ODP at sea: work aboard JOIDES Resolution.
The drillship JOIDES Resolution is outfitted with the most modern laboratory, drilling, and navigation equipment. The ship is 143 meters long and 21 meters wide, and its derrick rises 61.5 meters above the water line. A computer-controlled system regulates 12 powerful thrusters in addition to the main propulsion system to stabilize the ship over a specific drill hole located in water as deep as 8,235 meters. The drilling system can handle 9,150 meters of drill pipe, enough for drilling in all but the deepest parts of the world ocean.
In the most common drilling sequence, the four-coned tungsten-carbide roller bit that will cut into the seafloor is attached to the drill pipe along with its stabilizing weights. This assembly is lowered from the drill floor to the "moonpool," a seven-meter-diameter hole in the bottom of the ship, where it passes through a funnel-shaped guide horn into the water. The seven-member drill-floor crew employs various mechanical and hydraulic devices to extend the drill string down toward the seafloor. Twenty-eight and a half meter lengths of pipe weighing 874 kilograms are moved from their racks, lifted by the drawworks at the base of the drilling tower, threaded onto the drill string, and then lowered. In 5,500 meters of water, it takes 12 hours for the drill bit to reach the seafloor. Just before its arrival, an electric motor begins to rotate the drill string to drive the core bit into the sediment. Surface seawater is pumped down the drill pipe to remove cuttings and cool the bit. The drill string is decoupled from the surface motion of the ship by a heave compensator, a huge shock absorber built into the derrick so that cores can be cut and lifted smoothly.
An inner core barrel just above the bit at the bottom of the drill string is retrieved by a wire cable that travels down the center of the drill pipe. When the bit has advanced by an interval that matches the length of the inner core barrel (9.5 meters), the core barrel is pulled up through the drill string and delivered to the laboratory. Another core barrel is then lowered to receive the next core. It takes an hour and 40 minutes for a core barrel to make the round trip in 5,500 meters of water.
Drilling technique and equipment vary as different types of material are cored. When the target is soft sediment that would be considerably altered by the rotation of the drill bit, water pressure is used to drive the hydraulic piston corer developed by DSDP through the bit and into the sediment. When alternately hard and soft materials are encountered, a rotating extended core barrel pushes ahead of the bit in soft sediment and then retracts within the drill string when the core bit is needed to cut through harder material.
An important recent advance in technology now allows drilling in bare rock. Previously, at least 50 to 100 meters of soft sediment were required to stabilize the bottom of the drill string before hard rock could be drilled. With the new technique, a guide base filled with cement stabilizes the drill string, and specially designed drilling motors drive the bit without rotating the entire string. This process reduces damaging vibration and drill-string fatigue that would otherwise occur in coring young rock that has no sediment cover.
Each scientific cruise (called an ODP leg) lasts about two months. A normal shipboard party includes approximately 24 scientists, half from the US and two each from the other ODP partners. The scientific party typically includes the following:
* paleontologists who provide age determinations for cored sediment, and rock and environmental descriptions for the time of deposition based on the fossils found in the cores,
* sediment geologists who describe cores and provide compositional, environmental, and tectonic interpretations,
* petrologists who describe and classify the rocks recovered,
* magnetics specialists who study the magnetic reversals Earth has experienced as they are recorded in seafloor sediments and basement rock,
* geophysicists who consider the physical properties, such as density and heat flow, of the sediments and rocks and also interpret the general geologic setting of the site, and
* geochemists who study fluctuations of organic and inorganic material in the cores and monitor recovered samples for the presence of hydrocarbons.
An ODP technical support staff is responsible for collecting, recording, and preserving core materials and archiving routine scientific data. They also operate the shipboard computer system and maintain and repair laboratory and other equipment.
ODP shipboard operations run 24 hours a day with members of the scientific party standing 12-hour watches so that someone from each scientific discipline is always available. A well-established routine is initiated, night or day, when a core arrives in the laboratory in its plastic tube or "liner." It begins with measuring the length of the core, cutting it into sections for study and storage, coding the top and bottom of the core with colored caps, and clearly marking the liner with the core's original location on the seafloor.
The paleontology staff on duty immediately begins to examine fossils found at the bottom of the core to determine the age of the oldest material sampled. A chemist checks for gas pockets, bubbles, or frothing within the liner, indications of hydrocarbon presence. If these are found, drilling at the site is reevaluated and perhaps terminated. For safety reasons, every effort is made to avoid drilling into hydrocarbon accumulations that might erupt through the drillstring.
The core is then taken to the Physical Properties Laboratory where the Gamma Ray Attentuation and Porosity Evaluator (known as "the GRAPE") measures density by determining the amount of radiation able to pass through the core. Other physical measurements include determination of the strength of the cored material and of thermal conductivity for studies of the earth's heat flow.
When the whole-core analyses are completed, the core is split lengthwise, and the halves are moved to separate tables. One half becomes the working section and the other is preserved as the archive section. Small samples of the working half are removed according to the cruise sampling plan and the dictates of direct observation. The archive section is photographed and a geologist writes a rigorously detailed description of it before it is boxed for long-term storage under refrigeration.
As initial analyses of each core are completed, the data are entered into the computer for display on terminals throughout the laboratory complex. Scientists working anywhere on the ship can track the arrival of new samples and become immediately involved in their analysis if appropriate.
Depending on the hardness of the sediments or rocks being cored and the depth of drilling, cores are delivered from the drill floor to the laboratories at intervals ranging from 20 minutes to five or six hours. Each one follows the routine described above before core samples are taken to specialized laboratories for intensive study.
In the Paleomagnetics Laboratory, a state-of-the-art magnetometer reads the record of Earth's magnetic field changes, information that helps determine the ages of rocks cored and at what latitude they were originally formed.
Paleontologists retrieve microfossils from sediment samples with sieves, chemicals, filters, and centrifuges, in some cases recovering millions of tiny skeletons from a sample smaller than your thumb. Light microscopes are used to identify and examine fossil species at magnifications up to 2,000 times. This analysis provides information on the age of the sediment and climatic conditions at the time of its deposition. The climate and water conditions preferred by certain species can be inferred from the preferences of their living relatives. Then the conditions of ancient ocean represented by a section of core can be determined by identifying the proportions of similar fossil species found there. As shell forms common to certain periods of earth history become known, the fossils can be used to determine the age of the sediments in which they are found.
The finest details of rock and consolidated sediments are studied with thin sections of these materials cut with diamond saws and polished to a high gloss for study under special microscopes. Two of the four shipboard petrological microscopes can photograph the minerals, and each of the microscopes can be connected to a video camera so that scientists can view the thin section on a screen. Petrologists also study and classify mineral structures with an x-ray diffractometer, which identifies minerals by characteristic scattering patterns of x-rays passing through cored samples.
The Chemistry Laboratory is equipped for detailed analyses of the elements contained in sediments and rocks and in the water they contain. Determination of elemental variations along a core helps to reveal the history of the ocean recorded as the sediments were deposited over millions of years.
Once all core has been recovered from a particular drill site, the resulting borehole usually becomes a geochemical and geophysical laboratory itself. Characteristics of the layers of sediment and rock penetrated by the drill bit are determined with sophisticated instruments specially designed for this downhole work, which is called "logging." A discussion of downhole measurements begins on page 129.
Vicky Cullen is manager of publications, graphic services, and public information as well as Editor of Oceanus for the Woods Hole Oceanographic Institution. She also does occasional publication work for other oceanographic organizations and agencies; this description was written for a brochure on the Ocean Drilling Program published in 1987 by Joint Oceanographic Institutions Inc.
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|Title Annotation:||25 Years of Ocean Drilling; Ocean Drilling Program; Joint Oceanographic Institutions for Deep Earth Sampling research ship|
|Date:||Dec 22, 1993|
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