LIGHTING THE TUNNEL: MULTI-MODE GEORADAR IMAGING OF BIOTURBATION SCENARIOS

Ground-penetrating radar (GPR) imaging is increasingly used as an effective non-intrusive method for detecting near-surface biogenic structures. Mid-high frequency GPR (500-900 MHz) allows for in situ detection and analysis of vertebrate (rodent, reptile) and macroinvertebrate (decapod) structures at sub-decimeter resolution within the upper 2-4 m, spanning a diverse suite of substrate (= media) types without the need for excavation or casting. Variations in background (matrix) and burrow fill types exhibit different dielectric properties, which create predictable electromagnetic signal response. Transitions between sediment and subsurface fluids are manifested by predictable patterns in signal velocity and polarity trends at successive interfaces: normal (e.g., +/– ...+/–) vs. reversed (+/– ... –/+). This study explores a series of burrowing scenarios, including both end-member (maximum contrast) and intermediate expressions in traditional and time-lapse modes. Potential scenarios include: void space (air, ice, water, partial submergence), active vs. passive fill (compaction contrast), fill type (sediment, phytodetritus, etc.), fill volume (partial vs. full, collapse), presence of animal(s) and their position within the tunnel (minor signal scattering) or shaft (complex scattering), wall lining, roof vs. floor characteristics (bio-secretion, compaction). Differences in water retention will be the primary cause of dielectric changes and can be used for detecting temporal changes (weeks-years) in subsurface conditions. In contrast, rapid shifts (seconds-minutes) in time-triggered images will result from animal activity or event-scale geological causes (burrow collapse, flooding). Other conditions being equal, actively filled systems are expected to produce a weaker roof reflection, due to similarities between the compact fill and overlying matrix. Laboratory and field simulation experiments are especially important, where controlling the activity of large animals is often not feasible. In addition to paleo- and neo-neoichnological research, the information obtained through 2D and 3D GPR visualization has widespread applications to soil science and paleopedology, zoogeomorphology, hydrogeology, and conservation ecology.