New analysis of the Apollo 17 surface electrical properties experiment

The Surface Electrical Properties (SEP) experiment deployed at the Apollo 17 landing site in the Taurus-Littrow valley was a continuous-wave, radiofrequency (1-32 MHz) interferometer designed to probe the subsurface from depths of meters to kilometers. Signals were transmitted by orthogonal electric dipoles laid out near the lunar module and three components of the magnetic field were measured on the lunar rover at useful distances up to 1.6 km. The gross range decay of the signals is well fit by a simple equation combining spherical spreading, first order interference, and attenuation. The derived loss tangents similar to 0.01 are consistent with laboratory-measured radiofrequency absorption in Apollo 17 basalts. Thus the attenuation in the SEP data can be explained by absorption with negligible small-scale scattering; the latter can be quantified as a dearth of 10-m scale lateral heterogeneity and a mean-free path of kilometers. Numerical modeling was used to compare predicted vs. observed waveforms and to invert for vertically varying dielectric structure. The dielectric constant is readily converted to density and porosity for lunar rocks. The preferred model shows a sharp decrease in porosity in the top 20 - 30 m, with little change below 300 m. This likely tracks the transition from fully gardened regolith, to impact-fractured rock, to largely intact 3.7-Ga basalt. The bottom of the basalt was not detected by the SEP to a depth of similar to 2 km or more, which is consistent with reanalysis of Apollo 17 seismic and gravity data.