GEOCLUTTER ON THE BACKSHORE: EXPERIMENTAL HIGH-FREQUENCY GEORADAR IMAGING OF DYNAMIC SEDIMENT DEFORMATION

Supratidal settings along sandy coastlines, which include upper berm/foredune area and their ancient counterparts in ridge-swale strandplains, contain a diversity of 3D features, such as small channels, heavy-mineral lenses, biogenic structures (nests, burrows, tracks, tramplegrounds), buried vegetation, and anthropogenic objects (engineering structures, unexploded ordnance, hydrocarbon-impregnated sands, artifacts, and debris). Ground-penetrating radar (GPR) imaging has been used to identify some of these features above the fresh-saltwater interface, however, discrimination between the targets, which can contribute to geoclutter, remains a challenge. This study focuses on decimeter-scale laboratory GPR experiments using both traditional odometer wheel-triggered mode (OTM) and a time-triggered mode (TTM), with air- and liquid-filled balloons simulating a buried target (e.g., small animal). The substrate consisted of dry, well-mixed, moderately sorted, medium sand common for upper beach (berm/foredune) and aeolian settings. Targets were placed on a basal layer (L1) buried by ~20-cm-thick cover horizon (L2), both with near identical mean grain size (1.69 and 1.65 ϕ, respectively). A slightly less positive near-symmetrical skewness of the upper layer (0.056 vs 0.061 for L1) likely resulted from a loss of minor silt fraction during manual excavation and burial phases. High-frequency (2,300 MHz) surveys were captured in TTM while manually extracting the target (time interval: 20 seconds), as well as before and after extraction in OTM. Due to higher dielectric contrast, the air-filled target showed a stronger signature in all three survey phases. The flat antenna base (plexiglass cover) eliminated topographic effects, however minor post-emergence disruption of sediment surface has to be rectified during post-processing or avoided through surface planation prior to OTM re-survey. Our study demonstrates promise in non-invasive in situ identification of small targets (e.g., sea turtle egg chamber) in areas with signal-noise ratio, as well as potential for real-time monitoring of dynamic changes in sediment properties on very short time scales (e.g., hatchling emergence), with implications to geological, archaeological, ichnological, and conservation research along sandy coasts.