B

Banded Contour Map  – a residual magnetic map where color (or gray-scale) bands are used instead of standard contour lines, and each color band represents the same magnitude values. The width of a contour band equals a preselected interval between mapped values. B.C.M. can be very helpful in delineation of faults and determining their upthrown/downthrown sides by the criterion of an abrupt color bandwidth change. [75]. See also Interruption Zones and Basement Fault Block Pattern.

Band-Pass Filter  – a spectral domain filter that retains (passes) a pre-selected range of wavelengths or wavenumbers (spatial frequencies). Wavelengths between two specified Cutoff values will be retained. [99, 201, 223]. See Band-Pass Filtering.

Band-Pass Filtering  – a procedure that enhances a pre-selected range of the potential field components, regional or residual, based on their wavelengths or wavenumbers (spatial frequencies). The wavelength of anomaly is partially dependent on the depth to the source and, hence, B.F. can be used to separate qualitatively the deeper sources from the shallower ones. [201, 223]. See also Energy Leakage.

Bandwidth  – a range of spatial frequencies or wavelengths contained in a profile or grid of data; also the range of spatial frequencies over which a given filter is designed to operate in the pass or reject mode, or the range of frequencies containing significant energy for a given anomaly. [223, 257]. See Bandpass Filter and Wavelength.

Barometric Altimeter  – an instrument to measure and record the elevation above sea level with common accuracy of about 0.3 m (1.0 ft). Corrections must be made for temporal and spatial variations caused by weather. Temperature and humidity corrections are also needed for precise ground elevations. [57].

Bartlett Filter  – an edge-smoothing space domain grid and line filter which modifies the original data values (line curves or grid surface) within Rolloff Window to ensure their smooth transition to zero at the ends of survey lines or edges of a grid. B.F. is also referred to as Triangular Filter. [201].

Base Station  – the reference magnetic or gravity observation station. In airborne magnetic surveys, it is used to monitor and record the Diurnal Variations (which should be removed from the observed magnetic data) and magnetic storms as well as provide GPS correction data. B.S. should be set up in the magnetically quiet area, away from power lines, pipelines, roads or other sources of noise. In gravity surveys, B.S. is used as a reference point to anchor geodetic survey data as well as to determine an instrumental Drift. The local gravity base stations are tied to the national gravity base stations with a known Absolute Gravity value to allow the merging of different survey datasets as well as correct application of Latitude Correction. See also International Standardized Gravity Network, Ground Magnetometer and Differential GPS and Magnetic Storm.

Base Station Magnetometer  – see Ground Magnetometer.

Basement  – a term used primarily in oil and gas exploration to refer to the crystalline rocks beneath the sedimentary section. Usually this term is synonymous with Crystalline Basement. However the term Economic Basement is used when metamorphic rocks exist which are of high enough grade that no hydrocarbons are expected within them. This may be different from Magnetic Basement, which is the top of the first continuously magnetic layer below the sedimentary section. As a rule, B. is composed of igneous and metamorphic rocks. In many regions, B. is of the Precambrian age but it also may be younger. Magnetic susceptibility of B. rocks is much higher than that of the sedimentary section, up to 100-1000 times and even more. In addition, heterogeneity of B. yields very high susceptibility contrasts. Because of all that, for magnetic exploration B. is the dominant regional subsurface structure that contributes to the total intensity of the magnetic field after applying IGRF Correction. [13, 18, 75, 76, 107, 210].

Basement Fault Block Pattern  – an assemblage of Crystalline Basement blocks of varying shapes, sizes and often depths, separated by alignments of weakness zones, i.e., intra-basement faults. B.F.B.P. may exert a profound control on the structure and stratigraphy of the sedimentary section, both during and after the deposition of sediments. B.F.B.P. also controls the topography (i.e., surface relief) of the Crystalline Basement which, in turn, controls additionally the structure and stratigraphy of the sedimentary section through the mechanism of gravitational compaction. [74, 75, 76]. See also Banded Contour Map and Basement.

Basement Relief  – a range of the basement surface depths in the survey area. The rough basement has a “high relief” and the flat basement has a “low relief”. See also Anomaly Relief.

Bay  – a transient magnetic disturbance with the period of about an hour.  It originates outside the Earth and has the appearance of  “a bay along the sea coast” on the geomagnetic field curve recorded at Base Station or other stationary point of measurements. [223].  

BHGM  – 1) borehole gravimeter model; 2) borehole gravity measurements using a remote-controlled gravimeter lowered into cased or uncased wells. The maximum achievable amplitude and wavelength resolution of BHGM is estimated as 0.002–0.005 mGal and 7–12 m. [36, 123]. See Borehole Gravity, Amplitude Resolution, and Wavelength Resolution.

BHGM Density  – a rock density directly measured in a borehole (i.e., apparent Density) using a remote-controlled gravimeter lowered into a cased or uncased well. See BHGM.

Bi-Cubic Spline Method  – a grid resampling method which involves two steps: 1) using the  spline curves along the rows of the original grid to calculate interpolated values that will correspond to the columns in a new (resampled) grid; and 2) using the column values obtained in the first step to calculate a spline curve along each resampled column. B.-C.S.M. is very effective for resampling datasets with relatively small changes of values between adjacent grid cells and for smoothing noisy grids. See Resampling.

Bi-Directional Gridding  – a gridding method that, at first, interpolates the data along the flight lines and then re-interpolates across the flight lines to create a rectangular Grid. Generally, B.G. enhances anomalous trends that are orthogonal to the direction of the flight lines. [126]. See Gridding and Minimum Curvature.

Big G  – see Universal Gravitational Constant.

Bipole Map  – a map that represents the line-by-line image of the traverse line data as the sequence of bar-graphs with their heights proportional to the amplitudes of horizontal derivatives computed along each traverse line. The polarity of bar graphs is depicted with a color: for example, red for positive values, blue for negative values. Each bar graph is plotted at the location of the computed value of a horizontal derivative. A wavelength-dependent Automatic Gain Control (AGC) filter is applied to control the dynamic range of horizontal derivatives so that both small- and large-amplitude anomalies could be displayed. Before applying a horizontal derivative operator, the traverse line data are filtered with a low-pass filter in order to suppress high-frequency noise. B.M. of the 1-st, 2-d, 3-d and 4-th horizontal derivatives can also be calculated. [163]. See Shaded Stacked Profiles.

Bird  – a streamlined cylindrical housing (suspended from an aircraft by a cable) where the sensors are mounted in some airborne surveys. To eliminate magnetic effects of aircraft, B. is towered by cable at a distance of 50-100 m below and behind the aircraft. Fixed Wing Survey is commonly flown with sensors mounted in the aircraft tail stinger and (Gradiometry option) wings, while helicopters are more likely to tow B. [223, 238].  See also Fixed Wing Survey and Helicopter Survey.

Blackman Filter  – an edge smoothing spectral domain filter that smoothes the grid data values at the edges of a grid to ensure their smooth transition to zero. B.F. is an alternative to Hanning Filter, Hamming Filter and Bartlett Filter. [201]. See Edge Smoothing Filters.

Blakely Test  – an interactive calculation of the maximum values of the gridded magnetic or gravity data using Blakely-Simpson Method. See also Gridding.

Blakely-Simpson Method  – an automated polynomial fitting method of locating maxima on contoured maps of the gridded gravity or magnetic data. The calculating algorithm compares each grid intersection with its eight nearest neighbors in four directions (along the row, column and both diagonals) to determine the presence of a maximum. This comparison is tested with the system of inequalities. For each satisfied inequality requirement, the horizontal location and magnitude of the particular maximum are found by interpolating a second-order polynomial through the trio of grid intersection points. B.-S.M. requires no assumptions about the sources except the direction of magnetization in the magnetic case. At the horizontal gradient maps, abrupt source edges and near-vertical contacts are more precisely delineated than non-vertical contacts. Along with other methods, B.-S.M. is used in Boundary Analysis. [26].

Borehole Gravimeter  – a remote reading Gravimeter which can be lowered into a cased or uncased well.  The difference between the gravity readings at two different true vertical depths gives Apparent Density value (See Borehole Gravity). The density computed is a bulk density value with its sourve coming mostly between the depth interval measured.  Depending on the size of this interval, B. G. can detect effects of sources located tens to hundreds of feet/meters away from the well. [36, 223].  See also Airborne Gravity Meter  and Shipboard Gravimeter.

Borehole Gravity  – a method and instrumentation for gravity observations (measurements) with the gravimeter being lowered into a cased or uncased well with the purpose of direct rock density measurements. Two gravity measurements are needed to calculate the density of the interval between the measurement points:

p = 3.686 – 128.5   )g / )h1 = 3.686 – 39.18 )g /)h2 ;

            where “p” is Apparent Density; in g / cm3; “)g” is the gravity difference in mGal;“)h1” is the depth difference in meters; “)h2” is the depth difference in feet. Precise measurement of the vertical distance between stations is necessary to compute accurate densities over small intervals. Generally, borehole conditions have little effect on B.G. data. Maximum wellbore deviations should not exceed 15º. [211, 223]. See also BHGM.

Borehole Magnetics  – a method and instrumentation to provide measurements of the magnetic properties of rocks in boreholes. B.M. acquisition system usually incorporates both Susceptibility and Total Magnetic Field measurements by using three orthogonally oriented fluxgate magnetometers. See Fluxgate Magnetometer, Magnetic Stratigraphy, and Magnetic Susceptibility Logging.

Bottom Gravity  – a method and instrumentation to collect and process measurements of the Earth’s gravity field with Gravimeter being lowered on the sea bottom.  A shipborne-type gravimeter is modified to fit inside a pressure case mounted on a self-balancing platform.  B. G. provides high resolution of sourve bodies and detection of subtle anomalies which often cannot be detected by conventional sea surface measurements.  See also TOWDOG.

Bott-Smith Method  – a method of estimating a depth “H” to the source of the gravity anomaly:

H = 0.86 g-max/hoz-max,

            where “g-max” is the maximum gravity value (i.e., peak of anomaly) and “hoz-max” is the maximum value of Horizontal Derivative. Coefficient 0.86 is applied for 3-D source bodies, and 0.65 is applied for 2-D bodies. B.-S.M. is also referred to as Smith Rule or Maximum Depth Rule. [27]. See Depth Rules.

Bouguer Anomaly  – a) a local anomaly observed in Bouguer Gravity. B.A. represents the sum of gravity responses of all bodies that have densities deviating from the accepted correction model; b) more often, the value obtained after the known values of the gravity field at that location are removed, i.e. “B. A. = observed gravity-corrected Earth’s model”.   These known values (Earth’s model) include:  1) latitude effect, i.e. Latitude Correction; 2) elevation effect which includes both free-air and Bouguer effects, i.e. Free-Air Correction and Bouguer Correction; and, usually, 3) terrain effects, i.e. Terrain Correction.  The resulting B. A. gravity field is not the same as the gravity field which would have been observed at the datum elevation, because the shape of anomalies due to remaining density irregularities still are appropriate to the elevation of the point of measurement rather than to those of the datum elevation. [32,  34 ,  215, 223].

Bouguer Correction  – a correction which is applied to the gravity data to eliminate the gravitational effect of the subsurface mass between the point of measurement  (i.e. Station) and datum (usually, the sea level). B.C. formula can be presented as

B.C. = – 0.04192  Dh1 = – 0.01278  Dh2    

            where “D” is the assumed average rock density between the station and datum elevations or, in the case of stations below the datum elevation, the assumed density of rock that is missing between the station and datum in g/cm3; “h1” is the elevation above sea level (or datum) or thickness of Bouguer Slab in meters; “h2” is of the same meaning as “h1” but in feet.  For land and airborne surveys, B.C. is always subtracted from the observed data. Two main assumptions are made here: 1) the elevation difference between points of measurements and Datum can be “filled-in” with a simple infinite slab (for this reason, B.C. sometimes is referred to as Slab Correction); 2) the “fill-in” has a reasonable density distribution. If the estimate of the correction density is wrong, it will result in either undercorrecting or overcorrecting of data and, thus, accentuate the topography. If proper average densities for rocks above sea level were used, there should be little or no correlation between Bouguer Gravity and topography. There are two B.C. types: a) Simple Bouguer Correction which involves making a slab correction based on the station elevation; b) Complete Bouguer Correction which involves making Terrain Correction as well as the slab correction. In marine (shipborne) gravity surveys, B.C. replaces the sea water with rock, and “D” in the preceeding formula is the difference in specific gravity of this Replacement Rock and that of the sea water and it is added to the observed data. [34, 54, 134, 137, 173, 215, 223, 234, 238]. See also Bullard B Correction,  Free-Air Correction and Nettleton Test.

Bouguer Density  – a density value for the infinite slab of a finite thickness (Bouguer Slab) that is used for the calculation of Bouguer Correction. Often, B.D. equals 2.67 g/cm3. [19].

Bouguer Flat Plate  – see Bouguer Slab.

Bouguer Gravity  – the gravity field obtained after Free-Air Correction, Bouguer Correction, Theoretical Gravity Correction, and, usually,  Terrain Correction have been applied to the observed (measured) gravity data. B.G. is often referred to as Simple Bouguer Field for the gravity field before applying the terrain correction and Complete Bouguer Field for the gravity field after applying the terrain correction. In concept, B.G. is the residual gravity field that is left after removal of all possible components of the Earth model to represent the effects of density inhomogeneities due to the local geology. In practice, gravity anomalies observed in B.G. are caused by lateral density contrasts within the sedimentary section and Basement (i.e., within Crust) and sub-crust of the Earth. If proper average densities for rocks above sea level were used in Bouguer Correction, there should be minimal correlation between B.G. and topography. Sometimes, such correlation is expected and unavoidable when density changes quickly. For land surveys above sea level, B.G. formula can be presented as

B.G. = “observed data”  + “free-air correction” –

– “Bouguer correction” + “terrain correction” – “theoretical gravity correction.”

[25, 34, 134, 173, 215]. See also Bouguer Anomaly and Bouguer Slab.

Bouguer Gravity Field  – see Bouguer Gravity.

Bouguer Plate  – see Bouguer Slab.

Bouguer Reduction  – see Bouguer Correction

Bouguer Slab  – an infinite length slab of a finite thickness and assumed density (often, 2.67 g/cm3) between the point of measurement and Datum (usually, the sea level). B.S. is used in the calculation of Bouguer Correction. B.S. is also referred to as Bouguer Plate. [54, 223].

Bouguer Spherical Cap  – the Earth’s segment that includes all rocks above Datum (usually, the sea level) between the observation station and the arc distance along the curved Earth’s surface out to 166.735 km. B.S.C. has the height “h” (observation station elevation) which is the same as the thickness of Bouguer Slab. The gravity effect of B.S.C. at its apex is

B.S.C. = 2pGrh,

            where “G” is Universal Gravitational Constant; “h” is the station elevation; “r” is the assumed rock density (usually, 2.67 g/cm3). The gravity effect of B.S.C. can also be expressed by the formula

B.S.C. = A + B,

            where “A” is Flat-Plate Bouguer Factor; “B” is Bullard B Correction. [137, 234].

Boundary Analysis  – a methodology that delineates and maps structural discontinuities associated with Density and Magnetization boundaries. Originally, B.A. was based mainly on Blakely-Simpson Method in application for horizontal gradient maps. At present, the concepts of Analytic Signal Derivative and Enhanced Analytic Signal as well as other techniques can also be used for the same purposes. [26, 52, 116].

Box Filter  – a processing algorithm which calculates values for the interpolated cells in Grid using local averaging. B.F. tends to reduce a magnitude and size of grid anomalies.

Boxcar Filter  – a theoretical approximation of the filter response curve presented by a rectangular window function which retains (passes) the range of data components within the window length and rejects all data components beyond the length of this window. [223]. See also Filter Cutoff and Filter Order.

Bulk Density  – the weight of a rock material divided by its volume including the volume of a pore space. [33, 63]. See also Density.

Bull’s Eye Effect  – a result of Undersampling in the direction of a strike of the short-wavelength anomaly. B.E.E. appears in the form of a trend of small separated circular or elongated contours (“bull’s eyes”) on Contour Map or Color-Scaled Map. The source of this effect is the inability of gridding algorithms to interpolate data perfectly when Traverse Line spacing is longer than an extent of a short-wavelength anomaly. For example, the surface interpolated by Minimum Curvature algorithm in the absence of data point constrains is a circle in two dimensions. In Bi-directional Gridding anomalies perpendicular to traverse lines are usually well represented, while those at acute angles are also mapped as bull’s eyes. Sometimes, B.E.E. is referred to as Pill Box Effect. [126].

Bullard B Correction  – an adjustment to Bouguer Correction which accounts for the curvature of the Earth’s surface. It is recommended that B.B.C. be applied to the data when Terrain Correction is calculated for radii greater than 100 km from the observation station. B.B.C. can also be defined as the difference between the gravity effects of Bouguer Spherical Cap and equally thick infinite flat plate (Bouguer Slab). [135, 136, 234].

Butterworth Filter  – a spectral domain filter which retains (passes) one end of the data spectrum according to the specified cutoff (central) frequency or wavenumber. B.F. is characterized by a very flat amplitude response within the passband. There are low-pass and high-pass options available. The transition between the passed and rejected spectrum portions can be made steeper or smoother depending on the pre-selected degree of B.F. (the higher degree—the steeper transition and vice versa). B.F. is usually applied in Microleveling. Mathematically, B.F. can be presented as

F(k) = 1 / [1 + (k / kc ) n ],

            where “kc” is Central Wavenumber; “n” is a degree (order) of the Butterworth filter function. [67, 124, 201, 223]. See also Stabilized Downward Continuation.