Glossary of Geophysical Terms
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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.
