Using Ground Penetrating Radar (GPR) to identify the sediment morphology and buried large wood in excess pyroclastic deposits, Blanco River, southern Chile

Abstract

Snow avalanches, landslides and debris flows dominate wood delivery to streams in steep forested headwater catchments, while tree mortality and bank erosion are relatively more important in recruiting large wood (LW) in larger watersheds. Tephra fall, pyroclastic density currents and dome collapses associated with the 2008 explosive eruption of the Chaitén volcano (southern Chile) filled the Blanco River valley and obliterated massive areas of forests, generating deposits several meters thick with volcanic sediments and dead wood. Post-eruption fluvial reworking and channel adjustments are eroding these deposits and buried dead wood enter the channel generating an additional LW recruitment process. Considering that external visual identification and quantification of buried wood are not possible, the prospecting noninvasive geophysics technique based on Ground Penetrating Radar (GPR) was used to perform continuous scanning of the lithic-rich gravelly sand volcanic deposits along the Blanco River, as to be able to quantify this recruitment process little studied until now. Three research questions were addressed: i) Is it possible to describe the internal morphology and the stratigraphic structure of volcanic deposits by using the GPR technique? ii) Is it possible to identify the signature of wood pieces buried in these volcanic deposits using the GPR technique? iii) Is it possible to determine the spatial distribution of buried LW based on high-resolution 3D subsurface mapping of the GPR reflections? After initial calibration, the GPR scanning generated radargram profiles that were post-processed to produce 2D and 3D representations of the sediment deposits, which finally allowed identifying the presence of buried LW. As the second stage, the spatial distribution of LW by means of 3D GPR mapping was obtained. Results of this research show that the use of the GPR geophysical technique constitutes a fast noninvasive and precise strategy to characterize the morphology and the internal structure of the sediment layers of rivers affected by volcanic events, also permitting the identification of LW pieces into these deposits. Depending on the aim of the investigation, 2D or 3D surveys are necessary, in which the local field calibration and an adequate post-processing chain are mandatory. The exhumation of buried wood pieces can increase downstream risks associated to the transport of wood during floods.