Downflow amplitude decrease of ground penetrating radar reflections in base surge deposits

Ground penetrating radar (GPR) data were collected parallel to a flow direction in the base surge deposits of the Ubehebe hydrovolcanic field (Death Valley, California). These data showed a downflow amplitude decrease of the reflections that was displayed by computing Fourier transforms and the average square of the sample points of the traces. This trend probably reflects some aspects of the lateral facies variation of base surge deposits, and could be explained by the concomitant downflow increase in sorting and decrease in grain size with a consequent reduction of the differences between the beds, which, in turn, causes a reduction of the reflected energy. These results suggest that, although the GPR profiles are traditionally used merely as 2-D images of the subsurface stratigraphy, they could be useful in obtaining information about the heterogeneous distribution of subsurface physical properties. The downflow decrease of the amplitude of the signal could be used as a flow direction indicator when the position of the vent is unknown, although other geologic information and field constraints are probably necessary.