Fig. 3. Each set of cumulative-sensitivity simulations was based on a two-layer soil. Electrical conductivity of the upper layer in soil model Type 1 and the lower layer in soil model Type 2 were fixed at 0.1 mS m^–1 for the electrically resistive case or 100 mS m^–1 for the electrically conductive case. The variable electrical-conductivity layer was given values ranging from 0.1 to 200 mS m^–1. For a given two-layer soil model in each set of simulations, the thickness of the upper layer increased from 0.001 to 18 m. Tx indicates transmitter coil; Rx indicates receiver coil; and s is the intercoil separation.