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

Ground penetrating radar (GPR) data were collected parallel to a Row direct ion in the base surge deposits of the Ubehebe hydrovolcanic field (Death Va lley, California). These data showed a downflow amplitude decrease of the r eflections that was displayed by computing Fourier transforms and the: aver age square of the sample points of the tracts. This trend probably reflects some aspects of the lateral facies variation of base surge deposits, and c ould be explained by the concomitant downflow increase in sorting and decre ase in grain size with a consequent reduction of the differences between th e beds, which, in turn, causes a reduction of the reflected energy. These r esults suggest that, although the GPR profiles are traditionally used merel y as 2-D images of the subsurface stratigraphy, they could be useful in obt aining information about the heterogeneous distribution of subsurface physi cal properties. The downflow decrease of the amplitude of the signal could be used as a Row direction indicator when the position of the vent is unkno wn, although other geologic information and field constraints are probably necessary. (C) 2001 Elsevier Science B.V. All rights reserved.