Natural cavities of all sizes play a fundamental role in the subsurface movement of environmental fluids. Examples include the movement of ground water in both pore space and fractures, and magma in lava tubes. The presence of these cavities may be indicated by surface expression such as lava or limestone caves. However, only a small proportion of cavities are likely to intersect the surface. We present some preliminary 100 MHz pulsed radar data from both surface and cross-borehole datasets. These demonstrate the potential for detecting natural water and air filled cavities with sizes spanning the wavelength of the probe radiation. We discuss some of the cavity search strategies and data interpretation methods that may prove useful.

The difficulties in 'ground truthing' field data are often significant. In order to investigate the reflection and transmission of radar pulses interacting with surfaces and cavities, we have carried out some laboratory experiments using 500 MHz radiation in a water filled tank. A variety of cavity sizes and shapes were used with differing dielectric contrasts. We present the initial results and a theoretical model of aspects of these experiments, and use the data to aid development of field based detection strategies and data interpretation.