Technical Report No. 48
DIRECT DETERMINATION OF THE ELECTROMAGNETIC REFLECTION PROPERTIES OF SMOOTH BRACKISH WATER TO THE CONTINUOUS SPECTRUM FROM 10 8 TO 4 x 10 9 HERTZ
Larry K. Lepley, William M. Adams
A new geophysical technique for the remote measurement of the electromagnetic properties of natural surfaces was investigated. Theoretical and laboratory work has produced evidence for the validity of the use of ultra-high frequency radio reflection spectrometry for remote sensing of salinity of brackish water. A preliminary study of the electromagnetic properties of aqueous sodium chloride, computer-generated frequency dispersion curves of 1) the dielectric coefficients, 2) power reflectance, 3) brightness temperature, and 4) skin depth of water as a function of eight different normalities of sodium chloride, corresponding to a salinity range from pure to ocean water and as a function of five different temperatures from 0oC to 40oC to the radio frequency range from 106 to 3 x 1010 Hertz were constructed. These graphs indicate that the frequency dispersion of the reflectance of radio energy in the 106 to 109 Hertz band at normal incidence to a smooth water surface is strongly influenced by the salinity of the water, and that the spectral signature could be used as a measure of water salinity as distinguished from water temperature. In the range of interest, 20oC to 25oC, 0.3 ppt to 30 ppt dissolved solids, the dominant physical factor explaining the change of reflectance with salinity changes is the ionic conductivity due to the lateral flow of ions induced by the imposed electromagnetic field. The ionic conductivity of brackish water is a monotonic function of the salt content. A laboratory prototype radio refection spectrometer was designed and assembled from commercially available swept-frequency UHF-microwave test equipment, a single horn antenna, and a hand-made coaxial sample cell. Reflectance spectra from 2.5 x 109 to 4.0 x 109 Hertz of fresh and sea water were obtained with a free-wave (horn antenna and pool) system and reflectance spectra of brackish and sea water from 108 to 2.0 x 109 Hertz and of fresh water from 0.8 x 109 to 2 x 109 Hertz were obtained with a coaxial waveguide system. The measured spectral signatures appear to agree with the computed reflectance of aqueous sodium chloride solutions.