Resources > Glossary of Geophysical Terms

Note: The definitions given in this glossary refer to the common terminology as used in airborne geophysics.

a | b | c | d | e | f | g | h | i | j | k | l | m | n | o | p | q | r | s | t | u | v | w | x | y | z

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.

apparent- : 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.

amplitude: The strength of the total electromagnetic field. In frequency domain it is most often the sum of the squares of in-phase and quadrature components. In multi-component electromagnetic surveys it is generally the sum of the squares of all three directional components.

analytic signal: The total amplitude of all the directions of magnetic gradient. Calculated as the sum of the squares.

anisotropy: 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.

anomaly: A localized change in the geophysical data characteristic of a discrete source, such as a conductive or magnetic body: something locally different from the background.

B-field: 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.

background: 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-level: 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.

base frequency: The frequency of the pulse repetition for a time-domain electromagnetic system. Measured between subsequent positive pulses.

bird: A common name for the pod towed beneath or behind an aircraft, carrying the geophysical sensor array.

bucking: 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.

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.

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 localized conductors. (See also coplanar coils)

coil: 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.

compensation: 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.

component: 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).

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 also stripping.

conductivity: [s] The facility with which the earth or a geological formation conducts electricity. Conductivity is usually measured in milli-Siemens per metre (mS/m). It is the reciprocal of resistivity.

conductivity-depth transform: A process for converting electromagnetic measurements to an approximation of the conductivity distribution vertically in the earth, assuming a layered earth. (Macnae and Lamontagne, 1987; Wolfgram and Karlik, 1995)

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 mineralization will be equivalent to 5 metres of 40 S/m; both have 200 S conductivity thickness. Sometimes referred to as conductance.

conductor: 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 mineralization, 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.

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.

culture: 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.

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). 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.

dB/dt: 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.

decay: 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 gamma-ray spectrometry, the radioactive breakdown of an element, generally potassium, uranium, thorium, or one of their daughter products.

decay series: In gamma-ray spectrometry, a series of progressively lower energy daughter products produced by the radioactive breakdown of uranium or thorium.

depth of exploration: 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.

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)

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.

diurnal: The daily variation in a natural field, normally used to describe the natural fluctuations (over hours and days) of the earth’s magnetic field.

dielectric permittivity: [ε] The capacity of a material to store electrical charge, this is most often measured as the relative permittivity [ε_r], 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.

drape: 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.

drift: Long-time variations in the base-level or calibration of an instrument.

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.

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.

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.

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.

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.

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.

fixed-wing: Aircraft with wings, as opposed to “rotary wing” helicopters.

footprint: 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.

frequency domain: An electromagnetic system which transmits a primary field that oscillates smoothly over time (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 also time-domain.

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.

gamma-ray: A very high-energy photon, emitted from the nucleus of an atom as it undergoes a change in energy levels.

gamma-ray spectrometry: Measurement of the number and energy of natural (and sometimes man-made) gamma-rays across a range of photon energies.

gradient: 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 also analytic signal.

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.

half-space: 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.

HEM: Helicopter ElectroMagnetic, This designation is most commonly used for helicopter-borne, frequency-domain electromagnetic systems. At present, the transmitter and receivers are normally mounted in a bird 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.

homogeneous: This is a geological unit that has the same physical parameters throughout its volume. This unit will create the same response to an HEM system anywhere, and the HEM system will measure the same apparent resistivity anywhere. The response may change with system direction (see anisotropy).

HTEM: Helicopter Time-domain ElectroMagnetic, This designation is used for the new generation of helicopter-borne, time-domain electromagnetic systems.

in-phase: 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.

induction: Any time-varying electromagnetic field will induce (cause) electrical currents to flow in any object with non-zero conductivity. (see eddy currents)

induction number: also called the “response parameter”, this number combines many of the most significant parameters affecting the EM response into one parameter against which to compare responses. For a layered earth the response parameter is mwsh² and for a large, flat, conductor it is mwsth, where m is the magnetic permeability, w is the angular frequency, s is the conductivity, t is the thickness (for the flat conductor) and h is the height of the system above the conductor.

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.

infinite: 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.

inversion, or inverse modeling: A process of converting geophysical data to an earth model, which compares theoretical models of the response of the earth to the data measured, and refines the model until the response closely fits the measured data (Huang and Palacky, 1991).

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.

magnetic permeability: [µ] This is defined as the ratio of magnetic induction to the inducing magnetic field. The relative magnetic permeability [m_r] 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 susceptibility: [k] A measure of the degree to which a body is magnetized. In SI units this is related to relative magnetic permeability by k=µ_r- 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. In HEM data this is most often apparent as a negative in-phase component over high susceptibility, high resistivity geology such as diabase dikes.

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.

model: 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 modeled as being infinite in horizontal and depth extent, and very thin. The earth is generally modeled 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.

noise: 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 judgment, 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 also drift.

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, 1987).

off-time: In a time-domain electromagnetic survey, the time after the end of the primary field pulse, and before the start of the next pulse.

on-time: In a time-domain electromagnetic survey, the time during the primary field pulse.

overburden: 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.

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).

physical parameters: 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.

primary field: the EM field emitted by a transmitter. This field induces eddy currents in (energizes) the conductors in the ground, which then create their own secondary fields.

pulse: 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.

quadrature: 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.

radon: 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.

receiver: 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 also, transmitter)

resistivity: [ρ] The strength with which the earth or a geological formation resists the flow of electricity, typically the flow induced by the primary field of the electromagnetic transmitter. Normally expressed in ohm-metres, it is the reciprocal of conductivity.

resistivity-depth transforms: similar to conductivity depth transforms, but the calculated conductivity has been converted to resistivity.

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.

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.

sferic: 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)

signal: That component of a measurement that the user wants to see – the response from the targets, from the earth, etc. (See also noise)

skin depth: A measure of the depth of penetration of an electromagnetic field into a material. It is defined as the depth at which the primary field decreases to 1/e of the field at the surface. It is calculated by approximately 503 x

(resistivity/frequency ). Note that depth of penetration is greater at higher resistivity and/or lower frequency.

spectrometry: Measurement across a range of energies, where amplitude and energy are defined for each measurement. In gamma-ray spectrometry, the number of gamma rays are measured for each energy window, to define the spectrum.

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.

stacking: Summing repeat measurements over time to enhance the repeating signal, and minimize the random noise.

stripping: Estimation and correction for the gamma ray photons of higher and lower energy that are observed in a particular energy window. See also Compton scattering.

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 also vertical plate)

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 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 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-domain: Electromagnetic system which transmits a pulsed, or stepped electromagnetic field. These systems induce an electrical current (eddy current) in the ground that persists after the primary field is turned off, and measure the change over time of the secondary field created as the currents decay. See also frequency-domain.

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.

transient: Time-varying. Usually used to describe a very short period pulse of electromagnetic field.

transmitter: 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 also 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.

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 also thin sheet)

waveform: The shape of the electromagnetic pulse from a time-domain electromagnetic transmitter.

window: 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.

k	Magnetic susceptibility
ε	Dielectric permittivity
µ, µ_r	Magnetic permeability, relative permeability
ρ, ρ_a	Resistivity, apparent resistivity
σ, σ_a	Conductivity, apparent conductivity
σt	Conductivity thickness
	Tau, or time constant
Ω.m	ohm-metres, units of resistivity

AGS	Airborne gamma-ray spectrometry
CDT	Conductivity-depth transform, conductivity-depth imaging (Macnae and Lamontagne, 1987; Wolfgram and Karlik, 1995)
CPI, CPQ	Coplanar in-phase, quadrature
CPS	Counts per second
CTP	Conductivity thickness product
CXI, CXQ	Coaxial, in-phase, quadrature
FOM	Figure of Merit
fT	femtoteslas, normal unit for measurement of B-Field
EM	Electromagnetic
keV	kilo electron volts – a measure of gamma-ray energy
MeV	mega electron volts – a measure of gamma-ray energy 1MeV = 1000keV
NIA	dipole moment: turns x current x Area
nT	nanotesla, a measure of the strength of a magnetic field
ppm	parts per million – a measure of secondary field or noise relative to the primary
pT/s	picoteslas per second: Units of decay of secondary field, dB/dt
S	siemens – a unit of conductance
x	the horizontal component of an EM field parallel to the direction of flight
y	the horizontal component of an EM field perpendicular to the direction of flight
z	the vertical component of an EM field

Huang, H. and Fraser, D.C, 1996. The differential parameter method for muiltifrequency airborne resistivity mapping. Geophysics, 55, 1327-1337

Macnae, J. and Lamontagne, Y., 1987, Imaging quasi-layered conductive structures by simple processing of transient electromagnetic data: Geophysics, v52, 4, 545-554.

Sengpiel, K-P. 1988, Approximate inversion of airborne EM data from a multi-layered ground. Geophysical Prospecting, 36, 446-459

Yin, C. and Fraser, D.C. (2002), The effect of the electrical anisotropy on the responses of helicopter-borne frequency domain electromagnetic systems, Submitted to Geophysical Prospecting