Methods

Methods

Using Ground Penetrating Radar (GPR) to Determine Dune Stratigraphy

GPR maps the interior of any medium that can be penetrated by radio waves. The system detects waves that are reflected by changes in the penetrated medium, including buried objects, stratigraphic changes such as grain size or composition, and changes in moisture content.

GPR was used in this study to aid in the following:

	Identify dune sediment packages and paleosol locations that represent different phases of dune activity.
	Examine the orientation of strata in each dune package.
	Develop a sampling strategy for dating soils and dune sediment.

GPR Equipment

	Computer system and console (backpack ensemble).
	Radar transmitter and receiver with antennae connected to computer ensemble by fiber optic cables (100 MHz antennae shown).
	Laser surveying tool (on tripod) to include topographic features in GPR profiles so reflections have the right geometry.

gpr ensemble.jpg (69870 bytes)

How GPR Works
GPR involves a step-like procedure of repetitive moves of both the transmitter and receiver at a constant spacing. The transmitter sends a short pulse of electromagnetic energy (approximately radio frequency) into the ground which is reflected by boundaries in the penetrated medium and received by the receiver antennae 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).

For a more detailed explanation, read What is GPR?

gprschematic.gif (4703 bytes)

Ground-truthing GPR Lines

To ensure correct interpretation of reflections, augering was done where reflectors were near to the surface. The photograph on the left shows the auger used in this study. The auger is 1.6 meters long and can extend to approximately 3 meters. The position of the water table was confirmed in each dune by this method. Some soil horizons were also confirmed using an auger.

Trenches were also used to confirm dune reflectors, and proved to be valuable in understanding GPR resolution with respect to dune stratigraphy. The photograph on the top right shows dune strata in a trench on Jockeys Ridge with approximately 12 potential reflectors (changes in sediment texture or composition) per meter. The figure below is from a GPR line taken near the location of this trench. Because this line was sampled at 0.5 meter intervals with antennae spaced 1 meter apart, only 2 reflections can be expected in every meter of GPR data. However, the radar pulses consistently traced reflectors that were continuous over the 0.5 meter sampling intervals, revealing the same dip angle of dune strata in both the trench and the GPR data as illustrated on these figures.