Ground-penetrating radar (GPR) operates generally on the same principle as
aircraft radar in that a controlled pulse of electromagnetic (EM)
energy is transmitted, reflected off an object, and recorded by a
receiver.  The time required for the pulse of energy to travel to the
object and back is related to the distance traveled by the propagation
velocity of the EM pulse.  Unlike aircraft radar where the propagation
velocity of EM waves in air is known, the velocity at which the EM
energy travels in the ground depends on the material through which it
is traveling.  Otherwise, the same principles hold-EM reflections
arise when encountering material with different EM properties.

The GPR unit is wheeled across the ground (with the antennas pointing
downward) and the reflected pulse originating from the transmitting
antenna is recorded by the receiving antenna.  The data are initially
plotted in time, but may be subsequently converted to depth once the
velocity is calculated (by analyzing subsurface point diffractions
within the data).  Once the depth is calculated, depth maps or "depth
slices" can be generated to show the reflectivity at various depths in
the subsurface.  This is useful for identifying deeper structures
located below more shallow structures.

The GPR survey was designed to detect changes between loose, sandy
soil and sandstone building materials.  The dielectric permittivity
for a sandy soil ranges from 4 to 6, and the electromagnetic (EM)
velocity ranges from 0.12-0.17 m/ns.  Sandstone has a dielectric
permittivity between 2 and 3, and EM velocity of 0.17-0.21 m/ns.

A Sensors & Software Inc., Noggin Plus 250 GPR unit was used in the
survey.  The unit has 250 MHz antennas housed in a small case below
the cart frame.  The antenna orientation is fixed with a broadside EH
polarization. An odometer wheel controls the rate of data collection.
Data points were collected every 0.25 m along the profile lines.