Ground Penetrating Radar (GPR) can be used for geological surveys to detect features such as the bedrock, water table, and stratigraphic layers. In these cases the GPR frequencies in typical use in the UK, (200MHz and higher) are not suitable and a lower frequency system should be used.
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The bedrock was expected to be found within 3-5m of the surface, which in the UK is usually beyond the penetration of a 200MHz GPR (despite manufacturers advertising otherwise) so KB GPR Surveys proposed the use of a 100MHz GPR to ensure that the required penetration was achieved. In this case the antenna was a large, ground coupled antenna with an external wheel that was pulled on straps.
After consultation with our client, we proposed to use a very low frequency, 25MHz GPR which is designed for detecting layers in geological applications. The GPR itself consisted of a pair of large dipoles mounted on a fiberglass frame. The electronics were transported in a backpack and connected to the dipoles by cables. Instead of a wheel for triggering, a piece of cotton was attached to an object at the beginning of the survey line and unravelled from a reel mounted on the surveyors hip as the GPR moved forwards (carried by hand). This allows the survey to be conducted over scrub without clearing the land first.
The Earth is made up of layers. The top layer, the crust, is formed of organic matter, topsoil, subsoil, parent material, and bedrock. Below bedrock is the Mohorovicic Discontinuity, a boundary between the crust and mantle. Below this is the upper mantle, then lower mantle, outer core, and inner core.