So we have had a few set backs recently with a sheared pipe and BHA (Bottom Hole Assembly). We have had ice and wind to contend with and we have done seismic profiling to find new drill sites. But, we have successfully recovered many meters of core. What I want to write about is what we do with these cores after they are recovered. When we are on site and coring, several things occur in the forward and aft dry labs. We all anxiously wait for those words over the radio "Core on deck". That's our signal to go up to the helo deck and recover the plastic liner with the drilled ocean sediment from the inner core barrel. Immediately the liner ends are capped using different colored end caps to designate the top from the bottom. The liner comes into the helo hanger where we have the sectioning saw located. The liner is wiped down and labeled.
A quick sample from the very bottom of the section is obtained (usually in the core catcher). This is bagged and the bug people take it to make a smear slide. Bug people is an acronym we use for the micropaleontologists who can help date the sediment from knowing about the various species that have lived and died throughout geologic history. They look for diagnostic diatoms, foraminifera, or nanofossils that can tell us the age of the sediment. Back at the helo deck the liner is sectioned into 1.5 meter segments and taken down to the aft lab to be run on the MSCL (Multi sensor core logger). Here the liner is left for about 4 hours as we let the sediment temperature rise to around 20°C. When the sediment comes out of the bottom of the ocean, it often is around 1 to 2°C and it needs to be room temperature to be run on the MSCL.
Fred or Matt runs the core liner through the MSCL. This is the device that Isaiah and Dan fixed for us recently. As Isaiah stated in his journal, this device measures some physical parameters of the sediment by passing it through several different sensors. The linered core is laid on a track which slowly pushes the core centimeter by centimeter through the various sensors. The first sensor is the gamma-ray detector. By counting the number of gamma decays that pass through the core over a specific time period, the bulk density can be calculated. Bulk density is the ratio of the sediment weight to the volume. The next sensor measures the p-wave velocity, or how fast sound travels through the sediment. These two physical properties are important for understanding changes in lithology (sediment material) and porosity. Another method to obtain porosity (the ratio of the volume of open spaces in the sediment to the total volume) is by measuring electric resistivity. The linered core now passes over an induced electric field and the sensor measures the drop in voltage as the core crosses this field. The drop in voltage is converted to resistivity. Electric resistivity is a measure of how resistant the sediment is to letting an electric current be transmitted through it. The final measurement made is magnetic susceptibility. The core has now reached its last sensor, a white plastic square with a hole in the center. Inside the white plastic is electric wire in a loop (coil) that an electric current is passed through generating a magnetic field. As the core passes through the sensor it affects this magnetic field and from that we can determine the magnetic susceptibility of the sediment or the measurement of the amount of magnetic material in the sediment. MS (magnetic susceptibility) is another way of identifying slight changes in lithology as well as allowing us to correlate core-to-core in a region.
Finally the core sections are split into two halves and labeled with their name and depth downcore. One half is marked archive, photographed, and is given to the sedimentologists to describe and the second half goes to the sampling table. Samples are only taken from the sample half of the core. Once all is done, the core sections are covered in plastic and placed in special tubes for storage. In the meantime there has already been another "Core on Deck" announced and we all rush to the helo hanger to . . .
Pat Manley
Middlebury College
Photo by Pat.
Photo by Joel Cubley.
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