Core sample

Core sample

Overview

A core sample is a cylindrical section of (usually) a naturally occurring substance. Most core samples are obtained by drilling with special drills into the substance, for example sediment or rock, with a hollow steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 tube called a core drill
Core drill
A core drill is a drill specifically designed to remove a cylinder of material, much like a hole saw. The material left inside the drill bit is referred to as the core....

. The hole made for the core sample is called the "core hole". A variety of core samplers exist to sample different media under different conditions. More continue to be invented on a regular basis.
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A core sample is a cylindrical section of (usually) a naturally occurring substance. Most core samples are obtained by drilling with special drills into the substance, for example sediment or rock, with a hollow steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 tube called a core drill
Core drill
A core drill is a drill specifically designed to remove a cylinder of material, much like a hole saw. The material left inside the drill bit is referred to as the core....

. The hole made for the core sample is called the "core hole". A variety of core samplers exist to sample different media under different conditions. More continue to be invented on a regular basis. 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.

Core samples can also be taken to test the properties of manmade materials, such as concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

, ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

s, some metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

s and alloys, especially the softer ones. Core samples can be taken of living things, including human beings, especially of a person's bones for microscopic examination to help diagnose diseases.

Methods



The composition of the subject materials can vary from almost liquid to the strongest materials found in nature or technology, and the location of the subject materials can vary from on the laboratory bench to over 10 km from the surface of the Earth in a borehole. The range of equipment and techniques applied to the task is correspondingly great. Core samples are most often be taken with their long axis oriented roughly parallel to the axis of a borehole, or parallel to the gravity field for the gravity-driven tools. However it is also possible to take core samples from the wall of an existing borehole. Taking samples from an exposure, albeit an overhanging rock face or on a different planet, is almost trivial. (The Mars Exploration Rovers carry a Rock Abrasion Tool
Rock Abrasion Tool
The Rock Abrasion Tool is a grinding and brushing installation on NASA’s twin Mars Exploration Rovers, Spirit and Opportunity , which landed on Mars in January 2004...

 which is logically equivalent to the "rotary sidewall core" tool described below.)

Some common techniques include :
  • gravity coring, in which the core sampler is dropped into the sample, usually the bed of a water body, but essentially the same technique can also be done on soft materials on land. The penetration forces, if recorded, give information about the strength of different depths in the material, which may be the only information required, with samples as an incidental benefit. This technique is common in both civil engineering site investigations (where the techniques tend towards into pile driving) and geological studies of recent aquatic deposits. The low strength of the materials penetrated means that cores have to be relatively small.
  • vibracoring, in which the sampler is vibrated to allow penetration into thixotropic media. Again, the physical strength of the subject material limits the size of core that can be retrieved.
  • drilling exploration diamond drilling
    Exploration diamond drilling
    Exploration diamond drilling is used in the mining industry to probe the contents of known ore deposits and potential sites. By withdrawing a small diameter core of rock from the orebody, geologists can analyze the core by chemical assay and conduct petrologic, structural and mineralogic studies...

     where a rotating annular tool backed up by a cylindrical core sample storage device is pressed against the subject materials to cut out a cylinder of the subject material. A mechanism is normally needed to retain the cylindrical sample in the coring tool. Depending on circumstances, particularly the consistency and composition of the subject materials, different arrangements may be needed within the core tools to support and protect the sample on its way to surface; it is often also necessary to control or reduce the contact between the drilling fluid
    Drilling fluid
    In geotechnical engineering, drilling fluid is a fluid used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also used for much simpler boreholes, such as water wells. Liquid drilling fluid...

     and the core sample, to reduce changes from the coring process. The mechanical forces imposed on the core sample by the tool frequently lead to fracture of the core and loss of less-competent intervals, which can greatly complicate interpretation of the core. Cores can routinely be cut as small as a few millimeters in diameter (in wood, for dendrochronology
    Dendrochronology
    Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

    ) up to over 150 millimeters in diameter (routine in oil exploration
    Oil exploration
    Hydrocarbon exploration is the search by petroleum geologists and geophysicists for hydrocarbon deposits beneath the Earth's surface, such as oil and natural gas...

    ). The lengths of samples can range from less than a meter (again, in wood, for dendrochronology
    Dendrochronology
    Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

    ) to around 100 metres in one run (in oil exploration
    Oil exploration
    Hydrocarbon exploration is the search by petroleum geologists and geophysicists for hydrocarbon deposits beneath the Earth's surface, such as oil and natural gas...

    ), and many runs can be made in succession if "quick look" analysis in the field suggests that the zone of interest is continuing.
  • percussion sidewall coring coring uses robust cylindrical "bullets" explosively propelled into the wall of a borehole to retrieve a (relatively) small, short core sample. These tend to be heavily shattered, rendering porosity
    Porosity
    Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...

    / permeability measurements dubious, but are often sufficient for lithological
    Petrography
    Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. Petrographic descriptions start with the field notes at the...

     and micropalaeontological
    Micropaleontology
    Micropaleontology is the branch of paleontology that studies microfossils.-Microfossils:...

     study. Many samples can be attempted in a single run of the tools, which are typically configured with 20 to 30 "bullets" and propulsive charges along the length of a tool. Several tools can often be ganged together for a single run. The success rates for firing a particular bullet, it penetrating the borehole wall, the retention system recovering the bullet from the borehole wall, and the sample being retained in the bullet are all relatively low, so it is not uncommon for only half the samples attempted to be successful. This is an important consideration in planning sample programmes.
  • rotary sidewall coring where a miniaturised automated rotary drilling tool is applied to the side of the borehole to cut a sample similar in size to a percussion sidewall core (described above). These tend to suffer less deformation than percussion cores. However the core-cutting process takes longer and jams are common in the ancillary equipment which retrieves the sample from the drill bit and stores it within the tool body. This complicates the planning of a coring programme.

Management of cores and data


Although often neglected, core samples always degrade to some degree in the process of cutting the core, handling it, and studying it. Non-destructive techniques are increasingly common (e.g. the use of MRI scanning to characterise grains, pore fluids, pore spaces (porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...

) and their interactions (constituting part of permeability) but such expensive subtlety is likely wasted on a core that has been shaken on an unsprung lorry for 300 km of dirt road. What happens to core between the retrieval equipment and the final laboratory (or archive) is an oft-neglected part of record keeping and core management.
Equally, a core sample which cannot be related to its context (where it was before it became a core sample) has lost much of its benefit. The identification of the borehole, and the position and orientation ("way up") of the core in the borehole is critical, even if the borehole is in a tree trunk - dendrochronologists
Dendrochronology
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

 always try to include a bark surface in their samples so that the date of most-recent growth of the tree can be unambiguously determined.

If these data become separated from core samples, it is generally impossible to regain that data. The cost of a coring operation can vary from a few currency units (for a hand-caught core from a soft soil section) to tens of millions of currency units (for sidewall cores from a remote-area offshore borehole many kilometres deep). Inadequate recording of such basic data has ruined the utility of both types of core.

Different disciplines have different local conventions of recording these data, and the user should familiarise themselves with their area's conventions. For example, in the oil industry, orientation of the core is typically recorded by marking the core with two longitudinal colour streaks, with the red one on the right when the core is being retrieved and marked at surface. Cores cut for mineral mining may have their own, different, conventions. Civil engineering or soil studies may have their own, different, conventions as their materials are often not competent enough to make permanent marks on.

It is becoming increasingly common to retain core samples in cylindrical packaging which forms part of the core-cutting equipment, and to make the marks of record on these "inner barrels" in the field prior to further processing and analysis in the laboratory. Sometimes core is shipped form the field to the laboratory in as long a length as it comes out of the ground; other times it is cut into standard lengths (5m or 1m or 3 ft) for shipping, then reassembled in the laboratory. Some of the "inner barrel" systems are capable of being reversed on the core sample, so that in the laboratory the sample goes "wrong way up" when the core is reassembled. This can complicate interpretation.

If the borehole has petrophysical
Petrophysics
Petrophysics is the study of the physical and chemical properties that describe the occurrence and behavior of rocks, soils and fluids . Petrophysics mainly studies reservoirs of resources, including ore deposits and oil or natural gas reservoirs...

 measurements made of the wall rocks, and these measurements are repeated along the length of the core then the two data sets correlated, one will almost universally find that the depth "of record" for a particular piece of core differs between the two methods of measurement. Which set of measurements to believe then becomes a matter of policy for the client (in an industrial setting) or of great controversy (in a context without an overriding authority. Recording that there are discrepancies, for whatever reason, retains the possibility of correcting an incorrect decision at a later date ; destroying the "incorrect" depth data makes it impossible to correct a mistake later. Any system for retaining and archiving data and core samples needs to be designed so that dissenting opinion like this can be retained.

If core samples from a campaign are competent, it is common practice to "slab" them - cut the sample into two or more samples longitudinally - quite early in laboratory processing so that one set of samples can be archived early in the analysis sequence as a protection against errors in processing. "Slabbing" the core into a 2/3 and a 1/3 set is common. It is also common for one set to be retained by the main customer while the second set goes to the government (who often impose a condition for such donation as a condition of exploration/ exploitation licensing). "Slabbing" also has the benefit of preparing a flat, smooth surface for examination and testing of profile permeability, which is very much easier to work with than the typically rough, curved surface of core samples when they're fresh from the coring equipment. Photography of raw and "slabbed" core surfaces is routine, often under both natural and ultra-violet light.

History of coring


The technique of coring long predates attempts to drill into the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

’s mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

 by the Deep Sea Drilling Program
Deep Sea Drilling Program
The Deep Sea Drilling Project was an ocean drilling project operated from 1968 to 1983. The program was considered to be successful as evidenced by the data and publications that have resulted from it and is now supported by Texas A&M University, although for the years of its operations these were...

. 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 ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

s and many of its inland waters.

Access to many of these samples is facilitated by the Index to Marine & Lacustrine Geological Samples
Index to Marine & Lacustrine Geological Samples
The Index to Marine & Lacustrine Geological Samples is a collaboration between twenty institutions and agencies that operate geological sample repositories...

,
"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
Lithography
Lithography is a method for printing using a stone or a metal plate with a completely smooth surface...

, texture
Texture (geology)
Texture in geology refers to the physical appearance or character of a rock, such as grain size, shape, arrangement, and pattern at both the megascopic or microscopic surface feature level. This includes the geometric aspects and relations amongst the component particles or crystals which is called...

, age, principal investigator, province
Geologic province
A geologic or geomorphic province is a spatial entity with common geologic or geomorphic attributes. A province may include a single dominant structural element such as a basin or a fold belt, or a number of contiguous related elements...

, weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

/metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...

, glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 remarks and descriptive comments"

Informational value of core samples


Scientific coring began as a method of sampling the ocean floor. It soon expanded to lake
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

s, ice
Ice core
An ice core is a core sample that is typically removed from an ice sheet, most commonly from the polar ice caps of Antarctica, Greenland or from high mountain glaciers elsewhere. As the ice forms from the incremental build up of annual layers of snow, lower layers are older than upper, and an ice...

, mud
Mud
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...

, soil
Loss
Loss may refer to:*A negative difference between retail price and cost of production**Loss leader a deliberate commercial loss made in the expectation of recouping it by profitable sales of other lines...

 and wood
Dendrochronology
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

. Cores on very old tree
Tree
A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...

s give information about their growth rings without destroying the tree.

Cores indicate variations of climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

, species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 and sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

ary 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
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 and rainfall.

There are many ways to date a core. Once dated, it gives valuable information about changes of climate and terrain
Terrain
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...

. For example, cores in the ocean floor, soil and ice have altered the view of the geologic history of the Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

entirely.

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