KIRTLAND AIR FORCE BASE ALBUQUERQUE, NEW MEXICO ...

APPENDIX B
Ground Penetrating Radar
The strength of a radar signal is a complex function of the distance traveled through the medium and the
dielectric constant, the magnetic permeability and the electrical conductivity of the medium. Radar
signals are attenuated rapidly in materials with high electrical conductivity. The attenuation of radar
signals in subsurface media is a strong function of the mineralogy and the water content. Thus, materials
such as dry sands and gravels are least absorptive of radar signals, whereas wet mineralogic clays are
saline soils can be highly absorptive. The absorptive properties of the medium limit the penetration
depth, i.e., the depth at which targets can be detected.
GPR surveys are usually performed by establishing a grid of parallel survey lines across the site and
moving the radar antenna along each of these lines. A suitable means must be provided for determining
the location of the radar unit along each of the lines, and for documenting this information on the
recording medium. Typical systems measure the time and velocity of antenna motion, or determine the
position of the antenna by synchronization signals from the wheels or tracks of the vehicle used to tow
the antenna. Simpler systems have used aluminum foil strips placed across the path of the antenna to
provide unambiguous location information directly on the radar traces. Global positioning system (GPS)
measurements can be integrated with the radar data in ―open‖ areas void of dense canopy.
To determine the depth of anomalies noted on radar traces, the travel time data actually recorded are
corrected to depth using measured or estimated wave velocities. The velocity of electromagnetic waves
in the subsurface medium at the site can be determined by excavation to observed targets and measuring
their depths, or by acquiring data over objects at a known depth (e.g. underground utilities). The velocity
should be determined at several points in the area of interest. Alternatively, the velocity can be estimated
based on the subsurface medium‘s characteristics in comparison to other media with known velocities.
For determination of large-scale subsurface geologic features or to detect larger buried targets,
reconnaissance-type, low-resolution surveys are typically performed with a track spacing of 5 to 15 ft or
more. Surveys to detect small discrete targets may require a track spacing as close as 2 to 5 ft.
Reflected radar signals are electronically processed and displayed as an intensity-modulated time
spectrum, where the time of the signal response is related to the target depth. Typically, the data are
digitally recorded on the internal hard drive or small field computer, which is commonly built into the
radar systems.
Although much of the data obtained in a GPR survey are automatically recorded by the instrumentation,
additional information to unambiguously identify and to interpret each profile should be recorded on
standard data sheets. As a minimum the data sheet should contain the following information:
Project name and location with state planar coordinate information
Company or organization
Date and time of day
Operator(s) name
Instrument make, model, serial number, and calibration date/frequency (if applicable)
Weather, surface topography, vegetation, and soil descriptions
Kirtland AFB
SOPs for Field Investigations B-66 April 2004