At Sea Report from the Bismarck Sea:
Trying to drill a hole
December 11th, 2000
Our attempts to put casing in Hole 1188F have been adventurous to say the least. We tried to install thefirst set of casing (60 m long) to prevent the upper part of the hole from collapsing. The casing was assembled (it looks like long strings of wide pipe with threads that connect to each other) and a new developmental tool called hammer drill was attached at the end of it to guide its way. The hammer is a drill system that looks like a flat drill bit and acts like jack hammer with under-reamer arms (wings that open up when pressure is applied) that help cut the formation and pulls the casing inside the hole rightbehind it. Once the casing is all the way in, the hammer is unlatched and it slides inside the casing so it can be retrieved to the surface.
Everything went well as the casing went all the way in but at the end, the hammer would not fit back in the casing and it got stuck. It seems that the under-reamer arms did not close all the way back and it got stuck in the lower portion of the casing. After rotating several times and trying to pull on the hammer, the situation got worse because we could not rotate anymore therefore, the only alternative was to begin pulling until something would break loose. Finally, after pulling very hard, the hammer and the entire casing came back to the surface. Luckily, the casing was not attached to the reentry cone otherwise it would have effectively terminated drilling operations in this hole.
Once the hammer and casing were brought back to the surface, we noticed that the arms had carved a sizeable groove on the bottom of the casing and we could even see striations from when the pulling began. After examining the situation, we decided to try again but this time they took off the under-reamer arms. During the second attempt, the hammer was doing a good job until about 2 m from the final depth. At that point the casing got wedged and there was no more forward progress. The engineers decided to drop a second reentry cone on top of the first one because the casing was sticking out of the first cone, making it useless. This was the first time that this operation was ever attempted and it worked. Now the reentry structure looks like an inverted Christmas tree on the bottom of the sea - how appropriate.
After all these operations, we were able to place a second casing string and now we are coring ahead and getting fairly good core recovery. Lets hope that we can drill deeper so we can get some work trying to log the lower portion of the hole.
This is the sixth hole drilled in this area and the reason that we keep trying is because we have been somewhat successful in getting considerable depth in several holes and this area has a lot of scientific interest for understanding the hydrothermal system. The name of Snow Cap comes from the appearance of the seafloor where it is covered with a white bacterial mat that resembles snow. The microbiologists onboard are interested in interpreting the differences encountered in biological diversity and mass in terms of the nutrient supplied by the entire hydrothermal system. They are also interested in analyzing levels of adenosine triphosphate (ATP) to determine the biological activity of the subsurface biosphere. ATP is a molecule that serves as a proxy for living organisms and for the source of metabolic energy indicating subsurface biological activity. So, the microbiologists are looking for signs of life and they want to determine how deep these organisms live within the oceanic crust.
On the other hand, the geologists are interested in understanding the different mechanisms controlling fluid flow, dissipation of heat, and mineralization that are often associated with hydrothermal systems. Hot fluids are less dense (lighter) so they tend to rise whereas cold seawater is heavier and tends to sink. In a hydrothermal system, cold seawater penetrates through fractures into deeper parts of the crust where it warms up and begins finding its way back to the seafloor. The hot fluids rise up either very fast at localized areas forming black smokers (fluids with temperatures ranging between 200 and 400º C) or at a slower rate as they percolate through permeable sediments (temperatures < 100º C). Snow Cap is currently an area of slow diffuse flow characterized by sulfide mineralization, which may even include precipitation of precious minerals such as gold (Au), silver (Ag), and copper (Cu).
Diffuse venting typically occurs throughout the life of a hydrothermal system. It may occur at the flanks of a new high temperature system as the earliest form of fluid discharge where hydrothermal fluids mix with cold seawater. However, diffuse venting may also represent the last stages of activity in a hydrothermal system as a cooling volcanic intrusion generates high temperature upflow. The presence of sulfide and sulfate mineralization in this area suggests a past history that is different to the present conditions because most of these minerals precipitate at much higher temperatures that the ones recorded at present time (~ 60º C). Therefore, this area may help us understand the evolutionary processes that these systems go through.
We have 18 more days before we begin or journey to Australia and back home. Yes, I am looking forward to going back home. I will talk to you again next week.
Gerry
P.S. For those of you that spent time decorating your styrofoam cups, you are now the proud owners of minicups. They were placed inside a meshed bag and attached to the VIT camera frame in one of the multiple reentry attempts. The water depth was 1653 m. Congratulations!!!
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