Transient electromagnetic field method has high sensitivity to low-resistance abnormal bodies and strong adaptability to construction site, so transient electromagnetic field method of coil device has been widely used (Park Huarong,1990; Jiang Bangyuan,1998; Li Xian, 2002). In practical application, the side length of the transmission line frame of the large loop device is generally 200 ~ 800 m. For the transmission line frame with a length of several hundred meters, only observing the center of the transmission line frame and moving its position will greatly reduce the working efficiency of the TEM method. In this way, in the actual production later, the loop transient electromagnetic measuring device (commonly known as the large loop source transient electromagnetic method) gradually replaced the central loop and overlapping loop devices (Xue Guoqiang et al., 2007; Shi Xianxin et al., 2009), that is, only one-third to two-thirds of the rings were observed. In this way, the problem to be solved in theory becomes the calculation problem of non-common center point response. In the case of noncentral point, it is necessary to solve the problem of calculating the integral of formal double Bessel function (Raiche,1987; Xue, 20 10). Because it is not easy to solve the integral equation with multiple Bessel functions and there is no special formula for calculating the apparent resistivity of each field point in the large loop, the data processing and interpretation of the apparent resistivity calculation in practical application are still based on the central loop formula.

With the development of geophysical exploration methods to fine exploration, it is found that the deviation of induced electromotive force between the edge of observation area and the central point in the wireframe is 15% ~ 25%, which can not be ignored compared with some anomalies caused by geological targets, such as collapse columns hundreds of meters deep, small water-conducting faults and metal deposits thousands of meters deep (Shi Xianxin et al., 2009). For the case of non-central point, the integration is complicated, which can not be directly converted into polynomial form, let alone Fourier transform, which is one of the main reasons why the interpretation technology of transient electromagnetic method with large loop source can not be improved. Therefore, the theoretical formula of large fixed source loop is introduced into the central loop device, and the two are unified to form a trend; On this basis, the research of apparent resistivity algorithm has become the focus of current research.

The responses of non-central point field and central point field of transient electromagnetic field of large loop source are compared and discussed. It is considered that kernel functions have similar functional forms and properties, so the components of non-central points can also be expressed as polynomial forms similar to the vertical components of central points. The definition of apparent resistivity is completed by undetermined coefficient method. The model test shows that the method is feasible and the result is reliable. This technology has detected two mines in Shannan region of Tibet, such as Pan 'an and Nuri (Xue Guoqiang et al., 2065438)