Magnetometric resistivity (MMR) is an electromagnetic (EM) exploration method that has been used successfully to investigate electrical-resistivity structures below the sea-floor. Apparent resistivity, derived from the observed azimuthal component of the magnetic field, often is used as an approximation to the resistivity of a layered earth. Two commonly used formulas to compute the apparent resistivity have their own limitations and are invalid for a deep-sea experiment. In this paper, we derive an apparent-resistivity formula based upon the magnetic field resulting from a semi-infinite electrode buried in a 1D layered earth. This new formula can be applied to both shallow and deep marine MMR surveys. In addition, we address the effects that arise from the transmitter-receiver (Tx-Rx) depth difference and the choice of the normalized range (the radial distance between transmitter and receiver, divided by the thickness of seawater) on data interpretation and survey design. The performance of the new formula is shown by processing synthetic and field data.