Scientific Coring

Chapter 8.  Scientific Coring – Hows and Whys

      Geologists can study the earth with a number of different techniques.  In the 1800s when “geologists” evolved from a broader group known as “naturalists” they could simply climb over mountains and along river beds taking rock samples with a small pick.  The approach must be different with marine geology, which is dedicated to studying the ¾ of the earth covered by the oceans.  Under shallow or deep water a major and very widely used technique is seismic profiling or seismic surveying.  This is echo sounding,  generally using sound sources and geophone receivers near the sea surface.  The sounds travel a good distance into the subsea floor and return echoes that give a picture of the subterranean geology.  The images below are examples of 2-D seismic profiles typically used by DSDP and IODP for previewing sites that are candidates for drilling.  The sea floor and sub-layers can be seen.  The ocean surface would be miles away above.

     The offshore oil industry has improved the technology of seismics to include 3-D tomography from which truly amazing 3-dimensional images of sub-terranean geologic structure can be displayed on computer screens with stunning results.  Most large offshore oil and gas fields are confirmed by these techniques long before 100s of millions of dollars are invested in drilling and recovering the new reserves.  It is a point of irony that the actual oil or gas (or any other specific substance, say, coal or valuable minerals) cannot be seen and identified as such by the seismic imaging.  The presence of the oil or gas must be inferred when the physical structure of the terrain matches formations where similar valuable resources have been found in the past.  This is a highly evolved science in its own right within the oil and gas industry as well as among research geologists.

     There is a lot of educated guesswork involved in the interpretation of seismic images, especially when using the 2-D variety.  All those reflected dark and light pieces represent something, but what specifically?  Answering that question is where drilling comes in.  Whether on land or under the ocean, when a borehole is drilled the result is, well, a hole in the ground.  The hole itself has value as an access passage into which instruments can be lowered.  These “logging tools” can measure an astounding variety of geologic and geochemical properties in rocks and soils surrounding the actual borehole.

     But the real value of drilling for research is to bring back samples.  And the best way to do that is by coring, which is drilling a hole without destroying the innermost section – the core.  The drawing at right shows one of many types of core bits; this one being a specialized roller cone core bit used today by IODP from the JOIDES Resolution drillship.  The bit is rotated by turning the entire drillstring with a rotary drive mechanism located on the drillship.  (Much oil and gas drilling is done with downhole motors that turn only the bit but that is not compatible with wireline coring used in science research drilling.)  Note that the roller cones that cut the hole as the bit rotates preserve the inner core section as it passes the bit and enters a “core barrel” housed a few inches inside the drillstring just above the face of the bit.  The general dimensions here are bits with an outside diameter of 10-12 inches and cores with diameters about the same as a beer can.  Cores are cut in 30-ft lengths or at least that is the goal, not always successfully achieved.

     With skill and some luck a drill crew operating on a floating drilling vessel can retrieve cores of rock and soils from thousands of meters below the seafloor, operating from the surface of the ocean in waters 5000m deep or more.  The photo below shows some lovely hard rock cores that have been split lengthwise and laid out in the onboard core lab for inspection.  Note that many sections of the rock core have been captured relatively intact, many are broken into smaller pieces, and some sections appear to have been ground into virtual gravel.  Or --- were the gravel sections in that condition as they existed downhole? – one more thing for the geologists to ponder.

     Coring while drilling does not have to be continuous but most often it is when conducting science research drilling.  “Continuous coring” means every foot of depth in the hole that is penetrated is cored, striving for 100% core recovery – a foot of captured core for every foot the hole is made deeper.  Continuous coring is made possible by using “wireline core barrels”.  These clever devices lock into place inside the lower end of the drillstring immediately above the core bit and capture the incoming core inside a clear, tough plastic liner within a steel core barrel mechanism.  When the core barrel is deemed to be full drilling/coring is stopped and the core barrel is released from its position and retrieved through the hollow drillstring to the ship using a wire rope (wireline) on a high speed winch.  The core is prevented from falling back out the bottom of the core barrel by one-way “core catchers”.  The bit and drillstring remain in place in the hole while the full core barrel is retrieved and an empty core barrel is sent down to replace it.  Then the process repeats.

     Some core barrels are “passive” and do nothing more than provide a receptacle for the incoming core.  Some core barrels help cut or trim the core, others are “active” in projecting ahead of the drill bit and cutting the core with a core bit or “cutting shoe”. 
The picture here shows a nearly perfect core section cut by the diamond cutting shoe depicted.  The core barrel in this case is known as the extended core barrel (XCB) because the 4-inch cutting shoe and the lower end of the core barrel extend ahead of the main 12-inch bit and precuts the core before the main bit comes along to drill the main hole. 
     In soft sediments the best way to core is with the “hydraulic piston corer”, or APC, depicted here.  This tool shoots its core-capturing section 30-ft ahead of the stationary main core bit and cuts the core with a sharp-nosed, non-rotating cutting shoe.  This coring technique can be repeated as the hole is deepened until sediments become too stiff for penetration by the sharp cutting shoe - in some cases to depths of 700-800 feet below the seafloor.  In the diagram the yellow section is the bottom of the drillstring assembly.  It appears truncated but would actually extend all the way to the rig floor of the drillship. The technique is close to perfect with 100% core recovery generally expected.  It is also the best means to avoid disturbance of the sediments.  The comparison core section picture below shows identical sections at the same depth in the same sediments captured using a standard rotary core barrel (RCB) versus an APC core barrel. 

     In the upper image the RCB core has been "scrambled" by the rotary coring process - the sediment was not sufficiently rigid to remain undisturbed as the drill bit passed over it.  In the lower image the lines of undisturbed differing sediments in the APC core can be seen in the picture just as they were deposited on the seafloor 5, 10 or 20 million years ago.  The improved quality and quantity of the APC-type cores retrieved for scientific research drilling literally revolutionized the science of marine sedimentology and enabled scientists to make great strides in the interpretation of the earth’s climate history.

    Because APC-type coring does not involve rotation during the actual cutting of the core (which takes about 2-5 seconds for 30-ft) the cores can also be oriented – which is to say the north side of the core can be identified and labeled even after the core has been extracted and laid down on deck.  This orientation capability is vital to paleomagnetists who study the reversals of the earth’s magnetic field over time.  Entire science careers have been spent doing exactly those types of climate and magnetic field research using deep sea cores.

     On a typical IODP expedition 1000s of meters of cores are retrieved, cut into 1.5m long sections, then split and examined, all within a few hours of being extracted from miles below the surface of the sea.  In almost all cases the cores remain inside the clear plastic core liners.  The liners, also split lengthwise with the cores, become the permanent display and storage vessels for the cores.  The two split halves of any given core section are designated as “working” or “archive”.  Archive sections are immediately stored away in shipboard reefers maintained at about 40-45 deg F.  The working sections go to the onboard core lab to photographed, visually examined, and subjected to dozens of measurements depending on the type of core material and the specific science objectives.  The working sections are also subjected to sampling by the onboard scientists who take hundreds or thousands of small carefully labeled samples back to their university labs for later exhaustive examination.  The working sections are then boxed and stored away in the shipboard reefers alongside the untouched archive sections.

     At the end of a voyage the chilled core sections are boxed and sent by refrigerated trucks to one of three major core repository facilities, located in Texas, Germany and Japan.  The picture here shows a rack of the cores in their final cold storage room at one of the repositories.  Those repository core collections are considered national scientific treasures and both the measured data and the actual cores are available for further research by qualified earth scientists.  

 

 

 

Anecdote -- “Pine Wine and Vodka”

 
    Two months at sea as part of a research expedition on a scientific drillship is a LONG time.  You are away from family and friends, out of your normal comfort zone, and denied many of the comforts of modern living.  It is not iron men in wooden ships, certainly, but there is no TV, no telephone (or at least not in the past, things are better now), no sofa to take nap on, no fridge to grab a snack from, no mall to go shopping, very little recreation beyond movies in the crew lounge and maybe a game of chess, darts or cribbage.  And it is boring for much of the time.  Add all of these little anti-comfort details to the fact that both the Glomar Challenger and JOIDES Resolution were declared “dry” ships (no alcohol) and the results are obvious – smuggling, bootlegging and home brew.  The dry ship policy is a serious safety measure.  All U.S. Navy ships adhere to the same policy as well as most offshore drill rigs (although I was onboard a French drillship that offered beer and wine in the mess – but maybe that was only when the ship was in port as it was during my visit).  The safety issue is legitimate -- an alcohol-impaired roughneck or driller on the rig floor of a drillship is roughly equivalent to a drunk driver on the freeways – somebody stands a chance of getting killed.
     So did this serious anti-alcohol program work for the scientific drillships on their long expeditions?  Well, no.  Enforcement was a problem.  On more than one voyage alcohol ban enforcement was actually my thankless job.  What that mostly meant was that I, virtually alone, did not get invited to the secret drinking parties held quietly in selected staterooms and out-of-the-way labs corners.  It was pretty much understood (though officially denied, of course) that the enforcement policy was don’t ask-don’t tell, keep it under control, don’t get a rig floor hand drunk right before his time on duty, don’t be blatant, (and don’t invite the enforcer individual to any parties).
     Alcohol showed up on board by one of two means – contraband smuggled onboard in private luggage, or home brew.  The private luggage option is obvious and probably 50% of the invited scientists for any voyage brought a bottle or two of liquor or wine with them.  For them it was a long-standing tradition to drink a little at sea because most of their seagoing time was on research vessels that were not drillships, and thus not dry because they were not as inherently dangerous to life and limb as a ship with a full scale drilling rig.  The scientists tended to share their alcohol with a few old colleagues or new friends in the privacy of their staterooms after hours and no harm was ever done along these lines that I ever heard about.
     The home brew option was a lot more interesting and fun.  The Glomar Challenger crew included a veteran marine technician named Pine who was famous for routinely brewing up a batch of Pine Wine, ready for consumption by the brave and foolhardy near the end of each voyage.  Pine brewed his concoction secretly in a locked closet near the chemistry lab, although the secret was in name only because by about the 5^th or 6^th week of any voyage the secret was out when the atmosphere in the lab became heady enough to get a mild buzz – like visiting a winery in the Napa Valley.  He used grape juice, sugar, and who knows what other ingredients purloined from the galley, took a good long time to allow proper fermentation (about 6 weeks) and never failed to make enough for everybody to have a good shot at a “secret” end-of-cruise party held a night or two before arrival at the final port.  Picture the moonshine drinking scene with Steve McQueen in “The Great Escape”.  We always said Pine’s latest batch of wine was a “good year” – because it tasted like an old tire.  But that is not a fair assessment, really – it was worse than that.
     But Pine Wine was not the most ambitious home brew experiment I was ever aware of.  On one DSDP expedition the scientific contingent included a veteran scientist from Russia named Boris.  Boris was well known and well liked, onboard as well as ashore at the end of a cruise when he was always the life of the drinking parties at the pub nearest the dock.  Given that he loved his brew and he was Russian it was perhaps inevitable that he was asked if he thought the clandestine drinking enthusiasts on one voyage could produce some home brew vodka.  Would we be likely to have the proper ingredients at hand?  Boris answered in classis Russian fashion, “In Russia we have old saying – you can make vodka from old shoe!”  Figuring they could do better than an old shoe they pressed Boris for more useful information until he proceeded to detail how to use potatoes, cook them, ferment the mash, then heat it up and distill off the good stuff.  Sounded easy enough for a bunch of chemistry grads with a full lab at their disposal, albeit done secretly and requiring some sugar and potatoes “liberated” from the galley or food storage holds.  Needless to say, the vodka distillers stayed busy in their off hours making the best hooch they could devise.  On the appointed evening near the end of the voyage, deep in a locked storeroom, the conspirators gathered and ceremoniously presented Boris with their first glass of experimental vodka for his expert critique and appraisal.  Boris took one sip, made a deathly face, barely swallowed, and declared, “You should have used the shoe!”