Coring is not boring

Meanwhile, things have been busy and a lot of work has been done on Lake Hamana by the offshore team. About time for a new update on how everything went.

Geopulse raft strikes a pose...

Similar to our surveys on the Fuji lakes, reflection seismic profiling and lake bottom coring has been executed on Hamana-ko (ko = lake in Japanese). Due to the larger size of our survey boat, a slightly different coring setup had to be constructed, which consisted of the usual winch mechanism and, in this case, a higher elevated, over-the-side pulley. The advantage of this setup resides within the fact that heavy, full liners do not have to lifted out of the water by direct man/woman-power, saving a whole lot of efforts. 33 cores from 14 different locations were collected during 2 days of hard work, providing us with interesting study material for exploring the sedimentary infill of Lake Hamana.

Survey boat, with cabin for seismics in rainy weather and huge deck for easy coring!

New coring setup with high over-the-side pulley.

The reason why we are interested in the bottom sediments of this lake, is related to its capacity to collect deposits through time. Every year, a certain amount of sediment accumulates on the lake floor. Whenever an earthquake and/or tsunami occurs, this process of gradual sedimentation can be disrupted. Earthquakes are able to remobilise instable slope deposits, after which these move downslope rapidly in the shape of a density current (turbidity current), travelling and settling down into the deepest parts of the lake. Tsunami waves, in turn, are supposed to wash in a lot of coarser grained material (coastal dunes), organic fragments (trees and other plant), marine organisms (shells, plankton) etc. Hence, large earthquakes, often accompanied by tsunami, are supposed to have left a distinct imprint in the cored lake sediments. These event-deposits can be interpreted in terms of paleoseismics in order to improve the existing understanding of source mechanisms and recurrence patterns of earthquakes in the study area and along the Nankai Trough.

Our collection of Hamana lake cores, nicely lined up.

However, at this point of the project we can only guess what is in the liners... until they are brought back to Belgium where they will be opened. Meanwhile, our lake samples as well as the samples collected by the on-land team have left Hamana and will be shipped soon.

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