Mapping of alluvial deposits in Sierra Leone
February 9, 2011
Works on the alluvial deposits mapping were carried out from the surface of the river in Sierra Leone with GROT 12 type ground penetrating radar.
The GROT 12 georadar with 2 meters long antennas was placed on a raft and raised above the water surface.
The measurements were carried out both in manual and automatic mods, when the operator was in a boat nearby and on the river bank accordingly (the wireless connection of the georadar with the computer via Bluetooth was used). The control via Bluetooth is possible for use from a distance of up to 100 meters from the georadar.
The search object was alluvial deposits with the content of diamonds and gold.
To get the coordinates of the measuring points, we used GPS receiver data. The data collection and processing program allows the operator to combine the real-time GPR scanning data with GPS information. The picture below shows the GROT program window with the scanning tracks displayed:
The track 9, which is highlighted with red dots, is presented on the radargram below in its “min-max” version with averaged dielectric constant equal 81 (the value, which matches water):
That radargram is a copy of a radargram, which was displayed on the controlling computer screen and seen by the GROT 12 operator.
Below is an alluvial deposits section, based on the track 9 radargram, with the migration in GROT program performed:
Contemporary sediments – silts are marked in brown in the figure.
Contemporary sediments – sandstones, siltstones are marked in grey.
The riverbed, "ancient" sediments, most likely consists of marine sedimentary deposits and is marked in beige in the figure.
Crystalline basement is marked in dark grey.
It is possible to suggest that the productive formation includes sandstones and siltstones from contemporary sediments lying in the range of 65-95 meters of the track.
Below are the scheme of the tracks and a radargram from another track, marked on the scheme in red, in the “min-max” representation:
Convenient GROT 12 control system and its good depth and track resolutions allow caring out the prospecting online.