Electrical properties of rocks and their significance for lunar radar observations

Abstract

A wide variety of terrestrial rocks has been investigated to determine the range of variation of the high-frequency electrical properties. Both solid rocks and powders of various types have been measured at frequencies of 450 MHz and 35 GHz; some measurements have been made at elevated temperatures. Solid materials show wide variations in permittivity and absorption length, but apart from a small trend with silica content it is not clear that there is any pattern to the variations, and it is unlikely that measurements limited to these radar frequencies will be useful for identification. With powdered rocks, on the other hand, the variation from rock type to rock type is much smaller, but depends on the powder density in • quite predictable way. Consequently, radar information can be employed to make precise estimates of density and of density profiles even in the absence of information about composition. The long absorption lengths in powdered rocks indicate that radar reflections may occur at considerable depths in a regolith of the type found on the moon. INTRODUCTION Notwithstanding the growing body of planetaw radar and radiometric measurements, there are available few reliable determinations of the electrical properties of terrestrial rocks and minerals in the frequency range between 100 and 100,000 MHz. The relatively extensive compilations of electrical characteristics at low frequencies [see, for example, Parkhomenko, 1967] rarely include measurements above 10 MHz. Measurements on a few materials over a very wide frequency range have been made by the Massachusetts Institute of Technology Laboratory for Insulation Research [von Hippel, 1954, 1957; Westphal, 1963]. Griffin and Marovelli [1967] have published data on a large number of samples of rocks of six types, in the frequency range 20 to 100 MHz. Data on tektites and chondrites between 420 and 1800 MHz are included in a paper by Fenslet et al. [1962]. The only radar frequency determinations with any claim to comprehensiveness are those of Krotikov [1962], who studied both solid and powdered rocks at 3.0, 9.4, and 38 GHz. We are unaware of any data at higher frequencies X Now at School of Physics, University of Sydney, Australia. Copyrlght ¸ 1969 by the American Geophysical Union. than Krotikov's, although measurements have been made on isolated materials in the far infrared by using quantitative spectroscopic techniques.In this paper we report measurements of the dielectric constant and loss tangent of a wide variety of terrestrial rocks and minerals at two frequencies, 450 MHz and 35 GHz. These frequencies, corresponding to wavelengths of 67 cm and 8.6 mm respectively, lie near the extremes of the range spanned by current radar and radiometric observations of the moon and planets. Although our chosen frequencies themselves delimit a range of almost two decades, it will appear from the results we report that the electrical properties, permittivity and loss tangent, at the higher frequency differ as a rule only slightly from those at the lower frequency...

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