D

Data Conditioning  – a general definition of preliminary processing operations applied to the observed data to prepare them for subsequent purpose-oriented computer processing and interpretation. D.C. includes Database Creation, Leveling, Line Resampling and/or Grid Resampling, Gridding, Microleveling, applying reductions and other operations (not necessarily in this order).

Data Enhancement  – a sequence of selective procedures applied to the observed data to increase lateral resolution and relative amplitudes of short-wavelength (high-frequency) components of potential fields in order to highlight subtle features of exploration interest. There are two key factors which determine the effectiveness of D.E.: removal of regional long-wavelength high-amplitude components and suppression of regular and background noise. ^{[59, 115, 216].}

Data Extension  – a procedure which is applied to the line or gridded data in order to create the extended regions at survey edges where values of the first or last original data point can be smoothly rolled off to zero. D.E. is used to ensure the most effective performance of Spatial Domain Filters, Hilbert Transform, Fourier Transform as well as to minimize Edge Effects. See also Taper.

Database Creation  – a data storing procedure which organizes line oriented acquisition data (line datasets) in the standard order suitable for subsequent computer processing operations: Gridding, Microleveling, Filtering, etc.

Datum  – a reference level to which the observed data are referred or corrected. All depth estimates, as well as the magnetic and gravity survey datasets, are referred to a specified D. Sea level is the most common D. Sometimes, it’s called Reference Elevation. ^[223]. See Depth and Survey Datum.

Datum Transformation  – a procedure to compute coordinates expressed in one Datum into another.

DC Level  – a constant value of the gravity or magnetic field used as one of the input parameters in Inverse Modeling.

DC Power  – the highest value in Power Spectrum of the observed potential field data assigned to zero value of Spatial Frequency or zero value of Wavenumber. Map Power Spectrum plots DC P. at the center of map, Radial Power Spectrum – at the left side of the spectrum graph.

De-Aliasing Filter  – see Alias Filter.

Declination  – a deviation of Geomagnetic Field vector from the true (geographic) North. D. can also be defined as the azimuth of Magnetic Meridian:

D. = arcsin B_y / (B_x² + B²_y) ^½,

where “B_x” and “B_y” are horizontal components of Geomagnetic Field Vector in directions “x” and “y” respectively. By convention, D. is positive to the east and negative to the west. D. generally ranges between –25º and +25º. ^[25]. See also Inclination.

Decomposition  – a procedure to separate the observed gravity or magnetic field (usually presented as Grid or map) into its regional and residual parts. ^{[223 ].}  See also Residualizing.

Deconvolution  – a signal enhancement procedure which helps to extract (resolve) original anomalies produced by causative bodies from the observed potential field created by their superposition and interference. The term of D. originates from the mathematical approximation of the observed potential field as Convolution of the sources’ parameters with the basic function that describes the geometry of their subsurface distribution relative to the observation surface. Fundamental ambiguity in relationship between the potential field and its sources precludes any D.-based solutions without clearly specified assumptions about the particular model for the D. process. D. concept is the basis for automated estimation of magnetic source parameters such as depth, location, dip and susceptibility. D. has the same conceptual meaning as Inversion with the latter term being used in a more generalized sense. See Werner Deconvolution and Euler Deconvolution.

Decorrugation  – a microleveling procedure that eliminates residual errors due to line– related noise (Corrugations) remaining after the standard Leveling and Gridding of the observed data. D. is based on Directional Filtering which is applied along the traverse and control lines in the space domain or in the frequency (wavenumber) domain. In space domain, Naudy Filter and Fuller Filter can be applied in a tandem. In frequency domain, Butterworth Filter and Directional Cosine Filter are often used. ^[230]. See also 1-D Directional Filtering and 2-D FFT Filtering.

Deculturing  – see Cultural Editing.

Definitive Geomagnetic Reference Field (DGRF)  – a predictive version of International Geomagnetic Reference Field (IGRF) on a 5-year epoch. DGRF is updated based on DGRF models that represent the official record of how the geomagnetic field has behaved in epochs. At present, nine DGRF models are in effect: DGRF 1945 through DGRF 1985, each representing a 5-year epoch since 1945. ^[25].

DEM  – see Digital Elevation Model.

Demagnetization  – the effect of magnetization distribution that results in reducing the magnetic moment “M” by a factor which varies with a shape of the magnetized body. Generally, this effect is small. D. can be significant in a massive pyrrhotite and in rocks containing more than 5–10% magnetite. ^[238].

Demagnetization Temperature  – see Curie Point.

Density  – the mass of a rock per unit volume. D. is usually expressed in grams per cubic centimeter (in cgs units) or kilograms per cubic meter (in mks units). ^{[33, 63].} See Density Contrast.

Density Basement  – See Gravity Basement.

Density Contrast  – the difference in density of one rock mass relative to another. D.C can be either positive or negative. Gravity anomalies of exploration interest are caused by lateral density contrasts in the subsurface and they are analyzed and interpreted as lithologic and/or structural boundaries. For expample, local areas of relatively high values in Bouguer Gravity are usually interpreted as structural highs and vice versa.  Maximum D.C. between different rocks (i.e., range of density values from the lowest to the highest) is approximately two. This as a very small range compared to that of magnetic Susceptibility (about 10⁵). ^{[63, 215].} See Density.

Density Factor  – a value of rock density selected for calculating Elevation Correction.  The most appropriate D.F. is the one that minimizes the correlation of the gravity values with appreciable topographic relief features.  ^{[223 ].} See Nettleton Test.

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

Density Model  – a model of the geologic structure in which layers or bodies of given lithologies are replaced by specified bodies of assumed density distribution. See also Gravity Model.

Density-Depth Function  – a relationship between changes in the rock Density and increasing Depth.

Density Profile  – see Nettleton Test.

Depocenter  – an area of the maximum deposition (i.e., laying down of a rock-forming material) in the sedimentary basin. On regional-scale gravity or magnetic maps, D. is represented as the gravity minima trend (zone) or the lowered intensity Total Magnetic Field area. ^{[13, 141].}

Depth  – a vertical distance from the specified Datum (Earth’s surface, sea level or derrick floor) to the top of the magnetic or gravity source body or structural surface like Basement.

Depth Controller  – a ship borne magnetic survey device that maintains Fish at a pre-determined depth below sea level.

Depth Estimate Cosine Correction  – see Cosine Correction.

Depth Estimate From Spectral Analysis  – a calculation methodology which is based on the concept that for simple magnetic models the depth to magnetic source is related to a slope of the logarithm of Power Spectrum of the observed data. The minimum area, for which this power spectrum is computed, must be about 8-10 times the expected depth to magnetic source. For the total intensity magnetic field, depth estimate can be presented as

depth = slope of the log power spectrum / 4p,

where the slope value is defined as a ratio of the difference between two log power spectrum values to the difference between two corresponding Wavenumber values. For gradiometer data:

depth = 1 / 2pf_o ,

            where “f_o” is the frequency (wavenumber) of the log power spectrum maximum value. ^{[48, 96, 169, 215, 228].} See also Radial Power Spectrum, Spectral Analysis and Spector-Grant Method.

Depth of Compensation  – a term derived from the concept of Isostasy. It defines the theoretical depth level above which rocks are rigid and below which there is a slow movement of plastic rocks to adjust to changes in the overburden load. Lateral density variations below this level are assumed to disappear.  Sometimes, it is taken as a top of Asthenosphere. ^{[25, 223 ].} See also Hayford Modification.

Depth Rules  – the rules which are used in the graphic methods of estimating the depth to the top of a source of the observed isolated anomaly. D.R. relate this depth to some measured features of the anomaly shape, such as horizontal distance between points on the side of anomaly where its slope is a half the maximum slope (Peters Half-Slope Method) or horizontal distance between points corresponding to the half amplitude and maximum amplitude (Half-Width Method) and others. D.R. are more successively applied to specific source shape approximations (such as dikes, point masses or horizontal line masses) and for isolated anomalies which appear to be not interfering with adjacent anomalies. The profile of the selected anomaly should be perpendicular to the contours of the potential field map (i.e., orthogonal to the anomaly trend). ^{[53, 216, 223].} See also Hannel Method, “Quick-And-Dirty” Peters Method, Sokolov Method, Steenland-Vaquier Method, Tiburg Method.

Depth Slicing  – a method that uses specifically designed filters in order to isolate anomaly contributions from source bodies in a certain qualitatively estimated depth range based on Power Spectrum and wavelength criteria. D.S. is more often referred to as Pseudo-Depth Slicing, as it only enhances particular parts of Power Spectrum frequency (wavelength) content of the data, rather than attempting to completely separate them and make a quantitative depth estimate. ^{[18, 79, 175, 215].} See also Matched Filtering and Ensemble.

Depth-Slice Filtering  – see Matched Filtering and Depth Slicing.

Derivative  – a rate of change of the observed potential field in a particular direction (“x”, “y” and “z”). The first D. represents the slope or gradient of the field in a specified direction. The second D. represents the slope or gradient of the first D., which is the curvature of the original potential field. In general, D. has the properly of discriminating against large-scale regional trends and accentuating mid-wavelength and short-wavelength (high wavenumber) anomalies of exploration interest as well as short-wavelength noise components of the observed gravity or magnetic field. See also Horizontal Derivative and Vertical Derivative.

Derivative Filtering  – a spectral or space domain procedure which is based on applying Derivative Filters in order to enhance short-wavelength components of the observed data, accentuate subtle changes in the total intensity or filtered potential fields as well as reduce the effects of interference and superposition of anomalies generated by closely spaced sources. See Vertical Derivative, Second Vertical Derivative, Vertical Derivative Order and Horizontal Derivative.

Derivative Filters  – a group of the spectral or space domain processing operators which calculate the derivatives of the observed potential field in the directions of “x”, “y” and “z” coordinates. The effects of D.F. and Residual Wavelength Filter for appropriate wavelengths can be quite similar because both of them are constrained by Grid Spacing. See also Gradient Filter and Derivative Filtering.

Derivative Map  – a map of one of the derivatives of the gravity or magnetic field, usually First Vertial Derivative and/or Second Vertical Derivative. ^{[223 ].}  See also Fractional Vertical Derivative, Horizontal Gradient and Total Gradient.

Despiking  – a removal of spikes from the observed data (line or grid datasets) in order to improve the performance of filter operations both in space and spectral domains. See also Pre-Filter Transformation.

Despiking Filter  – a filter that removes spikes (i.e., noise outbursts) from the line or gridded data, usually, by the use of Running Window averaging.  Median Filter is often used as a simple D.F.

Detrending  – a removal of overall trends in one (line dataset) or two (grid dataset) dimensions from the observed data before Fourier Transform. Usually D. is performed by Polynomial Fitting. Sometimes, when data exhibit no evident trends, D. is performed by subtracting the dataset mean (i.e. average) from all values in the dataset. ^{[223 ].}  

Detrital Remanent Magnetization (DRM)  – one of five main types of the remanent (residual) magnetization. D.R.M. originates in the rock material from exposure of its detritous component to an external magnetic field during a relatively slow settling of the fine-grain particles. Some clays have a strong magnetization of this type, but the magnitudes are generally too small to affect exploration interpretation. ^{[33, 238].} See also Chemical R.M., Isothermal R.M., Thermal R.M. and Viscous Remanent Magnetization.

DGPS  – see Differential Global Positioning System.

DGRF  – see Definitive Geomagnetic Reference Field.

Diagenetic Magnetic Anomaly  – a shallow low amplitude short-wavelength anomaly generated by relatively high concentrations of strongly magnetic diagenetic minerals (like diagenetic magnetite and/or pyrrhotite) which were formed as a result of diagenetic changes caused by various factors including hydrocarbon seepage. ^{[55, 61, 110, 151].} See also Aeromagnetic Hydrocarbon Indicators and Syngenetic Magnetic Anomaly.

Diagenetic Magnetite  – a strongly magnetic mineral formed as a replacement mineral or through a hydrothermal alteration. Often, D.M. is accumulated above hydrocarbon reservoirs. High concentrations of D.M. can generate a detectable Diagenetic Magnetic Anomaly. ^{[55, 77].} See also Chimney, Magnetite and Syngenetic Magnetic Anomaly.

DiaMag^Ô  – a diamond and gold exploration helicopter-supported magnetic gradient acquisition system based on the use of UXO Detection technology. DiaMag^Ô integrates high-accuracy DGPS positioning (1–5 m) with high magnetic Amplitude Resolution (0.001 nT) and 0.05 nT sensitivity. A fixed-boom multi-sensor transverse gradient array is mounted directly to the helicopter. High-speed sampling at 1 kHz (summed to 20 samples per second) from a slow-moving and low-flying helicopter provides sampling density of 1 ´ 12 m or higher. Standard line spacing is 12 m. Trademark of Fugro Airborne Surveys.   See Multi-Sensor Gradient Array.

Diamagnetics  – magnetic rock materials having relatively small negative values of Susceptibility. The most common diamagnetic rocks are graphite, gypsum, marble, quartz and salt. The strongest diamagnetic anomalies are observed over large salt domes. ^{[33, 223].}

Differential Curvature  – a gravity equipotential surface parameter which is the difference between the maximum curvature of a given surface and its minimum curvature (which is measured perpendicular to the direction in which the surface curves the most), multiplied by the gravity constant “G”. D.C. is a measure of the curved shape of the gravity field surface in the vicinity of a gravity anomaly. Mathematically, D.C. can be presented as the horizontal gradient of a horizontal derivative component of the gravity field. ^[223].

Differential Global Positioning System (Differential GPS or DGPS)  – a system which provides the most accurate continuous information on the current position in three dimensions based on signal received from the space satellites and GPS Base Station. The records from the GPS base station are used to differentially correct the GPS data for the moving platform to remove signal dither and correct for atmospheric effects. ^[181]. See also Global Positioning System (GPS).

Differential GPS  – see Differential Global Positioning System.

Differential Upward Continuation Filtering  – see Separation Filtering.

Differentiation  – a term which sometimes is used to define the calculation of derivatives of the observed potential fields presented in the form of a map (grid). For example, double D. means calculation of Second Derivative.

Digital Elevation Model (DEM)  – see Digital Terrain Model.

Digital Image Processing  – a concept that is based on a computer-aided manipulation and transformation of the image data stored in a digital format in order to enhance the visual presentation of the objects of exploration interest. ^[132]. See also Pixel and Raster Grid.

Digital Terrain Model (DTM)  – a digital topography model of the survey area which is used for the pre-programming of the aircraft drape flying and in the calculation of terrain corrections for gravity. D.T.M. also called Digital Elevation Model. See Drape Survey.

Dike  – a tabular body of Igneous Rock in the form of a slab of a finite thickness which is longer in the vertical dimension than in the lateral dimension (i.e., as opposed to Sill) on its cross-section.  D. may arise up to the Earth’s surface cutting older rocks. D. is one of the basic geological sources of magnetic anomalies and model approximations in magnetic exploration. The response Magnetic Anomaly Amplitude and Magnetic Anomaly Wavelength of D. are controlled by its width and the depth to its top:  the narrower and shallower D., the higher amplitude and the shorter wavelength of anomaly.  Rectilinear geometry and above-average amplitudes of generated magnetic signals often make dikes easily recognizable on magnetic anomaly maps. ^{[13, 140 , 223 ].} See Dike Model and Goussev Filter.

Dike Model  – a model which is used to describe some linear features of the subsurface structure (like magnetized faults or block contacts), based on the assumption that corresponding causative bodies can be presented as wide/thick, narrow/thin, vertical or inclined dikes. See also Dike and Prism.

Dimension  – a property which generally defines length, area and volume. For example, two-dimensional (2-D) modeling defines magnetic or gravity model in “x” and “y” dimensions. D. can also represent any basic quantity (distance, time, wavenumber, etc.) through which it becomes possible to define all other quantities. See Time-Lapse (4-D) Gravity Survey.

Dip  – an angle that a stratum, or anomalous magnetic or density body, or fault plane makes with the horizontal plane. D. is measured perpendicular to Strike and in the vertical plane. ^[13].

Dipole  – see Magnetic Dipole.

Dipole Field  – an approximation of the major component of Earth’s Magnetic Field originating at the Earth’s center.  The intersections of axis of this dipole with the Earth’s surface are Geomagnetic Poles. ^{[223 ].}

Directional Cosine Filter  – a spectral domain grid filter which rejects or retains the components of the observed data oriented along the user-specified azimuth (angular direction). As compared to conventional directional filters, D.C.F. creates a smooth curve in Rolloff Range (i.e., from pass to reject ranges) to prevent short-wavelength (high-frequency) Ringing on the space domain images of the filtered data after Inverse Fourier Transform. D.C.F. is often used in Microleveling. ^[67].

Directional Filtering  – a two-dimensional (2-D) procedure which is used to a) highlight structural trends on maps of the gridded data using Artificial Sun Illumination methods; b) pass, attenuate or suppress regular noise components oriented in a certain direction by the processing in the spectral domain. Spectral processing is based on the fact that directional features in the space domain have the equivalent presentation in the corresponding sector of the spectral domain and this sector’s image on Map Power Spectrum is oriented orthogonally to the direction of given space features. ^{[216, 240, 254].} See also 1-D Directional Filtering and 2-D FFT Filtering.

Directional Filters  – a group of the spectral domain grid filters which pass, attenuate or reject components of the potential field oriented in a certain space direction. User specifies low-cutoff angle and high-cutoff angle (in degrees) for the low and high limits of the reject/pass range. D.F. may produce a short-wavelength (high-frequency) Ringing which appears on the image of the space domain data after Reverse Spectral Transform. ^{[40, 240].} See Directional Cosine Filter, Strike Filter, Strike Balance Filter, Strike Wiener Filter.

Directional Spectral Analysis  – a four-step procedure applied to the gridded magnetic or gravity data in order to determine from Power Spectrum what directions contain predominant portions of the total map energy to constitute linear anomalies associated with structural-tectonic trends. These steps include: 1) removal of the regional gradient from the observed data (Detrending); 2) 2-D Fourier Transform of the space domain data to the spectral domain with subsequent replacement of each spectral sample with the square of its magnitude to obtain a power spectrum for display; 3) determination of directions which contain predominant energy peaks; 4) directional (fan) filtering to separate selected lineaments associated with peaks in the power spectrum from other map features for independent examination. ^[240].

Discrete Fourier Transform  – a version of the standard Fourier Transform for sampled data. D.F.T. can also be performed for the wavelets over a specified finite space interval so that equivalent spectral domain values are obtained for Dominant Wavenumber and its harmonics. ^{[25, 44, 177, 201, 223].} See also Inverse Fourier Transform.

Display  – a visual presentation of data on a screen of the workstation.

Distortion  – an undesired change of the actual space waveforms of the potential field components caused by interference with and superposition of noise events.

Diurnal Correction  – a correction applied to the observed magnetic data to compensate for Diurnal Variations due to solar radiation activity. D.C. involves the point-for-point subtraction of the diurnal values, recorded by Base Station, from the airborne magnetic values. Prior to D.C., the raw diurnal data is: a) “softly” filtered to remove high-frequency noise fluctuations; b) interpolated to match the airborne Sampling Interval; c) adjusted to ensure synchronization with airborne measurements. ^[187].  See also Ground Magnetometer.

Diurnal Drift  – a level of Diurnal Variations recorded by Ground Magnetometer during the time of the airborne data acquisition. Using the GPS time as a synchronization parameter, the D.D. data are examined on a line-by-line basis in order to determine whether D.D., measured while each specific line was flown, exceeded the specified tolerances. Lines, for which D.D. did not meet the planned specification, are marked for re-flight. See Diurnal Variations.

Diurnal Variations  – daily short-period temporal fluctuations of the external magnetic field due to the solar radiation activity. D.V. must be removed from the observed magnetic data prior to processing and interpretation. Ground Magnetometer records are used to decorrelate the external magnetic field signal from the airborne magnetometer’s total magnetic field signal. D.V. are often referred to as Diurnals. See also Diurnal Drift.

Diurnals  – see Diurnal Variations.

Domain  – a mathematical set of values that define Dimension as the independent variable value and the corresponding values of a certain function as dependent variable values. For example, Space Domain defines the observed or processed potential field data as a set of values which vary depending on three geographic dimensions (i.e., independent variable values) of “x”, “y” and “z”. D. is also used with the meaning of the volume of a mineral with uniform (i.e. single direction with magnetic moments parallel) magnetization. ^[223]. See Spectral Domain and Magnetic Domain.

Dominant Frequency  – a local or area average Spatial Frequency which is estimated by measuring the distances between successive peaks or troughs of the gridded or line potential field data. ^[223]. See also Harmonics.

Double Bouguer Correction  – a two-step Bouguer Correction that is applied to the gravity measurements made by ocean-bottom gravimeters to compensate for: a) upward gravitational attraction of sea water above gravimeters; and b) the difference between sea water density and Replacement Rock density. For mine and borehole measurements, D.B.C. is referred to applying the Bouguer correction to the gravity readings obtained both above and below a target layer in a mine or at two different depths in a borehole. ^[223]. See also Replacement Density.

Double Differentiation  – see Differentiation.

Downward Continuation  – a procedure to recompute the observed potential field at the level which is lower than the level of actual measurements. Generally, D.C. is considered stable as long as no sources exist between a selected level and the level of measurements, i.e., it is assumed that the new level should always be selected above the Earth’s surface. The most general formula of D.C. can be presented as

F(w) = e^hw,

            where “h” is the level of continuation in meters. D.C. enhances short-wavelength (high-frequency) components of the potential field which are generated by relatively shallower sources. In practice, noise can be greatly amplified also. To minimize this effect, D.C. is  cascaded with a standard low-pass filter or other noise-suppressing procedures. ^{[39, 94, 215, 223].} See also Stabilized Downward Continuation and Upward Continuation.

Drape Survey  – an airborne survey flown as close as possible to a constant height (constant Ground Clearance) above the Earth’s surface. D.S. is recommended in order to maximize the lateral resolution of short-wavelength anomalies of potential fields. As a rule, Digital Terrain Model is constructed with peaks as “anchor points” for the programmed flight surface that incorporates maximum slopes relating to the climb performance of the aircraft. Alternatively, the pilot flies using Radar Altimeter without a preplanned flight surface.

Draped Image  – a composite image where one grid image, obtained in Pseudocolor, is overlaid (draped) on another grid image, obtained in a gray scale, or vice versa. See Composite Image.

Drift  – a gravity instrument characteristic which defines a gradual change in the reference level in respect to which actual gravity measurements are made. D. takes place because gravimeter springs or torsion fibers are not perfectly elastic but subject to slow creep over long periods of time as a result of the thermal expansion, elastic aging or excessive movement. Often, D. is calculated as the linear approximation between Base Station observations recorded at the start and at the end of each Gravity Traverse. ^{[223, 238, 255].} See also Drift Curves and Drift Correction.

Drift Correction  – a time-varying correction applied to the observed gravity data to compensate for the gravimeter’s drift using Drift Curves. D.C. can be presented as

D.C. = D (t_o – t_b),

            where “D” is the gravimeter’s Drift Rate, “t_b” is the base-in time, and “t_o” is the time of observation. Often, D.C. incorporates the correction for the Earth tides. ^[255]. See Drift and Time Variation Correction.

Drift Curves  – a graphic presentation of repeated gravity measurements at the same observation station (Base Station) at intervals during the day. D.C. are obtained in order to calculate corrections to compensate for the gravimeter drift as a function of time. Adjustment of readings at other stations is made by taking differences from D.C. and using various available techniques. It is critical to record the time of a given measurement to relate it to D.C. See Drift Correction.

Drift Rate  – a gravimeter instrument characteristic, which is calculated using Drift Curves. See Drift and Drift Correction.

DTM  – see Digital Terrain Model.

Dynamic Gravity  – a general definition of methods and instrumentation for measurements of the Earth’s gravity field using a moving platform such as a ship or near-bottom towed pressure case (marine gravity survey) and aircraft or helicopter (airborne gravity survey). D.G. is opposed to Station Gravity. ^[253]. See Towed Deep Ocean Gravimeter and HeliGrav^Ô.

Dynamic Range  – 1) a data parameter which defines the range of measured potential field values over the whole survey area or its local part. Usually, D.R. is defined as the ratio of the maximum measured amplitude to the minimum measured amplitude; 2) an instrument characteristic which defines the ratio of the maximum reading to the minimum reading (often, noise level) which can be recorded by and read from Gravimeter or Magnetometer without change of scale. ^[223].