Fig. 7.

a GPR survey being undertaken along profile A–C (Fig. 2) on breakout fan I. GPR is based on the measurement of the subsurface response to high-frequency electromagnetic waves transmitted by an antenna placed on the ground surface. The pulses are reflected, diffracted, or scattered at interfaces where the relative permittivity, the conductivity, and/or the relative magnetic permeability change. b GPR results where fractures are observable in the subsurface. Boundaries were determined using higher-resolution data and in a larger format than that represented in the figure; for a thorough description, see Cassidy et al. (2009)