G

G  – see Universal Gravitational Constant

Gal  – a unit of the gravity field acceleration (attraction) used in gravity measurements: 1 Gal = 1 cm/sec². The nominal value of the Earth’s gravity field acceleration at the Earth’s surface is 980 Gal. In gravity exploration, the unit of measurement is milliGal: 1 mGal = 10^–3 Gal and, sometimes, microGal: 1 :Gal = 10^–6 Gal. ^{[25, 182, 223].}

Gamma (g)  – a unit of the magnetic field measurements. It defines the magnitude of the magnetic field vector represented by the number of lines of Magnetic Induction passing through a unit area perpendicular to the vector direction. Magnetic survey maps were often contoured in gammas. 1 gamma = 10^–9 tesla = 10^–5 gauss. The term of G. is now replaced in formal usage by the SI unit of nanotesla:

1 nanotesla = 1 gamma = 10^–9 tesla.

^{[25, 223].} See Nanotesla (nT).

Gardner’s Equation  – an empirically derived equation which describes the relationship between bulk densities and acoustic velocities of rocks:

D = 0.23 V ^0.25,

            where “V” is the interval velocity; “D” is Bulk Density. ^[72].

Gas Chimney  – see Chimney.

Gate  – see Window.

Gauss  – the cgs-emu unit of Magnetic Induction (or flux density) “B”.  It is a measure of the number of magnetic lines of force per unit area. 

1 gauss = 10⁵ gamma = 10 ^–4 tesla. ^{[223 ].}

Gauss Filter  – a spectral domain grid-based pass filter that retains a low-frequency or high-frequency range of Power Spectrum using the smooth curve in Rolloff Range. The steepness of the G.F. curve is determined by the user-specified value of Standard Deviation for the power spectrum. G.F. is commonly used to attenuate high frequencies in order to stabilize the output. G.F. formula can be defined as:

            where “F” is the power spectrum standard deviation; “+” corresponds to a high-pass option; and “–” corresponds to a low-pass option. ^[230].

Gauss Theorem  – a theorem that postulates the inherent non-uniqueness of the potential fields: if the field distribution is known only on a bounding surface, there are infinitely many equivalent source distributions inside the boundary that can produce the same observed field. In terms of the potential field Modeling, if there exists one model that reproduces the observed field, there are other models that will reproduce this field to the same degree of accuracy. ^[144]. See Inversion.

General Symmetric Filter  – a spectral domain grid filter, which can be custom-designed as High-Pass Filter, Low-Pass Filter, or Band-Pass Filter. At first, Power Spectrum is divided into a set of segments of the user-specified length and a coefficient is assigned to each segment. Then, G.S.F. operator multiplies the data within the segment by its coefficient and processes the whole dataset in accordance with the coefficient value: 1 – pass the dataset as it is; 0 – reject the dataset; between 0 and 1 – reduce the dataset energy before passing it. ^[230].

Geodetic Datum  – a reference level of Positioning which defines shape (i.e., specified Ellipsoid), size (as applicable regional extent), position with regard to the Earth’s center, and coordinate axes’ orientation (i.e., rotation of axes) of Reference Ellipsoid. Often, G.D. is referred to as Survey Datum. ^[223].

Geodetic Reference System 1967 (GRS67 ) – the Earth’s shape approximation (i.e. Spheroid) adopted for the gravity data reduction purposes in 1967.  It is based on Flattening Coefficient of 1/295.25, established by satellite measurements.^{[223 ].}  See International Gravity Formula.

Geographic Coordinates  – 3-D position (latitude, longitude, elevation), which is determined based on Reference Ellipsoid. A point on the Earth’s surface may have more than one set of G.C. associated with it depending on Geodetic Datum.

Geoid  – the equipotential surface of the Earth’s gravity (i.e. surface to which the direction of gravity is everywhere perpendicular) that is the best fit for the mean sea level. On land G. is defined as the surface which the sea water would assume if it could reach its own level everywhere. ^{[25 , 223].}

Geoidal Undulation  – the difference between Elevation (actual height above sea level) and ellipsoidal height at the point of measurement. ^[25]. See Reference Ellipsoid.

Geologic Map  – a map of the Earth’s surface where the distribution, nature and age relationships of various rock types, as well as the occurrence of related structural features, are shown. ^[13].

Geologic Stripping –  an iterative modeling process that involves modeling, at first, the short-wavelength anomalies produced by shallow sources, and then the deeper and longer wavelength anomalies from sources at depths of exploration interest. After removal of the modeled residual anomalies at shallower depths, the longer wavelength anomalies will become much more apparent, helping to make necessary changes in the deeper portions of the geologic model. ^[117]. G.S. concept can also be implemented in Data Enhancement processing in order to remove both shallow and very deep gravity or magnetic effects of geologic origin, which obscure anomalies of exploration interest (such as those of sedimentary or Basement structures), and obtain enhanced anomaly field emphasizing the gravity or magnetic anomalies from the target depth level.

Geomagnetic Correction  – see IGRF Correction.

Geomagnetic Field  – the Earth’s magnetic field that can be approximated by Magnetic Dipole at the Earth center. The intersections of the axis at this dipole with the Earth’s surface are Geomagnetic Poles. The entire G.F. is composed of three basic components: core magnetic field, external magnetic field, and crustal magnetic field. Prior to processing and interpretation of magnetic data, the core (main) and external components are commonly removed. ^{[25, 54 , 238].} See Earth’s Magnetic Field Components, and I.G.R.F. Correction.

Geomagnetic Field Vector  – a vector that defines the total intensity of Geomagnetic Field in terms of three orthogonal components in Cartesian Coordinates and two angles called Inclination and Declination. The absolute value (magnitude) of G.F.V. can be presented as:

T = (B_x² + B_y² + B_z²) ^½,

            where “B_x”, “B_y” and “B_z” are orthogonal components in the directions “x”, “y” and “z” respectively. Usually, the coordinate system for G.F.V. is oriented so that “x” increases to the North; “y” to the East, and “z” increases downward. ^[25]. See also Magnetic Meridian.

Geomagnetic Poles  – magnetic poles that are the best fit for the dipolar nature of the Earth’s magnetic field. G.P. represent the points of intersection of the axis of Magnetic Dipole (which approximates the main magnetic field of the Earth) with the Earth’s surface. These points are referred to as the North magnetic pole and South magnetic pole. G.P. locations differ from those of the geographic poles of the Earth and they move with time. G.P. are also referred to as Earth’s Magnetic Poles. ^{[54, 238].} See Secular Variation.

Geomagnetic Reference Field  – see International Geomagnetic Reference Field (IGRF).

Geomagnetic Reversals  – changes in the polarity of the Earth’s magnetic field which have occurred a number of times in the magnetic history of the Earth. ^[25]. See Sea Floor Spreading and Paleomagnetic Time Scale.

Geomagnetic Secular Variation  – see Secular Variation.

Geometrics G-822 Magnetometer  – see Optically Pumped Magnetometer.

Geospatial Imagery  – a technique that allows to obtain images combining cultural information (pipelines, wellsites, roads, etc.) with color or black-and-white satellite-borne data.

Gibbs Phenomenon  – an oscillatory effect in the form of short-wavelength artifacts which appear whenever data discontinuities are present (for example, at the ends of data grid). The same effect is observed around the cut-off frequency of all standard filters. By this reason, it is not recommended to apply narrow band-pass filters and high-order pass filters with steep transition from “pass” to “reject” frequency range. G.P. can be minimized using Edge Smoothing Filters and low-order pass filters. G.P. is often referred to as Ringing. ^{[201, 223].} See also Rolloff Range.

Global Positioning System (GPS)  – a system which provides the accurate continuous information on the position and velocity of a survey platform (aircraft or ship) in three dimensions based on signals received from a constellation of space satellites. ^{[57, 181, 223 ].} See also Differential GPS and GPS Base Station.

GLONASS  – Global Navigation Satellite System, the Russian equivalent of GPS.

Goussev Filter  – a space domain line-based or grid-based operator that calculates a scalar difference between Total Gradient and Horizontal Gradient of magnetic data. This difference peaks above Thin Dike causative bodies (like magnetized faults or fracture zones) and faults with offsetted magnetized layers (like normal faults or strike-slip faults producing “flower” structures).  It also suppresses irregular noise events. The best results are obtained when G.F. is cascaded with other data enhancement procedures, such as Separation Filtering, Band-Pass Filtering or Vertical Derivative calculations. ^[81]. See Magnetized Intra-sedimentary Fault.

GPS  – see Global Positioning System.

GPS Base  – see GPS Base Station.

GPS Base Station  – a reference station equipped with a multi-channel receiver to monitor GPS satellite correction data. The records from this station are used to differentially correct the GPS data acquired in the aircraft. See also Global Positioning System (GPS) and Differential GPS (DGPS).

GPS Positioning  – determining the location of a survey aircraft or survey ship using Global Positioning System (GPS) or Differential GPS (DGPS).

Gradient  – a difference in the potential field values per unit of distance, usually between horizontally or vertically separated sensors. G. can also be defined as a rate of the potential field change with a distance along the given directions “x”, “y” or “z”. By results of measuring the horizontal G., the vertical G. of the potential field may be computed with varying degrees of accuracy.  G. measurements have the advantage of removing non-geologic field signals such as those introduced by the normal accelerations of an aircraft in the gravity surveys or Diurnal Variations in the magnetic surveys. G. is also referred to as First Derivative. ^{[171, 223, 238].} See Gradiometer, Gravity Gradient, Magnetic Gradient and Vertical Magnetic Gradiometry.

Gradient Depth Estimation  – a 2-D method of estimating the depth of Causative Body with the use of Werner Deconvolution applied to Horizontal Derivative of the observed magnetic field (under assumption of Magnetic Contact model) or the vertical component of the Earth’s gravity field (under assumption of Horizontal Cylinder model). ^[215].

Gradient Dip Estimation  – a 2-D method of estimating Dip of magnetic or gravity Contact based on spatial relationship between the total and horizontal gradient maxima of the residual potential field. Total Gradient maxima is always located over the top of the contact irrespective to its dip. Horizontal Gradient maxima is always displaced down-dip (except for the vertical contact when it is coincident with the total gradient maxima). The following equation is used in G.D.E.:

A = B cotan D,

            where “A” is the horizontal distance between the total and horizontal gradient maxima; “B” is the depth to the top of a contact; and “D” is the dip of a contact. The horizontal derivatives are computed in Space Domain in two orthogonal directions (“x” and “y”) and then Fourier transformed for input to Nabighian’s Algorithm to compute Vertical Derivative. The derivatives are combined in the space domain after Inverse Fourier Transform to calculate a horizontal gradient and a total gradient. Maxima locations of calculated gradients are estimated by Blakely-Simpson Method. ^[103].

Gradient Filter  – a space domain line data operator that calculates the slope of the total intensity field curve with respect to Fiducials. G.F. is commonly used to identify high-frequency noise components of the observed potential field data. ^[230].

Gradient Vector  – a quantity that defines variations of three vector components of Gradient. See Vector and Full Gradient Tensor.

Gradiometer  – a device or set of devices (gravimeters or magnetometers) to measure the value of the potential field in at least two different points in space at the same time to estimate Gradient of this field. The gradient value is the difference in field values per unit of distance between the sensors in a given direction. ^{[46, 104, 112, 114, 142, 143, 223].} See also Magnetic Gradiometer Survey and Gravity Gradiometer Survey.

Gradiometry  – a method and instrumentation to collect and process measurements of gradients of the Earth’s gravitational or magnetic fields. See Gradient, Gradiometer, Gravity Gradiometer Survey, Magnetic Gradiometer Survey and Magnetic Gradiometry.

Grain  – an arrangement of regular patterns on the image of gridded potential field data or its filtered map, which shows the dominating anomalous trends in the area. ^[223]. See also Structural Grain.

Graticule  – a transparent template which is superposed over cross-sections of subsurface structures whose gravity effects are computed. ^[238].

Gravicline  – a gravitationally-induced subsurface compaction structure, which is closely related to the crystalline basement fault-block surface. G. is favorable for forming oil and/or gas traps. ^{[74, 76].} See Basement.

Gravimeter  – an instrument for measuring small variations in gravitational attraction (vertical acceleration) of the Earth’s gravity field. The gravity force on a unit mass in the measuring system of the exploration gravimeters is balanced by a spring arrangement. At each reading site (Gravity Station) the position of a unit mass is altered by a change in the gravitational attraction. ^[223].  Most gravimeters are of the unstable or Astatic Balance type. See Gravity Acceleration, Torsion Balance, Weight-On-Spring, Zero-Length Spring and Gravity Gradiometer.

Gravitational Acceleration  – see Gravity Acceleration.

Gravitational Constant  – see Universal Gravitational Constant.

Gravitational Field  – see Gravity Field.

Gravity  – the force of attraction between bodies because of their masses.  G.is usually measured as Gravity Field or its gradients. ^{[223 ].}  See Newton’s Law of Gravitation, Gradiometer and Gravimeter.

Gravitational Potential  – see Gravity Potential.

Gravitational Potential  – at a point in the gravity field, G.P. is defined as the energy required for the gravity force to move a unit mass from an arbitrary reference point (usually at an infinite distance) to the point in question. ^{[25, 54, 223, 238].} See Gravity Field.

Gravity  – the force of attraction between bodies because of their masses.  G. is usually measured as Gravity Field or its gradients. ^{[223 ].} See Newton’s Law of Gravitation, Gradiometer and Gravimeter.

Gravity Acceleration  – the acceleration (attraction) of the unit mass “m” (such as Proof Mass in the gravimeter) due to the presence of the Earth’s mass “M”, i.e., acceleration due to the Earth’s gravity field:

            where “F” is Gravity Force; “G” is Universal Gravitational Constant; “R” is the Earth’s radius at the point of measurements. The unit of G.A. is 1 cm/sec² = 1 Gal. The nominal approximated value of G.A. at the Earth’s surface is 980 000 mGal. In gravity exploration, the variations of a vector field of G.A. are measured and this vector field is called Gravity Field.  Differences in the measured G.A. values are related to the differences in the subsurface mass distribution and considered to be representing the changes in geological structures. ^{[25,  34,  54,  182,  238].} See Newton’s Gravity Law and Gravity Anomaly.

Gravity Anomaly  – a gravity signature of the geological interest generated by a lateral contrast in the subsurface distribution of rock densities. G.A. represents the departure of a corrected gravity value from the theoretical value of gravity at the latitude and longitude of the observation station, i.e., the difference between the observed gravity values and those of calculated from the Earth model. The type of anomaly depends on the corrections that have been applied to the observed data according to the model approximation (such as free-air G.A. or Bouguer G.A.). Positive G.A. trend indicates positive lateral density contrast and might be interpreted as structural high trend (uplift, horst, or other); negative G.A. trend indicates negative lateral density contrast and might be interpreted as a structural low trend (trough, graben, source rock Depocenter, or other). G.A. is always a composite quantity representing the sum of effects due to the superposition of anomalies from multiple sources at different depths. ^{[25, 34, 54, 182, 186, 238].} See also Free-Air Anomaly, Bouguer Anomaly and Observed Gravity.

Gravity Anomaly Amplitude  – a component feature of the Earth’s gravity field governed by two factors related to the subsurface structure: 1) degree of a lateral density contrast: the greater contrast – the larger amplitude; 2) depth of a source of the gravity anomaly: the larger depth – the smaller amplitude. ^[215]. See also Gravity Anomaly and Gravity Anomaly Wavelength.

Gravity Anomaly Wavelength  – a component feature of the gravity field governed by four factors related to the source of the gravity anomaly: source depth, source thickness and source lateral extent in “x” and “y” directions. Density contrast has no effect on G.A.W. ^[215]. See also Gravity Anomaly and Gravity Anomaly Amplitude.

Gravity Base Station  – see Base Station.

Gravity Basement  – a term that is generally referred to a major Density Contrast interface in the gravity survey area. Anomalies, generated by density contrasts below this interface, are effectively lost in the background noise.  Often, but not necessarily, G.B. corresponds to the density contrast between the whole sedimentary sequences and a top of the underlying crystalline (metamorphic) Basement. ^{[84, 223 ].} See also Magnetic Basement.

Gravity Corrections  – a series of corrections (or reductions) applied to Observed Gravity in order to isolate the anomalies caused by local density variations from all other Earth’s Gravity Field components that contribute to values measured by Gravimeter.  If G.C. have been properly applied (i.e. all components due to the motion and shape of a simple and virtually homogeneous Earth as well as other related effects have been removed from the observed data), then whatever remains would represent the anomalous gravity field due to local inhomogeneities that could be of exploration interest.  The procedure of G.C. is also referred to as Gravity Reduction. ^{[25, 34, 54, 182, 215, 238].} See Bouguer Correction, Free-Air Correction, Isostatic Correction, Terrain Correction and Theoretical Gravity Correction.

Gravity Curvature Correction  – see Bullard B Correction.

Gravity Elevation  – a height of Gravimeter sensor above sea level in the airborne gravity survey. See Aerogravity and Elevation.

Gravity Field  – a vector field of the acceleration (attraction) which exists between bodies. G.F. is directly proportional to the values of bodies’ masses and inversely proportional to the distance between them. In gravity exploration, G.F. is measured by the force “F” (also called Gravity Field Strength) exerted upon the unit (“proof”) mass “m” in the gravimeter:

g = F/m,

            where “g” is the Earth’s gravity field (or gravitational acceleration) representing the vector sum of the attracting effects of all masses below the Earth’s surface. Differences in gravitational acceleration “g”, measured on or above the surface of the Earth, are related to differences in the subsurface mass distribution. Since such variations reflect changes in a geological structure, these measured differences can be used to interpret the geology of the subsurface. ^{[25, 34, 54, 182, 215, 223 , 238].} See Gravity Anomaly and Gravity Acceleration.

Gravity Field Strength  – a measure of the gradient of Gravity Field. At a point in this field, G.F.S. is defined as force “F” that will be exerted upon a unit mass “m” if it is placed at the distance “r” from the center of mass “M” which is the source of the gravity field:

            where “G” is Universal Gravitational Constant. G.F.S. is a vector directed toward the attracting mass “M”. ^{[54, 238].} See also Newton’s Gravity Law.

Gravity Force  – a vector force of gravitation between two masses “M₁” and “M₂” which is directly proportional to the product of these masses and inversely proportional to the square of the distance between their centers of masses. G.F. is directed along the line connecting the centers of these two masses and mathematically defined by Newton’s Gravity Law. G.F. on the Earth’s surface is the integrated effect of the Earth’s mass attraction and the opposing centrifugal force caused by the Earth’s rotation. ^{[25, 54, 238].} See also Gravity Acceleration.

Gravity Gradient  – the first derivative or spatial rate of change of the gravity field with respect to a particular direction. G.G. contains the directional information which is absent in observations of the gravity field amplitudes and it can be used to infer the structure of a source of the gravity anomaly. G.G. units are mGal/m and Ëotvös (E). “E” is equal to 10^–6 mGal/cm or 0.1 mGal/km (i.e., 10E = 1 mGal/km). Generally, G.G. anomalies due to the sedimentary geological sources are in the range of about ± 200 E. As the gravity field is a vector, 3-D gravity gradient can be represented by a nine-component Tensor. ^{[30, 31].} See Gravity Gradient Tensor, Ëotvös Unit, and FTG Technology.

Gravity Gradient Tensor  – a nine-component vector that defines 3-D gravity gradient as three directional components “T_x”, “T_y”, “T_z” each varying in three directions “x”, “y”, and “z”. See Gravity Gradient.

Gravity Gradiometer Survey  – a gravity survey that provides measurements of the horizontal gradient and/or vertical gradient of the Earth’s gravity field. See also Gravity Gradient and Gravity Gradient Tensor.

Gravity Gradiometer System  – the gravity acquisition system, designed as an assemblage of horizontally and/or vertically separated gravimeters, to provide measurements of gradients of the Earth’s gravitational field. The difference between the gravimeter readings represents the spatial rate of change of the observed gravity field (i.e., Gradient) along the direction in which gravimeters are separated. As compared to the conventional gravimeter systems, G.G.S. can ensure significantly higher accuracy of measurements on moving platforms (marine or airborne gravity surveys), because it is much less sensitive to both vertical accelerations of the moving platform and velocity-dependent effects due to the rotation of the Earth. ^[101]. See E`tv`s Effect and Gravity Gradiometry.

Gravity Gradiometry  – a concept of 3-D measurements of the derivatives (gradients) of the Earth’s gravitational field using an acquisition system with horizontally and vertically separated gravimeters. G.G. is based on measurement of all or some of nine components of Full Tensor Gradient (FTG): “T_Xx” represents the horizontal gradient in the “X” direction of the “x” horizontal component of Gravity Acceleration (“G_x”); “T_Yy” represents the horizontal gradient in the “Y” direction of the “y” horizontal component of the gravity acceleration (“G_y”); “T_Zz” represents the vertical gradient in the ”Z” direction of the “z” vertical component of the gravity acceleration (“G_z”); “T_Yz” represents the horizontal gradient in the “Y” direction of the “z” vertical component of the gravity acceleration (“G_z”); “T_Yx” represents the horizontal gradient in the “Y” direction of the “x” horizontal component of the gravity acceleration (“G_x”), and so forth. The “z” vertical component of the gravity acceleration is the quantity measured by exploration gravimeters. All these gravity gradients provide much more precise and detailed information on the edges, shapes, and depths of dominant gravity anomalies. The present and future applications of G.G. include improved imaging of salt bodies, delineation of oil-water contacts, “time-lapse” monitoring of the oil field under deverlopment, and others. ^{[15, 117].} See also Time-Lapse (4-D) Gravity Survey and Gravity Gradiometer System.

Gravity Model  – a density model of a given or assumed geological structure. The subsurface geology can be modeled by representing lithologic layers and separate bodies as equi-density layers and bodies formed by contrast model boundaries that may or may not correspond to actual geological boundaries and bodies. Assumed densities for a given G.M. need not be constant, they also may be a systematic function of a depth and/or lateral dimension. G.M. can be effectively used to constrain seismic velocity models for both pre-stack and post-stack depth migration in areas of complex geological structures where seismic imaging becomes difficult. ^{[54, 215, 238].} See Model, Gravity-Seismic Velocity Modeling and Gravity Modeling Shapes.

Gravity Modeling Shapes  – basic theoretical shapes (horizontal or vertical cylinder, vertical or horizontal sheet, infinite slab, faulted slab and others), which are considered as the most simple of the geometrical forms to be useful for the calculation of the gravity effects in Forward Modeling and for the matching of a computed model with the observed field in Inversion. ^{[54, 238].} See Model.

Gravity Network Standardization  – a general term that refers to various data adjustment procedures which allow placement of separate gravity survey datasets on a common Datum, using Absolute Gravity data from the local gravity Base Station. See also International Standardized Gravity Network.

Gravity Nose  – see Nose.

Gravity Potential  – a mathematical function that describes, through its derivatives, Gravity Field at any given space point. Quantitatively, G.P. can be defined as the amount of energy (work) required for Gravity Force to move a unit mass from the arbitrary reference point at an “infinite” distance to the point in question. G.P. is also called Newtonian Potential.  ^{[25, 54, 223 , 238].}

Gravity Reduction  – a multi-step procedure of applying latitude, free-air, Bouguer, terrain or other corrections to the gravity measurements^{.  [223 ].}   See Gravity Corrections and Observed Gravity.

Gravity Reference Field  – a mathematical model of the Earth’s gravity field defined by International Gravity Formula. It is based on three main simplifying assumptions: 1) the Earth is homogeneous in the lateral density distribution; 2) observations are made at sea level; 3) the observation point is not moving with respect to the Earth. G.R.F. is also referred to as Theoretical Gravity because it gives the theoretical value of the Earth’s gravity field at any point on Reference Spheroid.  G. R. F. provides the largest contribution to the measured gravity values and its removal is the starting point for all subsequent Gravity Corrections. ^{[34, 54, 238].}

Gravity Repeats  – a quality control measure that defines the number of gravity observations repeated at the same Station during the day. R. can make up to 10% or more of the total number of the gravity survey observations. R. are used to calculate Standard Deviation of the survey measurements.

Gravity Standard  – see International Standardized Gravity Network.

Gravity Station  – a ground position at which Gravimeter is set up for making measurements of the gravity field.

Gravity Survey  – ground, shipborne or airborne measurements of the Earth’s gravity field and/or its gradients at various locations over the area of exploration interest. The objective is to associate variations of the measured values with distribution of density contrasts and, hence, of the rock types and subsurface structure.  G.S. results are usually displayed as Bouguer Anomaly or Free-Air Anomaly maps. ^[223]. See Fixed Wing Survey, Helicopter Survey,  FTG Technology and Satellite Gravity.

Gravity Survey Resolution  – an amplitude and wavelength resolution of the gravity survey achievable with the use of specific instrumentation and observation techniques. A generalized table representing the range of present-day G.S.R. estimates is shown below. ^{[36, 90, 123, 253].}

            See also Amplitude Resolution, Wavelength Resolution and Gravity Target Resolution.

Gravity Target Resolution  – an amplitude and wavelength resolution of the gravity data required for objective identification of geological targets of exploration interest. A generalized table of G.T.R. for typical geological targets is shown below. ^{[123, 253].}

See also Gravity Survey Resolution, Amplitude Resolution and Wavelength Resolution.

Gravity Traverse  – one cycle of the land gravity observations, which starts from the measurements recorded at Base Station, continues at the pre-planned stations in the survey area, and ends at the base station. In rough terrain or highly populated areas, G.T. observations are made at stations along the roads using four-wheel drive vehicles. Usually, there are several gravity traverses recorded during a working day. See Station.

Gravity Unit (g.u.)  – a unit of the gravitational acceleration equal to 10^-6 m/sec². Formerly, gravity units were widely used in gravity surveys, but now measurements in milliGals are more common. 1 milliGal = 10 gravity units. ^[223]. See Gal.

Gravity/Magnetic Topography  – a term sometimes used to refer to Sun Angle Images of the potential field maps where high and low intensity coherent anomalies look like elevations (ridges) and troughs (valleys) at the topographic map. See also Artificial Sun Illumination.

Gravity-Controlled Seismic Statics  – seismic static corrections calculated from the weathering layer model obtained from the land gravity measurements with Amplitude Resolution of 0.02 – 0.1 mGal and Wavelength Resolution of 100 – 200 m. ^{[36, 123].} See also Gravity Target Resolution.

Gravity-Derived Static Corrections  – see Gravity-Controlled Seismic Statics.

Gravity-Velocity Analysis  – a methodology that is applied to obtain more precise and less expensive seismic velocity and depth migration data based on Gravity-Velocity Modeling. ^{[5, 122].}

Gravity-Velocity Modeling  – a multi-step iterative procedure of refining and enhancing the seismic velocity model with the use of the high-resolution gravity data. G.-V.M. includes the following main operations: a) high-resolution gravity data is obtained in the area of seismic survey; b) seismic velocity data is used to create a density section (2-D case) or density volume (3-D case) based on Gardner’s Equation and other density-velocity relationships as well as available velocity and density well logs; c) gravity field of the obtained density model is calculated; d) calculated gravity field of a model is compared with the observed gravity field; e) density model is refined with both Forward Modeling and Inversion methods in order to minimize the misfits between the calculated field of a refined model and the observed gravity field; f) revised subsurface structure model is reconverted into velocity domain to obtain an improved starting point velocity model for depth migration. If necessary, this iterative procedure and feed-back loop can be continued throughout the seismic migration and interpretation process. G.-V.M. proved to be very efficient in areas of the salt tectonic deformations. ^{[5, 122].}  See Salt Dome Gravity Anomaly.

Green’s Equivalent Layer  – a gravity concept stating that the Gravity Potential caused by a three-dimensional density distribution is identical to the gravitational potential caused by a surface (thin layer) density spread over any of its equipotential surfaces. ^{[25, 183, 186].} See Equipotential Surface.

Grey-Scale Display Method  – a data imaging method that involves the dividing of the whole range of grid cell values in the obtained grid into equal parts corresponding to the gray-scale values within a fixed range of the gray shades and assigning these gray shades to the grid cell values to produce Grey-Scale Map. See also Pseudocolor.

Grey-Scale Map  – a potential field map image in which gray-color saturation from dark to light is modulated by the magnitude of the observed potential field: as a rule, the lowest values are the darkest and the highest ones are the brightest. See Grey-Scale Display Method.

GRF  – see Gravity Reference Field.

GRF Leveling Correction  – see IGRF Correction.

Grid  – a standard presentation form of the potential field data where each data value is assigned to the center of one of the regular spaced square cells of the same size. The whole set of such cells constitutes a continuous 3-D surface. See also Gridding, Grid Cell and Grid Interval.

Grid Cell  – one of the regular spaced and same size cells (usually, square by shape) which constitute Grid of the observed and/or processed potential field data. G.C. is often referred to as Cell. See also Gridding and Grid Interval.

Grid Expansion  – a preprocessing operation that assigns Rolloff Window to the edges of gridded data to reduce edge effects and improve performance of filters after Fourier Transform. See also Grid Filling.

Grid Filling  – a preprocessing operation that replaces grid gaps with values determined by interpolation from the valid grid points. See also Grid Expansion, Grid Gap and Maximum Entropy Prediction.

Grid Gap  – an interval between data points that is larger than the selected Grid Interval. G.G. can be eliminated using interpolation procedures during Initial Gridding. See also Grid and Gridding.

Grid Interval  – the distance between centers of the regular spaced square grid cells which represents Spatial Sampling of the gridded potential field data. G.I. is the basic parameter for Gridding and its value is usually limited to one third or, sometimes, one forth of Traverse Line spacing. G.I. is also referred to as Cell Size.

Grid Merging  – the process of combining two geographically adjacent grids of the potential field data into the single integrated grid without changing original grid intervals. G.M. creates a smooth merge zone for closely spaced or, more often, overlapping grids so that data transition from one grid to another becomes gradual and invisible in the subsequent data imaging and processing of the whole area of these two grids. G.M. is also referred to as Grid Stitching. See Grid and Grid Interval.

Grid Resampling  – a procedure that gives a new grid with a new Grid Interval and, if necessary, new location (origin) of grid cells. The new grid interval can be selected smaller or larger than that of the original grid. G.R. uses an interpolation process to calculate the potential field value for each new cell. See also Gridding.

Grid Residual Method  ­– a method of enhancing the anomalies of a certain size on the gravity or magnetic map.  G.R. is based on calculating the average of grid point values over a pre-selected area with the center at each grid point (like a perimeter of a circle of the user-specified radius) and then subtracting this average from corresponding center grid points.  The remaining part represents the residual component of this grid.   The size of enhanced anomalies depends on the extent of a selected area of averaging.  Sometimes, the procedure is called Map Convolution. ^{[223 ].}  See Griffin Method.

Grid Smoothing  – a method of Smoothing sharp irregularities in gravity or magnetic measurements caused by very shallow disturbances which are considered as Noise. Generally, G.S. is based on replacing the original grid point values with the calculated average of all values within a fixed small distance from each grid point. ^{[223 ].}

Grid Spacing  – see Cell Size.

Grid Stitching  – see Grid Merging.

Gridded Data  – potential field data arranged in the form of a grid of the regular spaced square cells of the same size. Each potential field data value is assigned to the center of one of these cells. See Grid, Gridding, Grid Interval and Line Data.

Gridding  – a spatial reconstruction of line or scattered data, i.e., a process of converting (or resampling) the observed potential field data recorded along the survey lines or at randomly distributed stations into a continuous set of regularly spaced (usually, square) cells each representing the potential field value assigned to the center of a cell. Grid Cell values derive from the original Line Data values or scatter Point Data values or, for cells located between data points, from the interpolated values of these data points. G. is a multi-step procedure which includes the following main operations: a) a mathematical surface is computed to represent the best fit with the observed data (Minimum Curvature or other algorithms are used); b) the obtained surface is sampled at the centers of cells; c) data values assigned to the centers of individual cells are smoothed to eliminate spikes. G. is always performed after Leveling of the original survey data. Applying Alias Filter to the line data is recommended before G. to prevent Grid contamination by Aliasing. ^{[28, 43, 68, 100].} See also Equivalent Source Continuation and Station.

Griffin Method  – a method of calculating the regional and residual gravity or magnetic fields from gridded data. G.M. calculates the average of grid points along the perimeter of a circle of the user-specified radius and assigns this average to the radius origin (centre of circle) as a regional value. The residual value is the difference between this average (i.e., regional value) and the observed value at the centre of a circle. The process is repeated for all points in Grid. Depending on the radius of averaging, the resulting grid is called, for example, 1.5 km radius regional (or residual) gravity. The magnitudes of regional and residual G.M. anomalies depend on the choice of radius: both very small and very large radii yield nearly the same anomalies as those of the original field and, hence, the optimum choise is between these two extremes. ^[88]. See also Gridding and Local Median Filter.

Ground Clearance  – the height of an aircraft flight above the Earth’s surface. Also abbreviated as AGL, i.e., above ground level. G.C. is often referred to as Terrain Clearance. See Drape Survey.

Ground Magnetic Intensity  – the total intensity (Magnitude) of the Earth’s magnetic field recorded by Ground Magnetometer at the survey Base Station. Profiles of G.M.I. are used for monitoring Diurnal Variations and period magnetic storms, as well as for applying Diurnal Correction to the observed magnetic data. See Magnetic Storm.

Ground Magnetometer  – a magnetometer set up at Base Station for continuous operation throughout the data acquisition stage to monitor and record Diurnal Variations and periodic magnetic storms. As a rule, G.M. is similar to that of the airborne or surface (mining exploration) recording system. See Ground Magnetic Intensity.

Ground Truth  – a general definition of the gravity or magnetic data obtained on the ground which are used as  a) reference for estimating Accuracy and Resolution of new methods with a moving Observation Platform (such as aircraft, helicopter and satellite); b) control over variations of the external component of the Earth’s magnetic field (such as Diurnals, Bay and Magnetic Storm). ^{[223 ].}

Ground Truth Gravity  – land or marine (shipborne) gravity measurements which are used as a reference for estimating the accuracy and resolution of new methods of Dynamic Gravity, such as helicopter or satellite gravity surveys. ^{[90, 253].} See also HGMS and HeliGrav^Ô.

GRS67  – see Geodetic Reference System 1967.

Gulf Gravimeter  – see Weight-On-Spring.

Gulf Magnetometer  – Fluxgate Magnetometer, where three mutually perpendicular fluxgate instruments and servomechanisms are used to minimize the magnetic field in two of these, thus maximizing the field intensity for the third. ^{[223 ].}

Gyromagnetic Ratio  – the constant of proportionality that relates Larmor Signal to the intensity of the total magnetic field. G.R. is measured in units of Hz/nT and equals 2B/23.4868 Hz/nT for protons.  See Cesium Magnetometer, Optically Pumped Magnetometer, Proton Precession Magnetometer, and Larmor Frequency.

Gyroscope  – a heavy disk mounted so that its axis can turn freely in one or more directions. Once G. is set spinning rapidly, it will continue to rotate in the same plane regardless of the way the supporting frame is turned. Because of its property to resist a change in the direction of its axis, G. is used in servo systems to maintain the orientation of Stabilized Platform. See also Servo System.