The following is a useful collection of geophysical terms. Search through using the alphabetical shortcuts or the “Find” function in your web browser.


M electrode

The potential electrode nearest to the A electrode in a resistivity device.

magnetic basement

Magnetic basement is usually equated to crystalline (felsic and mafic) or sometimes, metamorphic basement. It is the unconformity upon which an essentially non-magnetic sedimentary section has been deposited. large exposures of basement (e.g., the Canadian Shield) show it to be lithologically and magnetically heterogeneous. Very thick sequences of highly magnetic volcanics may sometimes be considered equivalent to a magnetic basement.

magnetic permeability (µ)

This is defined as the ratio of magnetic induction to the inducing magnetic field. The relative magnetic permeability (µr) is often quoted, which is the ratio of the rock permeability to the permeability of free space. In geology and geophysics, the magnetic susceptibility is more commonly used to describe rocks.

magnetic sedimentary section

A surface or zone within the geologic column where magnetic susceptibility contrasts are significant enough to generate magnetic anomalies which could delineate sedimentary geology. Susceptibility variations within the sedimentary column are generally considered near zero except where relatively magnetic sediments (e.g. pyroclastics, arkoses, some shales) are present.

magnetic susceptibility (k)

A measure of the degree to which a body is magnetised. In SI units, this is related to relative magnetic permeability by k=µr-1 and is a dimensionless unit. For most geological material, susceptibility is influenced primarily by the percentage of magnetite. It is most often quoted in units of 10-6 (SI). In frequency-domain EM data, this is most often apparent as a negative in-phase component over high susceptibility, high resistivity geology such as dolerite dykes.

magnetics, geomagnetics

Geophysical methodology for studying anomalies in the geomagnetic field due to non-uniform magnetisation of the subsurface. Uses magnetometers.

magnetisation

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 summarised 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.

migration

The movement of chemicals, bacteria, gases, etc. in flowing water or vapour 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.

milligal (mgal)

The unit of acceleration used with gravity measurements. 1 mgal = 10 µm/s2 = 10 gu.

model

Geophysical theory and applications generally have to assume that the geology of the Earth has a form that can be easily defined mathematically, called the model. For example steeply dipping conductors are generally modelled as being infinite in horizontal and depth extent, and very thin. The Earth is generally modelled as horizontally layered, each layer infinite in extent and uniform in characteristic. These models make the mathematics to describe the response of the (normally very complex) Earth practical. As theory advances, and computers become more powerful, the useful models can become more complex.

modelling

Modelling usually means the process of developing models of the Earth based upon measured geophysical data. It may be as simple as recognising that an anomaly is likely caused by a buried pipe, or it may involve sophisticated data processing and/or inversion to mathematically build a range of plausible models.

monitoring

Observing the change in a geophysical, hydrogeological or geochemical measurement with time.

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.