~10-3.The results of this paper show that the cone-shaped source device has important theoretical significance and application value to reduce the mutual inductance between the coils and enhance the shallow detection ability of the transient electromagnetic method.In order to enrich and perfect the method,the research emphasis in the future should be focused on the posterior effect correction of cone-shaped source response,the applicability of the filtering technique and the multi dimension forward and inversion imaging,etc..