GROUND PENETRATING RADAR

GPR profile across UST

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


GPR profile showing structure and water table

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

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

NAEVA Geophysics operates three GPR systems:
  • the Sensors & Software Conquest (1000 MHz)
  • the Sensors & Software Noggin Smart Cart (250, 500 and 1000 MHz)
  • the Sensors & Software Pulse Ekko 100 
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 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 Pulse Ekko 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. 

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 Pulse Ekko 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

EKKO 100 in action

 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.

GPR with operator

Sensors & Software Noggin 250 GPR used to aid in the detection of subsurface utilities and other targets.