Fig. 9. (a) Magnitude as a function of frequency for sand saturated with distilled water (0 dS m^–1) and NaCl solutions of three electrical conductivities (EC) ranging from 10 to 40 dS m^–1. The dashed line represents the manufacturer reported feed-through cutoff. The 25- and 40-dS m^–1 cases have –40-dB cutoff frequencies of 22.8 and 64 MHz, respectively. (b) Linearized phase difference as a function of frequency for sand saturated with distilled water and the three NaCl solutions shown in (a). For clarity, the results are only shown for the lowest 200 MHz. The cutoff frequencies for the higher EC cases are shown as dashed lines.