Glossary of Geophysical Terms
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WXYZ
S
SASW
Spectral analysis of surface waves. An in situ seismic method that analyzes dispersion of surface waves and inverts it in terms of mechanical properties of the soil.
S-wave
A body wave in which particles move perpendicular to the direction of propagation. Also known as secondary or shear wave.
Saturation
The percentage of the pore space occupied by a fluid, usually water in hydrologic applications.
Scintillation detector
An efficient detector used in nuclear-logging equipment; radiation causes flashes of light that are amplified and output in a crystal as electronic pulses by a photo multiplier tube to which it is coupled.
Secondary (magnetic field)
The magnetic field that is generated by currents that are induced to flow in the ground by time variations in the primary magnetic field of the transmitter.
Secondary porosity
Porosity developed in a rock after its deposition as a result of fracturing or solution; usually not uniformly distributed.
Seismic reflection
A surface geophysical method recording seismic waves reflected from geologic strata, giving an estimate of their depth and thickness.
Seismic refraction
A surface geophysical method recording seismic waves refracted by geological strata.
Self potential (SP)
A geophysical method measuring the natural, static voltage existing between sets of points on the ground surface.
Shale base line
A line drawn through the SP log deflections that represent shale; a similar technique can be used on gamma logs and can represent the average log response of sand.
Shear modulus
The stress-strain ratio for simple shear in isotropic materials which obey Hooke's law.
Shear wave
An acoustic wave with direction of propagation at right angles to the direction of particle vibration (S wave).
Short-normal log
One of a group of normal-resistivity logs usually with AM spacing of 16 in. or less.
Single-point resistance log
A single electrode device used to make measurements of resistance that cannot be used quantitatively.
Skin depth
The effective depth of penetration in a conducting medium of electromagnetic energy (when displacement currents can be ignored); the depth at which the amplitude of a plane wave has been attenuated to 1/e or 0.37.
Sort
Data in shot record form are sorted for display as common offset records, common shot records, common receiver records, or common depth point records.
Sounding
In geophysics, a survey method whereby the geometry and/or frequency of an array of sensors is varied so as to measure the physical properties of the earth as a function of depth beneath the configuration. The alternative is usually profiling.
Spacing
The distance between sources or transmitters and detectors or receivers on a logging probe.
Specific conductance
Strictly speaking identical to electrical conductivity; the term is used in hydrogeology to refer to the conductivity of surface and ground water and expressed in micro Siemens per centimeter. It is a direct function of the total dissolved solids in the water.
Spectral-gamma log
A log of gamma radiation as a function of its energy that permits the identification of the radioisotopes present.
Spectral induced polarization (IP)
See complex resistivity.
Spine and ribs plot
A plot of long-spaced detector output versus short-spaced detector output for a dual detector gamma-gamma probe; permits correction for some extraneous effects.
Spinner survey
A log made with an impeller flowmeter.
Spontaneous-potential log
A log of the difference in DC voltage between an electrode in a well and one at the surface; most of the voltage results from electrochemical potentials that develop between dissimilar borehole and formation fluids.
Stacking
Adding together two or more signals. This process is often used in geophysics to improve the signal to noise ratio. A common application is stacking seismic signals in seismic refraction data recording.
Standoff
Distance separating a probe from the wall of a borehole.
Statics
Time shift corrections to individual traces to compensate for the effects of variations in elevation, surface layer thickness or velocity, or datum references.
Streaming potential
A voltage resulting from flow of an ionic fluid.
Surface wave
A wave that travels along, or near to, the surface; its motion dropping off rapidly with distance from it. A distinct seismic mode from the body waves, P- and S.
Survey
Oil-industry term used for the performance or result of a well-logging operation.
