Ground penetrating radar
Gpr survey can prove effective at
locating archaeological features underneath a wide range of surfaces
that cannot be surveyed by any other geophysical technique. Good
results can be obtained underneath concrete, stone, tarmac and through
grass and soil. In addition, an indication of depth can be provided
which may prove highly useful where intrusive work is required.
Gpr survey records reflected radio waves that are pulsed into the
ground as the radar antenna is dragged along the ground surface.
Changes in the ground make-up, such as from soil to stone, provide
conditions where some of the transmitted energy is returned to the
surface. In this way, subsurface features can be mapped.
Careful timing of the returned radio waves along with calculations
of the ground's dielectric constant can allow an estimate of depth to
The transmitted radio energy is very wide in bandwidth but peaks at
the resonant frequency of the antenna. As resolution is a function of
the transmitted wavelength, a 400MHz system will have a higher
resolution than a 200MHz system; however, the lower frequency system
may have superior penetration which may be critical in certain
conditions. 200 and 400MHz systems are typical of those used for
Damp clay soils may present problems at times and it is not uncommon
for conditions to be so poor that no penetration below a few
centimetres is possible. It's also worth noting that rarely can gpr be
used to locate graves. Exceptions exist, particularly if disturbance is
recent or if there are buried voids or associated structural features.