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Lateral facies variation of base surge deposits in the Ubehebe hydrovolcanic field investigated by ground penetrating radar Cagnoli, Bruno

Abstract

Ground penetrating radar surveys were conducted in the Ubehebe hydrovolcanic field (Death Valley, California) in order to study the lateral facies variation of base surge deposits. Using low frequency antennae (50, 100 and 200 MHz), the lower strati graphic boundary of the pyroclastic deposits was imaged, showing that their thickness can be estimated. Furthermore, different radar responses were obtained from base surge deposits and underlying sedimentary rocks, which enable their recognition when outcrops are unavailable. GPR data also confirmed the presence of small, eroded craters, which are partially filled by alluvium. In this case an unconformity between the overlying, horizontally bedded alluvium and the underlying bowl-shaped base surge deposits was recognized in the radar sections. GPR images collected with higher frequency antennae (900 MHz) show the presence of wavy reflections and that these reflections become increasingly less wavy and more planeparallel away from the vent. These wavy reflections have been interpreted as GPR images of subsurface trains of climbing dune-forms, whose size decreases downflow. High-pass eigenimages of the 900 MHz profiles were computed in order to quantify the waviness of the reflections. The high-pass eigenimages act as a filter discarding the highly correlated parts of the traces and leaving the wavy reflections. For this reason, the energy of the eigenimages appears to be an index of the waviness of the reflections. The 900 MHz data showed also a downflow amplitude decrease of the reflections that can be used as a flow direction indicator when the vent position is unknown. This trend was displayed computing Fourier transforms and the average square of the sample points of the traces and probably reflects some aspects of the lateral facies variation of base surge deposits. This can be explained with a downflow increase of sorting and decrease of grain size and a consequent reduction of the differences between the beds, which, in turn, causes a reduction of the reflected energy. These results suggest that, although GPR profiles are traditionally used as 2-D images of the subsurface stratigraphy, they can be useful to obtain information about the heterogeneous distribution of subsurface physical properties.

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