Fabric
– a spatial
configuration of the dominant alignments or trends on the image of the gravity
or magnetic anomalous fields. This term is often used to compare anomalous
fields in adjacent terranes and tectonic provinces. Grain and Signature
have similar meaning. See Tectonic
Province, Terraine,
and Trend.
False Easting
– a
constant longitudinal distance of 500 000 m, assigned to the central meridian in
the UTM coordinate system to ensure positive values of “x”
coordinate within the given UTM
zone. See also False Northing and Universal
Transverse Mercator (UTM).
False Northing
– a constant
latitudinal distance of 10 000 000 m, assigned to the Equator in the UTM coordinate system in the Southern Hemisphere to ensure
positive values of “y” coordinate within the given UTM zone. In the Northern Hemisphere, F.N. value is zero. See also False Easting
and Universal Transverse Mercator (UTM).
Fast Fourier Transform (FFT)
– a
computer algorithm which performs high-speed Fourier
Transform. [124,
201, 223].
Fault Block
– an
intra-sedimentary or intra-basement structural unit bounded by faults, either
completely or in part. [13].
See also Faults.
Fault Zone
– a zone
of fracturing or microfracturing which surrounds the fault plane. F.Z. may vary in width from centimeters to several hundreds of
meters. [13].
See Faults and
Magnetized Intra-sedimentary Fault.
Faulted Slab
– one of
the basic geometric shapes used for the model calculation of gravity or magnetic
effects. F.S.
is a horizontal slab of the uniform thickness terminated in a vertical
plane along one side. The calculated effects are similar to those of faulted
horizontal beds having anomalous density or susceptibility values. [54,
238].
See Gravity Model
and Magnetic Model.
Faults
– joints and
fractures or surfaces which limit different blocks of strata in the sedimentary
section and blocks of metamorphic or igneous rocks in the basement that have
moved (or have not moved) relative to each other. F. play a key
role in the crustal fluid migration balance. They can be an effective barrier to
migration of water and hydrocarbons and can seal a hydrocarbon accumulation or
divide it into separate pools preventing hydrolic communication. Alternatively, F.
as zones of a finite width may be much more permeable than surrounding rocks
and, hence, F. may drain away hydrocarbons from a source rock and empty a reservoir.
As a rule, F.
focus macroseepage: surface hydrocarbon indicators are commonly located on
outcropping F. and water springs often are aligned on F. Similarly,
ore deposits are sometimes located along major F. As a result
of fluid migration, intra-sedimentary F. may be
magnetized sufficiently to produce low-intensity short-wavelength magnetic
anomalies. Such anomalies can be resolved and enhanced using HRAM methods. F.
create weak zones within the basement and sedimentary section which are natural
conduits for Intrusion of strongly magnetic igneous rocks. Such Intrusive
Rocks or Magma
infill of F. produce high-intensity short-wavelength and mid-wavelength
magnetic anomalies which are easily detectable and identified using various
processing methods. In magnetic processing and interpretation F.
are often approximated by Thin Dike or Magnetic
Contact models. [13,
81,
162,
203,
221].
See also Fault Block,
Fault Zone and
Magnetized
Intra-sedimentary Fault.
Faye Anomaly
–
see Free Air Anomaly.
Faye Correction
– see Free-air
Correction.
Ferrimagnetics
–
minerals in which magnetic domains are divided into sub-regions that originally
may be aligned in opposite to each other, but their net Magnetic Moment
is non-zero in the absence of the external magnetic field. Magnetite, titanium,
titanomagnetite, iron oxides, pyrrhotite, etc.—the great majority of magnetic
minerals are F.
[33, 238]. See also Ferromagnetics.
Ferromagnetics
– a small
group of elements (iron, cobalt and nickel) in which the atomic magnetic moments
tend to align parallel to one another because of the exchange interaction
energy. F. are easily susceptible to the external magnetic field and have very
large positive values of Susceptibility,
sometimes, up to 106 times larger than those of the usual rock
materials. [33, 238]. See also Ferrimagnetics.
Ferry
– an
airborne survey term that defines the distance from the nearest airport
facilities.
FFT
– see Fast
Fourier Transform.
Fiducials
–
reference points located at the equal spacing along the survey lines. Often
these are equivalent to time in tenths of second.
Figure–Of–Merit (FOM)
– a
characteristic value derived from an airborne performance test flown in order to
estimate the effectiveness of applied compensation of the magnetometer sensors
for the static and dynamic components of the aircraft magnetic field. This test
is usually flown at relatively high altitude in the survey area or over the area
of a known low gradient field. The aircraft flies in each of the cardinal
compass directions (east, west, north, south) and performs a specified Pitch, Roll
and Yaw maneuver. Maximum deviation of the compensated response of
magnetic sensors from the mean is measured for each of these 12 maneuvers. FOM is the sum (not an average) of all measured magnetic
deviations resulting from all 12 maneuvers. Generally, FOM level of 1-2 nT is appropriate for the modern survey
aircraft. Using on-board computer-based compensation systems, FOM of much less than 1 nT can be achieved. FOM is flown at the start of a survey and after every major
change of an aircraft equipment. [57].
See also On-Site Magnetometer
Calibrations and Real-Time
Magnetic Compensation System.
Filter
– a
spectral domain grid-based operator or space domain line-based operator which
modifies the amplitude and phase of a specified range of frequencies (wavenumbers)
from Power Spectrum of a grid in the spectral domain or an interval along a
pre-selected direction in the space domain. There is a great variety of filters
(more than 50 different types are described in this book) designed with the
purpose of enhancing certain data components which are thought to be of
exploration interest, suppressing Noise and other unwanted components as well as improving the
general presentation and processing properties of the observed data. See also Filtering.
Filter Cutoff
– a filter
parameter which defines the half-power (30% amplitude) point of the filter
response in terms of Wavelength
or Spatial Frequency (as reciprocal of wavelength). For example, a high-pass filter with 1200
m cutoff wavelength value will retain (pass) all wavelengths smaller than 1200 m
(i.e., components which have higher spatial frequency values than the cutoff
frequency) and reject all wavelengths larger than 1200 m (i.e., components which
have lower spatial frequency values than the cutoff frequency). In accordance
with the general theory of filtering, there is a transition zone between the
retained and rejected wavelength ranges and F.C. defines the value point at which corresponding data
components will be attenuated by a half of their maximum value. The length of
this transition zone is determined by Filter
Order. See also Rolloff
Range.
Filter Order
– a filter
parameter which defines the filter falloff rate, i.e., the steepness of the
filter response curve: the higher F.O.,
the faster the falloff. Higher orders of filter will approach the response of Boxcar Filter.
At the same time, higher order filters may introduce strong Ringing into the filtered data. See Filter Cutoff
and Rolloff Range.
Filter Size
– a
parameter that defines the total extent of Band-Pass
Filter in “x” and
“y” directions. For high-resolution application in terms of Wavelength
Filtering, the optimally minimum F.S. to avoid
strong Ringing and
other side effects should be about 2.0 times the short-wavelength Cutoff
value. F.S.
can be expressed in meters, wavenumbers or grid units (i.e.,
cell size numbers). For example, if short-wavelength cutoff is 800 m, then the
long-wavelength cutoff should be no less than 1600 m. [257].
Filtering
– a space
or spectral domain procedure used to separate anomalies by their wavelengths as
well as enhance high-frequency (short-wavelength) residual components of the
observed potential field by attenuating the dominant regional components and
suppressing regular and random noise. F.
is also used to resolve the interfering anomalies generated by closely spaced
magnetic/gravity sources. Residual-Regional
Anomaly Separation by F.
is based on the assumption that a given geologic
source’s spectral power, presented in Radial
Power Spectrum, is
attenuated more rapidly at high spatial frequencies (short wavelengths) than low
spatial frequencies (long wavelengths) as the source depth increases. [24,
223].
See also Energy Leakage and Filter.
Final Bouguer Gravity
– a gravity
field data obtained after applying Latitude
Correction, Free-Air
Correction. Bouguer
Correction, Inner
Zone Terrain Correction
and Outer Zone Terrain Correction
to the land gravity survey measurements. In order to make all data values
positive, a constant value (100 mGals or more) is usually added to corrected
data. F.B.G. can be calculated for several different assumed rock (Bouguer
Slab) densities. See Bouguer
Density.
Finite-Impulse-Response
Reduction-To-Pole Filter
– see FIR
RTP Filter.
FIR RTP Filter
– a
finite-impulse-response reduction-to-the-pole filter. This filter is designed
through modifying both the gridded magnetic data and conventional RTP
Filter as follows: a) RTP filter grid operator with initial
dimension “k H
l” is windowed to the smaller dimension “m H
n”; b) gridded magnetic data, assumed to have the same initial dimension “k H
l”, are padded with zero values to generate an extended grid with dimension
equal to the summation of dimensions of the initial data grid and the windowed
filter grid, i.e., “(k + m) H
(l + n)”; c) the windowed filter grid is also padded with zeroes to generate
an extended grid of the same dimension “(k + m) H
(l + n).” Both extended data and filter grids are Fourier transformed and
multiplied. The result is inverse Fourier transformed to produce the
reduced-to-the-pole magnetic map and then trimmed back to the initial data
extent. Above described filter design procedure prevents Circular
Convolution and,
hence, eliminates Wraparound Effect. The decreased filter size allows to attenuate both
edge effects of data boundaries and noise effects. [148].
See Reduction-To-Pole (RTP)
and Padding.
First Derivative
– see Gradient.
First Derivative Map
– usually, a map of First
Vertical Derivative (1VD) of the gravity and magnetic field calculated
after all proper corrections have been applied to the observed data.
Being less resolving and much less enhancing Noise than Second Derivative
Map, it tends to enhance mid- and
short-wave length “residual” components and delineate areas of high Vertical
Gradient values (i.e. fast decay of high frequencies)
associated with anomalies of relatively shallow origin.
See also First Horizontal Derivative.
First Horizontal
Derivative (1HD)
– see Horizontal
Derivative.
First Vertical
Derivative (1VD)
– see Vertical
Derivative.
First Vertical Gradient
–
see Vertical
Derivative.
First Vertical Integral
(FVI) – an approximation of the gravity field as
calculated from the reduced-to-pole magnetic field using Poisson’s
Relation, i.e. F.V.I.
represents Pseudogravity field. See
Reduction-To-Pole
(RTP)
Fish
– a
watertight housing where magnetometer sensors are mounted during shipborne
magnetic surveys. To eliminate magnetic effects of a vessel, F. is towed at a distance of about 200-300 m behind the vessel, and it
usually rides at the depth of 15‑20 m below the sea surface.
Fixed Wing Survey
– an
airborne survey with the use of an aircraft. In the case of a magnetic survey,
magnetometer sensors are mounted on the aircraft tail stinger and/or wings. See
also Bird and Helicopter
Survey.
Flat-Plate Bouguer
Factor
– the gravity
effect of Bouguer Slab
that is included in calculating Complete
Bouguer Correction. [234].
Flattening Coefficient
– see Polar
Flattening.
Flight Lines
– see Traverse
Lines and Control Lines.
Fluxgate Magnetometer
– a
magnetometer which measures the axial component of the magnetic field induced on
its coil. Properly oriented with the direction of a measured field,
F.M.
can provide the accuracy of about 0.2-1.0 nT, which is quite satisfactory for
regional studies and basement depth approximations. [57,
223].
See also Proton Precession Magnetometer,
Cesium Magnetometer and Tri-Axial Fluxgate
Magnetometer.
Folding Frequency
– see Nyquist
Frequency.
Folding Wavenumber
– see Nyquist
Wavenumber.
Forward Modeling
– a
three-step procedure: 1) initial model of the ensemble of surfaces and bodies of
contrasting densities or susceptibilities is constructed, based on available
geological and geophysical data; 2) magnetic or gravity field produced by the
initial model is calculated and compared with the observed field; and 3) initial
model parameters are adjusted to improve the fit between the observed and
calculated fields. This procedure is repeated until satisfactory fit is
obtained. [25,
178,
209,
238].
Four-Dimensional (4-D)
Gravity
– see Time
Lapse (4-D) Gravity Survey.
Fourier Amplitude
Spectrum
– see Amplitude
Spectrum.
Fourier Analysis
– a
methodology that maps potential field data as functions of space into functions
of their equivalent spatial frequencies (or wavenumbers). F.A. is based
on the fact that any periodic function can be synthesized by an infinite sum of
weighted sinusoids where the weights of these sinusoids are determined through
the analysis of a given periodic function. [223]. See also Fourier
Transform.
Fourier Domain
– a
frequency domain where the gravity and magnetic gridded data are presented after
Fourier Transform as weighted sums of spatial frequencies generated by
the ensembles of subsurface sources. The definition of F.D. is used with the same conceptual meaning as Frequency
Domain and Spectral
Domain. See also Space
Domain.
Fourier Methods
– various
methods of the potential field data processing that are based on the use of Fast
Fourier Transform (FFT)
algorithms.
Fourier Power Spectrum
– see Radial
Power Spectrum.
Fourier Spectrum
– see Radial
Power Spectrum.
Fourier Transform
– a
mathematical operation that converts the gridded gravity and magnetic data from
their original space domain to the equivalent frequency domain. After
F.T. ,
gravity and magnetic grids (maps) can be analyzed for their wavelength content. F.T.
represent the observed
potential field as the synthesis of elementary sine/cosine waves of different
frequencies, each having its own amplitude and phase. F.T. products such as “amplitude-versus-frequency” graph and
“squared amplitude-versus-frequency” graph are referred to as Amplitude Spectrum
and Power Spectrum respectively. In application to the gridded data, F.T. is also referred to as Two-Dimensional (2-D) Fourier Transform. [25,
44,
124,
201, 221,
223].
See also Fast Fourier Transform,
Discrete Fourier Transform, Inverse Fourier
Transform, Hartley
Transform and Hilbert
Transform.
Fractional Vertical
Derivative
– a
vertical derivative (V.D.) which has a non-integer value of “N” – the order of vertical derivative. For example, the first
V.D. (commonly referred to as Vertical
Gradient) has N = 1, and the second V.D. has N = 2.
Generally, F.V.D. allows one to choose a degree of data enhancement that will
represent a balance between the enhancement of target short-wavelengths and
avoidance of random noise amplifying. It often happens that a good result can be
obtained with the first V.D., but the second V.D. result may be unusable because
of a high-frequency noise being strongly amplified. In such cases, derivative
computations with N
= 1.5 will produce a result with a superior resolution to the first V.D., but
they will not amplify high-frequency random noise as much as the second V.D.
Fractional derivatives do not correspond to obviously measurable physical
parameters, but they can provide additional details that assist visual
(qualitative) interpretation of the potential field data. [94].
See also Vertical Derivative
and Second Vertical Derivative.
Free-Air Anomaly
– the
gravity field anomaly at sea level after applying Free-Air Correction and Latitude Correction to
the observed gravity data. Because of a direct dependence on the elevation, F.A.A.
maps are very reflective of a topographic relief.
Shipborne (marine) gravity measurements are presented and interpreted as F.A.A.
datasets and maps.
F.A.A. computation can be presented as
F.A.A.
= “observed data” + “free-air correction” – “latitude correction”
[25,
34,
54,
245].
See Gravity Corrections.
Free-Air
Correction
– a correction
applied to the observed gravity data to compensate for the difference “Dh”
in elevation of the observation point and the survey Datum
(usually, sea level), i.e., it is a correction for the additional distance
(“free-air” without any rock mass) of separation between the gravimeter and
the Earth’s center of mass. F.A.C. is based on Free-Air
Gravity Gradient
calculation, but in practice it is usually performed using a simplified linear
formula:
F.A.C.
= 0.3086 Dh1 = 0.09406 Dh2
where 0.3086 mGal/m (or 0.09406 mGal/ft) is the value of the normal Free-Air Gravity Gradient; “Dh1”
is in meters above the datum; “Dh2”
is in feet above the datum. If
elevation is above the datum, F.A.C.
is added to the
observed data since “Dg” increases
as separation distance decreases. This correction is also dependent on the
latitude because a) the distance between observation point and the center of the
Earth varies with the shape of the assumed ellipsoid; b) the value of
acceleration due to the gravity attraction is different at each latitude. [25,
34,
182,
223,
238].
See also Latitude Correction,
Bouguer Correction and
Terrain Correction.
Free-Air Gravity
– the gravity
field at sea level after applying Free-Air
Correction and Latitude
Correction, but without correction for the density of a rock
between sea level and the plane of measurements; i.e., without Bouguer
Correction. Usually, F.-A.G.
maps are obtained from the marine gravity survey data using on-board-the-ship
gravimeters. [54,
223, 245].
See also Edge Effect Anomaly and Satellite
Gravity.
Free-Air Gravity Field – see Free-Air Gravity.
Free-Air Gravity
Gradient
– a derivative
of the acceleration “g”
due to the gravity attraction with respect to elevation “h”
of the observation point above the reference ellipsoid (i.e., sea level). F.-A.G.G. at a
given latitude “N ” and elevation “h”
can be presented as
dgh/dh = 0.308768
– 0.000440 sin2N
– 0.0000001442 h (mgal/m).
The most commonly used F-A.G.G. value is dg/dh = 0.3086
mGal/m, which was originally obtained using parameters of the 1930 International
Gravity Formula and proved to be fairly acceptable after publishing the updated
Geodetic Reference System in 1971. [211].
See Free-Air Correction and
Theoretical Gravity Anomaly.
Free-Water
Gradient
– a
correction parameter applied to gravity measurements obtained with the use of
the Towed Deep Ocean Gravimeter (TOWDOG).
F.W.G. is
the free-air gradient (a
= 0.3086 mGal/m (or 0.09406
mGal/ft) modified by a water column:
(w
= (a – 4BGD
,
where “G” is Newtonian
Gravitation Constant
and “D”
is the water density. [256].
See TOWDOG.
Free-Water Gravity
– a
gravity anomaly which is obtained with the use of the Towed Deep Ocean Gravimeter (TOWDOG)
and corrected for the free-water gradient, latitude effect, vertical
acceleration, Ëotvös Effect and solid-earth tides. [256]. See Free-Water Gradient.
Frequency
– a
time-domain wavefield parameter that defines the number of wave cycles per
second. In potential field data processing and interpretation, the space domain
analog of F. is used, and it is referred to as Spatial Frequency or Wavenumber.
[223].
Frequency Analysis
– a
methodology that is based on transforming the potential field data from original
Space Domain into Frequency
Domain using Fourier
Transform. Any of the pass filters (high-, low-, band-pass,
etc.) can then be applied to enhance certain residual components of exploration
interest and suppress others that represent Noise or very
deep-sourced regional components of the potential field. F.A. has the same meaning as Wavelength Analysis, but F.A.
is traditionally used more often in exploration terminology. [174].
See also Filter,
Filtering and Cascaded
Filtering.
Frequency Content
– a
spectral domain characteristic that defines the dominant amplitude components of
the observed and/or processed potential field data. This characteristic is
widely used in both magnetic and gravity references because the space domain
potential field data have their equivalent presentation in the spectral
(frequency) domain through Fourier
Transform and due to
simple relation between Spatial
Frequency and Wavelength: the higher frequency – the shorter wavelength. [223].
Frequency Domain
– a
domain where Spatial Frequency
is an independent variable which defines Dimension
of this domain
transformed from distance values of meters (or km) to the spatial frequency
(wavenumber) values of cycles per meter (or km). The dependent variables here
are the magnitude and phase of each spatial frequency. Definition of F.D. is
used with the same conceptual meaning as Fourier
Domain and Spectral
Domain. See also Space
Domain.
Frequency-Depth Rule
– a
qualitative interpretation rule which is based on the physically plausible
assumption that the higher frequency (shorter wavelength) components of the
observed potential fields originate from the relatively shallower
magnetic/gravity sources and the lower frequency components originate from the
deeper sources. In magnetic interpretation it is also referred to as Continuation
Concept as the
magnetic anomalies become broader (i.e., exhibiting lower frequency content) as
the distance between the source of anomaly and magnetometer sensor increases.
For broad areas covering a wide range of depths, it is possible to separate the
magnetic map into smaller areas of relatively shallow, intermediate and deep
basement depths by identifying the areas of predominantly narrow, intermediate
and broad anomalies. However, it is not only the source depth that determines
the frequency content of anomalies, but the source width as well; often narrower
anomalies result from a narrower source body of the same or even deeper depth. [75,
173].
FTG
– see Full
Tensor Gradient and Gravity
Gradiometry.
FTG Technology
– an
exploration technology that is based on measuring Full
Tensor Gradient (FTG),
i.e., the gradient of the Earth’s gravity vector field in three principal
directions (“X”, “Y”, “Z”). Originally, it was developed for
“Trident” submarines. Super-high Sensitivity
provides opportunities
to map very subtle Density changes
in subsurface to assist in petroleum exploration at the prospect level,
including processing and interpretation of 3-D seismic data in the areas of
complex Salt and subsalt structures. At present, FTGT is used in marine (shipborne) gravity surveys.
Full Tensor Gradient
– a
nine-component Tensor which defines variations of three vector components of
the Gradient (i.e.,
Tx, Ty,Tz) in three directions
“x”, “y” and “z”, providing important directional information absent
in observations of amplitudes of the potential field. The nine F.T.G.
components can be presented as the following tensor matrix:
where Tyx
= Txy; Tzx = Txz; Tzy = Tyz;
Tzz = Txx + Tyy. See Gravity
Gradient Tensor and FTG
Technology.
Full Vector Sun Image
– a
potential field map image obtained with the use of multiple “sun”
illumination azimuths (up to 360º), simultaneously displayed with varying
colors of a varying saturation from dark to light. The resulting cone of colors
allows, for example, subtle changes in dip magnitude to modulate the darkness of
the image while dip direction modulates the color. See also Color
Wheel and Artificial Sun
Illumination.
Fuller Filter
– a
moving average space domain convolution filter with a set of window
coefficients, which detects high-frequency components of the observed data and
retains (passes) or rejects it. The user specifies the window length (in data
points or meters) for the convolution operator. The window length corresponds to
the maximum width of anomalies to be identified as high-frequency components. F.F. is applied to line datasets or gridded data as one of the
options in the process of Microleveling.
[70]. See also Decorrugation.