In general, the time derivative of vertical magnetic field is considered only in the data interpretation of transient electromagnetic (TEM) method. However, to survey in the complex geology structures, this conventional technique has begun gradually to be unsatisfied with the demand of field exploration. To improve the integrated interpretation precision of TEM, it is necessary to study the three-component forward modeling and inversion. In this paper, a three-component forward algorithm for 2.5D TEM based on the independent electric and magnetic field has been developed. The main advantage of the new scheme is that it can reduce the size of the global system matrix to the utmost extent, that is to say, the present is only one fourth of the conventional algorithm. In order to illustrate the feasibility and usefulness of the present algorithm, several typical geoelectric models of the TEM responses produced by loop sources at air-earth interface are presented. The results of the numerical experiments show that the computation speed of the present scheme is increased obviously and three-component interpretation can get the most out of the collected data, from which we can easily analyze or interpret the space characteristic of the abnormity object more comprehensively. 1. Introduction The transient electromagnetic method has shown great potential in hydrological and hazardous waste site characterization [1, 2], mineral exploration [3], and general geological mapping, and geophysical reconnaissance. However, the behavior of TEM fields is not yet fully understood [4]. The need for further theoretical insight is reflected by the increasing demands placed on transient electromagnetic methods for petroleum, mineral, and geothermal exploration. Forward modeling is one of the most common and effective methods that help us understand the physical significance of the electromagnetic responses [5, 6]. Computer solutions for this method have been mainly confined to the vertical component of magnetic field time derivative. Until now, a limited number of solutions have appeared in the literatures which are relevant to the TEM three-component responses of a 3D source over 2D earth, which is the so-called 2.5D. At present the 2.5D model represents the only way of interpreting controlled source electromagnetic data in terms of a complex earth, due to the prohibitive amount of computer time and storage required for a complex 3D model [7]. The first published theoretical finite element derivation for the 2.5D electromagnetic problem was by Coggon [8]. Stoyer and Greenfield
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