Technical Report No. 10
George R. Jiracek
The reported high values of resistivity in the near surface zones in semi-arid regions on the island of Hawaii motivated research into the feasibility of using radio waves to sound the depth to the ground water table. field tests using a 35 MHz ranging system (built in England for ice depth sounding) were made in areas of differing geology and climate, but in no instance was an echo identified as having originated from the water table. Measurements made of transmissions from within an inclined tunnel and received at the surface gave use to a signal which may have traveled through a water saturated rock column at a velocity of 27 m/usec while attenuated by about 3 db/m. Equipment ringing, due to antenna miss-matches, contributed to the lack of success in measuring water table echoes. However, subsequent laboratory dielectric measurements in the frequency range 102 to 6.2 x 107 Hz on representative Hawaiian rocks and soil indicate that even small counts of moisture result in prohibitive attenuation losses. For example, in a low density basset, the attenuation at 18 MHz is 0.26 CB/m when dry, but increases to 1.66 db/m with less than 4% water by volume. In a volcanic ash soil sample, the loss increases at 18 MHz from 0.04 db/m to 1.3 db/m as the soil water content is increased from zero to 19% by volume. Electromagnetic propagation velocities decrease markedly with increasing moisture content, an effect which is most striking at low frequencies. In situ moisture conditions above the water table in the semi-arid regions in Hawaii are expected to be approximately > 4% in rock and >19% in soil. Considering all factors, usable echoes at 35 MHz are consequently expected when sounding water table depths of ~ < 25 m. However, frequencies as high as 0.1 MHz may prove useful in sounding depths to many hundred meters. The use of VHF (30-300 MHz) waves to probe the depths of drier environments such as possibly exist on the moon is considered feasible.