NEOICHNOLOGY AT THE SPEED OF LIGHT: GEORADAR SIGNAL RESPONSE TO VARYING BIOTURBATION SCENARIOS

Ground-penetrating radar (GPR) has emerged as a successful near-surface imaging technique for ichnological research. In addition to applied studies aimed at identifying and mapping the impact of (semi-)fossorial organisms (e.g., environmental, geotechnical, and conservation research), a growing number of applications to neoichnology and zoogeomorphology demonstrate the need for understanding georadar signal response to a range of subsurface scenarios. Due to differences in dielectric constants of various sediment types, as well as a large contrast between those of air and water, there is a predictable pattern of electromagnetic signal response that is diagnostic of specific interface scenarios. Given the complexity of many bioturbation structures and potential for signal interference in (sub-) vertical elements (shafts), this study focuses on diagnostic GPR pulse (trace) structures through a large (>1/4 wavelength) horizontal burrow segment (tunnel). In this sense, animal burrows with different fill characteristics (sediment, vegetation, waste products, skeletal remains), degrees of saturation, differential lithification, presence of burrow lining, and position of the animal, serve as subjects for field and experimental survey designs. For example, air-filled (open) burrows show a polarity reversal relative to ground reflection due to sediment-air transition across the burrow roof. In addition, an increase in signal velocity within the tunnel results in a characteristic “pull up” of the basal interface, which can be rectified during post-processing. In contrast, burrows filled by materials with a higher dielectric constant (lower velocity) are recognized by normal polarity and exhibit an apparent enlargement in their vertical dimension. Field experiments with live reptile tracemakers and eggs show characteristic point-source diffractions, with the latter typically enclosed in a bowl-shaped reflection (shallow nest or egg chamber). We present examples from siliciclastic (fluvial, estuarine, coastal, aeolian), carbonate (coastal, aeolian), and evaporite (aeolian) settings to illustrate the predictable patterns of 500 and 800 MHz GPR signal response to vertebrate and invertebrate burrows.