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Glossary

This page contains a simple glossary of technical terms used in our web content.

For a more in depth glossary with some fundamental definitions and descriptions of many of the ways in which geophysics is used please click here to view a Microsoft Word document produced by the Environmental and Engineering Geophysical Society.

Apparent resistivity
The resistivity calculated from electrical resistances measured in the field. Varies from true resistivity due to subsurface heterogeneity. c.f. true resistivity.
Anomaly
 
Is indicative of variation in a physical property of an object or zone, relative to some background value when detected remotely.
Aquifer
 
A rock formation or soil zone that is saturated with ground water and has adequate permeability to produce water from a well.
Base station
 
An arbitrary point in the survey area where repeat readings are taken throughout the duration of the survey to allow correction of instrument drift and correlation between different surveys.
Bedrock
 
Consolidated mechanically competent rock underlying any loose superficial material such as soil, alluvium or marine sediments.
Bouguer anomaly
 
The resulting gravity values after tidal, latitude, free air, and Bouguer corrections have been applied to the data. It is commonly the final presented result, which is then interpreted, for a microgravity survey.
Bouguer correction
 
A correction applied to gravity data to account for the additional mass of rock between the survey point and the datum level.
Depth of investigation
 
The maximum depth at which a geophysical technique provides useful information about the subsurface. Dependant on the technique being used, the local conditions, and the type of target.
DGPS
 
Differential Global Positioning System. An enhanced version of GPS that allows great accuracy.
DNAPL
 
Dense Non-Aqueous Phase Liquid. A liquid that is more dense than water and does not dissolve in water, e.g. chlorinated solvents.. Can be detected by electrical resistivity imaging, EM ground conductivity, and GPR techniques. cf. LNAPL.
EM Electromagnetic.
ERT
 
Electrical Resistivity Tomography. Another name for Electrical Resistivity Imaging (ERI). A way of producing 2D and 3D models of the subsurface resistivity distribution.
Fourier analysis
 
The mathematical decomposition of a waveform into its constituent frequencies. Often used in the process of converting time domain (TD) data to the frequency domain (FD) and vice versa.
Free air correction
 
A correction applied to gravity data to account for the change in distance from the centre of the Earth between the survey point and the datum level.
Frequency domain (FD)
 
A representation of data to aid analysis of its variation over different frequencies. c.f. time domain.
Geophone
 
A sensor for detecting seismic waves in the subsurface. It works by producing an electrical response proportional to the velocity of the ground motion.
Groundwater
 
Subsurface water in the zone of saturation, including water below the water table and water occupying cavities, pores, and openings in underlying rocks.
GPR
 
Ground Penetrating Radar.
Hydrophone
 
A sensor for detecting seismic waves in water. It works by producing an electrical response proportional to changes in pressure.
Instrument drift
 
The variation of an instrument’s response to the same input over time; e.g. the change in repeat readings at the same base station.
Latitude correction
 
A correction applied to gravity data to account for the global variation of gravitational attraction with latitude.
Leachate
 
The liquid that accumulates in, and in some cases drains from, a landfill. Can be detected using Can be detected by electrical resistivity imaging and EM ground conductivity.
LNAPL
 
Light Non-Aqueous Phase Liquid. A liquid that is less dense than water and does not dissolve in water, e.g. petroleum hydrocarbons. Can be detected by electrical resistivity imaging, EM ground conductivity, and GPR techniques. cf. DNAPL.
MEC
 
Munitions and Explosives of Concern. Another term for UXO.
MASW
 
The multichannel analysis of surface waves (MASW) method is a seismic exploration method evaluating ground stiffness in 1D, 2D or 3D.
NAPL
 
Non-Aqueous Phase Liquid. A liquid that does not dissolve in water. Can be sub-divided into LNAPL and DNAPL.
NDT
 
Non-Destructive Testing.
P-wave
 
A seismic wave in which particle motion is in the direction of propagation. Usually the first type of seismic wave to reach a receiver. cf. S-wave.
Plume
 
Contaminant plume - a mixture of waste chemicals, or leachate, and groundwater usually in solution form.
Pseudosection
 
A 2D graphical representation of how a measured parameter varies with location and depth. Requires inversion of the data to be converted into a 2D model (cross-section).
Resistivity
 
An intrinsic property of a material which resists the flow of an electrical current within the material. Typically calculated from the inversion of apparent resistivity data collected in the field. c.f. apparent resistivity.
S-wave
 
A seismic wave in which particle motion is perpendicular to the direction of propagation. Usually the second type of seismic wave to reach a receiver. cf. P-wave.
Saturated zone
 
The saturated region of the subsurface beneath the water table.
Tidal correction
 
A correction applied to gravity data to account for the diurnal variation in gravitational attraction due to the relative positions of the sun and moon.
Time domain (TD)
 
A representation of data to aid analysis of its variation over time. cf. frequency domain.
True resistivity
 
See resistivity.
UST
 
Underground Storage Tank. Often found on former industrial sites, particularly petrol stations. Can be detected using GPR and EM ground conductivity techniques.
UXB
 
UneXploded Bomb. Explosive weapons dropped by aircraft that did not explode. Can be detected by magnetic, EM ground conductivity and radar techniques. cf. UXO.
UXO
 
UneXploded Ordnance. Explosive weapons that have not exploded. Includes artillery shells and UXBs. Can be detected by magnetic, EM ground conductivity and radar techniques.
Vadose zone
 
Also termed the unsaturated zone. The region of the subsurface between the ground surface and the water table.
Water table
 
The boundary between the vadose zone and the saturated zone. The form of the boundary is a function of the local topography, geology, and seasonal climatic variations.

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