Glossary of Geophysical Terms (M,N,O)

M electrode
The potential electrode nearest to the A electrode in a resistivity device.
Magnetics, geomagnetics
Geophysical methodology for studying anomalies in the geomagnetic field due to non-uniform magnetization of the subsurface. Uses magnetometers.
Magnetic permeability
acteristic of a material, it is proportional to the magnetism induced in that material divided by strength of the magnetic field used.
Magnetic susceptibility
A measure of the extent to which a substance may be magnetized; it represents the ratio of magnetization to magnetic field strength.
Magnetization
The magnetic moment per unit volume. It is a vector quantity. See also magnetic susceptibility.
Magnetometer
A device for measuring the earth’s magnetic geomagnetic field. Variations in the field strength may indicate changes in magnetic properties of soil and rock or presence of ferrous metals.
Mapping
Locating geological, chemical or geophysical information in space (as opposed to time, which is monitoring). The results are usually summarized as maps.
Matrix
The solid framework of rock or mineral grains that surrounds the pore spaces.
Mho
A unit of electrical conductance that is the reciprocal of ohm.
Micro-gravity survey
A surface geophysical survey method, undertaken on a very small scale (typically station spacings of a few meters), and requiring a high meter sensitivity. Measures the earth’s gravitational field at different points over an area of interest. Variations in the field are related to differences in subsurface density distributions, which in turn are associated with changes in soil, rock, and cultural factors. Typically used for cavern or fracture detection.
Microresistivity log
Refers to a group of short-spaced resistivity logs that are used to make measurements of the mud cake and invaded zone.
Migration
The movement of chemicals, bacteria, gases, etc. in flowing water or vapor in the subsurface. Also, a seismic/radar term whose general meaning is the correction of the recorded image for the effects of reflector dip. A very typical result of migration is the removal of hyperbolic events on the record resulting from diffractions from faults and other discontinuities.
Monitoring
Observing the change in a geophysical, hydrogeological or geochemical measurement with time.
Mud cake
Also called filter cake; the layer of mud particles that builds up on the wall of a rotary-drilled hole as mud filtrate is lost to the formation.
Mud filtrate
The liquid effluent of drilling mud that penetrates the wall of the hole.
Muting
Change in the amplitude of all or part of a trace before additional processing. Noisy or clearly erroneous traces are given zero amplitude. Data before the first break and the known refraction arrivals are also often reduced to zero amplitude.
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.
Ohm (Ώ)
The unit of electrical resistance through which 1 amp of current will flow when the potential difference is 1 V.
Ohm-meter (Ώm)
Unit of electrical resistivity; the resistivity of 1 m3 of material, which has a resistance of 1 ohm when electrical current flows between opposite faces; the standard unit of measurement for resistivity logs.
Open hole
Uncased intervals of a drill hole.
Optimum offset
Seismic reflection technique employing optimum window.