Theoretical estimates of low-frequency radar sounding performance and its potential for mapping moist subsurface interfaces in conductive environments on Mars are controversial, with predictions of ultimate penetration depth ranging from a few meters to kilometers. To address this issue, we conducted a broadband electromagnetic field survey in which we combined ground penetrating radar (GPR) operating at multiple low frequencies with the transient electromagnetic method (TEM) to investigate the dependence of radar penetration depth on ground resistivity. Surveys were performed in the frequency range 16--100 MHz at two locations on the northwest margin of the Amargosa Desert, Nevada, where numerous Mars-analog investigations have been performed. The surveys were conducted on a 20-m-high homogenous sand dune and on the flanks of a 20-m-high scoria cone and above a buried lava flow. A wet alluvial interface was located at the bottom of each structure. GPR detected the wet alluvium contact at the base of the sand dune, but failed to penetrate to the same depth at the scoria cone under similar residual moisture content. Depths of investigation for both the scoria cone and the buried lava flow were limited to approximately 10 m owing to the presence of conductive inclusions in the first few meters, which are below the radar resolution but dramatically decreased the dynamic of the radar-backscattered echoes and hence the penetration depth. Absorption models constrained by the TEM data are in good agreement with these observations. Depths of investigation varied weakly with frequency owing to substantial, frequency-independent absorption.