Using Ground Penetrating Radar to Help Delineate Lahar Hazard Zones at Cotopaxi Volcano, Ecuador

Cotopaxi (5897 m) is located in the Eastern Cordillera of the Ecuadorian Andes about 80 km south of Quito and is one of the most active Ecuadorian volcanoes. Over the last 2000 years, Cotopaxi has had at least one eruption of Volcano Explosivity Index (VEI) 3 or larger per century. Many of these past eruptions melted parts of the glaciers on Cotopaxi, generating large volume debris flows. These flows rapidly descended along the three main drainage systems of the volcano towards the North, East and Southwest resulting in considerable destruction. The focus of this work is the study of a debris flow fan covering an area of 2.8 km2. The fan is situated within the confluence area of three quebradas; the San Lorenzo, the San Diego and the Burrohuaycu, that form part of the southwest drainage system 13 km downslope from the volcano summit. In order to estimate extent and thickness of recent lahar deposits, we used a Ground Penetrating Radar (GPR) survey that covers the fan surface in a grid pattern. Transects are perpendicular and parallel to the flow direction of the quebradas. The total length of all transects is 10 km. Regular common mid-point surveys (CMP) at different sites along the lines ensure wave velocity control on wet and dry ground. Global Positioning System Rover measurements along the transects yield X, Y and Z topographic control with an accuracy of decimeters that are used to correct the vertical exaggeration of the GPR profiles. We chose 200 MHz antennas having an average penetration depth of 6 m assuming a velocity of 0.09 m/ns, helping us detect even relatively small changes in outcrop morphology and stratigraphy over short distances. Subsurface control of the surficial geology comes from outcrop sections, quarry faces and excavated trenches that are distributed across the fan area. The identification of a paleo-lahar topography and the integration of GPR data are primary components of this ongoing lahar flow modeling study.