[H-TT30] GEOMORPHOLOGICAL APPLICATIONS OF HIGH-DEFINITION TOPOGRAPHY AND GEOPHYSICAL DATA IN THE ANTHROPOCENE

[HTT30-P03] Ground-Penetrating-Radar Investigation of drifted wood trapped in river-sand – Laboratory Experiments

*Christopher A Gomez¹, Norifumi Hotta² (1.Kobe University Faculty of Maritime Sciences Volcanic Risk at Sea Research Group, 2.Graduate School of Agricultural and Life Sciences, The University of Tokyo)

Keywords:Ground Penetrating Radar, Drifted wood, Geophysics

The large majority of drifted-wood research has been performed in river corridors, where the recruitment occurs during high flow-stages and from bank erosion. However, there is less work that has been performed on the dynamics of drifted wood during mass-movements and debris-flow recruitment. Empirical evidences from river sediment deposits in Asakura (North Kyushu, Japan) and in the check dams above Kobe City (Kansai Region, Japan) have shown that a portion of the drifted wood could be trapped within sediments. Although the deposition process that traps material with a lower density than water within the sediment is not fully resolved yet, detecting and quantifying this material is a first step in resolving the geometric characteristics of the trapped drifted wood. To reach this overall objective, the present research has turned towards laboratory experiments to solve a first issue: detecting drifted wood inside sediments. Indeed, research investigating tree roots or buried timber (from buried building) all use the geometric characteristics of the buried material to define its nature (for instance tree roots radiating from a known tree), but there are no evidences that the hyperbola being detected are generated by a rock or a root, etc. When the geometric characteristic of the object is not defined, it is then essential to attempt to differentiate the material based on its dielectric properties. For this purpose, the authors have performed laboratory tests with a Mala Ramac GPR 800 MhZ antenna (with an effective pulse of 1.313 ns, and thus a central frequency of 921 MHz, and a /wavelength of 32.57 cm) triggered by a coding wheel over a 38 cm strip, recording data once a millimeter. A buried log of 16.5 cm diameter and 19 cm length was buried at 5 cm depth (not the same as on the photograph with the abstract), for a wet mass of 2910 g (density of 194 kg/m³) was used to simulate the buried wood. The radar was dragged over the surface with the buried target, several times with dry to fully saturated conditions. Based on the experiments results, the dry sand provided data within the expected range of dielectric constant for dry sand εDS=4, while the dielectric constant for wood was εwood=36. Saturated sand provided values within the range of 22 – 28 (also within the 20 – 30 common values). Providing that the common values of the dielectric constant for sandstones is commonly around 6 and the one of shales are of values between 5 and 15, the dielectric constant can be used to determine whether the detected material is made of wood or whether it is a rock, providing that the diameter of the drifted wood is at least (as a rule of thumb) a quarter of the wavelength, i.e. ~ 9 cm diameter, which is enough to detect buried drifted-wood in sediments, and differentiate it from rocks and other blocks.