A

Absolute Base  – a regional Base Station where Absolute Gravity measurements are made. A.B. is a part of the national standardized network, which, in turn, is a part of the world-wide International Standardized Gravity Network. See also Temporary Base.

Absolute Gravity  – the vertical acceleration due to the Earth’s gravity field. The Earth’s gravitational acceleration is often approximated as 9.8 m/sec2 or 980,000 mGal. A.G. value for the gravity survey area is obtained from the local Base Station, which is tied to  International Gravity Standarization Net. [34, 248]. See Gravity Acceleration.

Absolute Station  – a term sometimes applied to the local gravity Base Station or gravity observatories measuring Absolute Gravity.

Acceleration of Gravity  –  see Gravity Acceleration.

Accuracy  – a) an instrument characteristic that defines the highest deviation of obtained readings from the true measured value under normal conditions; b) an exploration method characteristic that defines the range of deviation of the target parameter estimate made by this method (such as space location of a fault, depth to the top of an ore body, or depth to the basement surface) from the true value established by drilling.

Aclinic Line  – see Magnetic Equator.

Acquisition Footprint  – See Corrugations.

Aerogravity  – a method and instrumentation to collect and process measurements of the Earth’s gravity field in a moving airborne vehicle (airplane, blimp or helicopter). Usually, data are collected in a grid pattern, composed of Traverse Lines and Control Lines, over a lease block in stable air conditions. Precise measurements are required for the aircraft X,Y,Z position to calculate the necessary corrections. Processing of A. data is based on two fundamental assumptions: 1) gravity signals of exploration interest dominate Noise in the long-wavelength spectrum range; and 2) noise is primarily restricted to the shorter wavelength spectrum range and can be removed by applying Low-Pass Filter. [37]. See also Aerogravity Corrections, Spectrum and Power Spectrum.

Aerogravity Corrections  – corrections applied to the airborne gravity data to compensate for a) aircraft vertical motion (Vertical Acceleration Correction); b) aircraft course changes or turbulence (Horizontal Acceleration Correction); c) gravimeter platform velocity (Ëotvös Correction); d) aircraft elevation above sea level (Free-Air Correction); e) variation of the Earth’s radius and the centrifugal force (Latitude Correction); f) Offleveling Errors. After correcting for aircraft-induced accelerations and standard gravitational effects, the applying Low-Pass Filter is the primary technique for removal of the residual noise from the obtained Free-Air Gravity. [37].

Aeromagnetic  – a term applied to the Earth’s magnetic field measurements made from an aircraft. [223].  See High Resolution AeroMagnetic (HRAM) Survey.

Aeromagnetic Gradiometer  – an airborne three-sensor magnetometer array: two sensors are mounted on each wing tip extension, and one is mounted in a tail stinger of the aircraft to measure horizontal gradients. Also, two vertically separated sensors can be used to directly measure Vertical Gradient. A.G. can measure the total magnetic field using a tail sensor, as well as the diurnal-free (because of the subtraction in the process of calculation) horizontal and vertical gradients using the data from all sensors. A.G. data are compensated in the same manner as a single sensor magnetometer, both in real time and by post-flight processing. [46, 104, 112, 114, 155]. See also Fixed Wing Survey and Horizontal Aeromagnetic Gradiometer System.

Aeromagnetic Hydrocarbon Indicators  – secondary magnetic effects that occur as a result of intra-sedimentary hydrocarbon seepage along faults and fracture systems. It is assumed that reducing zones may be formed above hydrocarbon accumulations with subsequent formation of diagenetic magnetite; its relatively high concentrations can produce low-amplitude, high-frequency magnetic anomalies detectable by the High Resolution Aeromagnetic (HRAM) Survey. See also Diagenetic Magnetic Anomaly and Chimney. [55, 61, 62, 110, 138, 150, 208, 209, 231].

Aeromagnetic Survey   – a method and instrumentation to collect and process measurements of the Earth’s magnetic field along the traverse and control (tie) lines using a specially equipped aircraft or helicopter. A.S. is the effective approach to the mapping of basin structures, lineaments, intra-sedimentary and intra-basement magnetized faults, basement surface, salt intrusions and sometimes reefs. A.S. also helps to focus the detailed 3-D seismic in a more cost-effective manner. See High Resolution Aeromagnetic Survey, Fixed Wing Survey, Helicopter Survey.

Aeromagnetic Survey Equipment  – a set of acquisition, navigation, and processing equipment on board the survey aircraft and on the ground. Standard airborne A.S.E. set includes: a) high-sensitivity Magnetometer (usually, cesium or a similar type); b) Tri-Axial Fluxgate Magnetometer; c) Radar Altimeter; d) Barometric Altimeter; e) Differential GPS receiver and navigation system; f) video camera and video recorder; and g) computerized data acquisition system.The ground equipment for the Magnetic Base Station includes, usually, the same type of magnetometer as that on board the aircraft as well as a time synchronization interface with the ground GPS receiver. The ground GPS Base Station equipment is often identical to the GPS receiver and antenna on board the aircraft. Ground GPS data are used for post-flight differential correction to the recorded flight path.

Aeromagnetic Survey Specifications  – a detailed description of the aeromagnetic survey parameters, which include: 1) scheduling (sequence and time duration); 2) survey area information (geographic location and extent, number of line km to be flown); 3) flight line spacing (both traverse and control/tie lines); 4) line direction (for example, E-W traverses and N-S ties or other, generally, orthogonal combination); 5) line length (minimum lengths are usually in the order of 5-10 km); 6) survey altitude (“constant altitude” or “drape” or “modified drape” at about 100-150 m above the ground level); 7) sampling interval (line interval between magnetometer readings in meters); 8) type of Base Station Magnetometer (usually, similar to the airborne system); 9) GPS system surveyed with the ground GPS base station, i.e., Differential GPS. [57, 78, 205].

Aeromagnetics  – see Aeromagnetic Survey and High Resolution Aeromagnetic Survey.

AGL (Above Ground Level)  – an airborne survey parameter that defines the altitude of a flight along the survey lines, i.e., the height above ground level. Sometimes AGL is referred to as Terrain Clearance.

AGRF  – Australian Geomagnetic Reference Field. See International Geomagnetic Reference Field (IGRF).

Airborne Gravimetry  – see Aerogravity.

Airborne Gravity Meter  – a gravimeter mounted on a Stabilized Platform, upgraded for airborne use and equipped with fully digital Real-Time sensor and platform control systems. These systems are required to minimize non-linear responses of A.G.M. to aircraft motion and provide accurate measurements of the Earth’s gravitational accelerations due to density contrasts related to geological formations and structures. See Horizontal Acceleration Correction, Vertical Acceleration Correction, Borehole Gravimeter and Shipboard Gravimeter.

Airborne Magnetometer  – an instrument and supporting system used for measurements of the Earth Magnetic Field from an aircraft.[223].  Commonly, Cesium Magnetometer is used as A.M. in the high-resolution aeromagnetic (HRAM) surveys. See Magnetometer, Compensation Test, Figure of Merit and Real-Time Magnetic Compensation System.

Aircraft Signature  – an aircraft-induced effect on the airborne magnetic measurements. Each aircraft has its own A.S. and, hence, the magnetic survey flown with two or more aircrafts will exhibit a level shift between data values recorded along the same survey lines flown by each aircraft. [187]. See Leveling.

Airy Hypothesis  – an hypothesis of gravitational (isostatic) equilibrium between Crust and Mantle. It assumes a variable crust thickness below sea level, but a uniform Density of crust, “floating” on a liquid mantle substratum of a higher density, so that the thicker crust parts are topographically higher than the thinner ones. Mountainous areas have deep compensation “roots” extending to depths of about 50–60 km, while ocean basins have “antiroots” (as they are much shallower than mountains “roots”) at 6–10 km below the ocean bottom. Vening Meinesz Hypothesis assumes that some of the gravitational balance is also accommodated laterally (i.e., not only vertically) by surrounding region. The approximated radius of a region over which the compensation is distributed is about 200 km. [25, 54, 223, 238]. See also Pratt Hypothesis and Isostasy.

Algorithm  – a step-by-step computer procedure for carrying out data processing operations. [223]. See also Data Enhancement.

Alias Filter  – a filter applied before Gridding of the line data to remove the potential field components having spatial frequencies higher than Nyquist Frequency to avoid Aliasing. Low-pass option of Butterworth Filter is often used as A.F. Sometimes, A.F. is referred to as Anti-Aliasing Filter. See also Channel Filters.

Aliasing  – a property of sampling data at discrete space intervals. When sampling is less than two samples per wavelength, the components having frequencies higher than Nyquist Frequency will produce the same artificial values (i.e., alias) as real lower frequencies from which they are indistinguishable. To avoid A., the frequencies above the Nyquist frequency must be removed before a spatial reconstruction of the data, i.e., before Gridding. Low-pass filtering (with Cutoff value equal to Grid Interval) is one of the most common procedures to prevent A. problems. [25, 223]. See also Alias Filter.

Altimeter  – on-board instrument that measures and records the airborne survey flight height above the ground surface (AGL). Laser or radar altimeters are used to keep the aircraft constantly within the range of the planned height. [57, 78]. See also Barometric Altimeter and Radar Altimeter.

Altimeter-Derived Gravity  – see Satellite Gravity.

Altimeter-Derived Terrain Model  – a by-product of the airborne magnetic or gravity survey, representing surface (topographic) expression of the main geological domains in the survey area. See Altimeter, Barometric Altimeter and Radar Altimeter.

Altitude  – a height of the survey aircraft flight above the Earth’s surface. See also AGL, Elevation and Sensel.

Amplitude  – the maximum local departure of the potential field signal from the average value in the area. A. of the total intensity field is a composite feature that represents the sum of individual responses of magnetic/gravity causative bodies from various depths. See Causative Body.

Amplitude Anomaly  – a local increase or decrease of the potential field Amplitude caused by changes in the subsurface distribution of susceptibility/density contrast values. As a rule, high A.A. is generated by magnetic or gravity Contact. In magnetic exploration, for example, the highest observed amplitude anomalies typically indicate lithologic boundaries (i.e., Susceptibility contrasts) of Igneous Rocks within the sedimentary section and upper Basement. Low amplitude anomalies usually indicate basement block structures (uplifts, horsts, etc.). Large regional amplitude anomalies (up to hundreds of “nT” and “mGal”) in the shelf and continental margin areas are caused mainly by the contact between oceanic and continental Crust. [23, 223]. See Density Contrast and Susceptibility Contrast.

Amplitude Envelope  – see Analytic Signal Amplitude and Energy Envelope.

Amplitude Ratio Method  – see Enhanced Analytic Signal Amplitude.

Amplitude Resolution  – a quantitative estimate of the smallest Amplitude of the correlatable magnetic or gravity signal which can be objectively detected (i.e., resolved) using specific instrumentation, acquisition techniques and Noise suppressing algorithms. Present-day software tools are capable of extracting target signals that have much less amplitudes than the levels of background noise and Regular Noise. Generally, A.R. requirements are directly related to the geological target in the survey area: the smaller and shallower the target, the higher A.R. is required. See also Wavelength Resolution.

Amplitude Spectrum  – the amplitude-versus-spatial frequency (wavenumber) relation of the Fourier transformed potential field data. A.S. is often referred to as Fourier Amplitude Spectrum. [9, 124, 164, 165, 166, 221, 223, 228]. See also Fourier Transform and Power Spectrum.

Analog Magnetic Depth Estimation  – a group of graphical methods to estimate the depth to a magnetic source body, which are based on: a) construction of the profile of an isolated anomaly (this profile should be orthogonal to the anomaly’s long axis); and b) determination of the various profile curve parameters, such as maximum slope, straight-slope distance, half-maximum distance, tangent points and others, which are then used in the depth estimation formulas. See Depth Rules.

Analytic Signal  – a mathematical concept derived from the complex variable theory. Generally, A.S. is defined as a complex function whose real and imaginary parts are the Hilbert Transform of one another. For 2-D case, A.S. of a function “f(x)” can be presented as

A(x) = f(x) – j fHi(x),

            where “f(x)” is the real or inphase component of A.S.; “j” – the imaginary number; “fHi(x)” – the imaginary component of A.S. which is often referred to as Quadrature. The real and imaginary components have the same Amplitude Spectrum but differ in phase by 90°. For the potential field, A.S. can be defined in terms of this field and its Hilbert transform:

A(x,y,z) = M(x,y,z) – j H{M(x,y,z)},

            where “M(x,y,z)” is the magnitude of the field; “H{M(x,z)}” – the Hilbert transform of the field; “x”, “y” and “z” – Cartesian Coordinates. For 2-D and 3‑D potential field anomalies, the real and imaginary components of A.S. are given by the horizontal gradient(s) and vertical gradient, respectively.

For 2-D case, as

A(x,z) = dM(x,z)/dx – j dM(x,z)/dz ;

For 3-D case, as

A(x,y,z) = x dM(x,y,z)/dx + y dM(x,y,z)/dy + z dM(x,y,z)/dz ,

            where dM(x,y,z)/dx and dM(x,y,z)/dy are the horizontal gradients in the directions “x” and “y” respectively; dM(x,z)/dz is the vertical gradient (Vertical Derivative); “x”, “y”, “z” are unit vectors. Since the real and imaginary components of A.S. are the Hilbert transform of each other, A.S. can be also defined in terms of Horizontal Derivative of the total field. For 2-D case, as

A(x,z) = dM(x,z)/dx – j H{dM(x,z)/dx}

            For 3-D case, as

A(x,y,z) = dM(x,y,z)/dx + dM(x,y,z)/dy – j [H{dM(x,y,z)/dx} + H{dM(x,y,z)/dy}],

where “H{dM(x,y,z)/dx}” and “H{dM(x,y,z)/dy}” are the Hilbert transforms acting on “x” component and “y” component respectively. Correlating the A.S. amplitude maxima and computation of A.S. derivatives is used for delineating geologic boundaries in the subsurface. The amplitude characteristics of A.S. are used, under some simplifying conditions, to estimate the depth to the top surface of causative bodies. It should be noted that such depth estimates are strictly valid only for two-dimensional bodies (i.e., strike extent is assumed to be infinite and interference from adjacent anomalies is negligible). For 3-D case, the shape and absolute value of A.S. are dependent on the body’s magnetization direction and the Earth’s magnetic field direction, as well as on the depth below the observation level and the dip of a given boundary. Since A.S. parameters stem from computation of derivatives of potential fields, all noise effects (like gridding artifacts, line corrugations, random noise, etc.) will be enhanced, sometimes, significantly to obscure short-wavelength, low-amplitude anomalies of exploration interest. A.S. is also referred to as Simple Analytic Signal. [7, 103, 147, 152, 161, 164, 165, 166, 201, 214]. See Analytic Signal Amplitude, Analytic Signal Derivative, Enhanced Analytic Signal and Two-Dimensional (2-D) Body.

Analytic Signal Absolute Value  – a square root of the sum of squared values of the vertical and two horizontal (in “x” and “y” directions) derivatives of the gravity or magnetic field:

*A(x,y,z)* = [(dM/dx)2  + (dM/dy)2 + (dM/dz)2]1/2,

            where “M” is the potential field anomaly. In case of isolated 2-D bodies, A.S.A.V. exhibits maxima over magnetization/density contrasts and, practically, independent of the direction of the ambient potential field. A.S.A.V. is invariant with respect to the coordinate system and, hence, the tracing of its maxima can be used for delineation of arbitrary striking geological boundaries. A.S.A.V. is also referred to as Analytic Signal Amplitude or Energy Envelope or Instantaneous Amplitude. [147, 152, 161, 214]. See also Analytic Signal.

Analytic Signal Amplitude  – a component feature of the magnetic and gravity anomalies, which is defined as the square root of the sum of squared values of the vertical derivative and two horizontal derivatives (in “x” and “y” directions) of the potential field:

*A(x,y,z)* = [(dM/dx)2 + (dM/dy)2 + (dM/dz)2]1/2 ,

            where “M” is the potential field anomaly. A.S.A. is used to image geological discontinuities associated with magnetization/density contrasts such as faults (thin dikes or thin vertical sheets) and contacts (edges of causative bodies). In 2-D case of isolated magnetic anomaly, the shape of A.S.A. is independent of the Earth’s magnetic field parameters (Inclination and Declination) as well as of the direction of magnetization, either induced or remanent. 2-D A.S.A. is a bell-shaped symmetric function that has its maximum directly over Thin Dike or vertical Magnetic Contact. The A.S.A. value becomes larger and its shape becomes narrower with the higher order of the calculated derivative of the analytic signal. For vertical 2-D Thick Dike, the locations of A.S.A. maxima are not directly over its both edges: the greater a depth to the top surface, the more an offset from side edges and vice versa – the shallower a depth, the closer maxima locations to the edges of a thick dike. For the most common 3-D cases, the A.S.A. maxima locations are always offsetting from the edges of Causative Body and the amount of offset is governed by several factors including depth to the top surface (the greater depth – the larger offset), interference from neighboring causative bodies, terrain effects (distortions by high-relief topography), magnetization direction and the Earth’s magnetic field parameters, body’s dip and others. However, A.S.A. is invariant with respect to the coordinate system and, for large-scale regional magnetic surveys or shallow depths of isolated causative bodies, the tracing of A.S.A. maxima is considered a useful method of delineating arbitrary striking geological boundaries in the subsurface. For 3-D case, A.S.A. is often referred to as Analytic Signal Absolute Value or Energy Envelope. [7, 103, 147, 152, 161, 164, 165, 166, 214]. See also Analytic Signal, Analytic Signal Derivative, Enhanced Analytic Signal and Two-Dimensional Body.

Analytic Signal Derivative  – the analytic signal of the N-order Vertical Derivative of the potential field anomaly. The amplitude of A.S.D. of the N-order can be expressed equally in terms of the vertical or horizontal derivative of the potential field anomaly “M” (magnetic field or vertical component of gravity) as

*An(x,y)*= [(dMzn/dx)2 + (dMzn/dy)2 + (dMzn/dz)2]1/2

or

*An(x,y)* = [(dMhn/dx)2 + (dMhn/dy)2 + (dMhn/dz)2]1/2,

            where Mzn and Mhn are the N-order vertical and horizontal derivatives of the potential field anomaly “M”. As the shape of 2-D A.S.D. amplitude function is independent of magnetization direction and source geometry, the location and depth of the magnetic source body can be estimated: maximum of A.S.D. amplitude function shows the location, while the width of this function is related to depth. A.S.D. provides highly efficient resolution of interfering anomalies from closely spaced sources and can be used for delineation of source edges and other geological boundaries in the subsurface. Prior to A.S.D. computation, the magnetic field should be reduced to the Pole. [52]. See Analytic Signal, Enhanced Analytic Signal, and Reduction-To-Pole.

Analytic Signal Filter  – a processing algorithm that calculates three orthogonal gradients (horizontal “X”, horizontal “Y” and vertical) of Total Magnetic Field to obtain Analytic Signal values which reach their maxima over corresponding source bodies, and, more often, over their edges, regardless of Remanent Magnetization. [230]. See Contact and Analytic Signal.

Analytic Signal Method  – a method that includes a variety of processing techniques based on the concept of Analytic Signal. A.S.M. produces a particular type of gravity or magnetic anomaly enhancement maps used for delineation of maxima values of anomalous density or magnetization distributions in the subsurface. Extensions to this method include, as additional solved parameter, depth estimates. A.S.M. is also referred to as Total Gradient Method, because the absolute value the analytic signal equals to the absolute value of the total gradient, and the analytic signal of the magnetic or gravity anomalous field is calculated by taking the square root of the sum of square derivatives in all three directions (“X”, “Y” and “Z”). See also Analytic Signal Amplitude, Analytic Signal Derivative and Enhanced Analytic Signal.

Annihilator  – a non-zero rock magnetization or density distribution that does not result in detectable magnetic or gravity anomalies for a particular source geometry. [25].

Anomaly  – a portion of magnetic or gravity data which is different in its Magnitude and/or Wavelength from the survey data in general and can be of exploration interest. A. can only be understood in the context of the regional or background field which has been defined for that particular case. In gravity measurements, A. is a difference between the observed (measured) value and a value predicted by some model, for example, Bouguer Anomaly or Free-Air Anomaly. [223]. See also Gravity Anomaly, Magnetic Anomaly and Regional Potential Field.

Anomaly Amplitude  – the maximum local departure of a target signal (magnetic or gravity) from the level of a background Noise remaining after suppression of short-wavelenth, high-amplitude noise components and applying other signal enhancement procedures. Present-time processing algorithms are capable of extracting a low-amplitude target signal from background noise levels which are often much higher in amplitude than this signal. See also Amplitude Anomaly.

Anomaly Frequency  – an estimation of the dominant Spatial Frequency in the magnetic or gravity anomaly. A.F. is a function of depth to Source Body: the shallower a source body, the higher A.F. See also Anomaly and Anomaly Wavelength.

Anomaly Offset  – a specific pattern or trend of anomalies that differs from the adjacent or surrounding anomalies. See Anomaly.

Anomaly Relief  – a difference between the highest and lowest magnitudes of the anomaly. See Anomaly and Magnitude.

Anomaly Resolution  – a quantitative estimate of the smallest Anomaly Size which can be objectively identified on gravity or magnetic maps. [253]. See also Wavelength Resolution.

Anomaly Separation  – a general definition of various processing methods of separating the gravity or magnetic effects of deep subsurface sources from shallow subsurface sources. See Regional-Residual Anomaly Separation.

Anomaly Size  – a combination of Anomaly Amplitude and Anomaly Width or apparent Anomaly Wavelength, where the wavelength is estimated as about twice the anomaly width. [253]. See Anomaly.

Anomaly Wavelength  – an estimation of the dominant spatial Wavelength in the anomaly calculated as the doubled horizontal distance from the peak value of anomaly to its trough on the map of the potential field data. A.W. is a function of depth to the magnetic or gravity Source Body: the deeper a source body, the longer A.W. See Anomaly and Anomaly Frequency.

Anomaly Width  – a quantitative estimate of a specific Anomaly approximated as one-half of apparent Anomaly Wavelength. See also Anomaly Size.

Antenna Offset  – a distance between the actual location of Gravimeter or Magnetometer and position of the navigation antenna. A.O. is used as a positioning correction factor applied to Channel recording “X” and “Y” coordinates during survey measurements. See Positioning and Global Positioning System (GPS).

Anti-Aliasing Filter  – see Alias Filter.

Antiferromagnetics  – magnetic rock materials (elements, compounds, etc.) in which the net magnetic moments of parallel and antiparallel subdomains almost cancel each other. The resultant Susceptibility is rather small. The most common example of A. is hematite. [25, 238].

Apparent Density  – a) rock density calculated from gravity measurements in a borehole; b) rock density calculated from the residual gravity field under the following assumptions: 1) Earth’s surface is nearly flat; 2) rock Density varies horizontally, but not vertically (i.e., steeply dipping rocks); 3) residual anomalies are caused by source bodies within the uppermost part of the Earth’s crust. A.D. computations are used for the first-order mapping of geological boundaries hidden under the cover of relatively young unconsolidated sediments, glacier debris, and dense vegetation. [95, 108, 223]. See also Crust, Density Filter, Source Body and Terracing.

Apparent Susceptibility  – magnetic Susceptibility of rocks calculated from the observed or residual magnetic field under the following assumptions: 1) rock magnetization is along the Earth’s magnetic field (i.e., Induced Magnetization); 2) source bodies have a rectangular shape and a size of Grid Cell with each Source Body centered at each grid point; 3) source bodies are approximated as Prism models of vertical dip and “infinite” depth extent. Aeromagnetic maps (grids) must be reduced to the pole and continued downward to the ground level. A.S. computations are used for the first-order (i.e., initial) mapping of geological boundaries hidden under the cover of relatively young unconsolidated sediments, glacier debris and dense vegetation. [108, 232]. See also Susceptibility Filter.

Artifacts  – artificial (i.e., false) potential field anomalies originating from Aliasing, Spiking, Cultural Noise, Acquisition Footprint, Grid Merging as well as residual errors in Gridding, Leveling and other data processing operations.

Artificial Magnetic Anomalies  – see Artifacts

Artificial Sun Illumination  – a directional filtering method of Image Enhancement which is based on the visual selection among images obtained under various directions and angles of the sun illumination. Such images, color-scaled or gray-scaled, are often referred to as Shaded Relief, Sun Angle or Azimuth Images since they present high values of the potential field as “hills” and low values as “valleys”. A.S.I. is used to highlight subtle linear trends and feature areas on grids of potential field data. Such trends are interpreted as potential field responses of structural boundaries, regional faults or large-scale tectonic features. Using A.S.I. the interpreter has a choice of different sun azimuths (declinations) and sun elevations (inclinations) available in the software. Some software packages allow the sun position to be controlled by the mouse, and the image is updated in real time as the sun angle is changed.

ASL  – a height above sea level. Elevation of the airborne survey area is usually specified in exploration reports in meters ASL (for example, 1000 m ASL) based on GPS, Barometric Altimeter, and Radar Altimeter data. See also AGL and Sensel.

Astatic Balance  – a non-stationary (i.e., unstable) balance. A.B. is the basic operational principle of the most exploration gravimeters, where the gravity force on a unit or “proof” mass is balanced by a spring arrangement. When, due to a change in the gravity attraction, the imbalance occurs, a design-provided third force (two other forces are gravity and spring) intensifies the effect of this gravity change and increases Sensitivity of a gravimeter. [223].

Astatic Magnetometer  – a magnetometer designed to measure Remanent Magnetization of rock samples. A.M. magnet system consists of two magnets of equal magnetic moments, rigidly mounted parallel to each other in the same horizontal plane with their poles opposed. The whole arrangement of magnets is suspended by a torsion fiber. The rock sample is placed below the magnet system in various orientations and the angular deflections of the magnets are measured. [238]. See also Astatic Balance.

Asthenosphere  – a layer of the relative weakness below Lithosphere where isostatic adjustments are assumed to take place and Magma may be generated. A. begins about 100 km below the Earth’s surface and extends to a depth of about 350 km. A. is thought to be involved in the plate-tectonic movements.[13, 223]. See Isostasy and Plate Tectonics.

Atlas Gravimeter  – see Torsion Balance.

Atmospheric Gravity Correction – a correction which is required in using International Gravity Formula based on the WGS84 Reference Spheroid because the WGS84 Earth’s Gravitational Constant includes the mass of the atmosphere:

A.G.C. = 0.87  -0.118[(h/1000)1.047],

where “h” is the elevation above sea level.

Attenuation Rate  – see Euler’s Structural Index.

Automated Anomaly Axis Correlation  – a grid-based procedure for tracking the axis of long linear and curvilinear anomalies. Axis of strong broad, and smooth anomalies of relatively deep origin are better correlated by Blakely-Simpson Method. Axis of “sharper” anomalies of a shallower origin, as well as those contaminated with Artifacts due to Terrain Clearance variations and Gridding oscillations are better correlated by Coherent Filtering. [8, 26, 80, 81, 94].

Automated Magnetic Data Interpretation  – a general definition of various methods which can be applied to profile (line) or gridded magnetic data to calculate estimates of depth, location, dip, and susceptibility contrast of a source of the observed magnetic Anomaly. All methods assume certain elementary models whose calculated effects are accepted as the best fit with successive segments of profile data or windows of gridded data. See Automated Magnetic Depth Estimation.

Automated Magnetic Depth Estimation  – a variety of techniques which analyze digital profiles or grids of the magnetic data to obtain estimates of causative bodies’ depths using computer automated algorithms which free the user from specifying key portions (special points) of anomalies. The result is presented in the form of depth section (for line data) or map of depth estimates (for gridded data). A.M.D.E. includes such methods as Werner Deconvolution, 2-D Euler Deconvolution, 3-D Euler Deconvolution, Naudy Method, Phillips Method and Analytic Signal Method. [47, 52, 107, 131, 152, 168, 169, 195, 207, 214, 224, 225, 226, 239, 242, 252].

Automatic Gain Control (AGC)  – an image and/or data enhancement procedure that attempts to balance the amplitudes of anomalies of differing dominant wavelengths. AGC governing parameters are the sliding window size over which the gain is computed and the gain function that controls a relative amplification of signals (anomalies) and noise. The standard gain function is the inverse root-mean-square value computed within a sliding window. For varying wavelength (spatial frequency) anomalies, the choice of a window size predetermines a space range of anomalies that will be enhanced in the output. The best match for a specific datasheet is found by testing several combinations of gain functions and window sizes. AGC can be carried out either on profile (line) data or gridded data. Before applying AGC, it is recommended to remove the regional component of observed data (i.e., the average base level of data) by Detrending. [202].

Azimuth Images  – a set of color-scale or gray-scale maps (images) that are “sun” illuminated from several different directions or azimuths simultaneously. See Artificial Sun Illumination.

Azimuthally Averaged Logarithmic Power Spectrum  – see Radial Power Spectrum.