N

N-km Regional (Residual) Gravity  – a map of the regional or residual gravity field calculated using the data averaging radius of N-km. See Griffin Method.

Nabighian’s Algorithm  – a mathematical procedure that calculates Vertical Derivative of the residual potential field from its horizontal derivatives in “x” and “y” directions using the generalized Hilbert Transform. ^{[164, 165, 166].}

NAD 83  – the North American Datum 1983, which is the geocentric Ellipsoid accepted as the official frame of reference for all locations in North America. ^[223].

Nanotesla (nT)  – a unit of Magnetic Flux Density stating 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 are now contoured in nanoteslas. 1 nT = 10^–9 tesla = 10^–5 gauss = 1 gamma. ^{[215, 223 , 238].} See also Gamma.

Natural Remanent Magnetization  – see Normal Remanent Magnetization.

Naudy Filter  – a space domain line-based or grid-based filter which operates by detecting anomalies of wavelengths shorter than the defined window length, and then removing these anomalies by the extrapolation over them from the nearest adjacent data values. The user specifies the window length (in meters or data points) which corresponds to the maximum width of anomalies to be identified as high-frequency components. N.F. is commonly used for elimination of high-frequency noise as well as spikes in the process of Microleveling. ^[56]. See also Decorrugation.

Naudy Method  – an automated profile-based depth estimation method wherein the anomaly type and location are indentified by cross-correlation of the observed magnetic profile with theoretical anomalies. Depth to a dike-like or plate-like source is then estimated from the relating source body’s anomaly parameters such as half-width or others. N.M. option for manual calculations involves measuring the slope, amplitude and distance features of the magnetic anomaly profile and looking for the best fit of obtained values to theoretical values calculated for vertical dikes. In this case, it is referred to as Inflection-Tangent-Intersection (ITI) Method. ^{[168, 223].} See also Depth Rules.

Nettleton Test – an evaluation of different values of the average rock density between Datum (usually, sea level) and Station elevation in order to select a “true” value (i.e. Density Factor) for Elevation Correction.  After Theoretical Gravity Correction has been applied to the observed gravity data, a set of the gravity profiles is constructed over a prominent topographic feature in the survey area.  Generally, profiles with accepted lower density values (as compared to the “true” density) have a curtain similarity with topography (undercorrection case); profiles with accepted higher density values have a mirror-like, i.e. inverted similarity with topography (overcorrection case).  The profile with a “true” density value has a very little or no correlation with topography.

Neural Network  – a general definition of the processing algorithms whose operational principles are similar to those of the brain’s nervous systems. Following the “training” with series of input values (patterns) and corresponding output values (definition of patterns), the N.N. algorithm is able to recognize the patterns through the generation of specific logical rules based on associative memory. Along with pattern recognition, classification and statistical evaluation of data, N.N. algorithms can be used effectively in the modeling of complicated geological structures. ^{[188, 215].}

Newton’s Gravitational Constant  – see Universal Gravitational Constant.

Newton’s Gravity Law  – see Newton’s Law of Gravitation.

Newton’s Law of Gravitation  – the expression of the force of gravitation, which is the basis for the gravity measurements. N.L.G. states that the force “F” between two masses “M₁” and “M₂” is directly proportional to the product of these masses and inversely proportional to the square of the distance “R” between the centers of masses:

F = – GM₁M₂ /R²,

where “G” is Universal Gravitational Constant and the minus sign indicates that this force is always attractive. The force “F” is a vector directed toward the attracting mass. ^{[25, 223, 238].}

Newtonian Potential  – see Gravity Potential

Noise  – any unwanted, usually, high-frequency (short-wavelength) components of the observed data. N. often appears due to more or less random inhomogeneities in the surface and near-surface distribution of density or magnetic materials as well as a result of regular processing errors, such as aliased Gridding and poor Leveling, and effects of a cultural (man-made) nature such as pipelines, wellheads, power lines etc. Sven Treitel made the observation that “there is no such thing as noise, it really is only uncorrelated data.” ^[219]. See also Aliasing.

Noise Grid  – a resulting Microleveling error grid, containing elongated anomalies (line Corrugations) and obtained after applying Directional Filters to the standard leveled magnetic grids. N.G. is subtracted from original grid to get a microleveled, corrugation-free grid. See One-Dimensional (1-D) Directional Filtering and Two-Dimensional (2-D) Directional Filtering.

Non-Dipole Field  – the portion of the Earth’s magnetic field not representable by Magnetic Dipole approximation.

Non-Magnetized Fault  – see Magnetically Transparent Fault.

Non-Stationary Filtering  – a filtering with the use of a filter operator whose impulse response varies in a controlled fashion over the area of data processing. See also Stationary Filtering.

Non-Stationary Transform  – a transform procedure that decomposes input signal (for example, gridded potential field data) on a grid of both space and wavenumber. N.‑S.T. is an alternative to the conventional Fourier Transform, which decomposes input signal only in a single dimension of frequency (wavenumber). Wavelet Transform is an example of N.-S.T.

Normal Gravity  – see Normal Gravity Field.

Normal Gravity Field  – an approximation of the Earth’s gravity field at Mean Sea Level based on International Gravity Formula. ^{[25,  211,  238].}  See Latitude Correction.

Normal Magnetic Field  – a smooth large-scale component of the observed Earth’s magnetic field that represents its main (core) component and is free of anomalies of exploration interest.  N.M.F. is often referred to as International Geomagnetic Reference Field or IGRF. ^[223].  See Earth’s Magnetic Field Components.

Normal Remanent Magnetization (NRM)  – a general term of the residual magnetization (R.M.) of rocks which basically includes five main types: Chemical R.M., Detrital R.M., Isothermal R.M., Thermal R.M. and Viscous R.M. Often, N.R.M. is referred to as Natural Remanent Magnetization. ^{[33,  238].}

Normalized Map Power Spectrum  – a map power spectrum after applying the radial sweep normalization which suppresses radial variations in the power spectrum and enhances its directional trends. See Map Power Spectrum.

Northing  – a component of the survey leg in the north direction from an east-west reference line, usually, the Equator.  On maps and grids in Cartesian Coordinates, it may be expressed as an “Y” value.  See also Easting, False Easting and False Northing.

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

Nose – a gravity or magnetic anomaly for which contours do not close, i.e. Gravity Nose or Magnetic Nose. ^{[223 ].}

Nuclear-Precession Magnetometer  – see Proton Precession Magnetometer.

Nyquist Fraction  – a unit parameter often used in displays of Radial Power Spectrum and Map Power Spectrum. This parameter defines decimal fractions of Nyquist Frequency, from 0 to 1, to describe spectral characteristics of the observed or filtered potential field data.

Nyquist Frequency  – a data sampling parameter which is equal to a half of Sampling Frequency (S.F.) which is the reciprocal of Grid Interval (G.I.):

N.F. = S.F./2 = 1/2 G.I.

            For gridded data sampled as one data point per Grid Cell and in accordance with the Nyquist theorem, N.F. defines the highest frequency (shortest wavelength) which can be indentified (resolved) with a selected grid cell size, i.e., grid interval. All wavelengths shorter than the selected grid interval (i.e., spatial frequency components of the potential field which are higher than N.F.) cannot be identified as there is no data between grid cell centers, and they may be aliased. Sometimes N.F. is referred to as Folding Frequency. ^[223]. See also Aliasing, Nyquist Theorem, Nyquist Wavenumber and Nyquist Wavelength.

Nyquist Theorem  – see Sampling Theorem.

Nyquist Wavelength  – a data sampling parameter used with the same meaning as Nyquist Frequency, because of the conceptual analogy between Frequency as a time-domain characteristic and Wavelength as a space-domain characteristic. Generally, N.W. is defined as a quantity equal to two times Sampling Interval. Similarly, for the gridded data it equals two times Grid Interval (G.I.):

N.W. = 2 G.I.

            Wavelengths smaller than this threshold are indistinguishable and may be aliased. Aliasing can be reduced if N.W. is decreased, i.e., sampling interval is made smaller. ^[25]. See also Nyquist Frequency and Nyquist Wavenumber.

Nyquist Wavenumber  – a sampling quantity used in Discrete Fourier Transform and defined as a half of Sampling Wavenumber:

N.W. = B / )x,

            where “Dx” is Sampling Interval. Sometimes, N.W. is referred to as Folding Wavenumber. ^[25]. See also Nyquist Wavelength and Nyquist Frequency.