Research Of All-time Apparent Resistivity Calculation For Large Fixed Loop Transient Electromagnetic Methods

Transient Electromagnetic Methods(TEM) is one kind of the widely used geophysical prospecting methods. And the Large Fixed Transmitting Loop is a commonly used in TEM. But because the position of the reciever ralative to the transmitting loop can be changed, to process this kind of data is more difficult than other kind of data. Now, there is only a few specialized software can process the data of Large Fixed Transmitting Loop. Operating these software is dificult, and the result is not satisfactory yet.The oretical basis of this dissertation is the formula of the response of a large polygonal transmitting loop. With this formula that was which was published by Raiche in 1987, the characteristics of the response decay curves and the response section curve that is very important to indicate wild field sounding work, is systematically analyzed in this dissertation.It is an important step to eliminate the influence of ramp turn-off. In order to get accurate apparent resistivity, an analytic formula of ramp turn-off response of a large polygonal fixed loop is derived in this dissertation through the formula of step turn-off response and Duhamel integral formula.To get the apparent resistivity is a process of solution of an implicit function. In several common inversion algorithms, the Marquette inversion algorithm is choosen in this dissertation. The method to resolve the problem of multiple resolutions is to get all results of apparent resistivity first and then choose the reasonable result. In order to get all two apparent resistivity from one response value, the Marquette inversion algorithm is been modified to limit the search process in the monotone interval that contain the original value. The part of magnetic field is brought up in this dissertation as the main criterion to choose the reasonable result during early and intermediate stage, and the result calculated through central loop formula as the main criterion during later stage. Finally the auxiliary criterion is the near trace rensult.To improve the availability of wild field data, the quaulity control module is developed in which the three point filter method is used.Eventually a set of software, the large fixed transmitting Data Process System V1.0 is developed. The tests by self-designed test cases illustrate that the algorithms used in this software have very strong adaptability. A satisfactory result has been obtained in processing the measured data of HongQiling by this software.