V

Vacquier Straight Slope Method  – a graphic method used for the isolated magnetic anomaly depth estimation. It is applied to the dike or vertical prism models and based on determination of a straight slope segment on the profile of a magnetic anomaly and its projection to the “x” direction (straight slope distance). Estimation of depth requires the knowledge of a structural index (straight slope index):

“depth” = “straight slope distance” H “straight slope index”

V.S.S.M. is also referred to as Steenland-Vacquier Method. [53, 215]. See also Analog Magnetic Depth Estimation and Straight Slope Index.

Variable Datum Gravity Inversion  – a 3-D gravity Inversion that incorporates Upward Continuation of Bouguer Gravity to an arbitrary level from Mean Sea Level in order to ensure the interpretation of the gravity-detectable structures of exploration interest that are above the mean sea level, especially in the areas where the topographic relief changes drastically. [84]. See also Reference Upward Continued Distance.

Variance  – a statistical accuracy estimate parameter which is the square of Standard Deviation. See Accuracy.

Vector  – a quantity defined by both Magnitude and direction. V. is usually shown as an arrow pointing in a certain direction and the arrow length is proportional to the magnitude of V. [223].

Vector Magnetics  – a methodology based on three-component magnetic field measurements to examine three orthogonal elements of the magnetic field “M”:

F = iMx + jMy + kMz ,

            where “i”, “j” and “k” are unit vectors.

Vening Meinesz Hypothesis  – see Airy Hypothesis.

Vertical Acceleration Correction  – a correction that is applied to the airborne gravity data to compensate for Stabilized Platform vertical acceleration due the aircraft motion. V.A.C. is calculated based on GPS data for three component (x, y, z) aircraft position and velocity data using various processing software. [37]. See also Horizontal Acceleration Correction.

Vertical Component Filter  – a spectral domain line-based filter which retains (passes) the vertical component of the measured magnetic field. [230]. See also Horizontal Component Filter.

Vertical Cylinder  – one of the basic geometrical shapes which is used for the model calculation of the gravity and magnetic effects. V.C. is a cylinder of the radius “R” and height “L” with its top buried at a distance “Z” from the Earth’s surface. This shape is often convenient for computing the gravity and magnetic model anomalies from salt domes, volcanic plugs, and isolated magnetic intrusions. See Gravity Modeling Shapes and Magnetic Modeling Shapes.

Vertical Derivative  – a rate of a space change of the potential field in the vertical (“Z”) direction. V.D. is also referred to as First Vertical Derivative. For magnetic data V.D. usually does not represent an actual survey-measured quantity (except for special vertical gradient surveys) and calculated as a point approximation of the magnetic field’s rate of change in the vertical direction using the Hilbert transform properties of potential fields. V.D. enhances the shallower (short-wavelength) anomalies and attenuates the longer wavelength regional components of the potential field. V.D. has its zero values over the vertical edges of thick source bodies, positive values over positive anomalies, and negative values over negative anomalies. V.D. calculation is generally a spectral domain procedure, and it is often cascaded with conventional low-pass filtering or Upward Continuation in order to suppress high-frequency noise components. [25, 52, 59, 166, 214, 222]. See also Vertical Gradient, Second Vertical Derivative (2VD) and Analytic Signal Derivative.

Vertical Derivative Magnetic Anomaly  – a general term for maps (grids) showing the calculated Vertical Derivative of the magnetic field, as a rule, after applying Reduction-To-Pole (RTP) to the gridded magnetic data. V.D.M.A. emphasizes magnetic effects caused by relatively shallow and local subsurface features, such as Magnetic Basement structures and intra-sedimentary magnetized Faults. The zero contour of V.D.M.A. map outlines the boundaries of offsetting fault blocks and blocks of contrasting Susceptibility values in the upper part of magnetic basement and, hence, this data can be used in studies of the tectonic elements. V.D.M.A. lines of discontinuity often indicate the presence of cross-cutting faults or shear zones. See also Horizontal Gradient Magnetic Anomaly and Grid.

Vertical Derivative Order  – a parameter (“n”) which establishes the degree of enhancement of high-frequency (short-wavelength) components of the potential field data during Vertical Derivative computation: the higher orders of vertical derivatives enhance the high-frequency content to the greater extent than the lower orders of derivatives. For example, Second Vertical Derivative (n = 2) provides much better resolution of data through a greater high-frequency content enhancement than First Vertical Derivative (n = 1). [94]. See also Fractional Vertical Derivative.

Vertical Geomagnetic Intensity  – a magnitude of the vertical component of Geomagnetic Field Vector at the point of measurement. [25]. See also Total Geomagnetic Intensity and Horizontal Geomagnetic Intensity.

Vertical Gradient  – the absolute value of Vertical Derivative. V.G. is known to be efficient in enhancement of relatively shallow short-wavelength potential field anomalies associated, for example, with magnetized faults or density-contrast faults. V.G. has its peak value over the top of the vertical Thin Dike and two peaks over Thick Dike, which are centered at equal distances from the thick dike axis. V.G. maxima can be offset from the tops of thin dikes in case of their non-vertical occurrence. [2, 12, 127, 154, 171, 177, 189, 203]. See also Measured Vertical Gradient, Vertical Derivative and Analytic Signal Derivative.

Vertical Gravity  – a derivative of Gravity Potential in the direction of the vertical axis, i.e., vertical component of the Earth’s gravity field. This is the quantity measured by gravimeters. [25, 238].  See Gravimeter and Gravity Force.

Vertical Integration  – a grid-based Spectral Domain procedure (filter) that calculates the vertical integral of the observed potential field data after applying Fourier Transform. See also Vertical Derivative and First Vertical Integral.

Vertical Magnetic Gradiometry  – direct measurements of Vertical Derivative of the magnetic field using a recording system with two vertically separated magnetometer sensors. V.M.G. has the following benefits: a) minimization of the diurnal effects, particularly, in high magnetic latitudes; b) suppression of the regional gradients of the Earth’s magnetic field; c) enhanced lateral resolution of anomalies generated by closely spaced sources. Areas with low intensity of target magnetic signals may require an increased vertical separation of sensors. With the instrument sensitivity of 1 nT, the vertical separation of sensors about 1.0–1.5 m is considered sufficient for stable measurements. The general formula of V.M.G. can be presented as

dF/dz = (F2 – F1)/Dz,

            where “dF/dz” is the vertical derivative; “F1” and “F2” are sensor readings; “Dz” is a sensor separation distance. [155].

Vertical Separation  – a separation of the potential field anomalies caused by source bodies located at different depths. Without V.S., determination of source parameters (depth, lateral extent, etc) suffers from distant sources superposition that yields mislocations. Commonly, V.S. is performed by Upward Continuation and Filtering. Because of the fundamental ambiguity in relationship between the potential field and its sources, V.S. can produce a reliable qualitative (not quantitative) result on condition that distinctly different depth levels of sources are present. See Matched Filtering and Lateral Separation.

Vertical Sheet  – one of the basic geometrical shapes used for the model calculation of the gravity and magnetic effects. V.S. is equivalent to the vertical set of many horizontal cylinders having their radii much smaller as compared to the depth of their horizontal axes from the Earth’s surface. V.S. (or Thin Dike) is the most common approximation of faults. [238]. See Gravity Modeling Shapes and Magnetic Modeling Shapes.

Video Viewing  – a procedure which is a part of the Cultural Editing process. Line profiles of the acquired aeromagnetic data are visually reviewed in order to detect high-frequency anomalies (spikes) and check them against cultural objects, such as houses, farms, power lines, wellheads, roads, etc. recorded by the on-board video camera during the flight. All identified non-geological anomalies are then removed by the digital editing. [57].

Virtual Spring  – a gravity meter design which is based on the use of Magnetic Levitation. The V.S. design provides direct electronic measurements of Gravity Field by measuring the voltage necessary to maintain the permanent magnet mass in magnetic levitation at the null point. The value of this voltage is proportional to Gravity Acceleration at the point of measurement. For this reason, the instrument based on this concept is called Magnetically Levitated Accelerometer or simply Magnetic Accelerometer. The same basic concept is used in Cryogenic Magnetic Levitation devices operating at near absolute zero temperatures. The physical property of superconductivity is used in the design of the very high-precision (0.0008-0.0015 mGal) Superconducting Gravity Meter, which is now in use at some gravity observatories measuring Absolute Gravity. [36].

Viscous Remanent Magnetization (VRM)  – a residual magnetization produced by a long exposure to an external magnetic field. VRM acquisition rates vary widely according to the mineralogy and grain size of the rock. VRM is an important consideration in paleomagnetic studies, but can be neglected in exploration scale projects. [33, 238]. See also Chemical R.M., Detrital R.M., Isothermal R.M. and Thermal Remanent Magnetization.

Voice  – a discrete portion of a signal derived by Wavelet Transform Filtering. See also Wavelet Transform (WT).

Volcanic Rocks  – finely crystalline and, as a rule, strongly magnetic Igneous Rocks originating from the volcanic action at or near the Earth’s surface and which were ejected explosively or extruded as Lava.  Thick layer of V.R. covering a large area may create Magnetic Basement in the subsurface. [13]. See Extrusive Rocks.