Abstract: The computation of apparent resistivity for large loop source observation devices is discussed. Late-time apparent resistivity values for central loop have distortion due to boundary effect and not satisfy late-time condition and thus it affects data interpretation for coal mine goaf and water abundance. In this paper, time-domain electric dipole source vertical component of magnetic field is used to compute full-region, all-time apparent resistivity, which solved transmitting loop boundary effect and the apparent resistivity computational problems for early channel. The results of theoretical model calculations and survey data processing show the effectiveness of the method.