I want to utilise a quick geophysical method to roughly detect underground tunnels in a sedimentary layered setting. However this area does have an abundance of weathered clay minerals on the top.
Would GPR be able to atleast show minor traces of a cavity?
Dear Ms Marshal,
Less resistive clays rich horizon over more resistive sedimentary layer can limit the GPR investigation depth. Clayey layers are conductive and attenuate the radar signals. GPR is not appropriate in areas where the surface layer resistivity is less than 100 Ohm.m, that is the case of most clay rich soil horizons. But, if the 2 m clay layer is less conductive (resistivity > 100 Ohm.m), you can try using the GPR method with 50-MHz antennas, spaced by 0.5 m. Regards, Anicet
If you wish, you can read the article published in Geoderma, 2004, available to downloading from my reaserachgate records
Sure! With the appropriate antenna, which conditions the penetration depth.
If water is present and particularly shallow, the energy is pretty much absorbed and the penetration depth is substantially reduced
Thank you very much everyone for your answers. They were of great help.
Regards,
Rachael
Dear Marshal,
Various geophysical techniques could be applied to locate an air filled void (e.g. seismic, ultra sonic, gravity, electromagnetic induction, gamma ray), but GPR seems to be the technique to fulfill most demands as a general surveying tool. Detection of air filled cavities with GPR is well known in various fields of geology and civil engineering (e.g. to locate limestone caverns, abandoned mine shafts, cavities under pavements, voids in concrete, in masonry or behind tunnel walls). Working with GPR on a paved surface is often straight forward, and it is possible to locate cavities near the surface and at some depth below the surface. By applying radar antennas with different frequencies it is possible to investigate smaller anomalies near a surface or bigger anomalies at greater depth. However, to locate a small anomaly at great depth from the ground surface is not possible.
Surely, can be used. We have successfully attempted very recently in a soil horizon of 5m thickness.
Yes , with appropriate sensors and signal frequency is required to receive signals over 2m, but there are some limitation in it. You can find these information in the manual that could give exact information.