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A' electrode
One of the current-emitting electrodes of a resistivity-logging system (A); the current return electrode is labeled B. Sometimes referred to as C1 and C2

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.

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.

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.

Having a physical property, which varies with direction.

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.

Refers to deviation from uniformity in a physical property.

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.

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.

Rocks or unconsolidated sediments that are capable of yielding a significant amount of water to a well or a spring.

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.

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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.

A general term referring to rock that underlies unconsolidated material.

Borehole television or video
A downhole television camera; see acoustic-televiewer definition.

Probes designed to reduce the extraneous effects of the borehole and of probe position are called borehole-compensated.

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.

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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.

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.

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.

A device designed to maintain a probe in the center of a borehole.

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.

A measure of the similarity of two oscillating functions.

The technique for forcing radiation, like gamma photons, into a beam.

Complex conductivity
See Complex Resistivity

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.

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.

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.

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.

Geophysical methods carried out between boreholes (see also tomography).

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.

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.

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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.

In nuclear physics, the process of disintegration of an unstable radioisotope by the spontaneous emission of ged particles or photons.

Forcing a logging probe against one side of the drill hole.

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.

Can be any kind of a sensor used to detect a form of energy, but usually refers to nuclear detectors, such as scintillation crystals.

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).

A multielectrode, contact-resistivity probe that provides data from which the strike and dip of bedding can be determined.

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.

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.

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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.

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 method
A method which measures magnetic and/or electric fields associated with subsurface currents.

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.

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.

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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.

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.

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.

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.

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.

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.

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Describes geophysical techniques that require direct contact with the ground in order to pass current. The alternative is to induce currents in the earth.

The common unit of magnetic field intensity, equal to one nanoTesla (a Tesla is the SI unit). The Earth's magnetic field strength is about 50,000 gammas (g) in mid-latitudes.

Gamma log
Also called gamma-ray log or natural-gamma log; log of the natural radioactivity of the rocks penetrated by a drill hole; also will detect gamma-emitting artificial radioisotopes (see spectral-gamma log).

Gamma ray
A photon that has neither mass nor electrical ge that is emitted by the nucleus of an atom; measured in gamma logging and output from a source used in gamma-gamma logging.

Geomagnetic field
The Earth's magnetic field.

Receivers used to record the seismic energy arriving from a source, in seismic geophysical methods.

Geophysical mapping
Locating geophysical anomalies in space (as opposed to time, which is geophysical monitoring).

Geophysical monitoring
Observing the change in a geophysical measurement with time.

Grain density
Also called matrix density; the density of a unit volume of rock matrix at zero porosity, in grams per cubic centimeter.

Ground electrode
A surface electrode used for SP and resistivity logging.

Ground penetrating radar (GPR)
A geophysical method in which bursts of electromagnetic energy are transmitted downwards from the surface, to be reflected and refracted by velocity contrasts within the subsurface. Also known as Ground Probing Radar.

Guard log
A type of focused resistivity log that derives its name from guard electrodes that are designed to focus the flow of current.

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Radioactively, half-life is the time required for half of a given quantity of material to decay. Chemically, it is the time required for half of a given quantity of material to undergo a chemical reaction.

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Imaging work station
Consists of a microcomputer with a high-resolution color monitor and accompanying software which allows the manipulation, enhancement and visual display of digital data.

That part of a periodic signal that has zero phase shift with a reference signal. See also quadrature.

Induced magnetization
Magnetization caused by an applied magnetic field. Contrast with remanent magnetization.

Induced polarization (IP)
A geophysical effect whereby electrical ge is momentarily polarized within a material, usually a disseminated ore or a clay. This effect is the basis for the IP method, in which a decaying voltage due to this polarization is measured following the turn-off of the activating current in time domain surveying. See also complex resistivity.

Induction (EM), induce
The process, described by Faraday's Law, whereby a variable magnetic field generates an electric field (voltage) that, in the presence of a conductor, will produce electric currents.

Induction log
A method for measuring resistivity or conductivity that uses an electromagnetic technique to induce a flow of current in the rocks around a borehole; can be used in nonconductive-borehole fluids.

Induction number
A quantitative measure of the quality of a target for EM methods. The formulation varies for different targets but in general it involves the product of target conductivity, magnetic permeability, frequency of the transmitter and a cross-sectional dimension of the target. Dimensionless.

A method to determine intermediate values from surrounding known values.

Transforming geophysical measurements into subsurface structure. More general term than inversion.

Interval transit time
The time required for a compressional acoustic wave to travel a unit distance (t); transit time usually is measured by acoustic or sonic logs, in microseconds per foot, and is the reciprocal of velocity.

Invaded zone
The annular interval of material around a drill hole where drilling fluid has replaced all or part of the native interstitial fluids.

Inversion, inverting
The process of deriving a model of the subsurface that is consistent with the geophysical data obtained. Generally refers to a more specific methodology than interpretation.

Atoms of the same element that have the same atomic number, but a different mass number; unstable isotopes are radioactive and decay to become stable isotopes.

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Topographic area which has been created by the dissolution of carbonate rock terrain. It is characterized by caverns, sinkholes, and the absence of surface streams.

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The distance a nuclear logging probe moves during one time constant.

Last reading
The depth of the shallowest value recorded on a log.

Lateral log
A multielectrode, resistivity-logging technique that has a much greater radius of investigation than the normal techniques, but requires thick beds and produces an unsymmetrical curve.

A focused-resistivity logging technique; see also guard log.

Light-non-aqueous phase liquids (LNAPLs)
Organic fluids that are less dense than water. They are capable of forming an immiscible layer that floats on the water table (e.g. petroleum hydrocarbons or other organic liquids). Also referred to as Floaters.

Long normal log
A resistivity log with AM spacing usually 64 in.; see normal logs.

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M electrode
The potential electrode nearest to the A electrode in a resistivity device.

Magnetic permeability
acteristic of a material, it is proportional to the magnetism induced in that material divided by strength of the magnetic field used.

Magnetic susceptibility
A measure of the extent to which a substance may be magnetized; it represents the ratio of magnetization to magnetic field strength.

Magnetics, geomagnetics
Geophysical methodology for studying anomalies in the geomagnetic field due to non-uniform magnetization of the subsurface. Uses magnetometers.

The magnetic moment per unit volume. It is a vector quantity. See also magnetic susceptibility.

A device for measuring the earth's magnetic geomagnetic field. Variations in the field strength may indicate changes in magnetic properties of soil and rock or presence of ferrous metals.

Locating geological, chemical or geophysical information in space (as opposed to time, which is monitoring). The results are usually summarized as maps.

The solid framework of rock or mineral grains that surrounds the pore spaces.

A unit of electrical conductance that is the reciprocal of ohm.

Micro-gravity survey
A surface geophysical survey method, undertaken on a very small scale (typically station spacings of a few meters), and requiring a high meter sensitivity. Measures the earth's gravitational field at different points over an area of interest. Variations in the field are related to differences in subsurface density distributions, which in turn are associated with changes in soil, rock, and cultural factors. Typically used for cavern or fracture detection.

Microresistivity log
Refers to a group of short-spaced resistivity logs that are used to make measurements of the mud cake and invaded zone.

The movement of chemicals, bacteria, gases, etc. in flowing water or vapor in the subsurface. Also, a seismic/radar term whose general meaning is the correction of the recorded image for the effects of reflector dip. A very typical result of migration is the removal of hyperbolic events on the record resulting from diffractions from faults and other discontinuities.

Observing the change in a geophysical, hydrogeological or geochemical measurement with time.

Mud cake
Also called filter cake; the layer of mud particles that builds up on the wall of a rotary-drilled hole as mud filtrate is lost to the formation.

Mud filtrate
The liquid effluent of drilling mud that penetrates the wall of the hole.

Change in the amplitude of all or part of a trace before additional processing. Noisy or clearly erroneous traces are given zero amplitude. Data before the first break and the known refraction arrivals are also often reduced to zero amplitude.

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N electrode
The potential electrode distant from the A electrode in a resistivity device.

Neutron log
Neutrons from an isotopic source are measured at one or several detectors after they migrate through material in, and adjacent to, the borehole. Log response primarily results from hydrogen content, but it can be related to saturated porosity and moisture content.

Any unwanted signal; a disturbance that is not part of signal from a specified source. In electrical or induced polarization (IP) surveys, noise may result from interference of power lines, motor-generators, atmospheric electrical discharges, etc. See cultural noise.

Non-aqueous-phase-liquid (NAPL)
Elements or compounds in the liquid phase other than water. This phase is immiscible in water. Examples include petroleum hydrocarbons, like gasoline, and solvents such as trichloroethylene.

In geophysical interpretation and mathematical modeling, a problem for which two or more subsurface models satisfy the data equally well.

Normal log
A quantitative-resistivity log, made with four electrodes, which employs spacings between 4 and 64 in. to investigate different volumes of material around the borehole; see also long-normal log and short-normal log.

Normal moveout corrections
Time shift corrections to reflection arrivals because of variation in shotpoint-to-geophone distance (offset). The amount of shift depends on 1) the length of the raypath from shot to reflection point to receiver, and 2) the velocity of the material traversed. Deeper reflections are corrected using velocities indicative of the deeper section.

Nuclear log
Well logs using nuclear reactions either measuring response to radiation from sources in the probe or measuring natural radioactivity present in the rocks.

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Ohm (?)
The unit of electrical resistance through which 1 amp of current will flow when the potential difference is 1 V.

Ohm-meter (?m)
Unit of electrical resistivity; the resistivity of 1 m3 of material, which has a resistance of 1 ohm when electrical current flows between opposite faces; the standard unit of measurement for resistivity logs.

Open hole
Uncased intervals of a drill hole.

Optimum offset
Seismic reflection technique employing optimum window.

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An elastic body wave in which particles move in the direction of propagation. It is the wave assumed in most seismic surveys. Also called primary or push-pull wave.

Percentage frequency effect (PFE)
The percent difference in resistivity measured at two frequencies (one high, one low). It is the basic polarization parameter measured in frequency domain resistivity surveys. Equivalent to chargeability in time domain surveys.

Perennially frozen ground in areas where the temperature remains at or below 0o C for two or more years in a row.

The property which enables a three-dimensional material to store electrical ge; i.e. its capacitivity.

Phase II study
Common nomenclature for the part of an environmental investigation that first involves on-site activities (i.e. geophysics, soil gas surveys and drilling)

Phase shift
A measure of the offset between two periodic signals of the same frequency. Measured in degrees or radians/milliradians.

Polarize, polarization, polarizable
Separation of ge, as in induced polarization or IP.

The ratio of the void volume of a porous rock to the total volume, usually expressed as a percentage.

The reproducibility of a measurement; the closeness of each of a set of similar measurements to the arithmetic mean of that set.

Primary (magnetic field)
The magnetic field generated by an EM transmitter. May induce a secondary magnetic field.

Also called sonde or tool; downhole well-logging instrument package.

Geophysically, to change data so as to emphasize certain aspects or correct for known influences, thereby facilitating interpretation.

In geophysics, a survey method whereby an array of sensors is moved along the Earth's surface without change in its configuration, in order to detect lateral changes in the properties of the subsurface (faults, buried channels, etc.) The alternative is usually a sounding.

The nucleus of a hydrogen atom; a positively ged nuclear particle with a mass of one; see neutron.

A cross section showing the distribution of a geophysical property, such as seismic travel time, from which the distribution of the geological property of interest (depth to bedrock, for example) can be interpreted.

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That part of a periodic signal that is 90 degrees out of phase with a reference signal. See also in-phase.

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A system whereby short electromagnetic waves are transmitted and any energy which is scattered back by reflecting objects is detected. Acronym for radio detection and ranging.

Energy emitted as particles or rays during the decay of an unstable isotope to a stable isotope.

The Resource Conservation and Recovery Act, passed by the US Congress in 1976 to regulate solid and hazardous waste disposal.

The part of an acquisition system that senses the information signal.

Reflection coefficient
A term used in seismic reflection and GPR to describe the ratio of the reflected to incident amplitudes of a pulse reflected from an interface.

Remanent magnetization (Remanence)
Magnetization remaining after the application of magnetic field has ceased.

Repeat section
A short interval of log that is run a second time to establish repeatability and stability.

Resistivity (electrical)
Electrical resistance to the passage of a current, expressed in ohm-meters; the reciprocal of conductivity.

Resistivity logs
Any of a large group of logs that are designed to make quantitative measurements of the specific resistance of a material to the flow of electric current; calibrated in ohm-meters.

Refers to the smallest unit of measurement that can be distinguished using a particular instrument or method; based on the ability to separate two measurements which are very close together.

A typical distortion of normal-resistivity logs opposite beds that are thinner than the AM spacing; the effect is an apparent decrease in resistivity in the center of a resistive unit.

Remedial investigation/feasibility study.

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A body wave in which particles move perpendicular to the direction of propagation. Also known as secondary or shear wave.

Spectral analysis of surface waves. An in situ seismic method that analyzes dispersion of surface waves and inverts it in terms of mechanical properties of the soil.

The percentage of the pore space occupied by a fluid, usually water in hydrologic applications.

Scintillation detector
An efficient detector used in nuclear-logging equipment; radiation causes flashes of light that are amplified and output in a crystal as electronic pulses by a photo multiplier tube to which it is coupled.

Secondary (magnetic field)
The magnetic field that is generated by currents that are induced to flow in the ground by time variations in the primary magnetic field of the transmitter.

Secondary porosity
Porosity developed in a rock after its deposition as a result of fracturing or solution; usually not uniformly distributed.

Seismic reflection
A surface geophysical method recording seismic waves reflected from geologic strata, giving an estimate of their depth and thickness.

Seismic refraction
A surface geophysical method recording seismic waves refracted by geological strata.

Self potential (SP)
A geophysical method measuring the natural, static voltage existing between sets of points on the ground surface. See also Spontaneous Potential.

Shale base line
A line drawn through the SP log deflections that represent shale; a similar technique can be used on gamma logs and can represent the average log response of sand.

Shear modulus
The stress-strain ratio for simple shear in isotropic materials which obey Hooke's law.

Shear wave
An acoustic wave with direction of propagation at right angles to the direction of particle vibration (S wave).

Short-normal log
One of a group of normal-resistivity logs usually with AM spacing of 16 in. or less.

Single-point resistance log
A single electrode device used to make measurements of resistance that cannot be used quantitatively.

Skin depth
The effective depth of penetration in a conducting medium of electromagnetic energy (when displacement currents can be ignored); the depth at which the amplitude of a plane wave has been attenuated to 1/e or 0.37.

Data in shot record form are sorted for display as common offset records, common shot records, common receiver records, or common depth point records.

In geophysics, a survey method whereby the geometry and/or frequency of an array of sensors is varied so as to measure the physical properties of the earth as a function of depth beneath the configuration. The alternative is usually profiling.

The distance between sources or transmitters and detectors or receivers on a logging probe.

Specific conductance
Strictly speaking identical to electrical conductivity; the term is used in hydrogeology to refer to the conductivity of surface and ground water and expressed in micro Siemens per centimeter. It is a direct function of the total dissolved solids in the water.

Spectral induced polarization (IP)
See complex resistivity.

Spectral-gamma log
A log of gamma radiation as a function of its energy that permits the identification of the radioisotopes present.

Spine and ribs plot
A plot of long-spaced detector output versus short-spaced detector output for a dual detector gamma-gamma probe; permits correction for some extraneous effects.

Spinner survey
A log made with an impeller flowmeter.

Spontaneous Potential (SP)
See Self Potential.

Spontaneous-potential log
A log of the difference in DC voltage between an electrode in a well and one at the surface; most of the voltage results from electrochemical potentials that develop between dissimilar borehole and formation fluids.

Adding together two or more signals. This process is often used in geophysics to improve the signal to noise ratio. A common application is stacking seismic signals in seismic refraction data recording.

Distance separating a probe from the wall of a borehole.

Time shift corrections to individual traces to compensate for the effects of variations in elevation, surface layer thickness or velocity, or datum references.

Streaming potential
A voltage resulting from flow of an ionic fluid.

Surface wave
A wave that travels along, or near to, the surface; its motion dropping off rapidly with distance from it. A distinct seismic mode from the body waves, P- and S.

Oil-industry term used for the performance or result of a well-logging operation.

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The object at which a survey sighting is aimed.

Temperature log
A log of the temperature of the fluids in the borehole; a differential temperature log records the rate of change in temperature with depth and is sensitive to very small changes.

Terrain conductivity
Geophysical method in which EM methods measure directly the average electrical conductivity of the ground. Operates at low induction number.

Thermal neutron
A neutron that is in equilibrium with the surrounding medium such that it will not change energy (average 0.025 eV) until it is captured.

Time constant
The time in seconds required for an analog system to record 63 percent of the change that actually occurred from one signal level to another.

Time domain
In geophysics refers to measurements analyzed according to their behavior in time. The usual alternative is frequency domain measurements.

Time Domain Electromagnetics (TDEM)
See Transient Electromagnetics.

Time domain reflectometry (TDR)
A device, which measures electrical characteristics of wideband transmission systems. Commonly used to measure soil moisture content.

A method for determining the distribution of physical properties within the earth by inverting the results of a large number of measurements made in three dimensions (e.g. seismic, radar, resistivity, EM) between different source and receiver locations.

Tracer log
Also called tracejector log; a log made for the purpose of measuring fluid movement in a well by means of following a tracer injected into the well bore; tracers can be radioactive or chemical.

Term used for the areas in the American Petroleum Institute log grid that are tandard for most large well-logging companies; track 1 is to the left of the depth column, and tracks 2 and 3 are to the right of the depth column, but are not separated.

Any device that converts an input signal to an output signal of a different form; it can be a transmitter or receiver in a logging probe.

Occurring when the system is still changing with time; i.e., a steady state has not been attained. Most groundwater flow systems are transient, not steady state.

Transient Electromagnetics (TEM)
An electromagnetic surveying method in which the waveform of the transmitted signal is a pulse, step function, ramp or other form which can be considered to be nonperiodic and in which measuremnts are made after the primary field has stopped changing (Shariff, 1973). Generally used as a sounding method for differentiating electricaly contrasting geologic media. Also referred to as Time Domain Electromagnetics (TDEM).

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Variabledensity log (VDL)
Also called 3-dimensional log; a log of the acoustic wave train that is recorded photographically, so that variations in darkness are related to the relative amplitude of the waves.

Velocity panels
A set of stacked test sections with a progression of assumed normal-moveout velocities applied. A powerful method for determining velocities if distinct reflection events are present, as the reflections will be coherent where the velocities are correct and be degraded in appearance at higher or lower NMO velocities.

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Well log
A record describing geologic formations and well testing or development techniques used during well construction. Often refers to a geophysical well log in which the physical properties of the formations are measured by geophysical tools, E-logs, neutron logs, etc.

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Z/A effect
Ratio of the atomic number (Z) to the atomic weight (A), which affects the relation between the response of gamma-gamma logs and bulk density

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