Ground penetrating radar (GPR) provides a high resolution, cross-sectional image of the shallow subsurface. A short pulse of electromagnetic energy is radiated into the subsurface. 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. 
 

GPR data collected at an experimental bacterial injection site clearly images the water table, a buried scarp, and fine scale litholigies to depths greater than 10 meters.

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. 

NAEVA Geophysics operates two GPR systems, a Sensors and Software Pulse Ekko 100 and a GSSI SIR-3. The SIR-3 system operates with a higher frequency range and is used when very near surface high resolution data is required. With shielded antenna to help eliminate above ground reflections, this system is well adapted for delineating features such as utilities and USTs in cluttered urban environments. The Pulse Ekko 100 system addresses the problem of ineffective penetration depth. 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. 

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
  

Data Processing and Presentation

The typical data output of the SIR-3 is generated, in real time, on thermally-etched carbon treated paper. This allows for immediate in-the-field interpretation. The Pulse Ekko 100 is a fully digital system, which allows for a great deal of post collection processing not possible with an analog system. 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. 

 

The EKKO 100 is extremely versatile with the electronics mounted
to a backpack and the antennae attached to a sled or a large-wheeled
cart with an integrated odometer.

Information provided by: http://www.naevageophysics.com