Tuesday, June 25, 2013

Geophysics at Mesas: Dualem again...

The Tabuaço and Mesas group share an island, that is an island of Quaternary sand "floating" on top of Cretaceous clay. The question is now where the catchment boundary is, or more precisely, where does the water end up that is infiltrating in that island of sand, is it in the Tabuaço or in the Mesas River? So the group called in Michel for some shallow geophysical measurements using the new Dualem-421S sensor, basically a 4-metre yellow pipe that you have to carry along. Here you see them plowing through a forest, and later through a fallow area to get their information about the thickness of the sand and the palaeotopography of the clay surface.

Doing shallow geophysical measurements with the Dualem in the relative cool of a pine plantation...
and Dualem group plowing their way through a rough patch, temperature above 30 degrees Celsius!
We did 600 m in less than 2 hours measuring every 5 steps. We had problems to convert the data from the instrument to the inversion program, but Fernando Santos from EMTOMO in Portugal, who seems to work day and night, really helped us out and we could use his EM4Soil program to invert the data. Seems that the sand layer was sometimes thicker than the Dualem could reach (about 7 m, see graph below), so back later with the Geonics EM34 to do deeper measurements of the resistivities. With EM4SOIL we can do the combined inverison both for DUALEM and EM34!

Graphical display of the Dualem measurements, values below 1 indicate clay, those around 2 groundwater and higher values represent dry sand.
There were several wells in the area, so their water levels will also be measured accurately with a level to determine flow direction, question answered!

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