H

Half-Maximum Method  – see Tiburg Method.

Half-Slope Method  – see Peters Half-Slope Method.

Half-Width Method  – a graphic method of estimating the source’s depth of an isolated gravity or magnetic anomaly. Depending on the assumed model, four options are available: 1) “depth to the gravity center of spherical mass” = 1.3 “half the horizontal distance across the peak at the level of a half the maximum amplitude”; 2) “depth to the gravity center of horizontal cylinder” = “half peak width” (i.e., half the horizontal distance across the peak at the level of half the maximum amplitude); 3) “depth to the center of spherical magnetic body” = 2.0 “half peak width” (i.e., half the horizontal distance across the peak at the level of a half the maximum amplitude; 4) “depth to the center of semifinite horizontal slab (fault anomaly)” = “half the horizontal distance between the points where anomaly is three-quarters and one-quarter its asymptotic amplitude”. ^{[53, 215, 223].} See Depth Rules.

Halo Effect  – an outer short-wavelength curvilinear edge of the opposite sign adjacent to the filtered anomaly. H. E. represents the artificial opposite field curvature around the periphery of that anomaly due to convolution with a filter operator. H.E. usually appears on the grid data images after applying high-resolution band-pass or derivative filtering procedures to the observed potential field data. H.E. is the side effect of an increased lateral resolution, which at the same time makes enhanced residual anomalies better visible and, hence, better correlatable. ^[223].

Hammer Chart  – see Terrain Chart.

Hamming Filter  – an edge smoothing spectral domain grid filter that modifies grid surface values to ensure their smooth transition to zero at the edges of a grid. ^{[99, 124, 201].} See Edge Smoothing Filters, Hanning Filter and Windowing.

Hannel Method  – a graphic method used for estimating the source depth of an isolated magnetic anomaly. For the magnetic pole model, “depth = half horizontal distance at the level of ¹/₃ maximum amplitude”. ^[53]. See Depth Rules and Half-Width Method.

Hannel Rule – see Hannel Method.

Hanning Filter  – an edge smoothing filter that can be applied as a space domain line filter or spectral domain grid filter. H.F. modifies line curve (line dataset) or grid surface (grid dataset) values to ensure their smooth transition to zero at the edges of lines or grid edges. ^{[124, 201].} See Edge Smoothing Filters and Windowing.

Harmonic Function  – a mathematical function that: a) satisfies Laplace’s Equation; b) has continuous single-valued first derivatives; and c) has second derivatives. The function which is harmonic throughout a region “R” will have all maxima and minima on the boundary of “R” and none within “R” itself where H.F. value at any point is the average of values at neighboring points. ^[25].

Harmonics  – spatial frequencies that are multiples of Dominant Frequency. For example, the fifth harmonic has a frequency five times that of the dominant value. H. are components of Fourier Transform. ^[223].

Hartley Gravimeter  – see Weight-On-Spring.

Hartley Transform  – a mathematical operation which is used to convert the observed Line Data from their original space domain to the equivalent spectral domain. ^[230]. See also Inverse Hartley Transform.

Hayford Modification  – a modification of a concept of the gravitational (isostatic) equilibrium between Crust and Mantle. It assumes that the gravitational load of accumulated sediments is balanced in all sedimentary basins at a certain Depth of Compensation: the heavier sedimentary load (i.e., the larger thickness of sediments), the larger depth of compensation. ^{[25, 223].} See also Airy Hypothesis, Pratt Hypothesis, and Isostasy.

Head Test  – see Heading Error Test.

Heading  – a direction of the survey aircraft or ship as indicated by the compass readings.

Heading Correction  – one of the leveling corrections applied to the observed magnetic data to compensate for the measurement variations due to the aircraft’s flight direction with respect to the Earth’s magnetic field. ^[85].

Heading Error Test  – one of On-Site Magnetometer Calibrations made by magnetic field test measurements along at least four flight lines, oriented in the direction of survey lines and flown above the area of a low gradient of the Earth’s magnetic field. The heading variations are determined from comparison of readings obtained during the flight along these four lines. See also Figure-Of-Merit (FOM) and Heading Correction.

Heiskannen Modification  – see Pratt Hypothesis.

Helicopter Gravity Measuring System  – see HGMS.

Helicopter Magnetic Survey  – helicopter-based magnetic measurements using a tail stinger mounted or towed high-sensitivity magnetometer. In rugged terrains, H.M.S. provides increased accuracy and data resolution due to a greater control on the flying height, flight path positioning and ability to fly very low level surveys with a close line spacing (less than 40-50 m), usually, for mineral exploration. See also Fixed Wing Survey.

Helicopter Survey  – the airborne potential field measurements using a helicopter. H.S. can be conducted for both magnetic and gravity exploration. See also Fixed Wing Survey and HGMS.

Helicopter-Suspended Gravimeter  – see HeliGrav^Ô.

HeliGrav^Ô  – a helicopter-based gravity measuring system where the fully automatic gravimeter module is mounted in the self-leveling tripod assembly suspended on a tow cable. The gravimeter sensor itself is suspended in a dual-gimbal arrangement and leveled by servomotors controlled by signals from the gravimeter. When all tripod legs are on the ground, the ground-contact sensor generates a signal to the data-acquisition system and the self-leveling process commences, followed automatically by gravity measurements. The estimated Standard Deviation (SD) of gravity measurements is less than 25 microGal. Dual frequency Differential GPS provides relative Elevation measurements with SD less than 10 cm. Trademark of Scintrex Ltd. ^[220]. See also HGMS.

Herringbone Effect  – an appearance of systematic deviations in contours or color-coded values on grid data images which is the result of the displacement of one or several survey lines from their correct positions or mis-leveling between survey lines. H.E. is better visible at the high gradient parts of the survey area. ^[223].

HGMS  – a helicopter gravity measuring system. A basic set of HGMS instrumentation includes on-board stable-platform Gravimeter, laser or radar altimeter, GPS navigation system, video camera recorder, digital data processor and analog recording monitors. The achievable amplitude and wavelength resolution is estimated as 1-2 mGal and 1-3 km. ^[90]. See also HeliGrav^Ô.

High Resolution AeroGravity (HRAG)  – airborne measurements of the Earth’s gravitational field, which incorporate advanced technologies for recording three-component Positioning and velocities with the purpose-designed processing software to compute the aircraft accelerations to a precision that is equal to or exceeds the measuring Sensitivity of airborne gravity sensors. See Aerogravity and Aerogravity Corrections.

High Resolution AeroMagnetic (HRAM) Survey  – a survey flown at low Terrain Clearance (80-150 m), with close line spacing (100–800 m), recorded at high sample rates (0.1-0.25 sec.), acquired with high-sensitivity magnetometers (0.001–0.005 nT), and using high-precision positioning systems (GPS or Differential GPS). HRAM survey can facilitate defining spatially complex geological structures and detecting subtle intra-sedimentary anomaly amplitudes. Sometimes, HRAM survey is referred to as Sedimentary AeroMagnetic (SAM) Survey. ^{[57, 78, 155].}

High-Cut (Low-Pass) Filter  – a space or spectral domain filter that substantially attenuates or removes wavelength components smaller than Cutoff Wavelength and retains (passes) all wavelengths longer than the cutoff value. H.-C.F. is often used to suppress short wavelength (high-frequency) noise components of the potential field. It is also used as Alias Filter prior to resampling and Gridding. The corresponding Cutoff units (meters or km in the wavelength mode or cycles per grid unit in the spatial frequency mode) are specified depending on the software option available. ^{[99, 201].} See Cutoff Frequency.

High-Pass (Low-Cut) Filter  – a space or spectral domain filter that enhances wavelength components smaller than Cutoff Wavelength and removes or substantially attenuates wavelength components longer than this cutoff. H.-P.F. is sometimes used to suppress long-wavelength (low-frequency) regional components of the potential field. The corresponding Cutoff units (meters or km in the wavelength mode or cycles per grid unit in the spatial frequency mode) are specified depending on the software option available. ^{[99, 201].} See Cutoff Frequency.

High-Pass Rolloff Filters  – a group of spectral domain grid or line filters that retain (pass) the high-frequency range of Power Spectrum using a smooth curve in Rolloff Range to prevent Ringing. ^[230]. See also High-Pass (Low-Cut) Filter.

Hilbert Transform  – a linear mathematical operation which is related to Fourier Transform. In 3-D case, H.T. is composed of two parts, with one part acting on the “x” component, and another part acting on the “y” component of the input signal. H.T. is used in the potential field data processing to: a) calculate Vertical Derivative of the potential field gridded data from Horizontal Derivative calculated in “x” and “y” directions; b) calculate the imaginary component of Analytic Signal from its real component (magnetic or gravity anomaly) which is then used to calculate Instantaneous Phase and Instantaneous Frequency. ^{[68, 164, 165, 166, 201, 223, 236].} See also Hilbert Transform Filter and Three-Dimensional (3-D) Hilbert Transform Operator.

Hilbert Transform Filter  – a space domain convolution filter which is applied in order to obtain the imaginary component of the observed potential field data from line datasets using a finite difference operator. For better results, Detrending is recommended before H.T.F. The filter result is combined with the real component of data to calculate Instantaneous Phase and Instantaneous Frequency. ^{[68, 230, 236].} See also Analytic Signal and Quadrature.

Histogram Equalization  – an image enhancement procedure to modify the color scale of the gridded data. The processing operator finds minimum and maximum values for grid contours and adjusts the color scale in such a way that the majority of color shades move into the range of the majority of data values. H.E. can enhance small anomalous zones of high or low values, which initially lie within a wide color range. ^[246].

Horizontal Acceleration Correction  – a correction applied to the airborne gravity data to compensate for the stabilized platform tilt and corresponding gravimeter reading errors caused by the aircraft’s horizontal accelerations. ^{[37, 106, 233].}

Horizontal Aeromagnetic Gradiometer System  – an airborne observation system where each measurement includes the calculation of Gradient between the readings of the wingtip-mounted sensors and combines this information with the gradient between the wingtip sensors and a sensor mounted in the tail stinger of the aircraft. H.A.G.S. gives a measure of the full horizontal gradient (i.e., both in “x” and “y” directions) of the total magnetic field. The full horizontal gradient is very sensitive to short-wavelength magnetic anomalies. ^{[46,  104,  114,  153,  155].} See also Magnetic Gradiometer.

Horizontal Component Filter  – a spectral domain Line Data filter that retains (passes) the horizontal component of the measured magnetic field along the direction of a specified survey line. ^[230]. See also Vertical Component Filter.

Horizontal Cylinder  – one of the basic geometrical shapes used for the model-approximated calculations of the gravity or magnetic effects. H.C. is an infinitely long cylinder of the radius “R” with a horizontal axis buried at a distance “Z” below the Earth’s surface. Subsurface ridges or elongated anticlines generate gravity anomalies similar to those computed on the basis of this approximation. Magnetic effects are similar to those of Magnetic Basement uplifted areas of large extent as well as of magnetic intrusions along the strikes of regional faults. ^{[54,  238].}

Horizontal Derivative  – a rate of a lateral change of the potential field calculated in “x” or “y” directions. The magnitude of the resultant of these two derivatives gives Horizontal Gradient of data. ^{[25,  156].} See also Vertical Derivative.

Horizontal Geomagnetic Intensity  – a magnitude of the horizontal component of Geomagnetic Field Vector at the point of measurement. H.G.I. can be defined as:

H = (B_x² + B_y²)^½

            where “B_x” and “B_y” are orthogonal components of the geomagnetic field vector in directions “x” and “y” respectively. ^[25]. See also Total Geomagnetic Intensity and Vertical Geomagnetic Intensity.

Horizontal Gradient  – the absolute value of Horizontal Derivative. In the general form H.G. magnitude can be presented as

H.G. = [(dM/dx)² + (dM/dy)²]^1/2,

            where M = M(x,y) is the potential field anomaly. H.G. is often calculated as a difference between adjacent grid values of the potential field. But this method generally enhances noise in data and, therefore, is not suitable for the subsequent calculation of the higher order horizontal derivatives. More robust calculation of H.G. can be made using a least-squares method, which fits a polynomial surface to the potential field data points in a particular window. H.G. value in any direction, assigned to the center of the window, can then be found from the coefficients of the fitting polynomial. H.G. reaches maximum wherever a strong slope (i.e., decrease or increase in data values) on the original gravity or magnetic data is present. This maximum will be located over a position where density or susceptibility of rocks are laterally changing the most quickly, such as over vertical contacts or source body edges. H.G. equals zero above Thin Dike. H.G. maxima will be displaced down-dip from tops of contacts in case of their non-vertical occurrence. Based on this phenomena, the spatial relationship between the horizontal and total gradient maxima can be used to infer magnetic or gravity contact location and Dip (see Gradient Dip Estimation). Analysis of the H.G. maps is known to be efficient for delineating both density and magnetization discontinuities in the exploration area. This analysis is often referred to as Boundary Analysis. ^{[60, 98, 103, 104, 112, 142, 153, 189, 199, 238, 241].} See also Horizontal Gradient Vector (HGV) Method and Magnetic Horizontal Gradient Intensity.

Horizontal Gradient Intensity (HGI)  – see Magnetic Horizontal Gradient Intensity.

Horizontal Gradient Magnetic Anomaly  – a general term for maps (grids) showing calculated or measured Horizontal Gradient of the magnetic field, as a rule, after applying Reduction-To-Pole (RTP). Large horizontal gradient variations indicate the offsetting fault block boundaries and contacts between Basement blocks of contrasting Susceptibility values (boundaires between blocks of differing composition and/or lithology). Hence, the correlation of H.G.M.A. maxima can be useful for locating structural and compositional discontinuities. Low-pass filtered versions of H.G.M.A. are often used for delineation of Magnetic Terrane boundaries. See also Vertical Derivative Magnetic Anomaly, Boundary Analysis, and Contact.

Horizontal Gradient Vector (HGV) Method  – an interpretation method based on obtaining HGV maps where Horizontal Gradient of the magnetic or gravity field is represented as an arrow whose orientation shows the direction of a gradient while the arrow length is proportional to the magnitude of a gradient. Arrows originate at grid nodes and point away from local maxima in the potential field. ^{[60,  150].} See Horizontal Gradient.

Horizontal Intensity  – see Horizontal Gradient.

HRAG  – see High Resolution AeroGravity.

HRAM  – see High Resolution Aeromagnetic Survey.

Hydrocarbon  – any organic or non-organic compound (gaseous, liquid or solid), consisting of carbon and hydrogen. Crude oil and natural gas are complex mixtures of hydrocarbons with admixtures of other chemical components. ^[13].

Hydrocarbon Seepage  – a migration of hydrocarbons within the sedimentary section along structural discontinuities, such as faults and fractures. It is assumed that H.S. may result in secondary magnetic effects, which (at least theoretically) are detectable by high-resolution magnetic surveys. ^{[6, 55, 61, 62, 110, 138, 151, 208].} See also Chimney and Diagenetic Magnetic Anomaly.

Hysteresis Loop  – a magnetization cycle graph which describes the relation between Magnetic Induction “B” and the magnetizing field “H” in ferromagnetic materials. H.L. clearly demonstrates the effect of Remanent Magnetization: when applied magnetizing field “H” declines to zero, the value of “B” is non-zero and the residual (remanent) magnetization is retained. ^{[33, 223].} See also Magnetization and Induced Magnetization.