S

Salt  – a general term for naturally occurring halite and other saline minerals deposited from aqueous solutions as a result of extensive or total evaporation. S. has a relatively low density (2.1 – 2.6 g/cm³ with the average of 2.22 g/cm³), high plasticity and featured with regional occurrence. Because of these properties, S. is often squeezed out from the depth level of its origin under the pressure of the overlaying sediments. This process results in the deformation of overlaying sedimentary strata and creation of diapiric and/or piercement structures like salt anticline or Salt Dome. S. is strongly non-magnetic (Susceptibility value is about –0.01 in units of 10³ SI) and, being accumulated in domes or lenses, produces intense negative anomalies of Total Magnetic Field. Salt dissolution, which is often controlled by deeper faults, causes collapse of the overlying sediments. ^{[13, 238).} See also Evaporites, and Salt Dome Gravity Anomaly.

Salt Dome  – a general term for a diapiric or piercement structure with a columnar salt plug at its core, a cap rock of anhydrite and upturned, complexly faulted sedimentary formations adjacent to the salt plug. ^[13]. See Salt.

Salt Dome Gravity Anomaly  – a large negative Anomaly on the gravity profile or local closed minima zone on the gravity map. Often, the uppermost part of a salt dome (i.e., salt cap rock composed of Anhydrite) has a higher Density than that of the surrounding Sedimentary Rocks at the same depth. Thick salt cap rock (50–100 m and more) can create lateral Density Contrast and, hence, give rise to a small positive anomaly within the large negative S.D.G.A. Sometimes, gas chimney anomalies may look like shallow S.D.G.A. ^{[117, 174].} See also Chimney.

Salt Residual – a residual gravity map obtained after the calculated effects of Salt Dome model have been subtracted from the original (Bouguer or free-air) gravity map ^{[223 ].}  See Salt Dome Gravity Anomaly.

Salt Wall  – similar to Salt Dome, but very elongated in one dimension. S.W. is usually associated with Fault or fracture zone of a regional extent.

SAM Survey  – see Sedimentary AeroMagnetic Survey.

Sampling  – discrete measurements of data at fixed space or time intervals, usually, along the survey lines. See also Sampling Interval, Spatial Sampling and Sampling Theorem.

Sampling Density  – a ground magnetic or gravity survey parameter that defines the number of magnetic or gravity points of observation (stations) per square kilometer. See Station.

Sampling Frequency  – the number of samples (data points) per unit of distance (or time) in a given direction “x” or “y”, i.e., “1 / Dx(y)” samples per unit distance. For the gridded data (i.e., one sample per grid cell), “Dx(y)” is equal to Grid Interval and, hence, S.F. is the reciprocal of the selected grid interval. See also Gridding.

Sampling Interval  – a) distance between two successive measurement points (instrument readings), usually along the survey line. For aeromagnetic surveys, S.I. is derived from the magnetometer Sampling Rate, which is about 8-10 readings per second, and the aircraft ground speed defined in meters per second; b) distance between the cell centers of the gridded potential field data, i.e., Grid Interval. See also Cell and Sampling.

Sampling Rate  – an instrument characteristic that defines the minimum time interval between successive readings (measurements). See also Sampling Interval. ^[223].

Sampling Theorem  – a theorem which postulates that no information contained in the observed data is lost by a regular sampling, provided that Sampling Frequency is greater than twice the highest frequency component in the waveform being sampled. In other words, there must be more than two samples per cycle for the highest Spatial Frequency component of the data. S.T. is also referred to as Nyquist Theorem. ^{[25, 223].}

Sampling Wavenumber  – the period of Discrete Fourier Transform defined as

k_s = 2B / Dx

            where “Dx” is the sampling interval, i.e., Grid Interval for the gridded data. See also Gridding and Sampling.

Satellite Altimetry  – a method and instrumentation to collect and process measurements of the Earth’s surface topography in orbiting satellites. S.A. over ocean basins provides data for Satellite Gravity as measurements of the sea surface reveal undulations of the geoid and, hence, permit to map the related gravity anomalies.  In gravity applications, the satellite altimeter uses a pulse-limited radar to measure the altitude of the satellite above the sea surface. High-accuracy global tracking of the satellite orbit and orbit dynamic calculations provide an independent measurement of the satellite’s position and height above the Earth’s Ellipsoid. The height of the sea surface (i.e., Geoid height) is calculated as the difference between these two measurements, minus corrections for the radar signal propagation. The major source of S.A. errors in exploration in marine areas is the roughness of the sea surface due to waves (about 1-6 m). The achievable accuracy of S.A. over ocean basins is estimated as 10-20 mm. ^{[223 , 253].}

Satellite-Derived Gravity  – see Satellite Gravity.

Satellite Gravity  – a method, instrumentation and software tools to calculate gravity anomalies (i.e., Free-Air Gravity) over ocean basins from Satellite Altimetry data. S.G. is based on the fact that Earth’s Gravity Field in marine areas is related to the equipotential shape of the sea surface, called Geoid, which can be measured by radar altimeters aboard orbiting satellites. These measurements are converted into the gravity anomalous field using various transform techniques. The resolving capacity of S.G. is estimated at 3–7 mGal and 20–30 km in width. S.G. is also referred to as Satellite-Derived Gravity or Altimeter-Derived Gravity. ^{[87, 253].}

Satellite Magnetic Anomalies  – super long-wavelength (about 400-500 km and more) anomalies representing cumulative magnetic effects within and across neighboring large-scale tectonic provinces in Lithosphere as well as localized effects of regionally extensive magnetic ore deposits (like the Kursk iron formation in Russia). As satellites measure the Earth’s magnetic field far away from anomaly sources, the satellite-borne magnetic data usually are not suitable for determining 3-D source body geometry. ^[204]. See Satellite Magnetics, Source Body and Tectonic Province.

Satellite Magnetics  – a method and instrumentation to collect and process measurements of Earth’s Magnetic Field in orbiting satellites, usually at the elevation of about 400 km and more above sea level. Cesium Magnetometer, Rubidium-Vapor Magnetotemeter and, sometimes, Fluxgate Magnetometer are used for making S.M. measurements. The resolving capacity (i.e., Resolution) of S.M. does not exceed 1-2 nanotesla. ^[204]. See Nanotesla and Satellite Magnetic Anomalies.

Scale Factor  – a map projection parameter that defines the coefficient of deviation of the real Earth’s surface from the map plane with respect to the UTM central meridian and a map scale.

Scatter Point Data  – a set of potential field (usually gravity) measurements obtained at observation stations, which are irregularly distributed over the study area.

Schuler Frequency  – a reciprocal of Schuler Period.

Schuler Period  – a gyroscope-based mechanical system parameter that defines the period (or precession rate) of gyroscopes in Servo System, and equals 84 minutes. S.P. value is determined by the ratio of the Earth’s radius to Gravity Acceleration and presented as:

S.P. = 2p (R/g)^½,

            where “R” is the Earth’s radius and “g” is the gravity acceleration. ^{[25, 223].}

Scintrex CS-2 Magnetometer  – see Optically Pumped Magnetometer.

Sea Bottom Gravity – see Bottom Gravity.

Sea Floor Spreading  – a part of the Plate Tectonics concept that postulates the creation of a new oceanic Crust and moving away (spreading) of this new crust material by the convective uprising of Magma along the mid-oceanic ridges. S.F.S. half rates are typically about 1‑10 cm per year. ^{[13, 223].}

Sea Level  – see Mean Sea Level.

Sea Tides  – sea level movements governed by the gravitational pull which is exerted on the Earth by the Sun and Moon. Due to the closer distance of separation between the Moon and the Earth relative to that of the Sun and the Earth, S.T. are dominated by the Moon’s influence. See also Earth Tides.

Second-Derivative Map – usually, a map of Second Vertical Derivative of the gravity or magnetic field calculated after all proper corrections have been applied to the observed data.  S.D.M. tends to enhance and increase Resolution of short-wavelength local anomalies (i.e. “residual” components of the measured field) and suppress the long wavelength “regional” components.  Ambient Noise is also strongly enhanced ^{[223 ].}  See Second Horizontal Derivative and First-Derivative Map.

Second Horizontal Derivative (2HD)  – a horizontal derivative of First Horizontal Derivative (1HD) or rate of change of a horizontal derivative component of the potential field. 2HD magnetic profiles calculated along observation lines can enhance high-frequency components of anomalies associated with boundaries between regional structures like fold belts or depressions. Large negative values of 2HD can occur when magnetic sources exhibit a strong Remanent Magnetization. 2HD of the upward-continued gravity profile is used to obtain values of location, dip and vertical extent of the truncated horizontal plate model in Zero-Crossover Method. Sometimes, 2HD is referred to as Differential Curvature or Second Horizontal Difference. ^{[39, 163].}

Second Horizontal Difference  – see Second Horizontal Derivative.

Second Vertical Derivative (2VD)  – a rate of change of the vertical component of the gravity field or rate of change (the slope) of First Vertical Derivative of the magnetic field. 2VD is a measure of the curvature of the potential field: positive values indicate the increasing Vertical Gradient and negative values indicate the decreasing vertical gradient of the potential field. 2VD enhances the mid-frequency and, especially, high-frequency components of the observed data much more effectively than the first vertical derivative and, therefore, it is used to resolve (separate) the interfering anomalies from closely spaced magnetic or gravity sources. 2VD also amplifies any noise. For this reason, 2VD calculation is often cascaded with a conventional Low-Pass Filter or Upward Continuation to suppress or attenuate high-frequency components of noise. Sometimes, 2VD can be used to delineate source body edges: its zero crossings can mark the outlines of a thick body. ^{[25, 39, 59, 201].} See also Vertical Derivative, Fractional Derivative and Modified Second Vertical Derivative.

Second Vertical Gradient  – see Second Vertical Derivative (2VD).

Secular Variation  – regional long-period (sometimes, up to hundreds of years), cyclic variation in the Earth’s magnetic field Inclination and Declination, which are presumably caused by changes in convection currents in the Earth’s core and the rotation speed of the Earth. For example, Geomagnetic Poles precess around the geographic poles with a period of about 7000 years. S.V. can be displayed by a contour map called Isoporic Map. Often, S.V. are referred to as Geomagnetic Secular Variation. ^[25].

Sedimentary AeroMagnetic (SAM) Survey  – see High-Resolution AeroMagnetic (HRAM) Survey.

Sedimentary Rocks  – layered rocks originating from the consolidation of sediments, e.g., clastic rocks such as sandstones; chemical rocks such as carbonates; and salt or organic rocks such as coal. Magnetization of S.R. is much less (10-1000 times) than that of Igneous Rocks and Metamorphic Rocks. S.R. very rarely show any appreciable Remanent Magnetization which can be observed in exploration survey.  See Sedimentary Rocks Density and Sedimentary Rocks  Susceptibility. ^{[13, 33, 238].}

Sedimentary Rocks Density  – the basic quantity that predetermines the gravity properties of sedimentary rocks. On average, S.R.D. is lower than that of Igneous Rocks and Metamorphic Rocks. In general, S.R.D. varies with composition from the lowest for conglomerates to the highest for dolomites with considerable overlap in the range values. Wide range of S.R.D. values is due primarily to porosity variations, pore fluids content as well as age and depth below surface. Typical Density Contrast between adjacent sedimentary formations is rarely more than 0.25‑0.30 g/cm³. Generalized table of common S.R.D. values is shown below (in g/cm³) ^{[33, 238]}:

                        Rock Type                   Range                          Average (wet)

                        alluvium                      1.96 – 2.00                  1.98

                        clay                              1.63 – 2.60                  2.21

                        dolomite                      2.28 – 2.90                  2.70

                        gravel                          1.70 – 2.40                  2.00

                        limestone                     1.93 – 2.90                  2.55

                        sand                             1.70 – 2.30                  2.00

                        sandstone                    1.61 – 2.76                  2.35

                        silt                               1.80 – 2.20                  1.93

                        shale                            1.77 – 3.20                  2.40

Sedimentary Rocks Susceptibility  – the basic quantity that predetermines the magnetic properties of sedimentary rocks. S.R.S. values are usually very low as compared to those of Igneous Rocks or Metamorphic Rocks, and generally related to the volume content of Magnetite. Generalized table of the common S.R.S. values is shown below (in units of 10³ SI):

                        Rock Type                   Range                          Average

                        clay                              0.0 – 0.3                      0.2

                        dolomite                      0.0 – 0.9                      0.1

                        limestone                     0.0 – 3.0                      0.3

                        mudstone                     0.0 – 1.0                      0.5

                        sandstone                    0.0 – 100.0                  0.4

                        shale                            0.01 – 15.0                  0.6

            Average of main types            0.0 – 18.0                    0.9

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

Sensel  – a graph (profile) of the aircraft height (“sensor elevation”) above the Earth’s surface during the flight along a specified survey line. S. is obtained from the Radar Altimeter data. See also AGL.

Sensitivity  – an instrument characteristic that defines a degree of response to changes in the measured values. High-sensitivity gravimeters and magnetometers can measure very small variations in the potential field magnitudes. See also Accuracy.

Sensor  – an electronic device that detects a change in the magnetic or gravity field and turns it into a signal which can be measured and recorded.

Sensor Compensation  – an instrument correction applied to the airborne magnetic measurements to compensate for the static and dynamic magnetic components of the survey aircraft. ^[57]. See also Figure-Of-Merit (FOM) and Compensation System.

Separation – see Regional-Residual Anomaly Separation.

Separation Filter  – see Jacobsen Filter and Separation Filtering.

Separation Filtering  – a filtering procedure that attempts to separate the residual and regional components of the potential field. S.F. is based on either applying Upward Continuation of the observed field or calculating the difference between two upward continuations. Generally, the objective of S.F. is to determine the residual field due to sources: 1) mainly above a pre-selected depth level; 2) mainly below a pre-selected depth level; 3) mainly between two pre-selected depth levels. Fundamental ambiguity of the relationship between a potential field and its sources precludes total and complete separation of the regional and residual components. There will always be some Spectral Leakage. The effectiveness of S.F. depends entirely on the differences in wavenumber contents of the potential fields to be separated. ^{[39, 48, 97, 118, 167].} See also Energy Leakage, Sounding Filtering and Strip Filtering.

Servo System  – an electronically controlled system used in stabilized instrument platforms to keep them as close as possible to horizontal level during airborne or marine gravity surveys. S.S. is coupled with Gyroscope and activated by signals from accelerometers to obtain the necessary amount of correction to immediately restore the horizontal level of Stabilized Platform. See also Schuler Period.

Shaded Relief  – see Artificial Sun Illumination.

Shaded Stacked Profiles  – the line-by-line (“stacked”) image of the processed traverse line data for the whole airborne magnetic survey area. Commonly, S.S.P. represent the calculated profiles of the second horizontal derivative (called Second Horizontal Difference-2HD) along each of the traverse lines. Positive 2HD values are shaded for better visualization of the data trends. The square-root function is applied to the absolute value of the computed 2HD so that both small and large amplitude features can be displayed without being obliterated by the larger amplitudes on adjacent traverse lines. Before applying the 2HD operator, it is necessary to filter the traverse line data with Low-Pass Filter in order to suppress high-frequency noise. S.S.P. image can effectively delineate anomalies generated by normally magnetized thin dikes as well as the edges of thick bodies. ^[163]. See also Bipole Map.

Shadow Manipulation  – an image enhancement procedure for selecting the optimal Sun Declination and Sun Inclination to highlight the image features of exploration interest. ^[115]. See also Artificial Sun Illumination.

Shadowed Total Field  – an image of the total field (magnetic or gravity) obtained with the use of Artificial Sun Illumination imaging techniques. ^[115].

Shadowgram  – a term which is sometimes used to define Shaded Relief or Artificial Sun Illumination image of the observed or processed potential field data. Depending on the azimuth of the “sun” position, S. may be referred to as the western S., the southeastern S., the northern S. and so on.

Shape Factor  – see Structural Index.

Sharpe Gravimeter  – see Torsion Balance.

Sheet  – a thin tabular model approximation which is used in estimates of location and depth of the magnetized faults or magnetite-bearing sedimentary formations. See Magnetic Body Shapes and Magnetite.

Shifting of Magnetic Anomalies  – see Reduction-To-Pole (RTP) and Reduction-To-Equator (RTE).

Shipboard Gravimeter – an instrument and supporting system for measuring Earth’s Gravity Field (i.e. the field of Gravity Acceleration) from a moving ship.  The supporting system includes  Stabilized Platform and provides the insulation of S.G. from many  accelerations to which the ship is subject and corrects the data for the effects of measuring in Real-Time mode, i.e. while moving. ^{[223 ].}  See also Airborne Gravity Meter and Borehole Gravimeter.

SI  – International System of Units.

Signature  – a general term for intensity of observed anomalies and dominant amomalous alignments on images of the gravity and magnetic fields (i.e., gravity S. and magnetic S.). Fabric and Grain have similar meaning. See also Inverted Gravity Signature.

Sill  – a tabular interlayer intrusion of Igneous Rocks which is longer in the lateral dimension than in the vertical dimension (i.e., as opposed to Dike). S. always parallels the planar structure of surrounding rocks. ^[13].

Similarity Theorem  – a theorem of Fourier Transform. For the one-dimensional case, S.T. states that if a function “f(x)” (where “x” is in spatial/wavenumber units) has its Fourier transform “F(u)” (where “u” is in spectral/wavenumber units), then another function “f(ax)” will have the Fourier transform “F(u/a) / *a*”. S.T. is the basis for designing filters with the same or similar filter response characteristics for different wavelengths. ^[257].

Simple Analytic Signal  – see Analytic Signal.

Simple Bouguer Correction  – see Bouguer Correction.

Simple Bouguer Field  – a gravity field obtained after applying Bouguer Correction to the observed gravity data, but before applying Terrain Correction.

Simple Fourier Method  – a potential field continuation method. S.F.M. is based on the fact that the Fourier transform F(u,v,0) of a potential field measured on a horizontal plane at z = 0, can be converted into the Fourier transform of the same field measured on the plane z = h by a simple multiplication:

F(u,v,h) = F(u,v,0) exp[h(u² + v²)^1/2]

            If “h” is negative the operation is Upward Continuation; if “h” is positive the operation is Downward Continuation. This technique assumes there are no sources in the region between the old and new levels of continuation. ^[196]. See also Fourier Transform.

Skewness  – a horizontal displacement of the maxima of magnetic anomalies with respect to their sources. S. results from the fact that the directions of the magnetization and geomagnetic field are not usually vertical. See Reduction-To-Pole (RTP).

Sky Map  – a circle in Sun Angle Image display window which emulates the “sky” above the image viewed from the “space”. The center of S.M. corresponds to Overhead Point. Trough testing different locations within S.M., the user selects the optimum position of the “sun” to “illuminate” the image. See also Artificial Sun Illumination.

Slab Correction  – see Bouguer Correction.

Slope Analysis  – a methodology that obtains magnetic and gravity source depth estimates from the profile parameters of observed anomalies, such as maximum slope, half-maximum slope, three-quarter maximum slope, straight-slope distance, half-maximum distance, tangent points and others. See Depth Rules.

Smith Rule  – see Bott-Smith Method.

Smoothing – a line-based or grid-based procedure of averaging adjacent values in order to filter out (i.e. suppress) very high frequencies and spikes generated by Noise.  Usually, S. accomplished by use of Running Window ^{[223 ].}  See also Grid Smoothing and Local Median Filter.

Sokolov Method  – a graphic method of estimating a depth to the top of a source of the isolated magnetic anomaly: “Depth” = “horizontal distance between points of intersection of the maximum-slope-line with a regional line (i.e., an asymptotic line for both ends of anomaly profile) and with a line which is parallel to a regional line and tangent to the maximum value of anomaly profile”. ^{[223, 227].} See also Depth Rules.

Solar Wind  – a stream of charged plasma emitted by the sun. Complex interaction between the Earth’s internal magnetic field and S.W., coupled with the Earth’s rotation, tidal forces and thermal effects, produces External Magnetic Field.  Transient magnetic disturbances on the curve of the measured Earth’s Magnetic Field are often correlated with S.W. variations ^{[ 25, 223 ].}  See Bay and Diurnals.

Sounding Filtering  – an anomaly separation procedure which amplifies the components of the gravity or magnetic field originating mainly from the depth85 interval between two pre-selected depth levels. Fundamental ambiguity of the potential field sources predetermines only qualitative separation of this kind, i.e., it is impossible to extract the anomaly and/or potential field components from the quantitatively defined depth interval based only on spectral content. S.F. result is obtained by calculating the difference between two upward continuations of the observed potential field. ^[118]. See also Separation Filtering.

Source Body  – see Causative Body.

Source Edge  – see Magnetic Contact.

Source Parameter Imaging (SPI^™) Method  – an automated grid-based method of computing and imaging the instantaneous (“local”) magnetic source body parameters developed by CGG-Geoterrex, such as contact positions, depth, dip and susceptibility contrast. Estimates of these parameters are obtained from the calculated Complex Attributes of the analytic signal. All estimates assume: a) either 2-D sloping Magnetic Contact model or 2-D dipping thin-sheet model; b) there is no interference from adjacent anomalies; c) there is no Remanent Magnetization assumed in the dip and/or susceptibility contrast computations. The estimate of the local depth is obtained from Local Wavenumber and considered to be independent of the magnetic Inclination, Declination, dip, strike and any remanent magnetization. Contact positions are determined from the maxima of the local wavenumber. Four separate color-coded images can be obtained for each of the above four source body parameters. ^[242]. See also Improved SPI (iSPI^™) Method, Analytic Signal and Pseudocolor.

South-Seeking Pole  – a negative pole of Magnetic Dipole, which is attracted towards the Earth’s south magnetic pole. ^[238]. See also North-Seeking Pole.

Space Domain  – a domain where a mathematical function describes the relationship between the distance coordinates “x”, “y” and “z” as independent variables and some quantity (milliGals, gammas, density, susceptibility, etc) as the dependent variable. In S.D., the potential field data are presented by their observed or processed values at locations which correspond to the geographic distances in three directions (“x”, “y”, “z”). See also Domain and Spectral Domain.

Space Domain Filters  – a general definition of filters which process both line and grid datasets using mainly convolution operators in the space domain. S.D.F. include the following filter types: Fuller Filter, Gradient Filters, Local Median Filter, Naudy Filter, and others. See also Spectral Domain Filters.

Space-Frequency Localization  – the analysis of a portion the frequency spectrum of the potential field signal over a pre-selected space domain window. ^[35]. See Spectrum and Wavelet Transform.

Spatial Aliasing  – an aliasing which appears as a result of Spatial Sampling. Generally, S.A. occurs when the spatial sampling interval is chosen too large compared to the short wavelengths contained in the observed data. These wavelengths will then be aliased and their energy will be folded back onto longer wavelengths when Inverse Fourier Transform is applied. To avoid S.A., the data should be high-cut filtered back to Nyquist Frequency associated with the selected Sampling Interval prior to Gridding. In other words, one should expect to meet Aliasing effects when the potential field components have more waves per unit of distance or per grid unit in a given direction “x” or “y” than Nyquist Wavelength for a given survey. ^[223].

Spatial Filters  – a group of processing operators which enhance certain pre-selected Space Domain components of the observed gravity or magnetic fields (assumed to be of exploration interest) by removing or attenuating the other components. A large class of data transformations can be considered as S.F., including regional-residual separation, first and second vertical derivative computation, upward and downward continuation, and others. ^[108]. See also Space Domain Filters and Spectral Domain Filters.

Spatial Frequency  – a number of wave cycles per unit of distance or per grid unit in a given direction “x” or “y”. S.F. is also referred to as Wavenumber. ^[223]. See also Grid.

Spatial Resolution  – see Wavelength Resolution.

Spatial Sampling  – discrete measurements of the data at separate regular or irregular spaced locations (stations) over the survey area. S.S. may involve Spatial Aliasing problems. ^[223]. See also Sampling and Station.

Spector-Grant Method  – a magnetic source depth estimation method which is based on the use of the data Power Spectrum and assumes a statistical model of Causative Body as an ensemble of blocks or vertical prisms. The power spectrum is examined and linear slopes (“gradients”) of the log power curve are identified. The average ensemble depth “h”  is defined as:

h = “gradient” / 4B

            Sometimes, only two or three source ensembles with different slopes (“gradients”) can be identified. Non-linear spectra, which do not exhibit distinct linear slopes of the spectral curve, cannot be correctly interpreted. ^{[48, 228].} See also Depth Estimate From Spectral Analysis.

Spectral Analysis  – the examination of the Fourier domain spectra of the magnetic and gravity data with the purpose of identifying characteristic slope breaks on the plot of a power spectrum logarithm as a function of wavenumber (i.e., Radial Power Spectrum) and examining the noise characteristics of data using both the radial power spectrum and Map Power Spectrum. These slope breaks correspond to source ensembles of different depths and/or lateral extents. S.A. is performed for either gridded data or individual line profiles. S.A. results are used in preliminary estimates of the depth to magnetic source ensembles, the effect of Deculturing and in designing standard data filters as well as matched filters. ^{[48, 83, 169, 228].} See also Power Spectrum, Radial Power Spectrum, Map Power Spectrum and Matched Filtering.

Spectral Depth Estimation  – a method of estimating the depth of ensembles of source bodies based on the calculated Radial Power Spectrum of the observed potential field. S.D.E. is also referred to as Depth Estimate From Spectral Analysis. ^{[48, 96, 228].} See Spector-Grant Method and Pseudo-Depth Slicing.

Spectral Domain  – a domain in which frequencies and azimuth directions (along which frequencies gradually increase) are independent variables, while amplitudes of the data components are dependent variables. The observed data are presented in S.D. by their equivalent spatial frequencies (wavenumbers) with corresponding amplitudes after applying Fourier Transform to the gridded data or Hartley Transform to the line data. In S.D. both line and gridded data are processed in regard to their energy (i.e., square of amplitude) at each frequency rather than their field intensity value at the corresponding space position. Certain ranges of frequencies (wavenumbers) can be removed or attenuated, as well as certain wavelengths that are oriented in a particular direction. S.D. is often referred to as Fourier Domain. See also Domain, Space Domain and Wavenumber.

Spectral Domain Filters  – a general definition of filters which process both line and gridded data according to their “energy” at each frequency (wavenumber) rather than their space point values. S.D.F. include the following filter types: Bandpass Filters, Continuation Filters, Derivative Filters, Directional Filters, Magnetic Component Filters, Pass Filters, and others. Inverse Fourier Transform for the gridded data or Inverse Hartley Transform for line data are used to return data back to the space domain to locate features that have been enhanced using S.D.F. See also Space Domain Filters.

Spectral Filtering  – a directional filtering procedure based on calculation of Map Power Spectrum. ^{[240, 254].} See also Directional Filtering.

Spectral Leakage  – see Energy Leakage.

Spectral Matched Filtering  – see Matched Filtering.

Spectral Overlap  – see Energy Leakage.

Spectral Slope Method  – see Spector-Grant Method.

Spectral Transform  – a transformation of the observed data from their original space domain to the equivalent frequency (wavenumber) domain. See Fourier Transform, Space Domain and Frequency Domain.

Spectrum  – the amplitude characteristic of data components as a function of their respective wavenumbers (spatial frequencies). S. of the gridded data is calculated using Fast Fourier Transform (FFT). S. is the basic characteristic of the data which provides information on the main trends in the area and wavelengths involved as well as guides the computer processing in order to enhance the components of the exploration interest. S. constitutes Frequency Domain. ^{[223, 228].} See also Spectrum Calculation, Spectral Analysis, Spatial Frequency and Wavenumber.

Spectrum Calculation  – a multi-step procedure that generally includes the following processing operations (not necessarily all of them): a) Detrending, i.e., removal of the long wavelength trends and biases from the observed data; b) transformation of the residual field to the wavenumber domain using Fast Fourier Transform; c) applying a sliding window function; d) assigning the tapers for the grid ends or lines ends; e) expansion to square dimensions of the power (energy) components. S.C. can be performed in 1-D (one dimension) for line datasets and in 2-D for gridded data. See also Spectral Analysis, Spectrum and Power Spectrum.

Spherical Cap  – see Bouguer Spherical Cap.

Spheroid  – an approximation of the Earth’s shape by an ellipse of revolution. S. is symmetrical through its center, and also symmetrical about the axis of rotation.  See Reference Spheroid. ^[25].

SPI^Ô Method  – see Source Parameter Imaging(SPI^™) Method.

SPI Structural Index  – a model parameter which is used in Improved Source Parameter Imaging (iSPI^™) Method to discriminate between depth estimates of basic models. SPI S.I. is defined as

N = [ k₁ /(k₂ – k₁)] –1,

            where “k₁” and “k₂” are the first-order and the second-order local wavenumbers, respectively. For three SPI basic models, “N” gives the following integer values: 0 –magnetic contact (fault); 1 – thin sheet; 2 – horizontal cylinder. ^{[226, 242].} See also Local Wavenumber.

Spike – an irregular noise outburst.  See Noise.

Spike Filter – see Despiking Filter.

Spiking  – a term that is sometimes used to define the procedure of calculating the higher order derivatives of the original function, i.e., potential field or its horizontal and/or vertical derivatives. Due to the inherent noise, S. can yield Artifacts and only geological constraints can filter them out. S. can also be achieved through some versions of Downward Continuation but this procedure, as a rule, results in strong amplification of irregular noise. See Noise.

Spline  – a polynomial operator that creates a continuous curve by interpolation between discrete data values obtained at fixed points. S.–based functions are used in the gridding algorithms. See Gridding. ^[223].

Spring Balance – a sensor type in some Gravimeter models (like BHGM) where the weight of a hinged beam with Proof Mass on its free end is balanced by the tension of a spring.  As Gravity Acceleration changes, the spring tension (calibrated in the gravity units of measurement) also changes and, hence, allows to measure the gravity field change.

SQUID Magnetometer  – a high-sensitive magnetometer equipped with “superconducting quantum interference device (SQUID)” sensors. Its very low electronic noise level allows to detect subtle magnetic field changes using a superconducting loop. SQUID M. is capable of measuring the magnetic fields on the order of 10^–5 nanotesla (nT) and used for high-precision measurements of the Tensor components of the magnetic field as well as in paleomagnetic studies of sedimentary rocks. ^{[170, 223].} See Magnetic Gradient Tensor and Cryogenic Magnetometer.

Stabilized Downward Continuation  – a data enhancement procedure which combines the traditional Downward Continuation with a high-cut filtering technique using Butterworth Filter in order to provide the option of stripping high frequencies which cause instability in the downward continuation process from the magnetic data. The general formula of S.D.C. process can be presented as:

F(w) = e ^hWs ,

            where “h” is the continuation distance; Ws = W(1 / (1 + W/W_c)ⁿ; W = (u² + v²)^1/2 “W_c” is the median wavenumber; “n” is the exponent order; “u” and “v” are angular frequency coordinates. S.D.C. enhances deep magnetic anomalies covered by the shallow magnetic material. In some situations, the continuation distance required to enhance deeper magnetic sources may be below the top of shallow magnetic sources and, hence, the continued field cannot be considered as a theoretically true magnetic field. Nevertheless, it may be indicative of valuable qualitative information that is not obvious in the original magnetic field contaminated with the effects of shallow magnetic sources. ^[94].

Stabilized Platform  – a platform on which gravimeters are mounted during airborne and marine gravity surveys. S.P. keeps a nearly horizontal level despite tilts of the platform support because it is mounted on gimbals and controlled by Servo System on each gimbal axis. Generally, S.P. behaves like a damped long-period pendulum. When subjected to horizontal accelerations, S.P. may tilt resulting in gravimeter reading errors. The amount of tilt depends on the ratio of the period of the horizontal motion to that of the platform. The tilt is considered negligible if this ratio is less than 0.1. ^[223]. See also Horizontal Acceleration Correction, Vertical Acceleration Correction and Schuler Period.

Stacking  – a procedure of averaging over repeated measurements at the same observation stations. Often, S. can reduce Random Noise (i.e., the noise that is uncorrelated) and provide a relative enhancement of correlated signals. ^[219].

Standard Deviation (S.D.)  – a statistical parameter which represents an estimate of the accuracy of measurements, i.e., deviation of readings from their true value under normal conditions or the deviation of the potential field data values from the dataset mean value for the whole area after Detrending. Generally, S.D. is defined as

S.D. = [(1/n)S(x_i – x_m )²] ^1/2,

            where n is a number of readings or points in the dataset; x_i is the reading value or the observed value after detrending; x_m is the reading mean or the dataset mean. See also Accuracy and Variance.

Station  – a ground position at which Gravimeter or Magnetometer is set up for making measurements of the gravity or magnetic field. ^[223]. See Station Spacing.

Station Gravity  – a general definition of methods and instrumentation for ground (conventional or Microgravity Survey) and underwater (sea bottom gravity survey) measurements of the Earth’s gravity field at pre-planned gravity stations (i.e., ground or sea bottom positions at which Gravimeter is set up for measurements) over an area of exploration interest. S.G. is opposed to Dynamic Gravity. ^[253].

Station Spacing  – a distance between two consecutive points of the gravity or magnetic field measurements along the survey line. S.P. should not be more than ½ the lateral extent of the smallest wavelength anomaly which is expected to be resolved with a planned survey design. See Station.

Stationary Filtering  – a filtering procedure with the use of a filter operator which is the same (i.e., constant) all over the area of data processing. See also Non-Stationary Filtering.

Steenland-Vacquier Method  – see Vacquier Straight Slope Method.

Stopband  – see Cutoff.

Straight Slope Index  – a structural index used in Vacquier Straight Slope Method of magnetic anomaly depth estimation. S.S.I. value is based on the source geometry assumption. The following indices are commonly applied: vertical thin sheet – 1.9; horizontal thin sheet – 1.7; thick sheet – 1.4; plug-like body – 1.3; wide body (block) – 1.2; single interface (contact) – 1.2; default option -1.5. ^[215].

Straight Slope Method  – see Vacquier Straight Slope Method.

Strakhov Filter  – a linear filter that gives a best least-squares fit of a smoothed spectral estimate to the actual spectrum of the observed data over the whole frequency range while suppressing Random Noise components by a pre-selected amount. S.F. concept can be applied for Regional–Residual Anomaly Separation. ^{[82, 118, 167].} See also Separation Filtering.

Streaks  – see Corrugations.

Strike  – a space direction taken by a structural surface (fault plane or tabular intrusive body) as it intersects the horizontal plane. S. is always perpendicular to Dip. ^[13].

Strike Balance Filter  – a spectral domain grid-based filter that is applied to suppress Corrugations estimated from Map Power Spectrum. Any frequency bin in the map power spectrum whose radially normalized power is greater than a pre-selected amplitude ratio balance limit can be set to this limit. See also Strike Wiener Filter.

Strike Filter  – a spectral domain grid directional filter designed to pass or attenuate the components of the observed potential field along a pre-determined directional angle or azimuth, i.e., Strike. In the reject option, S.F. zeros out the Fourier domain segment (“pie-slice”) which corresponds to the specified space-domain trends irrespective to their wavelengths. S.F. can be used to remove the dominant Structural Grain from the gridded data so that subtle features may be seen at different azimuths. See also Directional Filtering.

Strike Wiener Filter  – an optimum-type spectral domain grid-based filter that is applied to suppress high-amplitude short-wavelength noise falling within a narrow azimuth range. S.W.F. design is based on assumption that Map Power Spectrum can be divided into two main segments: a noise-free segment where power spectrum represents only the signal “S²(r)” and a noise segment (noise fan) where power spectrum represents both signal and noise “S²(r) + N²(r)”. The optimum S.W.F., designed to suppress the noise, is defined as:

W(r) = S²(r) / (S²(r) + N²(r))

            In S.W.F. design, the user should define the maximum wavelength limit to be used with this filter. See also Strike Balance Filter.

Striping  – see Corrugations.

Stripping Filtering  – a separation filter procedure which amplifies regional components of the potential field based on calculating the consecutive upward continuations. S.F. represents the potential field data after the “removal” (“stripping”) of more and more of the crust overburden. ^{[42, 97, 118].} See also Separation Filtering.

Structural Grain  – alignments or trends that represent dominant structural features (for example, interpreted faults and associated horsts or grabens) at the specified subsurface level. See also Grain.

Structural Index  – see Euler’s Structural Index and SPI Structural Index.

Structural Model (2-D or 2.5-D)  – a model that is calculated to compare a magnetic or gravity response of an assumed geological structure, approximated by a density or susceptibility model, with the actual magnetic or gravity field. Each specific S.M. may or may not correspond to the actual geological structure. For 2-D modeling, the density and susceptibility models and their responses are assumed to be two-dimensional and semi-infinite. For 2.5-D modeling, the third dimension “y” (“in” and “out” of the plane of a profile) is approximated by one or more given distances providing a quasi-3-D model. ^[215]. See also Model, Gravity Model and Magnetic Model.

Subsurface  – an exploration term meaning the space below the Earth’s surface, in particular, Crust.

Sun Angle Image  – a gray-scale or color-coded image that creates the illusion of the sun shining from a chosen angle and highlighting the patterns of variously illuminated areas. ^[115]. See Artificial Sun Illumination.

Sun Declination  – a display parameter of Sun Angle Image which defines the azimuth of the straight line from Overhead Point to the sun location. For example, S.D. = 0º means that sun is at the North from the overhead point; S.D. = 90º means that sun is at the East; S.D. = –90º means that sun is at the West; S.D. = 180º means that sun is at the South. ^[115]. See also Sun Inclination.

Sun Inclination  – a display parameter of Sun Angle Image which defines the angle of the sun between the horizon and Overhead Point, i.e., sun elevation. For example, S.I. = 0º means that sun is on the horizon. S.I. = 90º means that sun is at the overhead point. ^[115]. See also Sun Declination.

Sunshaded Image  – an image of the magnetic or gravity map with artificial “illumination” under pre-selected “sun” azimuth and elevation. S.I. is also referred to as Sun Angle Image or Shadowed Image. ^[115]. See Artificial Sun Illumination.

Superconducting Gravity Meter  – see Virtual Spring.

Superposition of Anomalies  – a concept that defines the composite nature of the observed potential field anomalies: they represent the vector sum of individual effects of the ensemble of sources in the subsurface featured with their specific depths, lateral and vertical extents and density/susceptibility contrasts. ^[25].

Supra-basement Magnetic Anomaly Sources  – a general assumption about the nature of magnetic sources in the upper portion of Basement related to its topographic (relief) prominence rising above the average level of basement. There are two main types of S.M.A.S.: 1) fault-uplifted portion of a large block approximated by a thin horizontal sheet, i.e., fault throw is much less than the basement top depth; 2) large step (interface) in the basement block structure, i.e., fault throw is much larger than the basement top depth. ^[215]. See also Intra-basement Magnetic Anomaly Sources.

Surface Anomaly – a short-wavelength magnetic or gravity anomaly generated by variations of Density or Susceptibility at or near the Earth’s surface.  Such anomalies often produce spikes and usually are considered as Noise^{.[223 ].}  See also Regolith and Regolith Enhancement.

Surface Elevation  – a general term for a map (Grid) showing the topographic relief in the survey area. S.E. is also referred to as Terrain Elevation.

Surface Fitting  – see Trend Surface Analysis.

Surface-Ship Gravimeter  – see Shipboard Gravimeter.

Survey Altitude  – an airborne survey parameter that defines the flight height above ground level, i.e., AGL. HRAM surveys are usually flown as Drape Survey with the typical flight height about 100-150 m AGL. Before the advent of GPS, the altitude of airborne surveys was often defined as a constant barometric altitude. Safety and Aviation Authority Regulations are the major factors to be considered in planning S.A. ^{[57, 205].}

Survey Channel  – an instrument channel where airborne data (line datasets) are continuously recorded, usually, in a binary format. There are several separate channels for the recording of observed magnetic/gravity data, altitude measurements, GPS data, latitude and longitude, diurnals, fiducials, real time and other survey data. They can be added with channels containing pre-processed data: for example, in the aeromagnetic survey such channels are dc-mag channel (diurnal-corrected data), lev-mag channel (standard leveled data), edit-mag channel (culture-edited data), finlev-mag channel (leveled and culture-edited data).

Survey Datum  – see Geodetic Datum.

Survey Positioning  – determining the location of the survey area with respect to Reference Ellipsoid. See also Positioning and Survey Datum.

Susceptibility  – a magnetic property of rocks defining the degree to which the rock may be magnetized by an external (usually the Earth’s) magnetic field. For each specific rock type, S. is the proportionality constant “k” in the relationship between Intensity of Magnetization (“I”) and the external Magnetic Field Strength (“H”):

I = kH

            S. value is directly proportional to the volume percentage of highly magnetized minerals, like Magnetite, contained in a rock rather than a bulk rock property only. S. is a dimensionless unit, expressed in SI or cgs units (micro-cgs, i.e., cgs H 10 ^–6). The general range of S. values is about 10⁵. ^{[25, 158, 215, 223, 238].} See also Susceptibility Contrast and Koenigsberger Ratio.

Susceptibility Contrast  – the magnetic susceptibility difference between two adjacent rocks or geologic bodies. The degree of lateral S.C. primarily controls the amplitude of the magnetic anomaly. Theoretically, maximum S.C. between different rocks can be as high as 2000 (for example, peridotite dike within dolomite stratum). ^{[215, 238].} See also Susceptibility.

Susceptibility Filter  – a grid-based Spectral Domain operator (algorithm) that calculates values of Apparent Susceptibility in the subsurface. See also Density Filter.

Susceptibility Log  – the depth-versus-susceptibility curve obtained from the direct measurements of the magnetic Susceptibility of rocks in the borehole. Anomalous susceptibility values indicate the presence of magnetic minerals such as magnetite, pyrrhotite, etc. S.L. is not affected by mud resistivity, must be run in uncased boreholes, and can be run in dry boreholes. ^[22]. See also Magnetic Susceptibility Logging.

SVD  – see Second Vertical Derivative.

Swarm  – a group of sub-parallel tabular igneous rock bodies (usually, dikes) which can sometimes be correlated over hundreds of miles as intense short-wavelength elongated anomalies at the magnetic field maps. ^[198]. See Dike and Igneous Rocks.

Synergistic Correlation  – an integrated interpretation of different data types (aeromagnetic, seismic, airborne radar, and satellite radar) in order to delineate anomalies of exploration interest, which may not have been recognized in case of separate interpretation of each data type. Gravity, radiometry, well control, and surface geology data can also be integrated into the S.C. process to facilitate structural interpretation and evaluation of hydrocarbon and mineral prospects. ^{[6, 18, 29, 111, 130, 132, 157, 200, 252].}

Syngenetic Magnetic Anomaly  – a low amplitude short-wavelength anomaly generated by relatively high concentrations of Syngenetic Magnetite in the sedimentary formations. Syngenetic magnetic anomalies can be used to detect and map potential anticlinal reservoirs as well as stratigraphic traps in the areas of incised paleotopography. ^[77].

Syngenetic Magnetite  – a strongly magnetic mineral laid down contemporaneously with its sedimentary host formation at the time of deposition. S.M. can be derived directly from a magmatic source. Certain types of conglomerates, sandstones, tuffs, glacial till, “black sands” in stream channels, and other areas of incised topography can contain high concentrations of S.M. and, therefore, generate a detectable Syngenetic Magnetic Anomaly. ^[77]. See also Magnetite and Diamagnetic Magnetite.