Using Ground Penetrating Radar (GPR) to identify sediment morphology and buried large wood in volcanic deposits, Blanco River, southern Chile
Abstract
Snow avalanches, landslides, and debris flows are the primary mechanisms delivering wood to streams
in steep, forested headwater catchments. Tree mortality and bank erosion, in contrast, play a relatively major role in recruiting large wood (LW) in larger watersheds. The 2008-2009 explosive eruption of the Chaitén volcano in southern Chile caused widespread forest destruction, depositing several meters of volcanic sediments and dead wood in the Blanco River valley. Tephra fall, pyroclastic density currents, and dome collapses filled the valley, creating thick deposits of lithic-rich gravelly sand and buried wood. Post-eruption fluvial processes and channel adjustments have since eroded these deposits, mobilizing buried dead wood into the channel and creating an additional, previously understudied LW recruitment process. The exhumation of buried wood pieces could increase downstream risks during floods, emphasizing the importance of understanding this process. Given that buried wood cannot be visually identified, the prospecting noninvasive geophysics technique based on Ground Penetrating Radar (GPR) was used in this study to perform continuous scanning of the lithic-rich volcanic deposits along the Blanco River. Three key research questions were addressed: 1) Can the internal morphology and structure of volcanic deposits be described using GPR?; 2) Can GPR detect the signature of buried wood within these deposits?; and 3) Is it possible to determine the spatial distribution of buried LW using high-resolution 3D subsurface mapping of GPR reflections? Following initial calibration, GPR scanning produced radargram profiles that were post-processed into 2D and 3D representations of the deposits, successfully identifying buried LW, whose spatial distribution was subsequently mapped using 3D GPR analysis. This study demonstrates that GPR is a rapid, non-invasive, and precise tool for characterizing the morphology and internal structure of river sediments affected by volcanic activity, facilitating the identification of buried LW.
in steep, forested headwater catchments. Tree mortality and bank erosion, in contrast, play a relatively major role in recruiting large wood (LW) in larger watersheds. The 2008-2009 explosive eruption of the Chaitén volcano in southern Chile caused widespread forest destruction, depositing several meters of volcanic sediments and dead wood in the Blanco River valley. Tephra fall, pyroclastic density currents, and dome collapses filled the valley, creating thick deposits of lithic-rich gravelly sand and buried wood. Post-eruption fluvial processes and channel adjustments have since eroded these deposits, mobilizing buried dead wood into the channel and creating an additional, previously understudied LW recruitment process. The exhumation of buried wood pieces could increase downstream risks during floods, emphasizing the importance of understanding this process. Given that buried wood cannot be visually identified, the prospecting noninvasive geophysics technique based on Ground Penetrating Radar (GPR) was used in this study to perform continuous scanning of the lithic-rich volcanic deposits along the Blanco River. Three key research questions were addressed: 1) Can the internal morphology and structure of volcanic deposits be described using GPR?; 2) Can GPR detect the signature of buried wood within these deposits?; and 3) Is it possible to determine the spatial distribution of buried LW using high-resolution 3D subsurface mapping of GPR reflections? Following initial calibration, GPR scanning produced radargram profiles that were post-processed into 2D and 3D representations of the deposits, successfully identifying buried LW, whose spatial distribution was subsequently mapped using 3D GPR analysis. This study demonstrates that GPR is a rapid, non-invasive, and precise tool for characterizing the morphology and internal structure of river sediments affected by volcanic activity, facilitating the identification of buried LW.
Keywords
Ground Penetrating Radar; Buried wood; Depositional fluvial morphology; Pyroclastic deposits; Chaitén Volcano; Chile
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