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
A
A electrode
One of the current-emitting electrodes of a resistivity-imaging system (A); the current return electrode is labeled B.
API unit
The American Petroleum Institute (API) has established test pits for calibrating neutron and gamma logs. The API neutron unit is defined as 1/1,000 of the difference between electrical zero and the logged value opposite the Indiana limestone in the calibration pit with an average porosity of 19 percent. The API gamma unit is defined as 1/200 of the deflection between intervals of high and low radioactivity in the calibration pit.
Accuracy
Closeness of a measurement to the true value.
Acoustic impedance
Seismic velocity multiplied by density.
Acoustic log
Also called sonic log; a record of sound waves as they are transmitted through liquid-filled rock; a record of the transit time (t) is the most common; amplitude and the full acoustic-wave form also are recorded.
Acoustic televiewer log
A record of the amplitude of high-frequency acoustic pulses reflected by the borehole wall; provides location and orientation of bedding, fractures, and cavities.
Acoustic wave
A sound wave transmitted through material by elastic deformation.
Alluvium
A general term for unconsolidated material (e.g. clay, silt, sand, gravel) deposited from running water. Often a sorted or semi-sorted sediment in the bed of a stream or on its floodplain or delta. The deposit may be in the form of an alluvial fan.
Amplitude
The maximum departure of a wave from the average value.
Analog recording
Data are represented as a continuous record of physical variables instead of discrete values, as in digital recording.
Anisotropic
Having a variation in physical properties that are dependent on the orientation of the measurement.
Annulus
The space between the drill pipe or casing and the wall of the drill hole; in rocks saturated with hydrocarbons, the annulus is the transition interval between the invaded zone and the uncontaminated zone.
Anomaly
Refers to a deviation from uniformity in a physical property.
Apparent resistivity/conductivity
The resistivity of a homogeneous isotropic ground that would give the same voltage/current or secondary/primary field ratios as observed in the field with resistivity or EM methods. The apparent conductivity is the reciprocal of the apparent resistivity.
Aquifer
Rocks or unconsolidated sediments that are capable of yielding a significant amount of water to a well or a spring.
Aquitard
Geologic formation/s of significantly low hydraulic conductivity, typically saturated, but yielding a limited amount of water to wells. Also referred to as a confining unit.
Archie's Law
An empirical relationship linking formation resistivity ( rt), formation water resistivity (r w) and porosity. The form of the relationship is r t = a rw -m where a and m are experimentally determined constants.
Atomic number (Z)
The number of protons in the nucleus of an atom equal to the number of electrons in a neutral atom.
Attenuation, attenuate
A reduction in energy or amplitude caused by the physical characteristics of a transmitting system.
Automatic Gain Control (AGC)
A process for increasing signal amplitude of a signal through time, thus making all events on the trace appear to be of approximately the same amplitude. Note that this process will expand the amplitudes even if no data are present. Various window lengths are used; the appearance of the data may be greatly affected by the window used in the calculation.
