Wednesday, November 7, 2007

Biscuit with your tea?

The cracking or faulting of the core in the drilling and splitting process is to be expected and doesn't significantly interfere with the information that can be learned from the core. Another phenomena that can be caused by drilling is different. Biscuiting is the term that refers to what happens when the core sticks in the drill barrel and, instead of holding steady while the drill turns around it, spins with the drill. Somewhat like a piece of wood turned on a lathe, the spinning section is incised with circular grooves and rounded and smoothed at the surface where the section of core that is spinning meets a section of core that is not. Look at the top of the section in the photo to the left. Here the harder sediments were shaped and the softer sediment above was turned into a little flattened cookie of sediment. That is where the name comes from, a biscuit is the Brit/Aussie/Kiwi term for a cookie.

Biscuiting can be more subtle in areas of sediment that are only deformed or stressed by the turning pressure of the drill barrel around them. A repeated spin doesn't always happen so the scientists have to look closely at any area of softer sediment to make sure they are seeing evidence of geological features rather than drilling created artifacts. The picture on the right shows another way biscuiting can look. The middle section has had drilling muds infiltrate it as well as twisting. Drilling mud generally flows around the core between the liner and the barrel, but can seep in to the edges of the core, especially in soft sediment. We don't want to include drilling muds (used to lubricate and clear the drill of debris) in geochemical analysis so when we are in that type of sediment, samples will not include material all the way to the edge of the core. We also have a record of the chemical makeup of the muds throughout the drilling process that could be referenced if someone needed to check for mud contamination of their samples.

What other examples of circular turning objects can you think of? Do all of them use some type of liquid to lubricate and reduce friction?

1 comment:

Simon said...

Good post! I learned a lot...

While uncommon, biscuiting in definitely something to look out for in soft sediments.

:) Simon