APPLICATIONS OF HIGH-FREQUENCY GPR IMAGING TO ICHNOLOGICAL RESEARCH

The morphology and depth of biogenic structures, including the parts masked by overlying sediments, can be resolved using non-destructive high-resolution geophysical methods, such as ground-penetrating radar (GPR). To date, only a handful of studies have used mid-high frequency GPR imaging to locate animal burrows, mostly for biological and engineering applications. The present study addresses identification, mapping, and measurement of biogenic structures in the field and laboratory using a spectrum of GPR antenna frequencies (250, 500, 800, and 2300 MHz). The field surveys revealed subsurface expressions of a number of traces: 1) Psilonichnus-type Y-shaped ghost crab burrows (1 m long x 10 cm diameter) and large excavations (fox holes) in well-sorted backshore and aeolian sands, Assateague Island, MD; 2) mole tunnels (2-3 m long x 10-20 cm wide) and groundhog burrows (>2 m long x 15-30 cm wide) in silty-sand river bank deposits, Washington Crossing, PA, and 3) a 3-cm-wide near-surface tunnel among tree roots at Cape May, NJ. Mineralogical anomalies, such as heavy-mineral concentrations, provide the strongest dielectric contrast between the traces and surrounding sediment; however, these are not always associated with track-making activity and one must rely on more subtle lithological contrasts. Although fine-grained material may attenuate the electromagnetic GPR signal in unconsolidated sediment and soils, the dielectric contrast between burrow/track fill and surrounding sand-rich matrix is often sufficient for detecting biogenic structures. The air-sediment contrast is high enough to resolve the top and bottom of unfilled burrows. In addition to imaging large bioturbation structures, high-frequency antennas (800 and 2300 MHz) have sufficient resolution to detect small burrows (1-7 cm diameter) and buried tracks (2-10 cm long) in the laboratory. The contrast necessary to accentuate these structures was produced by changes in sediment saturation, thereby approximating the burial of tracks and burrow shafts by aeolian sand. In addition to demonstrating the viability of using high-frequency GPR imaging in neoichnological research, our study suggests that caution must be taken when interpreting irregular reflections at lower-frequencies (400-500 MHz) typically used in geological studies.