Currently, the use of Cagniard apparent resistivity and phase is still the main method and means to process and interpret controlled-source audio-frequency magnetotelluric (CSAMT) sounding data. The CSAMT data must meet the conditions in the far zone to use the magnetotelluric (MT) sounding inversion interpretation method. The conditions in the far zone are directly related to transceiver distance; Hence, the distance measured by the CSAMT method is directly related to the reliability and credibility of the data. In this study, based on the formation wave and ground wave of the analytical expression for the electric field in a uniform half-space condition, the response laws of the formation wave and the ground wave are analyzed. After rigorous mathematical derivation, the formula for determining the optimal transceiver distance of the CSAMT is obtained, and the steps for using this formula in practical application are given. The actual field application verifies the rationality of the derived formula. This study provides a reference for the design of a CSAMT field source, and effectively guides the determination of CSAMT transceiver distance in the field.

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