Central Loop Transient Electromagnetic (tem) Apparent Resistivity Computing Research

The apparent resistivity is a common parameters of expressing the underground electrical structure vividly in Transient electromagnetic methods, the calculation of the all-time apparent resistivity is particularly important in data processing and the inference and interpretation of transient electromagnetic signals. Many domestic scholars studied on the calculating the apparent resistivity of central loop transient electromagnetic method, such as Bai Denghai method, Su Zhu Liu method, Xiong Bin method, Wang Huajun method, Chen Qingli method etc, but all these methods have their advantages and disadvantages in terms of the speed, the accuracy of calculating and the application conditions, this paper through the study on central loop transient electromagnetic method puts forward a method of calculating apparent resistivity which has fast calculating speed, high precision, wide application condition.This paper analyzed thoroughly the transient electromagnetic field analytic expression in homogeneous half space, discovered that the kernel function exists a maximum value at z=1.61363, and is monotone increasing in (0,1.61363) interval as well as monotone decreasing in (1.61363,+∞) interval. Accouding to the monotone features in (0,1.61363) and (1.61363,+∞) interval,we proposed bisection method to calculate full time domain apparent resistivity. The calculating speed of this method is quick, we can get the solution of z by no more than55times calculations in every channel,the precision of bisection method can be set by yourself, the precision of apparent resistivity is high and can be evaluated, the algorithm is stable. We can get a smooth curve of apparent resistivity by piecewise cubic Hermite interpolation when there is no solution.This paper also analyzed thoroughly the kernel function of the center loop transient electromagnetic method in homogeneous half space, deduced the expression of the influence secondary potential errors exerts upon calculating all-time apparent resistivity, discovered that the small secondary potential errors will have a greater impact on calculating all-time apparent resistivity in later time than in early time. And This paper introduced the current methods and techniques of improving the quality of data collection and data processing in early time and later time.This is beneficial to the development of transient electromagnetic apparatus and the data processing of transient electromagnetic signals.Analyzing the expression of the center loop transient electromagnetic method in homogeneous half space can provide the forward datas, the method of inversion and can select the correct datas or delete the wrong datas. This paper programmed with matlab to calculate the accurate time of secondary potential attenuating to the level affected by noise seriously. This work can provide a reference for engineering design evaluating the calculation of all-time apparent resistivity and deleting the data affected by noise seriously.The accuracy of the secondary potential is directly related to the correctness of the interpretation of transient electromagnetic inversion. Researching on the factors affecting the surveying of secondary potential can promote the development of the transient electromagnetic method,can conducive to the development of precision transient electromagnetic apparatus,can contribute to get reliable data by collecting or processing data and make the inference and interpretation close to the objective reality, can help us to prospect more mineral resources,can conducive to better solve problems in the fields of hydrogeology, engineering geology, environmental geology and can make more contribution to the economic and social development. This paper summarizes and discusses the previous part of the research results on the factors affecting the secondary potential and the correct of secondary potential.Basing on the research and development of LTEM-1apparatus that been developed by Liwenyao teacher, author summarized the status of transient electromagnetic apparatus and the problems of transient electromagnetic apparatus in China, put forward to the scheme of solving these problems and the direction of domestic transient electromagnetic apparatus in the future, and introduced briefly the application of the study of calculating apparent resistivity of central loop transient electromagnetic method in the research and development of LTEM-1apparatus.