BIOTURBATION IN SPACE AND TIME: APPLICATIONS OF CONTINUOUS AND REAL-TIME GEORADAR IMAGING

Applications of ground-penetrating radar (GPR) to neoichnology are reviewed using recent examples of continuous (odometer-triggered) imaging of bioturbation structures of a variety of organisms: rodents, turtles, snakes, lizards, and large decapods. In all cases, the final product, such as a burrow, can be visualized in a 2D vertical cross-section, a depth slice, or in 3D space. The time dimension (4D) can be added through repetitive (time-lapse) imaging, although such surveys require rapid animal activity and frequent site visits. Furthermore, such surveys will capture active or passive modification of existing structures, rather than initiation of burrowing activity. Laboratory experiments have been successful at documenting bioturbation dynamics of small (semi)fossorial organisms, however the actual excavation mechanics may be obscured from direct video imaging. We present a new application of real-time (time- or operator-triggered) GPR data collection mode, which may shed light on the gross spatial and temporal aspects of bioturbation at a fixed point. Whereas time-domain reflectometry is used to ascertain the nature of subsurface interfaces, real-time GPR imaging relies simply on (near-)instantaneous shifts in velocity structure to detect and monitor changing subsurface conditions. Variations in electromagnetic signal velocity (v) at a stationary point produce shifts in the vertical position of key reflection(s) before, during, and after the passage of a tracemaker beneath a monostatic antenna. These shifts result from differences in dielectric properties of sediment (low background v), fluid-filled body (lowest v), and finally to air-filled burrow tunnel (highest v). In addition, deviations from background velocity are predicted from either loose inactive infilling or tighter packing (backfilling). A series of controlled experiments using 800 MHz imaging of shallow hand-made excavations in aeolian sands were successful at mimicking burrowing by snakes, which is common at nearby sites. Georadar thus emerges as a viable tool in a spectrum of field and laboratory neoichnological applications, using multiple scan time settings, spanning variable survey plane orientations, and covering frequency-dependent penetration and resolution scales ranging from sub-centimeter to meters.