Abstract: The borehole electromagnetic imaging technique is an advanced geophysical technology which is based upon the theory of medical CT to reconstruct the geological structure according to the absorption coefficients of the electromagnetic waves, and then reconstructs geological structure between the boreholes. In this paper, the method is discussed theoretically and computationally in detail for the different models, including low- and high-attenuation anomalies, and the inversion of the large-spare linear equations were solved by the Simultaneous Interactive Reconstructive Technique (SIRT) for the synthetic models to reconstruct the electromagnetic anomalies. Finally the method was applied to the observed data acquired in field to investigate the foundation of a to-be-built bridge in Guangxi province, and the result shows a few possible Karst caves which are fine consistent with the drilling data.