Introduction

Equipment

Ground penetrating radar (GPR) was used to remotely sense the internal structure of geomorphic features at Cape Henry. Each GPR transect was surveyed using either a laser theodolite or a Jacob Staff to add topographic features to the profiles.

The GPR used was a pulseEKKO 100 system with a 1000-volt transmitter (top right). Most surveys used 100 MHz antennae (bottom right), which has approximately a 1 meter wavelength, but 200 MHz antennae (1/2 meter wavelength) were used at one locality for higher resolution. Measurements were made at a constant interval for tens or hundreds of meters to ensure accurate representation of the area being examined. Data were processed and topography was corrected using pulseEKKO software.

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How GPR Works

GPR involves a step-like procedure of repetitive moves of both a transmitter and receiver at a constant spacing. The transmitter (#1 above) sends a short pulse of electromagnetic (EM) energy (approximately radio frequency) into the ground which is reflected by boundaries in the penetrated medium and received by the receiver antennae (#2 above) and sent to the computer via fiber optic cables (#3&4 above) before the ensemble moves. The two-way travel time of this process is measured and translated into depth using the electromagnetic velocity in the penetrated medium gained from a specialized GPR survey performed at each site (common mid-point, Jol and Smith, 1991).

GPR data are often ground-truthed at select points using augering, coring, or trenching, which was the intention of this study. Trenching would have been the best tool for maintaining the integrity of the subtle stratigraphic changes in this area. However, it was not possible during this field season to dig trenches.