M

Mac Rule  – a rule relating the depth of a source of magnetic anomaly to the width of its amplitude profile: “depth = ½ of the horizontal distance at the level of half the maximum amplitude”.  See also Half-Width Method  and Depth Rules.

MaFIC (Magnetic Fault Interpretation Cube ) – a 3-D data volume of the magnetic depth solutions obtained by Werner Deconvolution, Euler Deconvolution or other automated magnetic depth estimation methods and transformed into the standard SEG-Y format for interactive evaluation and interpretation using the seismic software. ^{[212 ].}

Magma  – a molten igneous rock material ascended into Crust. ^{[13 ]} Often, M. is a primary source of water, gas and ore-forming fluids which contribute significantly to magnetization of faults and fracture zones.  See Magma Chamber, Lava and Igneous Rocks.

Magma Chamber  – a reservoir of Magma in the shallow part of Lithosphere at the range of depth from several kilometers to tens of kilometers where magma accumulates. From here, magma may intrude into the upper Basement and sedimentary section and/or extrude onto the Earth’s surface through the system of faults that provide channels for the magma uprising. ^{[13 ]} See Extrusive Rocks and Intrusive Rocks.

Magmatism  – a development and movement of Magma and its solidification to Igneous Rocks. See also Magma Chamber, Intrusion and Extrusion.

Magnetic Accelerometer  – a high-precision Gravimeter designed on the basis of Magnetic Levitation. ^[36]. See Virtual Spring.

Magnetic Anomaly  – the difference between the observed Earth’s Magnetic Field and its theoretical or estimated components (such as IGRF, Diurnals, etc.).  In magnetic exploration, M.A. is a magnetic signature of geological interest generated by lateral contrasts a) in magnetic susceptibilities of rocks (i.e., rock composition); b) in rock subsurface structure (horsts, grabens, faults with offsets, magmatic intrusions, and salt diapiric bodies). Flat-layered subsurface structures will be transparent to the magnetic methods if there is no lateral variation in magnetic susceptibility or magnetite content. The presence of a vertical contact (such as fault) can also produce a lateral susceptibility contrast and corresponding M.A. detectable by Magnetometer. Observed M.A. is always of a composite nature, representing the superposition and interference of anomalies from variously spaced sources. M.A. is positive when the field of a buried source reinforces the Earth’s magnetic field, and it is negative when the source’s field opposes the Earth’s magnetic field (such as strong Remanent Magnetization) or has insignificant susceptibility value (such as salt diapiric bodies). Because of the great difference in Susceptibility values between sedimentary and metamorphic (basement) rocks, most of the anomalies observed at residual magnetic field maps stem from sources at or near the basement surface. ^{[25, 54,  158,  173,  215,  223 , 238].} See also Anomaly, Magnetic Anomaly Amplitude, Magnetic Anomaly Wavelength, and Magnetic Anomaly Shape.

Magnetic Anomaly Amplitude  – the relative change in the magnetic field intensity across the survey area governed primarily by two factors: 1) degree of a lateral magnetic susceptibility contrast (as a proportional scale factor: the greater contrast – the higher amplitude); 2) source body depth (as an inverse proportional factor: the deeper source – the smaller amplitude). ^{[215 ].} See also Magnetic Anomaly Wavelength and Magnetic Anomaly Shape.

Magnetic Anomaly Shape  – a component feature of the magnetic field governed by seven factors: 1) geometry of a source body; 2) susceptibility contrast created by a source body; 3) depth of a source body; 4) direction of the Earth’s magnetic field at the location of a source body (i.e., Inclination and Declination); 5) direction and intensity of Remanent Magnetization of rocks forming a source body; 6) orientation of a source body with respect to the Earth’s magnetic field; 7) azimuth of the observation line with respect to both source body and the Earth’s magnetic field. ^{[215 ].} See also Magnetic Anomaly Amplitude and Magnetic Anomaly Wavelength.

Magnetic Anomaly Wavelength  – the dominant wavelength in the power spectrum of an observed magnetic anomaly. M.A.W. is governed primarily by four factors: 1) source depth; 2) source thickness; 3) source lateral extent in “x” direction; 4) source lateral extent in “y” direction. These factors cannot be perfectly separated and, hence, there is no proven technique for uniquely determining the depth of a magnetic source based on its dominant wavelength. M.A.W. provides important insight into source geometry: shallow sources generate short wavelength (high-frequency) anomalies, deeply buried sources generate long wavelength (low-frequency) anomalies. M.A.W. is not affected by changes in the lateral magnetic Susceptibility contrast. ^[215]. See also Magnetic Anomaly Amplitude and Magnetic Anomaly Shape.

Magnetic Base Station  – see Base Station.

Magnetic Basement  – an approximation in the magnetic exploration which defines the unconformity upon which the essentially non-magnetic sedimentary cover has been deposited. As a rule, M.B. is magnetically heterogeneous and featured with high values of Susceptibility Contrast. For this reason, M.B. is the major regional magnetic structure that constitutes the dominant component to the observed magnetic field after applying IGRF Correction. Sometimes, the relatively thick sequence of highly magnetic Volcanic Rocks may be considered as equivalent to M.B., because the magnetic effects of deeper source bodies are not resolvable. Quite often, but not necessarily, M.B. is coincident with Crystalline Basement. ^{[173, 195, 215, 223, ].} See also Crust, Mantle and Gravity Basement.

Magnetic Compensation System  – on-board computerized system to compensate for the magnetic noise from both static and moving (dynamic) components of the survey aircraft. Compensation coefficients are derived from Figure-Of-Merit (FOM). ^[155]. See also Real-Time Magnetic Compensation System.

Magnetic Component Filter  – a space domain line-based filter which resolves the observed magnetic field into its horizontal or vertical components. See also Horizontal Component Filter and Vertical Component Filter.

Magnetic Contact  – a boundary or interface (usually Fault) between two basement blocks of contrasting magnetic susceptibilities or between the intrusive body and host rocks both in Basement and sedimentary section. M.C. position can be identified by zero values of Second Vertical Derivative or by maxima values of Horizontal Gradient, Analytic Signal Amplitude or Energy Envelope. Sometimes, M.C. is referred to as Source Edge. ^{[26, 215, 238, 241].}

Magnetic Corrections  – a series of corrections applied to Observed Magnetics in order to isolate the anomalies caused by the rock Susceptibility variations (i.e., anomalies of exploration interest) from all other Earth’s Magnetic Field Components, which contribute to values measured by Magnetometer. See also Correction of Magnetic Data.

Magnetic Dipole  – a pair of infinitesimally separated Magnetic Poles of opposite signs. Volume of magnetic material is considered to be an assortment of continuously distributed magnetic dipoles. M.D. is a basic approximation of the magnetically polarized nature of rocks and bodies as well as the Earth as a whole. M.D. is also a model approximation in various inversion methods of location and depth estimates for relatively small isolated bodies. ^{[25, 54, 119, 223, 238].} See Inversion.

Magnetic Domains  – microscopically small regions of differently oriented Magnetization within rock grains or crystals. M.D. are magnetized to saturation and represent elementary components of magnetic materials (i.e., magnetized rocks). The creation, size, shape, orientation, and stability of M.D. depend on such physical factors as rock grain size, shape, mechanical conditions (presence of microcracks, inclusions, etc.), and surrounding temperature. Relatively weak external magnetic fields can orient M.D. with this field’s present direction, creating Induced Magnetization of a rock material. The strong external magnetic field can align M.D. irreversibly, creating a permanent or, in terms of the geological history, Remanent Magnetization of a rock material. ^{[33, 223].}

Magnetic Elevation  – a height of Magnetometer sensor (or sensors) above sea level in the airborne magnetic survey. See Elevation.

Magnetic Equator  – the approximated line at the Earth’s surface where the magnetic lines of force are horizontal and nearly orthogonal to this line.  Sometimes, M.E. is called Aclinic Line. ^{[25, 223, 238].} See also Magnetic Poles.

Magnetic Field  – a vector field of Magnetic Dipole. Intensity of M.F. is inversely proportional to the distance from its source dipole. ^[25]. See also Earth’s Magnetic Field.

Magnetic Field Logging  – direct measurements of the Earth’s magnetic field in boreholes. M.F.L. is also used for the vertical gradient measurements and estimates of the depth of isolated magnetic anomalies. ^{[54, 238].} See also Magnetic Susceptibility Logging.

Magnetic Field of the Earth  – see Earth’s Magnetic Field and Earth’s Magnetic Field Components.

Magnetic Field Strength  – a measure of the gradient of Magnetic Field. At a point in this field, M.F.S. is mathematically defined as force “H” that will be exerted upon a small fictitious pole if it is placed at the distance “r” from another large and much stronger pole:

H = P/mr²,

            where “P” is the large pole strength, “m” is a medium constant called Permeability, which depends on the magnetic properties of the medium. ^{[25, 54, 238].}

Magnetic Flux Density  – see Magnetic Induction.

Magnetic Force  – a force “F” between two poles of strengths “P₁” and “P₂” separated by the distance “r”:

F = P₁P₂ /mr²,

            where “m” is a medium constant called Permeability, which depends on the magnetic properties of the medium where these poles are located. ^{[54, 238].}

Magnetic Gradient  – a vector quantity describing the spatial rate of change of the magnetic field with respect to a particular direction:

M.G. = i dM/dx + j dM/dy + k dM/dz,

            where “M” is the magnitude of the magnetic field; “i”, “j” and “k” are unit vectors. ^{[26, 46, 54, 142, 153, 199, 238].} See Horizontal Gradient, Vertical Gradient and Total Gradient.

Magnetic Gradient Tensor  – a 3-D vector quantity that defines a spatial rate change of the three vector components of the observed magnetic field along three mutually orthogonal axes. M.G.T. can be measured by an array of vector magnetometers or by modified SQUID Magnetometer. In airborne applications, M.G.T. components can be derived from either horizontal or vertical magnetic field data obtained with the use of airborne gradiometers or calculated from the observed total magnetic field. M.G.T. is independent of the direction of the Earth’s magnetic field. M.G.T. contour maps are capable of providing enhanced and more accurate delineation of the 3-D source body edges and can be used in Boundary Analysis. ^[170].

Magnetic Gradiometer Survey  – a magnetic survey that provides measurements of the horizontal gradient and/or vertical gradient of the Earth’s magnetic field. ^{[46, 142].} See also Horizontal Aeromagnetic Gradiometer System and Vertical Magnetic Gradiometry.

Magnetic Gradiometer System  – the magnetic acquisition system, designed as an assemblage of horizontally and/or vertically separated magnetometer sensors, to provide measurements of gradients of the Earth’s magnetic field. The difference between M.G.S. sensor readings represents the spatial rate of change of the observed magnetic field (i.e., Gradient) along the direction in which sensors are separated. See also Horizontal Aeromagnetic Gradiometer System and Vertical Magnetic Gradiometry.

Magnetic Gradiometry  – a method and instrumentation to collect and process measurements of the horizontal or vertical gradients of the Earth’s magnetic field. As compared to conventional magnetometry (i.e., measurements of Total Geomagnetic Intensity), M.G. has three main advantages: 1) elimination of noise effects due to temporal changes in the Earth’s magnetic field – Diurnals; 2) enhancement of signals due to relatively shallow magnetic sources over those due to deep-seated sources; 3) increased Lateral Resolution of closely spaced magnetic sources by observed gradient anomalies. ^[171]. See Magnetic Gradient and Magnetic Gradiometer Survey.

Magnetic Grain  – coherent and, mostly, mid-frequency and high-frequency magnetic field features on the map of gridded data, which represents magnetic responses of major structural elements in subsurface.

Magnetic Horizontal Gradient Intensity (HGI)  – the absolute value of Second Horizontal Derivative (2HD) of the residual magnetic field which is calculated as a difference between Total Magnetic Field and its long-wavelength component representing the regional magnetic effects of the crystalline Basement and deeper portions of Crust. ^{[142, 143].} See also Horizontal Derivative and Regional-Residual Anomaly Separation.

Magnetic Indicators of Hydrocarbon Seepage  – see Aeromagnetic Hydrocarbon Indicators.

Magnetic Inclination – see Inclination.

Magnetic Induction  – a vector quantity designated by the symbol “B” which defines the total magnetic field including the field within a body. M.I. is the vector sum of the external and internal field strengths:

B = m_o (H + M) = m_o(1 + k) H = m_oH,

            where “H” is Magnetic Field Strength (i.e., magnetizing field); “m” is a medium constant called Permeability; “m_o” is the permeability of free space; “k” is magnetic Susceptibility; “M” is Magnetization. The internal field is a body’s own field generated by the poles induced upon the surface of a magnetic rock material when it is placed in the external (Earth’s) field. In moderately magnetic materials, M.I. is directly proportional to the external field strength. SI unit of measure for “B” is tesla: 1 tesla = 10⁴ gauss = 10⁹ gamma = 10⁹ nanotesla (nT). ^{[25, 223, 238].}

Magnetic Intensity  – see Magnetic Field Strength.

Magnetic Latitude – the angle of Magnetic Inclination determined on a smoothed regional basis rather than locally at a point of measurement. ^{[223 ].}

Magnetic Levitation  – a basic physical principle of operation of gravimeters with Virtual Spring design, where permanent magnet (proof) mass is maintained in suspended position (“levitates”) in the electromagnetic field. The voltage value necessary to maintain the proof mass at the gravimeter’s null point is proportional to Gravity Acceleration. ^[36].

Magnetic Marker  – a sedimentary formation (bed) with a high concentration of Syngenetic Magnetite. Magnetic anomalies generated by M.M. can be used to map the sedimentary structures, including potential anticlinal reservoirs. ^[77]. See also Magnetite and Syngenetic Magnetic Anomaly.

Magnetic Material  – a volume assortment of continuously distributed magnetic dipoles within a given rock substance. Being exposed to any external magnetic field, M.M. becomes magnetized, and its originally random-oriented magnetic dipoles line up in the direction of the applied (external) magnetic field to produce a magnetic field of their own. See also Magnetic Dipole, Magnetic Domain, and Magnetization.

Magnetic Meridian  – the vertical plane running through Geomagnetic Field Vector and indicating the direction of the horizontal component of Earth’s Magnetic Field. ^{[25, 223].} See also Declination and Inclination.

Magnetic Model  – a susceptibility model of a given or assumed geological structure. Geology can be modeled by representing lithological blocks and bodies as equi-susceptibility model units (polygons) formed by contrast boundaries that may or may not correspond to the actual geologic blocks and bodies. Where high susceptibility contrasts exist in nature, M.M. may correspond closely to those geologic blocks and bodies. The condiuional criterion of such correspondence is the satisfactory fit of a model response with the observed magnetic field. See Forward Modeling and Inverse Modeling.

Magnetic Modeling Shapes  – basic Causal Body shapes (like horizontal or vertical cylinder, point dipole, monopole or sphere, thin or thick dike, contact and others), which are considered as the most simple of the geometrical forms to be useful for the calculation of magnetic effects in Forward Modeling and for the matching of a computed model with the observed field in the process of Inversion.

Magnetic Moment  – a vector parameter of Magnetic Dipole. For magnetic dipoles consisting of two poles of equal strength “P” and opposite signs that are separated by an infinitesimal distance “Dx”, M.M. is defined as:

M.M. = PDx

            M.M. direction is along the line connecting the poles and toward North-Seeking Pole. Continuous distribution of magnetic dipoles within a magnetized body results in a quantity defined as M.M. per unit volume and called Magnetization or Intensity of Magnetization. ^{[223, 238].}

Magnetic Nose – see Nose.

Magnetic Permeability – see Permeability.

Magnetic Polarization  – an effect of the lining up the magnetic dipoles of Magnetic Material by induction due to the presence of the external magnetic field. M.P. is also referred to as Polarization, Magnetization or Intensity of Magnetization. ^{[25, 33].}

Magnetic Poles  – two points near the opposite ends of a bar magnet toward which the magnetic lines of force are oriented and concentrated. M.P. always exist in pairs. See also Magnetic Dipole and Geomagnetic Poles. ^{[25, 223].}

Magnetic Potential  – a mathematical function that describes, through its derivatives, Magnetic Field at any given space point. M.P. is a scalar quantity that defines the work to be done in moving a fictitious unit magnetic pole against the magnetic field from the “infinity” to the point in question. ^{[25, 54, 238].}

Magnetic Sensor Cable Length Correction  – a correction applied to the marine magnetic data to compensate for the distance between the actual position of the magnetic sensor during survey measurements and the navigation point (as well as the position of the on-board gravimeter). M.S.C.L.C. is based on an interpolation procedure which artificially moves the magnetic sensor to the ship (i.e., navigation point) so that magnetic and gravity data can be correlated.

Magnetic Signal  – a measured component of the observed magnetic field.  M.S. at any observation point is the vector sum of contributions from all geologic source bodies (magnetic sources) that lie within some radius of that point.  The radius at which the magnetic source no longer contributes to the total M.S. depends on the size, shape, depth of occurrence and Magnetization of the source body.  See also Magnetic Anomaly. ^{[140 , 215 ].}

Magnetic Signature – a combination of a magnetic signal amplitude, Wavelength and response geometry as a distinctive constituent part of Magnetic Grain.  Also called Magnetic Anomaly Shape.  See also Magnetic Signal.

Magnetic Source Depth  – depth to the source of magnetic anomaly, calculated with the use of Depth Rules, or, more often, a set of depth estimates for the near vertical (2-D case) or lateral (3-D case) alignment of sources (approximating heterogeneously magnetized fault or fracture zone or Contact), obtained by applying various Inversion techniques, such as Werner Deconvolution, Euler Deconvolution, Phillips Method, Naudy Method, SPI^Ô Method, and others.

Magnetic Storm  – short-period irregular fluctuations of the external magnetic field which are greater in magnitude, more irregular and of higher frequency than Diurnal Variations. M.S. occurs during the unusually high sun radiation activity and can have amplitudes of 50 nT to 200 nT and, sometimes, up to thousands of nT. M.S. duration is often several days. During M.S., the exploration magnetic survey should be suspended. ^{[13, 25, 223].}

Magnetic Stratigraphy  – a method and instrumentation to identify Magnetization and stratigraphic parameters (such as Susceptibility values, thickness, age) of the sedimentary formations due to the presence of Magnetite or other magnetic minerals. M.S. is used for stratigraphic differentiation and cross-well correlation of the target formations based on direct measurements of magnetic susceptibility of rocks in boreholes. M.S. is often referred to as Magnetostratigraphy. ^[77]. See also Magnetic Marker and Magnetic Susceptibility Logging.

Magnetic Survey  – ground, shipborne or airborne (Helicopter Survey or Fixed Wing Survey) measurements of the Earth’s Magnetic Field and/or its gradients over the area of exploration interest.  Deviations from Normal Magnetic Field as well as various Residual components are attributed to the subsurface distribution of rocks having different Susceptibility and related to geological structures. ^[223].  See also Satellite Magnetics.

Magnetic Survey Resolution  – an amplitude and wavelength resolution, achievable with the use of specific instrumentation, acquisition techniques and noise-suppressing software tools. Present-day instrumentation and software capabilities can satisfy practically all requirements for Amplitude Resolution: magnetometers (most generally, Cesium Magnetometer) operate at a resolution up to 0.001–0.01 nT and a sampling rate 6–20 readings per second in the range of 20,000–100,000 nT; processing algorithms extract target signals from the background and regular noise levels of much higher amplitudes than those of a target signal. Hence, M.S.R. is mainly determined by Wavelength Resolution required for exploring the geological target in the survey area. The wavelength resolution of magnetic survey with the use of Moving Platform is defined as twice grid interval of the gridded data. The Grid Interval is usually selected as ¹/₃ or ¼ of Line Spacing. Generally, the line spacing is estimated as a function of depth to the subsurface target interval: the shallower the target interval, the closer the line spacing. In mineral exploration (land and airborne surveys) with relatively shallow targets, the line spacing is primarily derived from the expected target size, i.e., the size of the ore body, Kimberlite Pipe, etc. In petroleum exploration (marine and airborne surveys), the target interval is often defined as the whole sedimentary section and the upper part of Basement. For this reason, most HRAM surveys are flown at low heights (120–150 m above the Earth’s surface) with 400–800 m Traverse Lines spacing and 1200–2400 m Control Lines spacing. ^{[20, 78, 123, 155].} See also Magnetic Target Resolution.

Magnetic Susceptibility – see Susceptibility.

Magnetic Susceptibility Logging (MSL)  – measurements of rock susceptibility in boreholes with the purpose of improved stratigraphic differentiation and cross-well correlation. Both local and regional stratigraphic correlation is possible based on the differentiation of Sedimentary Rocks according to their measured magnetic Susceptibility. ^[22].

Magnetic Susceptibility Meter  – a portable, high-sensitivity electronic device to measure Susceptibility values on rock outcrops, core samples and laboratory samples, with Accuracy of about 10^–5 SI units (10^–6 cgs units). Measured values are stored in M.S.M. digital memory. An installed microprocessor controls a display (digital or graphic-analog), auto-ranging and auto-calibration. As an option, M.S.M. has an interface to store magnetic susceptibility readings on a computer disk for subsequent processing and analysis.

Magnetic Target Resolution  – an amplitude and wavelength resolution required for objective identification of anomalies caused by a geological target in the survey area. A generalized table of M.T.R. is shown below. ^{[20, 78, 123].}:

            See also Magnetic Survey Resolution.

Magnetic Terrane  – an area of regional extent where the anomalous lineament pattern differs from that of surrounding or adjacent areas. Magnetic terranes are assumed to be limited by the ancient plate tectonic boundaries.

Magnetically Levitated Accelerometer  – see Magnetic Accelerometer and Virtual Spring.

Magnetically Susceptible Intra-sedimentary Unit  – a local zone of high magnetization within the sedimentary section. M.S.I.U. is often represented by volcanics (i.e., Volcanic Rocks), magmatic intrusions (such as sills, batholites, etc.) and, sometimes, by porous hydrothermal dolomites or extensively fractured rock formations with a history of repeated migration of highly mineralized fluids, which resulted in the occurrence of significant concentrations of strongly magnetic minerals like Magnetite. ^{[51, 115, 191].} See also Magnetized Intra-sedimentary Fault and Plutonic Water.

Magnetically Transparent Fault  – an intra-sedimentary or, sometimes, upper basement fault without Magma infill or the history of repeated fluid flows.  M.T.F. may have a low magnetization which is not sufficient to generate magnetic anomaly detectable by HRAM surveys. Often, long M.T.F. can be detected using the criteria for Interruption Zones.  A fault detected by seismic data can often be a M.T.F.

Magnetics Filters  – a term used in some software packages to define processing operators that calculate and apply Reduction-To-Pole or Reduction-To-Equator to the observed magnetic data.

Magnetite  – a strongly magnetic mineral (Fe₃O₄), which is a very common and widely distributed accessory mineral in rocks of all kinds. M. has the highest Susceptibility value among all rocks and minerals ranging from 1200 to 19,200 with the average of 6000 (in units of 10³ SI). The volume content of M. in Sedimentary Rocks is the dominant parameter of their magnetic properties. There are four main types of M. defined based on its origin in the sedimentary section: 1) detrital M. deposited in sandstones; 2) diagenetic M. formed as a replacement mineral or through hydrothermal alteration; 3) M. formed as a part of the near surface weathering processes (i.e., place deposits in drainage systems or beach sands); and 4) M. formed in the medium-oxidation, low-pressure geologic environments of faults and fracture zones with a history of repeated vertical migration of highly mineralized fluids. Outcrops and unconformity truncations of the magnetite-bearing layers generate relatively intensive high-frequency (short wavelength) alignments on maps of the filtered magnetic data. ^{[1, 13, 55, 61, 62, 77, 110, 231].} See also Magnetized Intra-sedimentary Fault.

Magnetization  – a vector quantity that defines Magnetic Moment per unit volume. M. is a measure of a rock’s ability to be magnetized by the Earth’s magnetic field: originally random-oriented elementary dipoles within a rock body are lining up in the direction of the Earth’s magnetic field creating a magnetic field of their own, i.e., induced magnetic field. The extent of this lining up is directly proportional to the strength of the Earth’s field and/or any other applied magnetic field at the date of M.:

M = kH,

            where, “H” is the magnetizing field (Magnetic Field Strength); “k” is the magnetic Susceptibility. Generally, rocks are magnetized by two mechanisms: 1) Remanent Magnetization, which may date from any time and may point in any direction; 2) Induced Magnetization, which points in the direction of the present Earth’s magnetic field. As a result, M. of any rock is a vector sum of the two magnetization types – remanent M. and induced M. Sometimes, M. is referred to as Magnetic Polarization or Intensity of Magnetization. ^{[25, 33, 38, 59, 93, 223, 238].}

Magnetized Intra-basement Fault  – an intra-basement fault filled with intrusions of strongly magnetic igneous rocks. Such faults generally give rise to relatively high-intensity, high-frequency and mid-frequency (short-wavelength and mid-wavelength) magnetic anomalies, which are reliably identified and mapped with HRAM survey data. In magnetic processing and interpretation, M.I.F. is approximated by either Thin Dike or Thick Dike models. ^{[18, 20, 71, 215].} See also Fault, Intrusion, and Magnetized Intra-sedimentary Fault.

Magnetized Intra-sedimentary Fault  – an intra-sedimentary fault with a history of the repeated vertical migration of highly mineralized fluids, which resulted in the occurrence of high concentrations of strongly magnetic minerals, like magnetite, along and around the fault plane. M.I.F. usually generates low-intensity, high-frequency (short-wavelength) magnetic anomalies, which can be identified with HRAM survey data after applying appropriately designed Cascaded Filtering. In magnetic processing and interpretation, M.I.F. is approximated by Thin Dike model. ^{[1, 51, 81, 191, 192].} See also Fault and Magnetized Intra-basement Fault.

Magnetometer  – an instrument for measuring the total intensity and other components of the Earth’s magnetic field. In airborne and ground gradiometry surveys, the arrangements of magnetometer sensors are used to measure simultaneously Horizontal Gradient or Vertical Gradient as well as the total magnetic field. ^{[25, 223].} See Cesium Magnetometer, Fluxgate Magnetometer, Optically Pumped Magnetometer and Proton Precession Magnetometer.

Magnetometer Calibrations  – a set of tests of the airborne magnetometer readings performed by measuring: a) known magnetic field (pre-survey M.C.); b) effects of the aircraft and flight lines’ orientation on magnetometer sensors (on-site M.C.). See also Heading Test and Figure-Of-Merit (FOM).

Magnetosphere  – the Earth’s outer zone of interaction between Solar Wind and the Earth’s internal magnetic field. On the sunlit side M. is almost hemispherical, with the radius of about ten radii of the Earth under quiet conditions (it may be compressed to about six Earth’s radii by Magnetic Storm). On the shadow side, M. may have a “tail” extent of more than a hundred Earth’s radii. ^{[13, 25, 223].} See also Ionosphere.

Magnetostratigraphy  – see Magnetic Stratigraphy.

Magnitude  – a measure of the anomaly or field intensity assigned to a quantity so that it may be compared with other similar anomalies or fields. Commonly, M. is considered as the reference equivalent of Amplitude. ^[215].

MagProbe^™  – a profile-based automated inversion program of LCT Inc. that applies Werner Deconvolution, 2-D Euler Deconvolution and Phillips Method in a batch mode to obtain a cross-section of estimates of the magnetic sources’ locations and depths (“magnetic depth section”) along each of the magnetic survey lines. See also SPI Method and Improved SPI Method.

MagSlice Image  – see Pseudo-Depth Slice.

Maneuver Noise  – an aircraft motion-induced noise measured during Compensation Flight with Roll, Pitch and Yaw maneuvers.

Mantle  – the Earth’s subsurface interval between Crust and the Earth’s core. M. density is estimated about 3.3–3.4 g/cm³. M. is considered essentially non-magnetic, because temperatures at those depths exceed Curie Temperature. Below M. is Core. ^{[13, 25, 223].}

Map Convolution  – see Grid Residual Method.

Map Filtering  – applying variously designed two-dimensional filters to grids (maps) of the potential field data. The purpose of M.F. is the separation of anomalies of different sizes from each other to better define the anomalous trends, lineaments and other features that are not easily detectable or completely obscured on the original map. The most common M.F. is performed by wavelength filters applied either in Space Domain or Frequency Domain. See Lateral Separation, Vertical Separation and Wavelength Filter.

Map Power Spectrum  – a presentation of Power Spectrum of the Fourier transformed gridded magnetic data as a function of direction. M.P.S. plots the highest magnetic energy of an infinitely small Wavenumber or Spatial Frequency (i.e., zero frequency) at the center descending out to some pre-selected value of Nyquist Frequency (often 0.5) at the edges; instead of the Nyquist fractions, wavenumbers may also be used as distance units. M.P.S. is imaged in order to estimate the quality of Leveling and Cultural Editing, monitor the effectiveness of applied filtering procedures, control Aliasing effects, and identify the strike of dominant structural trends. It should be noted that linear features on the spectral-domain M.P.S. are orthogonal to the linear space-domain features to which they correspond. Sometimes, M.P.S. is called Energy Rosette. See also Power Spectrum and Radial Power Spectrum.

Map Projection  – a transfer of positions of the measurement points on the ellipsoidal surface of the Earth to corresponding points on a flat sheet, i.e., from spheroid (ellipsoid) points to the grid coordinates on a map. More than 250 different map projections are currently in use worldwide. See Universal Transverse Mercator (UTM).

Marine Gravimeter – see Shipboard Gravimeter.

Marine Magnetometer – see Overhauser-Effect Magnetometer.

Matched Filtering  – a spectral domain grid-based separation-type filtering method which uses Power Spectrum parameters to design the filter operator. Provided there are distinct linear slopes in the spectrum image representing the effects of different depth source ensembles, these effects can be separated by M.F.: the Fourier transformed data are multiplied by the calculated transfer function “W” and then converted back to the original space domain by Inverse Fourier Transform. Matched filter transfer function “W(r)” is defined as:

W(r) = 1/{1 + b/B[exp(H – h)r]},

            where “h” is an average depth of the shallow source ensemble defined by the linear slope of the high-frequency part of the spectrum curve ; “H” is an average depth of the regional (i.e., deep seated) ensemble defined by the linear slope of the low-frequency part of the spectrum curve; “b” and “B” are the amplitude constants (energy axis intercepts by the high-frequency and low-frequency lines) for the shallow and deep source ensembles respectively; r = (u² + v²)^½ where “u”, “v” are Cartesian Coordinates. Parameters “b”, “B”, “h”, and “H” are obtained from the power spectrum image.  M.F. is also referred to as Spectral Matched Filtering or Depth Slice Filtering. ^[48]. See also Separation Filtering.

Maximum Depth Rule  – see Bott-Smith Method.

Maximum Entropy Method  – an interpolation method applied to the gridded potential field data to calculate missing values for gaps in Grid using an algorithm of linear preduction from segments of rows and columns in the original dataset.

Maximum Entropy Prediction (MEP)  – a convolution method for interpolating the observed data within Grid as well as near grid edges. MEP calculates a data function with the same Spectrum as the original data, while reducing Ringing effects that might appear with traditional convolution methods. See also Maximum Entropy Method and Edge Effect Correction.

Mean Sea Level (MSL)  – the average height of the sea surface for all stages of the sea tide over a 19-year period. MSL is often referred to as Sea Level. ^[13].

Measured Vertical Gradient  – Vertical Gradient obtained during the magnetic survey with the use of two vertically separated magnetometer sensors on the same aircraft. M.V.G. value is obtained by subtracting the bottom sensor readings from the top sensor readings, and then dividing the result by the sensor separation distance. No diurnal correction of M.V.G. data is required, when acquired on a single aircraft. An alternative approach is to fly two surveys, both corrected for Diurnals, at different elevations. ^[155]. See also Calculated Vertical Gradient.

Median Filter  – see Local Median Filter.

MEP  – see Maximum Entropy Prediction.

Merging  – see Grid Merging.

Metamorphic Rocks  – rocks which have derived from the pre-existing rocks by mineralogical, chemical and/or structural alterations in response to changes in temperature, pressure, shearing stress and chemical environment. Magnetization and density of M.R. is higher (sometimes, significantly) than that of Sedimentary Rocks. The average estimate of 61 types of M.R. gives the range of susceptibility values from 0 to 70 with the average of 4.2 (in units of 10³ SI). ^{[13, 33, 238].} See also Igneous Rocks.

Metamorphic Rocks Density  – the basic quantity that predetermines the gravity properties of metamorphic rocks. Usually, M.R.D. values increase with the degree of metamorphism since this process fills pore spaces and recrystallizes the rock into a more dense form. Thus, metamorphosed sedimentary and igneous rocks have generally higher densities. M.R.D. variations are more diverse than that of sedimentary and igneous rocks because of their complicated geological history. ^{[33, 238].} Generalized table of the common M.R.D. values is shown below (in g/cm³)

                        Rock Type                               Range                          Average

                        amphibolite                             2.90 – 3.04                  2.96

                        gneiss                                      2.59 – 3.00                  2.80

                        quartzite                                  2.50 – 2.70                  2.60

                        schist                                       2.39 – 2.90                  2.64

                        serpentine                                2.40 – 3.10                  2.78

                        slate                                         2.70 – 2.90                  2.79

Metamorphic Rocks Susceptibility  – the basic quantity that predetermines the magnetic properties of metamorphic rocks. M.R.S. values are usually low as compared to those of Igneous Rocks but significantly higher (in average, up to 10 times and more) than susceptibility values of Sedimentary Rocks. Generalized table of the common M.R.S. values is shown below (in units of 10³ SI):

                        Rock Type                               Range                          Average

                        amphibolite                             –                                  0.7

                        gneiss                                      0.1 – 25.0                    –

                        quartzite                                  –                                  4.0

                        schist                                       0.3 – 3.0                      1.4

                        serpentine                                3.0 – 17.0                    –

                        slate                                         0.0 – 35.0                    6.0

                        Average of main types            0.0 – 70.0                    4.2

            S.I. susceptibility unit = 4B c.g.s. susceptibility unit. ^{[33, 238].} See also Igneous Rocks Susceptibility and Sedimentary Rocks Susceptibility.

Microgravimetry  – gravity measurements within the range of several dozens of microGals and less, i.e., about (5-100) 10^–3 mGal. M. can be used for detection of subsurface cavities, investigations of foundation conditions, exploring stratigraphic traps, as well as waterflood surveillance at oil fields under development. ^{[30, 32, 105, 255].} See also Time-Lapse (4-D) Gravity Survey and Earth Tides.

Microgravity Modeling  – a) a method of Inversion, which is based on 4-D time-lapse gravity survey data and used to obtain a reservoir density model with the purpose of waterflood surveillance; b) a method of Forward Modeling, which is based on reservoir simulations and used to estimate the change in anomalies observed at the surface over periods of years after the water injection was initiated. ^[105]. See Time-Lapse (4-D) Gravity Survey.

Microgravity Survey  – see Microgravimetry.

Microleveling  – a) variety of profile or grid-based processing schemes which minimize very small (about 1nT or less) residual line-to-line level shifts in the magnetic data remaining after imperfect removal of the time-dependent diurnal variations, uncertainties in the altitude of the survey flight and gridding errors; b) term used to describe the process of making precise elevation surveys to measure temporal changes in elevation due to faulting, volcanic activity or petroleum reservoir depletion. ^{[67, 160, 187].} See also Decorrugation and Leveling.

Microleveling Discontinuities  – short wavelength local artificial anomalies of about a grid cell size which are the residual errors of extrapolation procedures applied during Microleveling. As a rule, M.D. can be effectively smoothed out by high-cut Line Filtering.

Microleveling Filters  – a group of filters applied to assist in performing Microleveling. See Cascaded Filtering.

Micropulsations  – a time-varying external components of the observed Earth’s Magnetic Field having small amplitudes (usually, less than 10 nT) and frequency range from 0.01 to 3 Hz. M. with amplitudes up to tens of nanotesta result from interactions between Solar Wind and the Earth’s magnetic field. ^{[223 ].}  See also Bay and Magnetic Storm.

MilliGal (mGal)  – a unit of the gravity field acceleration used in the gravity exploration measurements. 1 mGal = 10^–3 Gal = 10^-3 cm/sec² = 10^-5 m/sec². State-of-the-art gravimeters provide measurements with Accuracy of 0.01 mGal or higher, which is a variation of less than one part in one hundred million of the total gravity field. ^{[174, 223].} See also Gal, Microgravimetry and Time-Lapse (4-D) Gravity Survey.

MilliGal Constant  – a gravimeter instrument parameter used to convert gravimeter readings from counter (dial) units to milligals. M.C. is determined by the manufacturer during gravimeter calibration. See also Interval Factor and MilliGal.

Minettes  – deeply sourced igneous intrusives that comprise dikes, sills and small extrusive vent complexes of Mafic Rocks. As a rule, M. have relatively high magnetization properties. M. are related to kimberlites, but they are not usually diamondiferous. See Kimberlite Pipe.

Minimizing Inversion  – a methodology that divides the subsurface into a large number of fixed size cell of unknown susceptibility or density values. Inversion algorithm produces a single 3-D model by minimizing Objective Function, which approximates the subsurface structure based on available geological and geophysical information. The minimizing algorithm initiates the iterative process constrained by pre-selected misfit between Model Response and the observed data. The final 3-D model represents a spatially dependent distribution of density or susceptibility value in subsurface. ^[144]. See Inversion and Parametric Inversion.

Minimum Curvature  – a gridding algorithm that computes the best fit surface passing through the observed data points by iteratively solving a set of difference equations to minimize the curvature of this surface. Original data values are used to control successive iterations and final surface construction. M.C. is also referred to as a method of the grid data smoothing. ^{[28, 68].} See also Grid, Gridding, and Bidirectional Gridding.

Misclosure  – see Mis-Tie.

Mis-Tie  – a difference in data values obtained at identical points on intersections of Traverse Lines and Control Lines. Mis-ties are minimized in the process of Leveling. Remaining residual errors are eliminated by Decorrugation. See Crossovers.

Model  – a conceptual approximation of the subsurface object or structure by one or an arrangement of simple geometric forms. Calculated effects from simple models help to develop better understanding of the observed magnetic or gravity fields. The concept of M. is based on the fact that, at depths comparable with horizontal dimensions of subsurface objects, the magnetic and gravity effects from bodies with irregular boundaries can be simulated by models with simple geometric forms. There are three main types of models: 1) 2-D (two-dimensional) M. meaning the “infinite” length in the strike direction; 2) 2.5-D (two-and-one-half dimensional) M. meaning each body is limited and symmetrical in the strike direction or 2.75-D (two-and-three-quarters) M. meaning the same as 2.5-D, but strike symmetry is not required; and 3) 3-D (three-dimensional) M. meaning all bodies are defined in three dimensions with the edges of the model space carried to “infinity.” Often, several different models give responses that match the observed gravity or magnetic field. However, much ambiguity can be eliminated by using available geological and geophysical data as model constraints. ^{[215, 223].} See also Gauss Theorem, Inversion, Forward Modeling, Magnetic Modeling Shapes, and Gravity Modeling Shapes.

Model Response  – a potential field calculated from the assumed distribution of densities or susceptibilities in subsurface. See Model and Forward Modeling.

Modified 2VD Filtering  – a second vertical derivative (2VD) filter method which uses an additional Gauss Filter operator to attenuate wavenumber components of the observed potential field data which are close to Nyquist Wavenumber. The degree of attenuation in the wavenumber domain is controlled by the Gauss filter amplitude factor: the higher this factor, the more of the higher wavenumbers are attenuated. ^[48]. See Second Vertical Derivative.

Modulus  – the positive square root of the sum of three squared spatial (in “x”, “y” and “z” directions) terms of a vector quantity. ^[223].

Monopole  – a model approximation of Magnetic Dipole by such a dipole where one of its poles is “infinitely” away from another pole. Like many other approximations, magnetic M. does not exist in nature. Magnetic M. is used in estimates of location and depth of near vertical sources, e.g., cased wells and igneous rock intrusions like Kimberlite Pipe. ^{[54, 125, 238].} See also Magnetic Body Shapes.

Motion Noise  – unwanted airborne survey signals caused by Pitch, Roll and Yaw of the survey aircraft. See Motion Sensor.

Motion Sensor  – an electronic device installed in aircraft to measure the aircraft’s motions during a flight for Post Flight Quality Control data analysis. In airborne magnetic survey, a three-component Fluxgate Magnetometer is often used in the capacity of a M.S. See Motion Noise.

Moving Platform  – a general definition of a ship, helicopter, fixed-wing aircraft, or orbiting satellite used as carriers in gravity or magnetic surveys. M.P. is also referred to as Observation Platform. See Stabilized Platform.

MSL  – see a) Magnetic Susceptibility Logging and b) Mean Sea Level.

Multi-Sensor Gradient Array  – a configuration of magnetometer sensors providing vertical, horizontal, and/or three-axis (XYZ) gradiometer measurements in, mostly, helicopter-supported surveys for ultra-detailed geological mapping (such as intrusive body zonation or alterations in the ore body), environmental, engineering, and military applications. See also DiaMag^Ô.