Monday, June 25, 2012

Geophysics, beach hydrology and tablets

With hydrochemistry out of the way, it was time to start the next phase. This is more focussed on hydrogeology. When you want to grow a cabbage for your Caldo Verde soup, you need some water and good soil. Us geologists call these good soils "Quaternary sands", as these are recent deposits of old rivers, beaches and dunes that the local farmers like very much. Below this stuff is usually some less permeable layer (Cretaceous clay) with a low electric resistance, and the question is at what depth it is, because this determines how much water you can extract for irrigation.

If it is shallow, we could use an auger to get the answer, but sometimes using geophysics is also a nice way to find out. Our Mesas group reached the same conclusion and reserved the vertical electrical sounding (VES) equipment for a morning session between aggressive flies in their area. Although everybody used this equipment in the Twente field course, it is always difficult at first to find out how to connect all wires. In the picture shown below it is clear that everybody tries different strategies to come up with the correct answers.

How to connect VES wires? Would drinking water help, or discussing the question in Spanish while looking at a bunch of wires, or is practicing yoga a better strategy?
Terrameter Valentina telling everybody to get their hands of the electrodes so that she can apply 2000 V to Mother Earth
With the answer found, the 2000 V power is switched on and a resistance measurement made, plotted on paper, electrode distance increased and measurements repeated. We also have a Geonics EM34 that uses a different method to achieve the same and it was compared to the VES.

EM34 measurement looking for Cretaceous clays below Quaternary sand
These measurements were done by all groups and a tablet computer, given to each group at the strat of the field course, was used to enter data, display the geologic map, find the piezometers that were installed in earlier field courses here, and so on. This works surprisingly well and it also helps us to find our way around.

Meanwhile, on the beach of Costa Nova, BSc students Evy and Leonie were still measuring water salinity on a mudflat. Now they used some real kind of industrial art, called a spiral auger (the Van Harlingen auger after its designer), to sample groundwater at different depths. The handle got stuck, was repaired, but now the thing starts at 3 m height, so they have to use the box to reach its top.

Leonie and Evy sampling groundwater with the Van harlingen auger.
The final week of our course has started today and it is time to determine changes in groundwater storage by measuring the water levels in the large diameter wells, as was done in the beginning. Below is the Sao Romao group group doing their thing as a well-oiled machine.

Sao Romao Group entering well data in their tablet computer.
More posts will follow soon...

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