Dual-Mode Georadar Imaging of Biogenic Structures in Sand-Dominated Substrates

Abstract

Recognition of large biogenic sedimentary structures (burrows, nests), their differentiation from physical structures (small storm-surge channels, synsedimentary deformation, buried objects), as well as imaging bioturbation in real time remain key challenges in sedimentology and ichnology. To address these issues, this study focused on laboratory and field ground-penetrating radar (GPR) experiments using both traditional time-lapse mode (TLM) and a time-triggered mode (TTM). In three sets of laboratory experiments, substrate consisted of dry, well-mixed, moderately sorted, medium sand common for upper beach (berm/foredune) and aeolian settings. Targets simulating burrowing organisms 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). Improvements were made to the experimental design, including an experiment with a saline balloon (vertical pull) and a ground-coupled antenna, at varied moisture levels (0%, 3.7%, and 29.5%). High-frequency (2300 MHz) surveys were captured in TTM while manually extracting the target (variable deformation rate; total time window: 20 seconds). Velocities of simulated deformation calculated from time-triggered radargrams have the potential to be used in the field and laboratory to quantify rates of subsurface bioturbation not available by direct observation. Sediment disruption was quantified using standard ImageJ-aided grayscale analysis to detect truncations (breaks in reflection continuity), with an increase of 10-28% relative to undeformed substrate. Similarly, area-based mean grayscale values increased between 8-16% for damp and saturated TLM surveys, respectively. Complementing the laboratory experiments, this research produced one of the first GPR databases of post-emergence sea turtle nests, ichnologically understudied and relatively complex biogenic structures. A simulated structure (Deauville Beach, DE) and two in situ post-emergence sea turtle nests (Sandbridge Beach, VA) were imaged with an 800 MHz antenna, complemented with sediment texture and magnetic susceptibility analyses. The Delaware experiment provided a reference dataset for a full ethological sequence of nesting and emergence, for comparison with few available studies. At Sandbridge, a clear anomaly was identified at the recent Kemp’s Ridley (Lepidochelys kempii) nesting site, including a V-shaped truncation (width: 0.3-0.5 m; depth: ~0.75-0.9 m). At another location, an older (2020) loggerhead (Caretta caretta) nest was imaged and characterized in a similar aeolian ramp setting, which is characterized by a unique combination of upper berm and aeolian granulometric statistics. Numerous ghost crab burrows, with some imaged during surveys, place sea turtle nests into the Psilonichnus ichnofacies, with overprinting representing a contemporary ichnocoenosis rather than a facies shift. This research has wide-ranging implications for: 1) nest recognition in ancient sequences through identification of diagnostic aeolian ramp packages with diagnostic deformation structures; 2) distinguishing nests from morphologically similar paleo-channels based on overall metrics (tiered components) and fill structure, and 3) conservation of endangered species, with novel applications for nest characterization and potential hatchling emergence monitoring.