Glossary of Geophysical Terms (S,T,U,V)

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.
Target
The object at which a survey sighting is aimed.
Temperature log
A log of the temperature of the fluids in the borehole; a differential temperature log records the rate of change in temperature with depth and is sensitive to very small changes.
Terrain conductivity
Geophysical method in which EM methods measure directly the average electrical conductivity of the ground. Operates at low induction number.
Thermal neutron
A neutron that is in equilibrium with the surrounding medium such that it will not change energy (average 0.025 eV) until it is captured.
Time constant
The time in seconds required for an analog system to record 63 percent of the change that actually occurred from one signal level to another.
Time domain
In geophysics refers to measurements analyzed according to their behavior in time. The usual alternative is frequency domain measurements.
Time domain reflectometry (TDR)
A device, which measures electrical characteristics of wideband transmission systems. Commonly used to measure soil moisture content.
Tomography
A method for determining the distribution of physical properties within the earth by inverting the results of a large number of measurements made in three dimensions (e.g. seismic, radar, resistivity, EM) between different source and receiver locations.
Tracer log
Also called tracejector log; a log made for the purpose of measuring fluid movement in a well by means of following a tracer injected into the well bore; tracers can be radioactive or chemical.
Track
Term used for the areas in the American Petroleum Institute log grid that are tandard for most large well-logging companies; track 1 is to the left of the depth column, and tracks 2 and 3 are to the right of the depth column, but are not separated.
Transducer
Any device that converts an input signal to an output signal of a different form; it can be a transmitter or receiver in a logging probe.
Transient
Occurring when the system is still changing with time; i.e., a steady state has not been attained. Most groundwater flow systems are transient, not steady state.
Velocity panels
A set of stacked test sections with a progression of assumed normal-moveout velocities applied. A powerful method for determining velocities if distinct reflection events are present, as the reflections will be coherent where the velocities are correct and be degraded in appearance at higher or lower NMO velocities.
Variabledensity log (VDL)
Also called 3-dimensional log; a log of the acoustic wave train that is recorded photographically, so that variations in darkness are related to the relative amplitude of the waves.