Thursday, 30 October 2014

Vibrocoring (+Video)

Our work on the coastal lowlands surrounding Lake Hamana involves investigating stratigraphy: layers of sediment laid down over time. We use coring equipment to recover sediments from many metres below the current ground surface. Most coring equipment essentially consists of a hollow metal pipe, which fills with sediment when it is driven into the ground. When extracted, the different sedimentary layers can be viewed. 

The simplest approach is to drive the corer into the ground and extract it again using muscle power. Sometimes this just isn't sufficient, but we have some useful motorised approaches to help. Today we spent time at a site where the anthropogenic fill at the surface and the very deep sedimentary sequence meant that the vibrocorer was the best tool for the job. 

Instead of pushing the corer into the ground under our own force, we use a modified jack hammer - the sort of thing more frequently seen being used to break up concrete and tarmac. Here's a demonstration from Svenja and Martin:

Of course, once you've hammered the corer into the sediment, it's pretty hard to pull it back out. Just the weight of the extension rods, which can be up to 15m in length for particularly deep sedimentary sequences, means that we need some more mechanical assistance. We let hydraulic pistons and a ball clamp do most of the pulling for us. Here's 6 minutes and about eight metres of extraction condensed into 25 seconds:

Once the core has been extracted, it can be cleaned, described and sampled. We're using a combination of open auger heads, which allow us to see the sediment in the field, and closed heads, which recover the sediment in a plastic liner, which can then be sealed and analysed in the laboratory later. 

Vanessa cleans a section of core

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