The following is a useful collection of geophysical terms. Search through using the alphabetical shortcuts or the “Find” function in your web browser.

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 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