Based upon a series of researches done by earlier investigators on problems concerning earth waves, an approximate formula for calculating the lateral waves arriving from the earth (limestone) -air interface is given in this paper, with special reference to the geoelectrical characteristics of the karstic terrain. This formula is explicit in physical sense, easy to calculate, and simple for numerical inversion. The result of the numerical calculati onshows good consistence with the asmptotic expansion solution. In addition, taking into consideration of the geoelectric features of the karstic terrain, some kind of digital modelling are made for the synthesis of earth waves in terms of different combinations of electromagnetic and geometric parameters, and whereby the dependence of the amplitude ratio of the lateral to the direct waves upon the freguency of electromagnetic waves, the distance between boreholes, the depth of the measured point, and upon the electrical paramenters has been studied, indicating thus the capability of the double-hole EMmethod to obtain interference curves with apparent extremes for direct and lateral waves in karstic terrain.All inferences in this paper are drawn primarily from the geoelectrical environment of the karstic terrain, but they can be valid for other areas with low absorption media.