|Ground Penetrating Radar is a geophysical technique which uses a short burst
of electromagnetic radiation that propagates downwards into the subsurface.
This pulse of energy is absorbed by the subsurface but is also reflected back to
the surface by objects which have contrasts in dielectric permittivity.
The depth of penetration obtained with GPR depends mainly on the antenna
frequency selected and the electrical conductivity of the material being imaged.
High frequency 800-1200 MHz antennas have excellent spatial resolution and are
useful for imaging things like rebar or pipes embedded
in concrete. Greater depth is achieved with lower frequency 100-500 MHz
antennas at the expense of resolution. The larger antennas are very useful
for imaging underground storage tanks, shallow groundwater interfaces, and
landfill or excavation boundaries.
Large flat objects like clay lenses within alluvial deposits show up as long flat reflectors in radargrams, but small objects like rebar, pipes or USTs often show up as
ECHOTECH has worked in a wide range of geologic environments in the
Northern US and have encountered great success and great failure with this
technique. Conductive clay rich soils attenuate radar signal more than
resistive materials like dry sand or igneous bedrock. Luckily, when soil
conductivities prohibit the use of GPR, we have often be able to use electrical
resistivity to accomplish the project goals.
ECHOTECH has successfully mapped sedimentary deposits as thick as 10
meters with our shielded 250 MHz antenna, though typical exploration depths in
valley soils range from 1 to 4 meters using our 250 and 500 MHz antennas.
Radar is being used to rapidly image an increasing variety of materials
and targets including:
- Locate rebar and pipes in concrete
- Locate metallic and nonmetallic utilities
- Locate subsurface voids, cavities and tunnels
- Locate septic tanks, sumps, and absorption fields
- Locate voids and irregularities in railroad ballast
- Locate underground storage tanks and associated buried piping
- Determine snow pack thicknesses
- Map stratigraphy of lake bottom sediments and locate sunken
objects (on frozen lakes)
- Map backfilled trenches and disturbed soils
- Map shallow sedimentary units and bedrock
- Map shallow groundwater and buried paleochannels
- Map contamination plumes in shallow soils
- Map cracks, fractures and delaminations in concrete structures,
airport runways, and road surfaces
- Forensic investigations
- Archaeological investigations