The following is a useful collection of geophysical terms. Search through using the alphabetical shortcuts or the “Find” function in your web browser.
- A electrode
One of the current-emitting electrodes of a resistivity-imaging system (A); the current return electrode is labelled B.
Closeness of a measurement to the true value.
- acoustic impedance
Seismic velocity multiplied by density.
- acoustic log
Also called sonic log; a record 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.
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.
- altitude attenuation
The absorption of gamma rays by the atmosphere between the Earth and the detector. The number of gamma rays detected by a system decreases as the altitude increases.
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.
- analytic signal
The total amplitude of all the directions of magnetic gradient. Calculated as the sum of the squares.
- analytic signal method
The analytic signal method, known also as the total gradient method, as defined here produces a particular type of calculated gravity or magnetic anomaly enhancement map used for defining in a map sense the edges (boundaries) of geologically anomalous density or magnetisation distributions. In exploration potential field applications, the term analytic signal loosely refers to the calculated modulus of the gravity or magnetic anomaly field’s three mutually orthogonal spatial (x, y, z) derivative terms. Mapped maxima (ridges and peaks) in the calculated analytic signal of a gravity or magnetic anomaly map locate the anomalous source body edges and corners (e.g. basement fault block boundaries, basement lithology contacts, fault/shear zones, igneous and salt diapirs, etc.). Analytic signal maxima have the useful property that they occur directly over faults and contacts, regardless of structural dip which may be present, and independent of the direction of the induced and/or remanent body magnetisations. Various extensions to the analytic signal method (as defined here) exist. For example, some extensions to the method include as an additional solved parameter the anomalous source body depth(s).
Having a variation in physical properties that are dependent on the orientation of the measurement.
Having different physical parameters in different directions. This can be caused by layering or fabric in the geology. Note that a unit can be anisotropic, but still homogeneous.
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 a deviation from uniformity in a physical property. A localised change in the geophysical data characteristic of a discrete source, such as a conductive or magnetic body.
- 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 with 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.
The physical parameters of the Earth measured by a geophysical system are normally expressed as apparent, as in “apparent resistivity”. This means that the measurement is limited by assumptions made about the geology in calculating the response measured by the geophysical system. Apparent resistivity calculated with HEM, for example, generally assumes that the Earth is a homogeneous half-space – not layered.
- 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 significantly 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 (rw) and porosity. The form of the relationship is rt = 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.
- attenuation, attenuate
A reduction in energy or amplitude caused by the physical characteristics of a transmitting system.
- automated depth estimation
A variety of techniques, which include Werner deconvolution, the Euler method, Naudy’s method, Phillips’ method, and the analytic signal method, which analyse digital analytic signal method magnetic profiles or maps to obtain estimates of source body depth without specific user identification of key portions of anomalies. This contrasts with profile techniques such as Peters’ method (half-slope) or Vacquier’s method (straight slope) which may be implemented as computer programs but require interactive identification of special points on anomalies.
- automatic gain control (AgC)
A process for increasing signal amplitude of a signal through 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.
In time-domain electromagnetic surveys, the magnetic field component of the (electromagnetic) field. This can be measured directly, although more commonly it is calculated by integrating the time rate of change of the magnetic field dB/dt, as measured with a receiver coil.
The “normal” response in the geophysical data – that response observed over most of the survey area. Anomalies are usually measured relative to the background. In airborne gamma-ray spectrometric surveys the term defines the cosmic, radon, and aircraft responses in the absence of a signal from the ground.
- base frequency
The frequency of the pulse repetition for a time-domain electromagnetic system. Measured between subsequent positive pulses.
The measured values in a geophysical system in the absence of any outside signal. All geophysical data are measured relative to the system base level.
A general term referring to rock that underlies unconsolidated material.
A common name for the pod towed beneath or behind an aircraft, carrying the geophysical sensor array.
- borehole television or video
A downhole television camera; see acoustic-televiewer definition.
Probes designed to reduce the extraneous effects of the borehole, casing, 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.
- Bouguer gravity field
The gravity field obtained after latitude, elevation, Bouguer, and terrain corrections have been applied to the measured (observed or raw) gravity data. The Bouguer (named after Pierre Bouguer, a French geodesist) gravity field is often noted as simple Bouguer for the gravity field before applying terrain corrections or complete Bouguer for the gravity field after applying terrain (and sometimes curvature) corrections. The gravity anomalies observed in the Bouguer field are caused by lateral density contrasts within the sedimentary section, crust and sub-crust of the Earth. A measured above sea level Bouguer gravity field and accurately corrected to sea level datum is not equivalent to gravity measured at sea level. Anomalies caused by mass inhomogeneities between station elevation and datum and which were measured at the original station elevations remain in the data unless special corrections are made.
- 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.
The process of removing the strong signal from the primary field at the receiver from the data, to measure the secondary field. It can be done electronically or mathematically. This is done in frequency-domain EM and to measure on-time in time-domain EM.
- bulk density
Bulk density is the mass of material per unit volume; in logging, it is the density, in grams per cubic centimetre, 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.
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.
- calibration coil
A wire coil of known size and dipole moment, which is used to generate a field of known amplitude and phase in the receiver, for system calibration. Calibration coils can be external, or internal to the system. Internal coils may be called Q-coils.
- caliper log
A continuous record of borehole 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 affects permeability.
A device designed to maintain a probe in the centre of a borehole.
The normalised (using the primary voltage) area under an induced polarisation (IP) decay curve, between two times, after the transmitted current is stopped in a time domain survey. Usually expressed in millivolt-seconds per volt.
- coaxial coils (CX)
Coaxial coils in an HEM system are in the vertical plane, with their axes horizontal and collinear in the flight direction. These are most sensitive to vertical conductive objects in the ground, such as thin, steeply dipping conductors perpendicular to the flight direction. Coaxial coils generally give the sharpest anomalies over localised conductors. See: coplanar coils
A measure of the similarity of two oscillating functions.
A multi-turn wire loop used to transmit or detect electromagnetic fields. Time varying electromagnetic fields through a coil induce a voltage proportional to the strength of the field and the rate of change over time.
The technique for forcing radiation, like gamma photons, into a beam.
Correction of airborne geophysical data for the changing effect of the aircraft. This process is generally used to correct data in fixed-wing time-domain electromagnetic surveys (where the transmitter is on the aircraft and the receiver is moving), and magnetic surveys (where the sensor is on the aircraft, turning in the Earth’s magnetic field.
- complex number
Comprised of a real and imaginary part.
- complex resistivity (CR)
A geophysical effect, also the basis of the CR method, in which polarisation 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 polarisation (IP) is one form of complex resistivity.
In frequency domain electromagnetic surveys this is one of the two phase measurements – in-phase or quadrature. In “multi-component” electromagnetic surveys it is also used to define the measurement in one geometric direction (vertical, horizontal in-line and horizontal transverse – the Z, X and Y components).
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 time
- compton scattering
Gamma ray photons will bounce off electrons as they pass through the Earth and atmosphere, reducing their energy and then being detected by radiometric sensors at lower energy levels. See: stripping.
The product of conductivity and thickness (Siemens, S). See: conductivity thickness.
- conduction currents
Electrical current resulting from the movement of free charges (contrast with displacement current).
- conductivity (s)
The ability of a material to conduct electrical current. In isotropic material, it is the reciprocal of resistivity. Units are Siemens/m or usually milli-Siemens per metre (mS/m). It is the inverse of resistivity.
- conductivity thickness (st)
The product of the conductivity, and thickness of a large, tabular body. (It is also called the “conductivity-thickness product”). In electromagnetic geophysics, the response of a thin plate-like conductor is proportional to the conductivity multiplied by thickness. For example a 10 metre thickness of 20 Siemens/m mineralisation will be equivalent to 5 metres of 40 S/m; both have 200 S conductivity thickness. Sometimes referred to as conductance.
- conductivity-depth imaging
See: conductivity-depth transform
- conductivity-depth transform
A process for converting electromagnetic measurements to an approximation of the conductivity distribution vertically in the Earth, assuming a layered Earth.
Used to describe anything in the ground more conductive than the surrounding geology. Conductors are most often clays or graphite, or hopefully some type of mineralisation, but may also be man-made objects, such as fences or pipelines.
- coplanar coils (CP)
In HEM, the coplanar coils lie in the horizontal plane with their axes vertical, and parallel. These coils are most sensitive to massive conductive bodies, horizontal layers, and the halfspace.
Determination of the position of stratigraphically equivalent rock units in different wells, often done by matching the character of geophysical logs; also the matching of variables, such as log response and core analyses.
- cosmic ray
High energy sub-atomic particles from outer space that collide with the Earth’s atmosphere to produce a shower of gamma rays (and other particles) at high energies.
- counts (per second)
The number of gamma-rays detected by a gamma-ray spectrometer. The rate depends on the geology, but also on the size and sensitivity of the detector.
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.
A term commonly used to denote any man-made object that creates a geophysical anomaly. Includes, but not limited to, power lines, pipelines, fences, and buildings.
The quantity of any radionuclide that produces 3.70 x 1010 disintegrations per second.
- current channelling
Channelling is a restriction of current flow due to an insulating barrier or narrowing of a conductor.
- current density
A measure of current flow through a given (oriented) area [Amperes/ m2].
- current gathering
The tendency of electrical currents in the ground to channel into a conductive formation. This is particularly noticeable at higher frequencies or early time channels when the formation is long and parallel to the direction of current flow. This tends to enhance anomalies relative to inductive currents (see also induction). The disproportionate influence of lakes and swamps on VLF surveys is a well-known example. Also known as current channelling.
- daughter products
The radioactive natural sources of gamma-rays decay from the original “parent” element (commonly potassium, uranium, and thorium) to one or more lower-energy “daughter” elements. Some of these lower energy elements are also radioactive and decay further. Gamma-ray spectrometry surveys may measure the gamma rays given off by the original element or by the decay of the daughter products.
As the secondary electromagnetic field changes with time, the magnetic field (B) component induces a voltage in the receiving coil, which is proportional to the rate of change of the magnetic field over time.
In time-domain electromagnetic theory, the weakening over time of the eddy currents in the ground, and hence the secondary field after the primary field electromagnetic pulse is turned off. In nuclear physics, including gamma-ray spectrometry, the radioactive breakdown of an element, generally potassium, uranium, thorium, or one of their daughter products.
- decay constant
See: time constant
- decay series
In gamma-ray spectrometry, a series of progressively lower energy daughter products produced by the radioactive breakdown of uranium or thorium.
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.
Mass per unit volume, expressed in grams per cubic centimetre. Rock or formation densities are usually measured as either saturated bulk densities or grain densities. For gravity interpretation, the contrasts between rock bulk densities are of primary interest since these contrasts are responsible for the anomalous gravity field. Rock bulk densities have been shown to vary as a function of geologic age, lithology and depth of burial. Rock densities typically range from 1.9 g/cm3 to 3.0 g/cm3.
- density contrast
The density of one rock unit relative to another. Density contrasts can be either positive or negative. For example, if Rock A = 2.30 g/cm3 and Rock B = 2.40 g/cm3 then the density contrast of Rock A relative to Rock B is -0.10 g/cm3. Conversely, the relative density contrast of Rock B relative to Rock A is +0.10 g/cm3. gravity anomalies caused by density contrasts within the Earth’s sedimentary section, crust and sub-crust can be analysed and interpreted as lithologic and/or structural anomalies.
- 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.
- density model
A model of the geology in which layers or bodies of given lithologies are replaced by equi-density layers or bodies. The equi-density layers or bodies may or may not correspond to specific geological formations.
- density-depth function
The relationship between the change in density with a change in depth. In many areas of the world with thick clastic sections the increase in density with an increase in depth has been shown to be primarily a function of compaction. However, age, lithology and porosity may also influence the relationship. The relationship is important in gravity modelling because a gravity anomaly may be caused by a gradational change in density rather than a relatively abrupt density contrast, such as that which may occur at a fault, contact, or unconformity.
- 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 exploration or depth of investigation (DOI)
The maximum depth at which the geophysical system can detect the target. The depth of exploration depends very strongly on the type and size of the target, the contrast of the target with the surrounding geology, the homogeneity of the surrounding geology, and the type of geophysical system. One measure of the maximum depth of exploration for an electromagnetic system is the depth at which it can detect the strongest conductive target – generally a highly conductive horizontal layer.
- depth reference or datum
Zero reference point for logs of a borehole.
- depth section
A cross section to which a velocity function has been applied, thus converting arrival times of reflections to depths.
- depth slicing
Generically, the use of linear filters to isolate (based on wavelength criteria) anomaly contributions to a map derived from source bodies in a certain depth range. Numerous techniques are used to carry out the isolation.
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 charge when an electric field is applied.
- dielectric permittivity (e)
The capacity of a material to store electrical charge, this is most often measured as the relative permittivity (er), or ratio of the material dielectric to that of free space. The effect of high permittivity may be seen in HEM data at high frequencies over highly resistive geology as a reduced or negative in-phase, and higher quadrature data.
- 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.
- differential resistivity
A process of transforming apparent resistivity to an approximation of layer resistivity at each depth. The method uses multi-frequency HEM data and approximates the effect of shallow layer conductance determined from higher frequencies to estimate the deeper conductivities (Huang and Fraser, 1996)
- digital log
A log recorded as a series of discrete numerical values (compare analogue recording).
A multi-electrode, contact-resistivity probe that provides data from which the strike and dip of bedding can be determined.
A pair of equal charges or poles of opposite signs. In resistivity, a pair of closely spaced electrodes approximating a dipole.
- dipole moment (NIA)
For a transmitter, the product of the area of a coil, the number of turns of wire, and the current flowing in the coil. At a distance significantly larger than the size of the coil, the magnetic field from a coil will be the same if the dipole moment product is the same. For a receiver coil, this is the product of the area and the number of turns. The sensitivity to a magnetic field (assuming the source is far away) will be the same if the dipole moment is the same.
- 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 frequency dependent. The basis of methods such as MASW in which seismic wave velocity is analysed as a function of wave frequency.
- displacement currents
The movement of charge within a material by polarisation, as opposed to the flow of free ions or electrons. Related to the applied electric field by the electric permittivity (dielectric constant).
The daily variation in a natural field, normally used to describe the natural fluctuations (over hours and days) of the Earth’s magnetic field.
- dose rate
See: exposure rate
To fly a survey following the terrain contours, maintaining a constant altitude above the local ground surface. Also applied to re-processing data collected at varying altitudes above ground to simulate a survey flown at constant altitude.
Long-time variations in the base-level or calibration of an instrument.
- dual laterolog
A focussed resistivity log with both shallow and deep investigation; usually gamma, SP, and microfocused logs are run simultaneously.
- dyke model
See: prism. Dyke model descriptions include wide, narrow, thin, vertical, and inclined.
- eddy currents
The electrical currents induced in the ground, or other conductors, by a time-varying electromagnetic field (usually the primary field). Eddy currents are also induced in the aircraft’s metal frame and skin; a source of noise in EM surveys.
- 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 charge [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 (typically stainless steel) acting as an electric contact with a non-metal substance.
- electromagnetic (EM)
Comprised of a time-varying electrical and magnetic field. Radio waves are common electromagnetic fields. In geophysics, an electromagnetic system is one which transmits a time-varying primary field to induce eddy currents in the ground, and then measures the secondary field emitted by those eddy currents.
- 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).
- elevation correction
The sum of the free-air and Bouguer corrections to observed or “raw” gravity. The Bouguer correction requires an estimation of bulk density to calculate and eliminate the gravitational effect of the subsurface mass between point of gravity measurement and a datum.
- energy window
A broad spectrum of gamma-ray energies measured by a spectrometric survey. The energy of each gamma-ray is measured and divided up into numerous discrete energy levels, called windows.
- equipotential map
A plot in which points of equal hydraulic head are connected.
- equivalent (thorium or uranium)
The amount of radioelement calculated to be present, based on the gamma-rays measured from a daughter element. This assumes that the decay series is in equilibrium – progressing normally.
- Euler method
A profile-based or map-based depth estimation method based on the concept that the magnetic fields of localised structures are homogeneous functions of the source coordinates and therefore satisfy Euler’s equation. This equation can therefore be solved parametrically for the source locations. In recent years, use of this method has become more widespread because it has been automated to work with either grid or profile data.
- exposure rate
In radiometric surveys, a calculation of the total exposure rate due to gamma rays at the ground surface. It is used as a measurement of the concentration of all the radioelements at the surface. Sometimes called “dose rate”. See: natural exposure rate
- fan shooting
A seismic refraction technique where the sensors (geophones) are deployed on a segment of a circle centred 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.
- fiducial, or fid
Timing mark on a survey record. Originally these were timing marks on a profile or film; now the term is generally used to describe 1-second interval timing records in digital data, and on maps or profiles.
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.
- Figure of Merit (FOM)
A sum of the 12 distinct magnetic noise variations measured by each of four flight directions, and executing three aircraft attitude variations (yaw, pitch, and roll) for each direction. The flight directions are generally parallel and perpendicular to planned survey flight directions. The FOM is used as a measure of the manoeuvre noise before and after compensation.
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.
Aircraft with wings, as opposed to “rotary wing” helicopters.
- 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.
- focussed 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.
This is a measure of the area of sensitivity under the aircraft of an airborne geophysical system. The footprint of an electromagnetic system is dependent on the altitude of the system, the orientation of the transmitter and receiver and the separation between the receiver and transmitter, and the conductivity of the ground. The footprint of a gamma-ray spectrometer depends mostly on the altitude. For all geophysical systems, the footprint also depends on the strength of the contrasting anomaly.
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.
- forward modelling
Forward modelling means calculating a data set that would occur if a survey were gathered over a known model of the Earth. This usage of the word “modelling” is essentially the reverse of the definition above, and this often causes confusion for new users of geophysics.
- free-air gravity field
The gravity field after the free-air correction. This correction is applied to observed or “raw” gravity readings to correct for the change in gravity due to the difference in elevation of the gravity station relative to datum elevation (usually sea level). The change in gravity with elevation is inversely related to the change in distance between the meter’s centre of mass (meter elevation) and the Earth’s centre of mass.
- frequency domain
1. An electromagnetic system which transmits a harmonic primary field that oscillates over time (e.g. sinusoidal), inducing a similarly varying electrical current in the ground. These systems generally measure the changes in the amplitude and phase of the secondary field from the ground at different frequencies by measuring the in-phase and quadrature phase components. See: time-domain
2. A domain is where a mathematical function (the independent and dependent variables x and y and maybe z and perhaps more) exists. In the frequency domain, the independent variable has been transformed from a distance such as miles (seconds in the case of seismic) to frequency like cycles/mile (a spatial frequency versus a temporal frequency like cycles/second). The dependent variables then become the strength and phase of that frequency.
- full-stream data
Data collected and recorded continuously at the highest possible sampling rate. Normal data are stacked (see stacking) over some time interval before recording.
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 charge that is emitted by the nucleus of an atom; measured in gamma logging and output from a source used in gamma-gamma logging.
- gamma ray spectrometry
Measurement of the number and energy of natural (and sometimes man-made) gamma-rays across a range of photon energies.
- Gardner’s equation
An empirically derived equation which describes the relationship between bulk densities and acoustic velocities of rocks: p = 0.23v0.25
- 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.
In magnetic surveys, the gradient is the change of the magnetic field over a distance, either vertically or horizontally in either of two directions. Gradient data is often measured, or calculated from the total magnetic field data because it changes more quickly over distance than the total magnetic field, and so may provide a more precise measure of the location of a source. See: analytic signal
A device or set of devices which measures the value of a field in at least two different points in space at the same time. The gradient is the difference in field values per unit of distance between the sensors. By measuring a field’s gradient (that is, its first derivative or rate of change with distance), the total field itself may be computed with varying degrees of accuracy. For potential fields, the direction of the measurement relative to the Earth is critical. Is the gradient being measured horizontally, vertically, and in the case of magnetics, what is the orientation relative to the Earth’s magnetic field? Even with these possible difficulties, measuring just the gradient has the advantage of removing non-geologic field signals, such as when measuring gravity, those introduced by the normal accelerations of the survey aircraft.
- grain density
Also called matrix density; the density of a unit volume of rock matrix at zero porosity, in grams per cubic centimetre.
- gravity unit
The SI unit of acceleration used with gravity measurements. Abbreviated as gu. 1 gravity unit = 1 μs/s2 = 0.1 milligal.
- ground effect
The response from the Earth. A common calibration procedure in many geophysical surveys is to fly to altitude high enough to be beyond any measurable response from the ground, and there establish base levels or backgrounds.
- 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.
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.
A mathematical model used to describe the Earth as infinite in width, length, and depth below the surface. The most common halfspace models are homogeneous and layered earth.
- heading error
A slight change in the magnetic field measured when flying in opposite directions.
Helicopter ElectroMagnetic. This designation is most commonly used for helicopter-borne, frequency-domain electromagnetic systems. The transmitter and receivers are normally mounted in a bird or on a loop carried on a sling line beneath the helicopter.
- herringbone pattern
A pattern created in geophysical data by an asymmetric system, where the anomaly may be extended to either side of the source, in the direction of flight. Appears like fish bones, or like the teeth of a comb, extending either side of centre, each tooth an alternate flight line.
Helicopter Frequency-domain ElectroMagnetic, This designation is used for helicopter-borne, frequency-domain electromagnetic systems. Formerly most often called HEM.
- high density basement
The deepest significantly thick, high density unit(s) within the geologic section of an area, which provide a major positive density contrast. The rocks above the major density contrast are usually younger sediments and/or volcanics, typically having densities ranging from approximately 1.9 g/cm3 to 2.6 g/cm3. Those below the major density contrast are usually older sedimentary, volcanic and/or crystalline rocks, typically having densities ranging from 2.6 g/cm3 to 3.0 g/cm3. High density basement may or may not be equivalent to crystalline and/or magnetic basement.
- high resolution aeromagnetics
This might more correctly be termed “high precision aeromagnetics”. The term has gained wide acceptance in the industry to denote surveys flown at low terrain clearance (30-150 m), with close line spacings (25-500 m), recorded at high sample rates (0.1-0.25 s), and acquired with high-sensitivity magnetometers (0.001-0.005 nT).
This is a geological unit that has the same physical parameters throughout its volume. The response may change with system direction (see anisotropy).
Helicopter Time-domain ElectroMagnetic, This designation is used for the new generation of helicopter-borne, time-domain electromagnetic systems.
- imaging work station
Consists of a microcomputer with a high-resolution colour monitor and accompanying software which allows the manipulation, enhancement and visual display of digital data.
The component of the measured secondary field that has the same phase as the transmitter and the primary field. The in-phase component is stronger than the quadrature phase over relatively higher conductivity. See also quadrature.
- induced magnetisation
Magnetization caused by an applied magnetic field. Contrast with remanent magnetisation.
- induced polarisation (IP)
A geophysical effect whereby electrical charge is momentarily polarised 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 polarisation 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. See: eddy 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.
- inductive limit
When the frequency of an EM system is very high, or the conductivity of the target is very high, the response measured will be entirely in-phase with no quadrature (phase angle =0). The in-phase response will remain constant with further increase in conductivity or frequency. The system can no longer detect changes in conductivity of the target.
In geophysical terms, an “infinite’ dimension is one much greater than the footprint of the system, so that the system does not detect changes at the edges of the object.
- international geomagnetic Reference Field (IGRF)
An approximation of the smooth magnetic field of the Earth, in the absence of variations due to local geology. Once the IGRF is subtracted from the measured magnetic total field data, any remaining variations are assumed to be due to local geology. The IGRF also predicts the slow changes of the field up to five years in the future.
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 or inverse modelling
A technique whereby a 2d or 3d density, susceptibility, or geometric (geologic) model is computed to satisfy (invert) a given observed gravity or magnetic field.
- 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.
Topographic area which has been created by the dissolution of carbonate rock terrain. It is characterised by caverns, sinkholes, and the absence of surface streams.
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.
- layered Earth
A common geophysical model which assumes that the Earth is horizontally layered – the physical parameters are constant to infinite distance horizontally, but change vertically.
- 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.
- M electrode
The potential electrode nearest to the A electrode in a resistivity device.
- magnetic basement
Magnetic basement is usually equated to crystalline (felsic and mafic) or sometimes, metamorphic basement. It is the unconformity upon which an essentially non-magnetic sedimentary section has been deposited. large exposures of basement (e.g., the Canadian Shield) show it to be lithologically and magnetically heterogeneous. Very thick sequences of highly magnetic volcanics may sometimes be considered equivalent to a magnetic basement.
- magnetic permeability (µ)
This is defined as the ratio of magnetic induction to the inducing magnetic field. The relative magnetic permeability (µr) is often quoted, which is the ratio of the rock permeability to the permeability of free space. In geology and geophysics, the magnetic susceptibility is more commonly used to describe rocks.
- magnetic sedimentary section
A surface or zone within the geologic column where magnetic susceptibility contrasts are significant enough to generate magnetic anomalies which could delineate sedimentary geology. Susceptibility variations within the sedimentary column are generally considered near zero except where relatively magnetic sediments (e.g. pyroclastics, arkoses, some shales) are present.
- magnetic susceptibility (k)
A measure of the degree to which a body is magnetised. In SI units, this is related to relative magnetic permeability by k=µr-1 and is a dimensionless unit. For most geological material, susceptibility is influenced primarily by the percentage of magnetite. It is most often quoted in units of 10-6 (SI). In frequency-domain EM data, this is most often apparent as a negative in-phase component over high susceptibility, high resistivity geology such as dolerite dykes.
- magnetics, geomagnetics
Geophysical methodology for studying anomalies in the geomagnetic field due to non-uniform magnetisation 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.
- manoeuvre noise
Variations in the magnetic field measured caused by changes in the relative positions of the magnetic sensor and magnetic objects or electrical currents in the aircraft. This type of noise is generally corrected by magnetic compensation.
Locating geological, chemical or geophysical information in space (as opposed to time, which is monitoring). The results are usually summarised 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 vapour 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.
- milligal (mgal)
The unit of acceleration used with gravity measurements. 1 mgal = 10 µm/s2 = 10 gu.
Geophysical theory and applications generally have to assume that the geology of the Earth has a form that can be easily defined mathematically, called the model. For example steeply dipping conductors are generally modelled as being infinite in horizontal and depth extent, and very thin. The Earth is generally modelled as horizontally layered, each layer infinite in extent and uniform in characteristic. These models make the mathematics to describe the response of the (normally very complex) Earth practical. As theory advances, and computers become more powerful, the useful models can become more complex.
Modelling usually means the process of developing models of the Earth based upon measured geophysical data. It may be as simple as recognising that an anomaly is likely caused by a buried pipe, or it may involve sophisticated data processing and/or inversion to mathematically build a range of plausible models.
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.
The degree of homogeneity in Euler’s equation, interpreted physically as the fall-off rate with distance and geophysically as a structural index (SI). Values vary from 1 to 3 according to magnetic or gravity source body geometry.
- N electrode
The potential electrode distant from the A electrode in a resistivity device.
- nanotesla (nT)
A unit of the magnitude of the magnetic field vector B represented by the number of lines of induction passing through a unit area perpendicular to the vector direction. 1 nanotesla = 1 gamma. See: tesla
- natural exposure rate
In radiometric surveys, a calculation of the total exposure rate due to natural-source gamma rays at the ground surface. It is used as a measurement of the concentration of all the natural radioelements at the surface. See: exposure rate
- naudy’s method
An automated profile-based depth estimation method wherein anomaly type and location are identified by cross-correlation of the observed magnetic profile with theoretical anomalies. The depth to a dike-like or plate-like source is then estimated from parameters relating source body half-width, depth, and data sampling interval.
- neural network
A member of a class of software that is “trained” by presenting it examples of input and the corresponding desired output. For example, the input might be a magnetic anomaly and the required output the depth to the source of that anomaly. Training might be conducted using synthetic data, iterating on the examples until satisfactory depth estimates are obtained. Neural networks are general-purpose programs which have applications outside potential fields, including almost any problem that can be regarded as pattern recognition in some form.
- 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.
That part of a geophysical measurement that the user does not want. Typically this includes electronic interference from the system, the atmosphere (sferics), and man-made sources. This can be a subjective judgement, as it may include the response from geology other than the target of interest. Commonly the term is used to refer to high frequency (short period) interference. See: drift
- 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 modelling, 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 (NMO) 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.
- observed gravity field
The term “observed gravity” is also often used in lieu of “raw gravity” or “measured gravity”. Incorrectly, but often, the term “observed gravity map” may be posted on the following maps: Bouguer, free-air, regional or residual gravity field.
- Occam’s inversion
An inversion process that matches the measured electromagnetic data to a theoretical model of many, thin layers with constant thickness and varying resistivity (Constable et al, 1987a).
In a time-domain electromagnetic survey, the time after the end of the primary field pulse, and before the start of the next pulse.
- ohm (Ώ)
The unit of electrical resistance through which 1 amp of current will flow when the potential difference is 1 V.
- ohm-metre (Ώm)
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.
In a time-domain electromagnetic survey, the time during the primary field pulse.
- open hole
Uncased intervals of a drill hole.
- optimum offset
Seismic reflection technique employing optimum window.
In engineering and mineral exploration terms, this most often means the soil on top of the unweathered bedrock. It may be sand, glacial till, or weathered rock.
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 polarisation 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.
See: magnetic permeability
The property which enables a three-dimensional material to store electrical charge; i.e. its capacitivity. See: dielectric permittivity
- 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.
- phase, phase angle
The angular difference in time between a measured sinusoidal electromagnetic field and a reference, normally the primary field. The phase is calculated from tan-1 (in-phase/quadrature).
- Phillips’ method
An automatic depth estimation method in which the source parameters are estimated from the autocorrelation function of the magnetic anomaly. like Werner deconvolution, the method uses a dike or contact model.
- physical parameters, physical properties
These are the characteristics of a geological unit. For electromagnetic surveys, the important parameters are conductivity, magnetic permeability (or susceptibility) and dielectric permittivity; for magnetic surveys the parameter is magnetic susceptibility, and for gamma ray spectrometric surveys it is the concentration of the major radioactive elements: potassium, uranium, and thorium.
A term used to describe a sheet-like magnetic source body with limited vertical dimension. That is, its thickness may range from 0.1 to 1.0 times its depth-to-top. Its anomaly character is similar to that of a set of dipoles.
- polarise, polarisation, polarisable
Separation of charge, as in induced polarisation or IP.
The ratio of the void volume of a porous rock to the total volume, usually expressed as a percentage.
- potential field
A field which obeys a differential equation known as Laplace’s Equation. Gravity and magnetic fields are both vector potential fields. Most exploration gravity work utilises the vertical component of the gravity field, while most exploration magnetic work utilises the scalar total intensity of the magnetic field.
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.
- primary field
The EM field emitted by a transmitter. This field induces eddy currents in (energises) the conductors in the ground, which then create their own secondary fields.
A term used to describe a magnetic source body which can be considered, for practical purposes, parallelepiped which is semi-infinite in vertical dimension. That is, its depth-to-bottom is at least four times its depth-to-top. Its anomaly character is similar to that of a monopole or line of poles. A two-dimensional prism (semi-infinite normal to the plane of section) is sometimes referred to as a dike model.
Also called sonde or tool; downhole well-logging instrument package.
Geophysically, to change data so as to emphasise 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 charged nuclear particle with a mass of one; see neutron.
An approximation of a gravity field derived from a magnetic field measured at, or transformed to, the magnetic pole. The process requires conversion of susceptibility values to density values and a vertical integration of the magnetic data.
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.
In time-domain EM surveys, the short period of intense primary field transmission. Most measurements (the off-time) are measured after the pulse. On-time measurements may be made during the pulse.
See: calibration coil
That component of the measured secondary field that is phase-shifted 90° from the primary field. The quadrature component tends to be stronger than the in-phase over relatively weaker conductivity. See also in-phase.
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.
This normally refers to the common, naturally-occurring radioactive elements: potassium (k), uranium (U), and thorium (Th). It can also refer to man-made radioelements, most often cobalt (Co) and caesium (Cs).
Commonly used to refer to gamma ray spectrometry.
A radioactive daughter product of uranium and thorium, radon is a gas which can leak into the atmosphere, adding to the non-geological background of a gamma-ray spectrometric survey.
- raw gravity
Also called measured gravity, or observed gravity. The gravity field measured at a gravity station before latitude, free-air, Bouguer or terrain corrections are applied.
The Resource Conservation and Recovery Act, passed by the US Congress in 1976 to regulate solid and hazardous waste disposal.
- receiver (Rx)
The signal detector of a geophysical system. This term is most often used in active geophysical systems; systems that transmit some kind of signal. In airborne electromagnetic surveys it is most often a coil. See: transmitter
- reduction to equator (RTE)
A mathematical transformation of the total magnetic intensity (TMI) field at its observed inclination (I) and declination (d) to that of the magnetic equator (i.e. I=0°).
- reduction to pole (RTP)
A mathematical transformation of the total magnetic intensity (TMI) field at its observed inclination (I) and declination (d) to that of the north magnetic pole (i.e. I=90°, d=0°).
- 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.
- regional gravity field
The long wavelength component of the usually attributed to Bouguer gravity field density variations considered to be deeper than general exploration interest; (e.g. the gravity component due to crustal density variations or undulations of the crust/mantle interface). A subjective regional can often be designed to enhance residual anomalies of primary interest.
- remanent magnetisation (remanence)
Magnetisation 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.
- residual gravity field
The shorter wavelength component of the attributed to density Bouguer gravity field contrasts within high density basement and/or the lower density overburden. Anomalies in the residual field are usually of exploration interest. A first residual is a difference field obtained by subtracting the regional gravity field from the Bouguer gravity field.
- resistivity (ρ)
Electrical resistance to the passage of a current, expressed in ohm-meters; the reciprocal of conductivity.
- resistivity depth dransforms
Similar to conductivity depth transforms, but the calculated conductivity has been converted to resistivity.
- 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.
- resistivity section
An approximate vertical section of the resistivity of the layers in the Earth. The resistivities can be derived from the apparent resistivity, the differential resistivities, resistivity-depth transforms, or inversions.
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.
- response parameter
Another name for the induction number.
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 centre of a resistive unit.
Remedial investigation/feasibility study.
Spectral analysis of surface waves. An in situ seismic method that analyses dispersion of surface waves and inverts it in terms of mechanical properties of the soil.
- second vertical derivative (2VD)
A second vertical derivative map of a potential field may be calculated by application of a frequency domain or space domain filter to a potential field grid file. The result is an anomaly enhancement or residual map related to the “curvature” of the input field. Inflection points on the anomalies of the input field will be zero values on the derivative map and may have special interpretation significance.
- secondary field
The field created by conductors in the ground, as a result of electrical currents induced by the primary field from the electromagnetic transmitter. Airborne electromagnetic systems are designed to create and measure a secondary field.
- sengpiel section
A resistivity section derived using the apparent resistivity and an approximation of the depth of maximum sensitivity for each frequency.
Lightning, or the electromagnetic signal from lightning, it is an abbreviation of “atmospheric discharge”. These appear to magnetic and electromagnetic sensors as sharp “spikes” in the data. Under some conditions lightning storms can be detected from hundreds of kilometres away. See: noise
That component of a measurement that the user wants to see – the response from the targets, from the Earth, etc. See: noise
- skin depth
The effective depth of penetration of an electromagnetic wave in a conductive medium. The skin depth is the distance in which the wave decays to 1/e (about 37%) of its value. It can be expressed as: δs=(2/σμω)½, where δs=skin depth in m, σ=conductivity in mhos/m, μ=permeability in henries/m, ω=angular frequency in radians/m. Note that depth of penetration is greater at higher resistivity and/or lower frequency.
- space domain
A domain is where a mathematical function (the independent and dependent variables x and y and maybe z and perhaps more) exists. In the space domain, distance (1 if by profile, 2 if by map measured in perhaps feet, kilometres, degrees, seconds, etc.) is the independent variable and some quantity (milligals, gammas, density, seismic amplitude, etc.) is the dependent variable. See: frequency domain
Measurement across a range of energies, where amplitude and energy are defined for each measurement. In gamma-ray spectrometry, the number of gamma rays is measured for each energy window, to define the spectrum.
In gamma ray spectrometry, the continuous range of energy over which gamma rays are measured. In time-domain electromagnetic surveys, the spectrum is the energy of the pulse distributed across an equivalent, continuous range of frequencies.
Summing repeat measurements over time to enhance the repeating signal, and minimise the random noise.
- strike fIlter (pass or reject)
A band-pass filter designed to pass or attenuate Fourier components of a potential field data set along a pre-determined angle (strike).
Estimation and correction for the gamma ray photons of higher and lower energy that are observed in a particular energy window. See: Compton scattering
- structural model
A gravity or magnetic structural model is a 2d or 2.5d density and/or susceptibility model of given or assumed geology. The geology of an area can be modelled by representing lithologic layers as equi-density and/or equi-susceptibility layers and/or blocks. The layers are formed by contrast boundaries which may or may not correspond to specific geologic formation boundaries. Where high density or susceptibility contrasts exist in nature, the model may correspond closely to those geologic formations. For 2-dimensional modelling, the density and susceptibility models of the geology and the observed gravity and magnetic anomalies for the model are assumed to be semi-infinite. For 2.5-dimensional modelling, the third dimension y (in and out of the plane of the profile) is approximated by one or more given distances, thus providing a quasi-3d model.
See: magnetic susceptibility
Often used as a name for the time constant.
- tau (τ)
Often used as a name for the time constant.
See: time-domain electromagnetic method
See: transient electromagnetic method
- 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.
- tesla (T)
The SI unit of magnetic field B, which is also known as “magnetic flux density” and “magnetic induction”. One tesla is equal to one weber per square meter
- 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.
- thin sheet
A standard model for electromagnetic geophysical theory. It is usually defined as a thin, flat-lying conductive sheet, infinite in both horizontal directions. See: vertical plate
- three dimensional (3D) model
A network or grid of values which models a geologic surface represented as a surface of (gravity) or susceptibility contrast (magnetics). The output of a forward model is based on the calculated gravity or magnetic effect of a specified input surface. The output of an inverse model is the geometry of an appropriate (but non-unique) surface calculated by inverting the input gravity or magnetic field.
- tie line
A survey line flown across most of the traverse lines, generally perpendicular to them, to assist in measuring drift and diurnal variation. In the short time required to fly a tie-line it is assumed that the drift and/or diurnal will be minimal, or at least changing at a constant rate.
- time channel
In time-domain electromagnetic surveys the decaying secondary field is measured over a period of time, and the divided up into a series of consecutive discrete measurements over that time.
- time constant
The time required for an electromagnetic field to decay to a value of 1/e of the original value. In time-domain electromagnetic data, the time constant is proportional to the size and conductance of a tabular conductive body. Also called the decay constant.
- tIme domain
In geophysics refers to measurements analysed according to their behaviour in time. The usual alternative is frequency domain measurements.
- time domain ectromagnetic method
See: transient electromagnetic method
- 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.
- total energy envelope
The sum of the squares of the three components of the time-domain electromagnetic secondary field. Equivalent to the amplitude of the secondary field.
- total magnetic intensity (TMI) anomaly
The total magnetic intensity anomaly field is the resultant field after correcting TF, the total magnetic (observed) field for a regional gradient field, such as an IGRF.
- 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 standard 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.
Time-varying. Usually used to describe a very short period pulse of electromagnetic field.
- transient electromagnetic method
A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. In the last two decades, TEM surveys have become the most popular surface EM technique used in exploration for minerals and groundwater and for environmental mapping.
- transmitter (Tx)
The source of the signal to be measured in a geophysical survey. In airborne EM it is most often a coil carrying a time-varying electrical current, transmitting the primary field. See: receiver
- traverse line
A normal geophysical survey line. Normally parallel traverse lines are flown across the property in spacing of 50 m to 500 m, and generally perpendicular to the target geology.
- variable density 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.
- vertical plate
A standard model for electromagnetic geophysical theory. It is usually defined as thin conductive sheet, infinite in horizontal dimension and depth extent. See: thin sheet
The shape of the electromagnetic pulse from a time-domain electromagnetic transmitter.
- 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.
- Werner deconvolution
An automated profile-based depth estimation method derived from S. Werner’s analysis of magnetic anomalies from sheet-like bodies. Polynomials representing a total field anomaly or its derivative (horizontal gradient) can be simultaneously solved to estimate the depth, dip, horizontal location, and susceptibility of the source body (thin sheet or interface).
A discrete portion of a gamma-ray spectrum or time-domain electromagnetic decay. The continuous energy spectrum or full-stream data are grouped into windows to reduce the number of samples, and reduce noise.
- 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