DETECTION OF ICE IN ANDEAN ROCK GLACIERS USING FIRST-ARRIVAL SEISMIC TOMOGRAPHY AND GROUND PENETRATING RADAR

We completed a geophysical pilot study on two rock glaciers in the Cordillera Principal of western San Juan Province, Argentina, in January 2012. The question being addressed is whether the rock glaciers are active features containing a significant volume of ice or relict ice-free periglacial features. Seismic data were acquired with two 24-channel Geometrics Geodes (24 bit A/D conversion), and shots were acquired every two receiver stations along each profile (1-m or 1.5-m spacing, depending on line length) using a variable number of coincident sledgehammer hits (4 to 16, depending on S/N) at each shotpoint. Seismic first-arrival tomography data were processed using Geotomo software. The GPR data were collected using 100 MHz antennae with a Sensors & Software PulseEkko Pro system mounted on a cart with odometer trigger, and processed using EkkoView Deluxe software. At the first site, results from all four seismic tomograms indicate a shallow (within 8-13 m of the surface) fast p-wave velocity layer (~3600 m/s). Regional bedrock includes highly weathered units of much lower expected velocity, and upward-extending bedrock topographic highs are not expected within the cirques and down valley. Based on our previous experience on active glaciers, the 3600 m/s velocity is indicative of relatively debris-free ice. The high-velocity layer extended through the bottom of all profiles, i.e., no bottom of the interpreted ice layer was detected. The GPR data at the first site were marginal due to severe heterogeneity of the surface debris resulting in poor antennae coupling. At the second site, results from all four seismic profiles similarly indicate a shallow (within 8-13 m of the surface) fast p-wave velocity layer (~3600 m/s), interpreted also as buried ice. One GPR profile coincident with a seismic profile along a dirt road ploughed across the rock glacier yielded good-quality results and the image shows a high-reflectivity target body coincident with the top-of-ice as interpreted using the seismic data. Results from both sites indicate the presence of buried ice beneath the debris cover of the rock glaciers, and also demonstrate the usefulness of the seismic first-arrival tomography technique for identifying buried ice within rock glaciers.