Micropaleontologist Tim Bralower explains what it’s all about….
Location, location, location. Recovery, recovery, recovery.
Coring is all about recovery. Recovery is defined as the percentage of the formation penetrated that is brought up from the sea bed. For instance, if the drill advances 3 meters and 1.5 meters of the core is retrieved, the recovery is 50%. Obviously the more core recovered the easier it is to do the science you plan to do with it.
There are a number of reasons why less than perfect recovery is attained. One of the most common is when there is a variation in hardness in the formation, the weight on the bit has to be increased to get through the hard layer, but then it literally blows away the soft layer, and it is not recovered. In other instances, rock that fractures often jams the core barrel so no other formation is recovered. I’ve been on expeditions where lack of recovery really hinders the science. It’s also bad for morale, day after day after day of few bits and crumbs with pay dirt left behind on the sea bed.
Recovery of harder rock is generally more difficult than very soft sediment. The softest sediment is typically cored with a piston corer, a sharp knife edge bit that is shot into the sediment typically netting near 100% recovery. In hard rock, a rotary coring bit is used which cuts the formation in a spinning fashion. Most times when I’ve rotary cored, we’ve recovered about 50% of the formation but often a lot, lot less. During a previous Expedition, we spent many days coring the nemesis of drillers, “chalk-chert” very soft-very hard, and we got a few chert pebbles here, a few pebbles there. Very dispiriting.
This brings me to the last two days of drilling. In excess of 98% recovery! Truly outstanding! The system we are using has a low weight on bit but fast rotation, and it has done an incredible job retrieving soft ash layers in between hard limestones and cherts. I’ve never seen anything like it. And the cores are large diameter too. What is equally spectacular is how beautiful the rock structures are, we have seen volcanic ash deposits, downslope gravity flows, varve-like laminations, scour marks and worm burrows to name a few. The cores are going to be truly glorious when they are split at the Onshore Science Party later in the year, exposing fresh faces ready for study.