LASER REMOTE OPTICAL GRANULOMETRY (LROG) – A NEW TECHNIQUE TO REMOTELY MEASURE TEXTURAL FEATURES OF SEDIMENTARY SEQUENCES WITH APPLICATION TO MOUNT ST HELENS

Granulometric and textural studies of pyroclastic sequences provide critical information regarding the fragmentation efficiency of an eruption and the mechanism of transport and deposition. However, traditional collection of granulometric and fabric data through sequences of thick volcaniclastic deposits, such as lahar and pyroclastic density current (PDC) deposits, are often limited by inaccessible outcrops, degree of deposit consolidation, or high concentrations of blocks. To address this limitation, we developed the Laser Remote Optical Granulometry (LROG), a new, cutting edge method based on telephotography, image analysis and stereology that permits remote textural study of sedimentary outcrops. The LROG system consists of a CCD camera, coupled to a small, high-resolution telescope. The system can be placed several tens of meters away from the outcrop. We project three light points forming an equilateral triangle on the outcrop's wall, perfectly aligned, using laser beams. The triangle is a scale, and allows the correction of deformations due to perspective or surface roughness. A series of scaled photographs are then taken, which ultimately provides accurate granulometric measurements of particles between 0.2 mm to several meters. We can also use the images to measure apparent clasts shape and fabric.

LROG was tested along the extensive outcrops across the pumice plain of Mount St Helens, which expose up to 40 meters of debris avalanche and pyroclastic flow deposits from the 18 May 1980 eruption. We analyzed 15 outcrops to construct 36 vertical granulometric profiles across the pumice plain. We will present our LROG data in comparison with data collected from traditional granulometric sieving. We will also present preliminary analysis of apparent fabric, which will ultimately help to constrain the influence of debris avalanche hummocks (surface roughness) on PDC dynamics and deposition.