Core sample

A core sample is a cylindrical section of a naturally occurring medium consistent enough to hold a layered structure. Most cores are obtained by drilling into the medium, for example sediment or rock, with a hollow steel tube called a corer. The hole made for the core sample is called a core hole. A variety of corers exist to sample different media under different conditions. More continue to be invented. In the coring process the sample is pushed more or less intact into the tube. Removed from the tube in the laboratory, it is inspected and analyzed by different techniques and equipment depending on the type of data desired. Analysis is generally non-destructive of most of the sample.

Methods

* gravity coring, in which the core sampler is dropped into the sample
* drilling exploration diamond drilling
* vibracoring, in which the sampler is vibrated to allow penetration into thixotropic media.

Management of cores and data

Coring is the method to retrieve cores samples from the ground. Coring is often utilised in ocean drilling and surveying. Scientist often using coring to acquire core samples for study.

Drilling equipment is often used for coring. Rock core is often taken during mineral exploration operations to help determine the rock type and amount of mineralisation present. A diamond impregnated core bit is used which is rotated to cut an annulus of rock, producing a rock core which extends through the bit into the core barrel. In a wireline system, the core barrel can be retrieved using a wire cable that is run inside the drill rods. Thus, the drill rods do not have to be removed from the borehole each time a core run is complete. Diamond coring can be carried out to depth of 2000m using conventional mineral exploration drilling equipment. Core can be recovered from deeper wells but the operation becomes more expensive.

The technique of coring long predates attempts to drill into the Earth’s mantle by the Deep Sea Drilling Program. The value to oceanic and other geologic history of obtaining cores over a wide area of sea floors soon became apparent. Core sampling by many scientific and exploratory organizations expanded rapidly. To date hundreds of thousands of core samples have been collected from floors of all the planet’s oceans and many of its inland waters.

Access to many of these samples is facilitated by the Index to Marine & Lacustrine Geological Samples,

"A collaboration between twenty institutions and agencies that operate geological repositories."

The above agency keeps a record of the samples held in the repositories of its member organizations. Data includes

"Lithography, texture, age, principal investigator, province, weathering/metamorphism, glass remarks and descriptive comments"

Layering

Any natural medium at or under the Earth’s surface or other body that is consistent enough to maintain a solid or semi-solid structure is layered. The layering comes from successive deposition or growth in time of structural or compositional variants of the medium.

Most familiar to us are the stratigraphic layers of the Earth’s surface on which the geologic history of the surface is based; for example, the Eocene, Oligocene, Miocene, etc. Each layer in this case contains distinctive fossils generated by the evolution of species. Layers often are divided into sublayers.

Layering is more pervasive than the broad outline of the Geologic Time Scale leads us to believe. Any change in environment causes a new layer to be deposited. A succession of plant species in a region, for example, causes a succession of layers containing different pollen in ice and mud. Variation in rainfall causes tree rings to be of different widths.

Informational value of core samples

Scientific coring began as a method of sampling the ocean floor. It soon expanded to lakes, ice, mud, soil and wood. Cores on very old trees give information about their growth rings without destroying the tree.

Cores indicate variations of climate, species and sedimentary composition during geologic history. The dynamic phenomena of the Earth’s surface are for the most part cyclical in a number of ways, especially temperature and rainfall.

There are many ways to date a core. Once dated, it gives valuable information about changes of climate and terrain. For example, cores in the ocean floor, soil and ice have altered the view of the geologic history of the Pleistocene entirely.

From http://en.wikipedia.org/