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Glossary- Geophysical Terminology (A - F)... |G - M | N - T | U - Z | Conversion Table |
A electrode. One of the current-emitting electrodes of a resistivity-logging system (A); the current return electrode is labeled B.
API unit. The American Petroleum Institute (API) has established test pits for calibrating neutron and gamma logs. The API neutron unit is defined as 1/1,000 of the difference between electrical zero and the logged value opposite the Indiana limestone in the calibration pit that has an average porosity of 19 percent. The API gamma unit is defined as 1/200 of the deflection between intervals of high and low radioactivity in the calibration pit.
Accuracy. Refers to closeness of a measurement to the true value.
Acoustic impedance. Reflects the ability of a boundary to reflect seismic energy. It is the contrast of density multiplied by velocity across the boundary. A measure of the seismic inertia of the medium.
Acoustic log. Also called sonic log; a record of changes in the acter of sound waves as they are transmitted through liquid-filled rock; a record of the transit time (t) is the most common; amplitude and the full acoustic-wave form also are recorded.
Acoustic televiewer log. A record of the amplitude of high-frequency acoustic pulses reflected by the borehole wall; provides location and orientation of bedding, fractures, and cavities.
Acoustic wave. A sound wave transmitted through material by elastic deformation.
Activation log. Also called neutron-activation logs; a record of radiation from radionuclides that are produced in the vicinity of a well by irradiation with neutrons; the short half-life radioisotopes usually are identified by the energy of their gamma radiation or decay time.
Alluvium. A general term for unconsolidated material (e.g. clay, silt, sand, gravel) deposited from running water. Often a sorted or semi-sorted sediment in the bed of a stream or on its floodplain or delta. The deposit may be in the form of an alluvial fan.
Amplitude. The maximum departure of a wave from the average value.
Analog recording. Data are represented as a continuous record of physical variables instead of discrete values, as in digital recording.
Anisotropic. Having a physical property, which varies with direction.
Annulus. The space between the drill pipe or casing and the wall of the drill hole; in rocks saturated with hydrocarbons, the annulus is the transition interval between the invaded zone and the uncontaminated zone.
Anomaly. Refers to deviation from uniformity in a physical property.
Apparent resistivity/conductivity. The resistivity of a homogeneous isotropic ground that would give the same voltage/current or secondary/primary field ratios as observed in the field with resistivity or EM methods. The apparent conductivity is the reciprocal of the apparent resistivity.
Aquifer. Rocks or unconsolidated sediments that are capable of yielding a significant amount of water to a well or a spring.
Aquitard. Geologic formation/s of low hydraulic conductivity, typically saturated, but yielding a limited amount of water to wells. Also referred to as a confining unit.
Archie's Law. An empirical relationship linking formation resistivity ( rt), formation water resistivity (r w) and porosity. The form of the relationship is r t = a rw -m where a and m are experimentally determined constants.
Atomic number (Z). The number of protons in the nucleus of an atom equal to the number of electrons in a neutral atom.
Atomic weight. The total number of protons and neutrons in the nucleus of an atom.
Attenuation, attenuate. A reduction in energy or amplitude caused by the physical characteristics of a transmitting system.
Automatic Gain Control (AGC). A process for increasing the amplitude of a trace with time, thus making all events on the trace appear to be of approximately the same amplitude. Note that this process will expand the amplitudes even if no data are present. Various window lengths are used; the appearance of the data may be greatly affected by the window used in the calculation.
Back-up curve. A curve on the analog record that displays log data on a new scale when deflections on the main curve exceed the width of the paper; usually displayed with a different pattern or color.
Bedrock. A general term referring to rock that underlies unconsolidated material.
Borehole-compensated. Probes designed to reduce the extraneous effects of the borehole and of probe position are called borehole-compensated.
Borehole television or video. A downhole television camera; see acoustic-televiewer definition.
Bottom-hole temperature. The bottom-hole temperature (BHT) usually is measured with maximum recording thermometers attached to a logging probe.
Bouguer correction. The process of correcting gravity data for the mass of the rock between a given station and its reference (base) station. Application of the Bouguer correction to the data set, as well as corrections for latitude, topography, meter drift and elevation, yields the Bouguer anomaly.
Brute stack. A common midpoint stack with only preliminary static corrections (often none) and preliminary normal-moveout corrections (often constant velocity). This stack is often done by field computers to verify the existence of actual reflections.
Bulk density. Bulk density is the mass of material per unit volume; in logging, it is the density, in grams per cubic centimeter, of the rock with pore volume filled with fluid.
Bulk modulus. A modulus of elasticity, relating change in volume to the hydrostatic state of stress. It is the reciprocal of compressibility.
Calibration. Determination of the log values that correspond to environmental units, such as porosity or bulk density; calibration usually is carried out in pits or by comparison with laboratory analyses of core.
Caliper log. A continuous record of hole diameter, usually made with a mechanical probe having from one to six arms.
Casing-collar locator. An electromagnetic device (CCL) that usually is run with other logs to record the location of collars or other changes in casing or pipe.
Cementation factor. The cementation exponent (m) in Archie’s equation relating formation-resistivity factor and porosity; cementation factor as relates to many aspects of pore and grain geometry that affect permeability.
Cement bond log. An acoustic amplitude log that is used to determine the location of cement behind the casing and, under some conditions, the quality of the bonding to casing and rock.
Centralizer. A device designed to maintain a probe in the center of a borehole.
chargeability. The normalized (using the primary voltage) area under an induced polarization (IP) decay curve, between two times, after the transmitted current is stopped in a time domain survey. Usually expressed in millivolt-seconds per volt.
Coherence. A measure of the similarity of two oscillating functions.
Collimation. The technique for forcing radiation, like gamma photons, into a beam.
Complex number. Comprised of a real and imaginary part.
Complex resistivity (CR). A geophysical effect, also the basis of the CR method, in which polarization within the medium results in the voltage and applied current being out of phase - that is, their ratio is complex. Also known as spectral IP. Induced polarization (IP) is one form of complex resistivity.
Compressibility. The relative volume reduction that geological material can undergo when a force is applied or water is removed from the vicinity by pumping.
Compressional wave. Compressional acoustic waves (P) are propagated in the same direction as particle displacement; they are faster than shear waves and are used for measuring acoustic velocity or transit
Compton scattering. The inelastic scattering of gamma photons by orbital electrons; Compton scattering is related to electron density and is a significant process in gamma-gamma (density) logging.
Conductance. The product of conductivity and thickness [Siemens].
Conduction currents. Electrical current resulting from the movement of free ges (contrast with displacement current).
Conductivity (electrical). The ability of a material to conduct electrical current. In isotropic material, it is the reciprocal of resistivity. Units are Siemens/m.
Correlation. Determination of the position of stratigraphically equivalent rock units in different wells, often done by matching the acter of geophysical logs; also the matching of variables, such as log response and core analyses.
Cross-hole. Geophysical methods carried out between boreholes (see also tomography).
Crossplot. A term used in log analysis for a plot of one parameter versus another, usually two different types of logs. Useful for the identification of lithology.
Cultural environment. The part of the environment which represents man-made features (e.g. roads, buildings, canals, bridges) as opposed to natural features.
Curie. The quantity of any radionuclide that produces 3.70 x 1010 disintegrations per second.
Current channeling/gathering. Channeling is a restriction of current flow due to an insulating barrier or narrowing of a conductor. Current gathering is a concentration of current in a locally, more conductive zone. The disproportionate influence of lakes and swamps on VLF surveys is a well-known example.
Current density. A measure of current flow through a given (oriented) area [Amperes/ m2].
Cycle skip. In acoustic-velocity logging, cycle skips are caused by only one of a pair of receivers being triggered by an arriving wave, which causes sharp deflections on the log.
Dead time. In nuclear logging, dead time is the amount of time required for the system to be ready to count the next pulse; pulses occurring during dead time are not counted.
Decay. In nuclear physics, the process of disintegration of an unstable radioisotope by the spontaneous emission of ged particles or photons.
Decentralize. Forcing a logging probe against one side of the drill hole.
Deconvolution. A data processing technique applied to seismic reflection data to improve the detection and resolution of reflected events. The process reverses the effect of linear filtering processes (convolution) that have been applied to the data by recording instruments or other processes.
Dense-non-aqueous-phase liquids (DNAPLs). Organic liquids that are more dense than water. They often coalesce in an immiscible layer at the bottom of a saturated geologic unit.
Density log. Also called gamma-gamma log; gamma photons from a radioactive source in the sonde are backscattered to a detector; the backscattering is related to the bulk density of the material around the sonde.
Departure curves. Graphs that show the correction that may be made to logs for some extraneous effects, such as hole diameter, bed thickness, temperature, etc.
Depth of invasion. Radial distance from the wall of the hole to which mud filtrate has invaded.
Depth of investigation. See volume of investigation, also called radius or diameter of investigation.
Depth reference or datum. Zero reference for logs of a well; kelly bushing may be used if the rig is still on the well; ground level or top of casing is frequently used.
Depth section. A cross section to which a velocity function has been applied, thus converting arrival times of reflections to depths.
Detector. Can be any kind of a sensor used to detect a form of energy, but usually refers to nuclear detectors, such as scintillation crystals.
Deviation. The departure in degrees between the drill hole or probe axis and vertical.
Dielectric constant. A measure of the ability of a material to store ge when an electric field is applied.
Dielectric permittivity. Describes the ge separation or polarization in a medium.
Differential log. A log that records the rate of change of some logged value as a function of depth; the differential log is sensitive to very small changes in absolute value.
Digital log. A log recorded as a series of discrete numerical values (compare analog recording).
Dipmeter. A multielectrode, contact-resistivity probe that provides data from which the strike and dip of bedding can be determined.
Dipole. A pair of equal ges or poles of opposite signs.
Directional survey. A log that provides data on the azimuth and deviation of a borehole from the vertical.
Dispersion. A property of seismic surface waves in which their velocity (as well as their penetration into the subsurface) is dependent on their frequency. The basis of methods such as SASW in which seismic wave velocity is analyzed as a function of wave frequency.
Displacement currents. The movement of ge within a material by polarization, as opposed to the flow of free ions or electrons. Related to the applied electric field by the electric permittivity (dielectric constant).
Dual laterolog. A focused resistivity log with both shallow and deep investigation; usually gamma, SP, and microfocused logs are run simultaneously.
Effective porosity. The amount of interconnected pore space through which fluids can pass. Effective porosity is usually less than total porosity because some dead-end pores may be occupied by static fluid.
Elastic moduli (elastic constants). Elastic moduli specify the stress- strain properties of isotropic materials in which stress is proportional to strain. They include bulk and shear moduli.
Electric field. A vector field describing the force on a unit electrical ge [newtons/coulomb = volts/meter].
Electrical logs. Provide information on porosity, hydraulic conductivity, and fluid content of formations drilled in fluid-filled boreholes. This record is based on the dielectric properties (e.g., electrical resistivity) of the aquifer materials measured by geophysical devices lowered down boreholes or wells.
Electrode. A piece of metallic material that acts as an electric contact with a non-metal. In chemistry, it refers to an instrument designed to measure an electrical response that is proportional to the condition being assessed (e.g. pH, resistivity).
Electromagnetic-casing inspection log. The effects of eddy currents on a magnetic field are used to provide a record of the thickness of the casing wall.
Electromagnetic method. A method which measures magnetic and/or electric fields associated with subsurface currents.
Electron volt. The energy acquired by an electron passing through a potential difference of one volt (eV); used for measuring the energy of nuclear radiation and particles, usually expressed as million electron volts (MeV).
Epithermal neutron. A neutron source emits fast neutrons that are slowed by moderation to an energy level just above thermal equilibrium, where they are available for capture; most modern neutron probes measure epithermal neutrons, because they are less affected by chemical composition than thermal neutrons.
Equipotential map. A plot in which points of equal hydraulic head are connected.
Fan shooting. A seismic refraction technique where the sensors (geophones) are deployed on a segment of a circle centered on the seismic source. Variations in the time of arrival are caused by radial variations in the velocity structure. Could be used, for example, to search for low velocity anomalies caused by buried waste.
Ferrimagnetic. Substances having positive and relatively large magnetic susceptibility as well as generally large hysteresis and remanence. This is due to the interaction of atoms and the coupling of magnetic moments aligned in opposition, which result in non-zero net moments. Ferrimagnetic minerals have this property.
Field. That space in which an effect, such as gravity or magnetism, is measurable.
Field print. A copy of a log obtained at the time of logging that has not been edited or corrected.
Filtering. a) The attenuation of a signal's components based on a measurable property (usually frequency). Filtering usually involves a numerical operation that enhances only a portion of the signal. b) Fluid passage through a material that retains particles or colloids above a certain size.
First reading. The depth at which logging began at the bottom of the hole.
Flexural Waves. Flexural waves occur in bars and refers to the ‘flexing, or bending, of a bar. Thus they can be created in shafts by impacting the side of a shaft. The velocity of flexural waves depends on their wavelength.
Flowmeter. A logging device designed to measure the rate, and usually the direction, of fluid movement in a well; most are designed to measure vertical flow.
Fluid sampler. An electronically controlled device that can be run on a logging cable to take water samples at selected depths in the well.
Flushed zone. The zone in the borehole wall behind the mudcake that is considered to have had all mobile native fluids flushed from it.
Focused log. A resistivity log that employs electrodes designed to focus the current into a sheet that provides greater penetration and vertical resolution than unfocused logs.
Formation. Used in well-logging literature in a general sense to refer to all material penetrated by a drill hole without regard to its lithology or structure; used in a stratigraphic sense, formation refers to a named body of rock strata with unifying lithologic features.
Formation-resistivity factor. Formation factor (F) is the ratio of the electrical resistivity of a rock 100 percent saturated with water (Ro) to the resistivity of the water with which it is saturated (Rw). F = Ro/Rw.
Frequency domain. In geophysics, refers to measurements analyzed according to their constituent frequencies. The usual alternative is time domain measurements.
