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Borehole measurements beneath the seafloor.

When a scientific borehole bore·hole  
A hole that is drilled into the earth, as in exploratory well drilling or in building construction.
 is drilled beneath the seafloor, there is an opportunity to measure rock properties in an environment that would otherwise have remained totally inaccessible to us. Although physicochemical physicochemical /phys·i·co·chem·i·cal/ (fiz?i-ko-kem´ik-il) pertaining to both physics and chemistry.

1. Relating to both physical and chemical properties.
 rock properties of recovered cores have been measured routinely in shipboard ship·board  
1. The condition of being aboard a ship: on shipboard.

2. Archaic The side of a ship.

 laboratories, it is desirable for several reasons to complement these data with measurements in the borehole, which constitutes a natural laboratory. First, core recovery can be erratic, leaving substantial sections of the bore-hole column unsampled, especially in hard sediments and basement rocks, which fragment easily and are too hard for piston-coring. Second, core measurements are usually made at surface conditions, whereas borehole measurements are necessarily made at in situ In place. When something is "in situ," it is in its original location.  conditions of temperature and pressure, thereby leading to a more realistic database of physicochemical rock properties. Third, downhole measurements are usually made at a scale that is many times greater than the core scale, and this attribute provides an important linkage between laboratory studies and surface geophysical surveys.

The Nature of Downhole Measurements

Downhole measurements used in scientific ocean drilling programs can be grouped into three categories. The most common are wireline logs, spatially continuous records of the physical and chemical properties of the formations penetrated by a borehole. The wireline is a cable that extends from a ship down to a probe or sonde in the borehole; it comprises one or more conductors that allow real-time communication between the probe and the surface. Logs or depth records are made as the probe is pulled up the length of the hole at constant speed to provide continuous measurements of the surrounding formation. Some tools are lowered on a mechanical line, or slickline, that provides no digital communication with the surface: These are known as memory tools, and they are deployed where cable specifications would be inadequate, for example, in very hot holes where temperatures are greater than 400 |degrees~ C. Logging tools are available for measuring a wide range of formation properties, including electrical resistivity Electrical resistivity

The electrical resistance offered by a homogeneous unit cube of material to the flow of a direct current of uniform density between opposite faces of the cube.
 (laterologs and induction logs), sonic velocity, natural radioactivity (gamma-ray log), porosity (neutron log), density, susceptibility, magnetic field orientation and strength, and temperature. The primary sources of tool technology have been the oil-field well-logging service companies, which have provided and run the majority of drilling program logging tools on a contractual basis.

The second category of downhole tools includes formation testers and fluid samplers, which provide spatially discrete data. These tools are designed to respond to formation-behavior-induced mechanical disturbance, such as an applied stress or a pressure drawdown Drawdown

The peak to trough decline during a specific record period of an investment or fund. It is usually quoted as the percentage between the peak to the trough.

 in the wellbore. They can be deployed on a wireline, a slickline, or as part of the drill string. Primary objectives of such tools are the in situ measurement of dynamic parameters such as permeability, temperature, and pressure; determination of stress distributions; and the acquisition of pristine pore-fluid samples.

The third type of downhole tool is a long-term sensor placed in a drill hole to record natural data over a period of time. In this case, the borehole is used not as a laboratory but rather as an observatory. Sensors can be designed to record variations in local microseismic activity or in fluid properties such as temperature, pressure, or chemistry. In some cases, the long-term measurement of fluid properties requires that the borehole observatory be sealed to prevent direct flow between the deeper pore fluids and the sea above.

The History of Downhole Measurements in Scientific Ocean Drilling

Although logs had been run in offshore wells in established petroleum sedimentary provinces such as the Gulf of Mexico Noun 1. Gulf of Mexico - an arm of the Atlantic to the south of the United States and to the east of Mexico
Golfo de Mexico

Atlantic, Atlantic Ocean - the 2nd largest ocean; separates North and South America on the west from Europe and Africa on the east
 for many years, the Years, The

the seven decades of Eleanor Pargiter’s life. [Br. Lit.: Benét, 1109]

See : Time
 deep water sites and basement rocks encountered by the scientific drilling Scientific drilling is a way to probe down into the Earth, allowing scientists and students to obtain samples of sediments, crust, and upper mantle. In addition to rock samples, drilling technology allows us to obtain samples of connate fluids and of the subsurface biosphere,  programs presented new operational and technological challenges that once again made borehole measurements a pioneering venture. The first DSDP DSDP Deep-Sea Drilling Project
DSDP Deflect, Segment and Drop Policy (IEEE) 
 borehole logs were run northwest of Bermuda in September 1968. These used natural gamma-ray and neutron-porosity tools run in the drill pipe. From this point the use of borehole logs increased erratically, drawing eventually on most of the branches of classical physics, but there were extended periods when no logs were run at all. Two basic open-hole tool suites gradually emerged. These can be described retrospectively as a seismic-stratigraphic tool suite (to make resistivity resistivity

Electrical resistance of a conductor of unit cross-sectional area and unit length. The resistivity of a conductor depends on its composition and its temperature.
, sonic, and gamma-ray measurements) and a lithology-porosity tool suite (for density, neutron porosity, and gamma-ray measurements). The gamma-ray log is run with all tool combinations to facilitate reconciliation of depth scales between logging runs.

When DSDP was succeeded by ODP ODP - Open Distributed Processing  in 1985, the logging program became more formalized for·mal·ize  
tr.v. for·mal·ized, for·mal·iz·ing, for·mal·iz·es
1. To give a definite form or shape to.

a. To make formal.

. The standard logging suite encompassed the seismic-stratigraphic and lithology-porosity tool sets from DSDP days, but also took advantage of subsequent developments in tool technology by oil-field service companies. A geochemical tool set was added to provide the elemental concentrations of formations surrounding a borehole. In 1988 the formation microscanner was added to the standard suite. This is a high-spatial-resolution microresistivity tool that provides an electrical image (Elec.) a collection of electrical points regarded as forming, by an analogy with optical phenomena, an image of certain other electrical points, and used in the solution of electrical problems.
See under Electrical.

See also: Electric Image
 of the borehole wall. The formation microscanner allows matching of cores and logs in terms of both depth and orientation.

In addition to the standard tools, several other oil-field tools have been deployed from time to time. These essentially comprise in-hole seismic tools for vertical seismic profiling, a borehole televiewer for obtaining an acoustic image of the borehole wall (analogous to the formation microscanner), and, more recently, a susceptibility/magnetometer tool for resolving magnetic reversals in sediments, which are weakly magnetized compared to basement rocks. All of these are examples of technology originally developed for oil-field applications. However, several downhole tools have been built by scientists in the DSDP/ODP community specifically for scientific use.

They include an ultra-deep-sensing resistivity tool for hydrogeological studies of rock porosity; a high-resolution temperature tool for heat-flow studies; a probe tool for taking water samples and measuring temperature and pressure in sediments ahead of the drill bit; and a thermistor Thermistor

An electrical resistor with a relatively large negative temperature coefficient of resistance. Thermistors are useful for measuring temperature and gas flow or wind velocity.
 string for long-term deployment in a borehole observatory.

Scientific Applications of Downhole Measurements

Downhole measurements have made a major contribution to all the principal scientific themes of DSDP and ODP. The first key area is that of global environmental change. Logs are especially well suited to addressing these problems because the solutions require a continuous depth record so that cyclic variations in sediment composition and texture can be evaluated in terms of paleoclimate and ocean circulation. The second area is crustal crust·al  
Of or relating to a crust, especially that of the earth or the moon.

Adj. 1. crustal - of or relating to or characteristic of the crust of the earth or moon
 composition and structure, which must be known in order to understand the origin and evolution of oceanic lithosphere lithosphere (lĭth`əsfēr '), brittle uppermost shell of the earth, broken into a number of tectonic plates. The lithosphere consists of the heavy oceanic and lighter continental crusts, and the uppermost portion of the mantle. . For example, the sharper spatial resolution (Data West Research Agency definition: see GIS glossary.) A measure of the accuracy or detail of a graphic display, expressed as dots per inch, pixels per line, lines per millimeter, etc. It is a measure of how fine an image is, usually expressed in dots per inch (dpi).  of logs relative to surface seismic data has led to a better understanding of the acoustic characteristics of different layers of Earth's crust. The third area is hydrogeology hy·dro·ge·ol·o·gy  
The branch of geology that deals with the occurrence, distribution, and effect of ground water.

, with all its implications for the global geochemical budget. The two key parameters are porosity and permeability. Porosity can be evaluated from density, neutron, sonic and (in the absence of hydrocarbons) resistivity logs. Permeability is determined from downhole pressure tests over an interval of the borehole that has been isolated using packers or seals. The fourth key area is the global stress regime. Our understanding of the forces that drive tectonic plates This is a list of tectonic plates on Earth. Tectonic plates are pieces of the Earth's crust and uppermost mantle, together referred to as the lithosphere. The plates are around 100 km (60 miles) thick and consist of two principal types of material: oceanic crust (also called  and determine their motions can be advanced through knowledge of in situ stresses. Stress orientations can be inferred from failures of the borehole wall, known as "breakouts," which can be imaged using the borehole televiewer or formation microscanner. Changes in stress orientation can be depicted with depth or mapped regionally.

Scientific borehole logging is entering a new era. It is no longer sufficient to rely on oil-field technology to meet ODP logging needs. A major ODP objective is to drill in the young brittle crust near spreading centers. This will require high-temperature tools that are less than 5 centimeters in diameter and rated to 400 |degrees~ C. Since these specifications exceed the capabilities of commercial logging tools, ODP will have to develop its own tools, possibly in conjunction with other scientific programs in order to share the considerable engineering costs. At present, resistivity, temperature, and fluid sampling tools are being developed for high-temperature slim hole deployment. In this respect, scientific borehole logging is at a watershed. The scientific community is responding to the technical challenges positively so that the downhole measurements of the future will constitute as effective a scientific legacy as their present counterparts.

Paul F. Worthington served as Chairman of the ODP Downhole Measurements Panel from 1987 to 1992. He is an environmental and resource evaluation consultant, based in Ascot, Great Britain Great Britain, officially United Kingdom of Great Britain and Northern Ireland, constitutional monarchy (2005 est. pop. 60,441,000), 94,226 sq mi (244,044 sq km), on the British Isles, off W Europe. The country is often referred to simply as Britain. , and a visiting research professor at the Lamont-Doherty Earth Observatory Lamont-Doherty Earth Observatory (LDEO) is a world-class research institution specializing in the Earth sciences and is part of Columbia University. The current director of Lamont is G. Michael Purdy.  of Columbia University.
COPYRIGHT 1993 Woods Hole Oceanographic Institution
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:25 Years of Ocean Drilling
Author:Worthington, Paul F.
Date:Dec 22, 1993
Previous Article:Technology developments in scientific ocean drilling.
Next Article:Polar Day Nine.

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