Ground Penetrating Radar

The Method

Ground penetrating radar (GPR) provides a high resolution, cross-sectional image of the shallow subsurface. A short pulse of electromagnetic energy is radiated downward. When this pulse strikes an interface between layers of material with different electrical properties, part of the wave reflects back, and the remaining energy continues to the next interface. Depth measurements to interfaces are determined from travel time of the reflected pulse and the velocity of the radar signal.

Equipment Used

Sensors & Software Conquest 1000 MHz
Sensors & Software Noggin SmartCart (250, 500 and 1000 MHz)
MALÅ Geoscience 250 MHz
Sensors & Software pulseEKKO 100 MHz

Equipment

The 1000 MHz systems are used when very near surface high resolution data is required, such as when locating rebar and/or conduits in concrete. The MALÅ 250 MHz and Noggin 250 and 500 MHz systems operate at intermediate frequencies, offering good depth penetration and resolution, and is well adapted for delineating features such as utilities and USTs in cluttered urban environments. The pulseEKKO 100 system addresses the problem of limited depth penetration. This highly advanced digital recording system is capable of operating at relatively low frequencies with very high pulsing power resulting in maximum depths of investigation.

The Conquest 1000 MHz GPR has the ability to image high resolution depth slices to locate conduits

Field Procedures

The GPR method provides a very rapid means of non-intrusive data collection. The system is most commonly moved along the surface at a consistent pace as data are collected continuously along profiles. The depth at which a feature can be imaged is largely dependent on subsurface material type (resistive versus conductive). With prior knowledge of expected subsurface materials and clearly defined objectives, an experienced operator can optimize data collection parameters to compensate for less than ideal geologic environments.

Data Processing and Presentation

The Conquest and Noggin generate real-time images on a LCD, which can later be uploaded to a computer. The pulseEKKO 100 is a fully digital system, which allows for a great deal of post collection processing. The data are presented as cross-sectional profiles in wiggle trace, color fill, or gray scale format. To carry processing further, the data can be formatted for importation into a variety of commercial processing programs including 3-D visualization packages.

Applications

• Map the location and burial depth of drums, underground storage tanks, and utilities
• Image man-made subsurface structures
• Delineate disposal pits, trenches, and landfill boundaries
• Locate voids and washouts along pipelines, under roadways, parking lots, and building floors
• Screen proposed borehole locations for subsurface interference
• Map water table and bedrock topography
• Delineate inorganic and organic free-phase contamination plumes
• Map stratigraphic layers
• Evaluate mine and quarry rock
• Investigate archaeological sites and cemeteries

A GPR profile across the apex of an underground storage tank. The two markers indicate the edges of the associated metal detector anomaly