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

- SASW
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.

- 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: 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

- 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 is 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.

- spheric
See: sferic

- stacking
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).

- stripping
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.

- susceptibility
See: magnetic susceptibility