WRRCTR No.10 Feasibility of Radio Sounding to the Groundwater Table in Hawaii

Abstract

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 rise to a signal which may have travelled through a water saturated rock column at a velocity of 27 m/μsec 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 10^2 to 6.2 x 10^7 Hz on representative Hawaiian rocks and soil indicate that even small amounts of moisture result in prohibitive attenuation losses. For example, in a low density basalt, the attenuation at 18 MHz is 0.26 db/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.