What is GPR?
GPR is based on sending high-frequency (typically several MHz up to a few GHz) electromagnetic waves into a material and recording the strength and the time required for the return of any reflected signal. Reflections occur whenever the radar signal enters into a material with different electrical conduction properties or dielectric permittivity from the material it left.
The strength of the reflection is determined by the contrast in the dielectric constants and conductivities of the two materials. This means that a pulse which moves from dry sand (dielectric of 5) to wet sand (dielectric of 30) will produce a very strong reflection, while moving from dry sand (5) to limestone (7) will produce a relatively weak reflection.
Higher frequencies allow better resolution but do not penetrate very deep whereas lower frequencies provide better depth penetration but less resolution. Thus, the choice of antenna frequency is crucial for best results.
The electric and dielectric properties of the subsurface material govern the attenuation of the radar signal. Very conductive material like clay and salt water attenuate the signal significantly and thus depth penetration will be very limited in these circumstances.
With NGI's system and standard antenna (200 MHz) a depth of about 9 m can be achieved under optimal conditions.
Principle of GPR operation: The antenna moves over the ground and produces a real-time image of the reflected signal, showing the signature of e.g. a buried object. From www.geophysical.com
Applications of GPR include:
- Depth to bedrock
- Rock quality
- Detection of buried objects, e.g. oil drums, pipes etc.
- Investigation of contaminated ground
- Archaeological investigations
- Layering in the subsurface
- Layering in snow: weak layers, snow Depth
- Void detection
- Non-destructive testing
GPR Equipment at NGI:
- GSSI- SIR 4000 (central unit) and 200 MHz antenna. Antennae with different frequencies (from 16 MHz to 2.6 GHz) can be operated for targets outside the 200 MHz range. (http://www.geophysical.com/products.htm)
- NGI's proprietary step frequency radar (300 kHz to 3 GHz), developed and produced at NGI since 1989 and exported worldwide. For applications that exceed the capabilities of commercial GPRs, the NGI radar can be adapted with purpose build antennas
- ReflexW (http://www.sandmeier-geo.de/reflexw.html)