Theoretical Study the Error Caused by Dipole Hypothesis of Large-loop TEM Response

Transient electromagnetic method (Transient Electromagnetic Field is TEM for short) is an time domain artificial sources of electromagnetic detection method based on the principle of electromagnetic induction. This method has higher sensitivity for the anomalous body of low resistivity , a stimulating can cover the required frequency bands and greatly improve the work efficiency by using a rich spectrum component pulse waveform . Also because send magnetic moment is relatively large, so the loop source system TEM has been very widely used in geological exploration, engineering, geothermal and environmental exploration which belong to medium depth ( less than 2000m) exploration [1], especially the central-loop system TEM has become the preferred method in Coalfield Hydrogeology Exploration and metal ore exploration . In order to obtain greater depth of exploration, the launch frame side of loop system commonly use large-scale form , Now the imported equipments with a large number of applications are large-loop system which shall be accounted for about 80% of the share of middle-depth exploration on the ground , sush as V-5, V8, GDP-32, PROTEM, SIROTEM, PEM, etc. If only one point be observed in the frame center to move the whole system when the side of loop system reach to hundreds of meters, the work efficiency of TEM will be greatly reduced. In order to increase the production efficiency, we will observe the points from center of loop to 0ne third of loop in the actual production, which is to think that the field in this region is approximately homogeneous. So, we can derive large-loop center-model TEM system which is different from the center-loop TEM system in the conventional sense as well as large-loop source TEM system. Fixed loop source TEM sysetm has been built based on the theory of dipole infinitesimal, which can not show more correct character in near-field area, even though this kind of theory has played more important guide in TEM study. There exist some error between dipole approximate solution and loop-source exact solutions. In this paper ,Exploratory Research of the error caused by dipole approximate has been finished. Because in Classical electromagnetic theory study, the formula corresponding to of harmonic magnetic dipole source and electric dipole source have been reached after Match the result of Constant current field to TEM response based on using the parameter of vector potential and scalar potential, the error between dipole approximate solution and loop-source exact solutions of magnetic dipole source and electric dipole source has been calculated in the condition of static field. Based on the error analyses, it has been concluded that the error behave big value in near-field of TEM, as a result ,the response of non-zero offset field point become distortion. This paper give the important idea that based on matching the study result of electrostatic field point charge to the result of time-domain field, the exact formula of whole zone and whole time for large-loop central-model TEM can be simulated by integral time-changing infinitesimal point charge along loop side.