Monday, 20 May 2013

Lake Ohrid borehole hits 568m by Jack Lacey

I arrived in Macedonia just over a week ago and travelled to Lake Ohrid in the south west of the Former Yugoslavian Republic. The drilling and science team had just completed a 568 meter hole!  

Drill tool showing bit and core catcher

The DOSECC drilling and science teams work in two 12-hour shifts, with each shift being able to drill around 30 meters on a good day (and up to 60 meters on a great day!). I work on the night shift so start my day at 7pm and spend my nocturnal hours waiting for core to arrive on deck. The drilling process starts by constructing a chain of outer pipework the length of the hole (over 650 meters of it when I joined!) and then a drill tool is dropped down ready to recover new sediment. The tool is attached to a 3 meter long pipe containing a plastic liner which houses the newly drilled core. At the base of the tool is a drill bit to churn through the lake bed, behind which is a core catcher that acts like a valve to allow the sediment to pass through but not fall out and also provides a sample for immediate analysis. After drilling a wire line is lowered down to recover the tool, complete with sediment core, which is then pulled back up to the surface – which takes around 20 minutes at depths of over 700 meters. This is where the science team’s job starts.

Plenty of cores- a successful nights drilling!
On the barge the main task is to separate the core into 1 meter lengths and accurately record from what depth they were recovered (really important for all later work). It is then secured in the plastic liner and labelled for identification and the depth recorded. The cores are transferred at the end of each shift back to the office where more data is recorded and a Multi-Sensor Core Logger (MSCL) is used to determine some sediment properties for preliminary analysis. Once everything has been completed that can be in the field, the cores are stored in a refrigerated unit and await transport back to the University of Cologne, Germany. 

Bentonite plumes in the water
The campaign is recovering the deepest lake sediment ever drilled, and as such some problems do occur. On several occasions the wire line used to recover the drill tool has broken, which to retrieve it, last week required 90 meters of the outer pipe work to be removed. Also at these depths maintaining a stable drill hole is really important so the pipes can rotate and stay free for drilling. This is achieved by pumping a mix of water and clay down the drill which acts to remove waste material and support the structure of the drill hole. On one night shift we used 56 bags of bentonite (clay) which weighed over 2000 kg, luckily the drillers have to load it!

The barge being moved to the SE of the lake
This week the barge has been towed into a new position to the south east of the lake where we will start coring shallower holes, with an aim to investigate lake level fluctuations and catchment dynamics (similar to the Lini core I am currently working on, see future blog posts!). A second location is planned to the north, nearer our hotel, to look at land slide deposits and water input into the lake via underground springs. 

For more frequent updates and a more in depth account of the drilling process follow me on Twitter @JackHLacey or take a look at my blog Jack is a BUFI-University of Leicester first year PhD student and is being supervised by Prof Mel Leng and Prof Randy Parrish at BGS/Leicester and Dr Bernd Wagner in Cologne.

Me @JackHLacey

1 comment:

Aasiyah Rene said...
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