Radar penetration of sand and dust as determined by laboratory and field measurements.

摘要

Radar imaging systems have the unique ability to penetrate sand or other loose material to reveal the subsurface bedrock geology. In anticipation of potential future radar missions to study Earth and Mars, radar backscatter and transmission experiments were conducted for dry and wet sand and for iron-rich dust. In the experiments, the change in radar signal was measured for various combinations of radar and target properties to determine the attenuation, or the decrease in signal per meter. Over the frequency range used for imaging radars, it was found that dry sand, sand with 5 percent water content, and dry iron-rich dust all cause an attenuation of less than 2 dB/m at 0.5 GHz frequency. Furthermore, sand with 11 percent water content results in attenuation of only ∼4 dB/m. It is expected that low-frequency energy will penetrate deeper, but the low attenuations for wet sand challenge previous claims that sand has to be extremely dry for penetration to occur. At higher frequencies, attenuation due to increased water content increases rapidly to values that prohibit penetration, but the dry sand exhibits a much slower increase in attenuation. At 9.6 GHz, the dry sand created an attenuation of only 5.9 dB/m. It is therefore expected that higher frequency radar energy can penetrate dry sand deposits, possibly as much as a meter depending on the sensitivity of the radar. The attenuation due to the iron content of the dust also increases with frequency. That attenuation is still low (5.8 dB/m for 1.24 GHz), but it rises to 67.4 dB/m at 9.6 GHz. Comparison of field data to radar images showed that the laboratory relationship between backscatter and sand thickness can be useful for future estimates of sand thickness. These results also support a multi-frequency Mars imaging radar, and they will aid in selection of other radar parameters to meet the science goals of future radar missions.