Glossary of Geophysical Terms
A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
L
M |
N |
O |
P |
Q |
R |
S |
T |
U |
V |
WXYZ
N
N electrode
The potential electrode distant from the A electrode in a resistivity device.
Neutron log
Neutrons from an isotopic source are measured at one or several detectors after they migrate through material in, and adjacent to, the borehole. Log response primarily results from hydrogen content, but it can be related to saturated porosity and moisture content.
Noise
Any unwanted signal; a disturbance that is not part of signal from a specified source. In electrical or induced polarization (IP) surveys, noise may result from interference of power lines, motor-generators, atmospheric electrical discharges, etc. See cultural noise.
Non-aqueous-phase-liquid (NAPL)
Elements or compounds in the liquid phase other than water. This phase is immiscible in water. Examples include petroleum hydrocarbons, like gasoline, and solvents such as trichloroethylene.
Non-unique
In geophysical interpretation and mathematical modeling, a problem for which two or more subsurface models satisfy the data equally well.
Normal log
A quantitative-resistivity log, made with four electrodes, which employs spacings between 4 and 64 in. to investigate different volumes of material around the borehole; see also long-normal log and short-normal log.
Normal moveout corrections
Time shift corrections to reflection arrivals because of variation in shotpoint-to-geophone distance (offset). The amount of shift depends on 1) the length of the raypath from shot to reflection point to receiver, and 2) the velocity of the material traversed. Deeper reflections are corrected using velocities indicative of the deeper section.
Nuclear log
Well logs using nuclear reactions either measuring response to radiation from sources in the probe or measuring natural radioactivity present in the rocks.
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