IGF
– see International
Gravity Formula.
IGSN Base Value – a
value of Absolute
Gravity measured at the local Base
Station tied to International Gravity Standarization Net.
Igneous Rocks
– rocks
that solidified in the process of the molten Magma
cooling either within the Earth (Intrusive
Rocks) or after eruption onto the Earth’s surface (Extrusive
Rocks). I.R.
constitute one of three main classes into which rocks are divided, the others
are Metamorphic Rocks and Sedimentary
Rocks. Magnetization
of I.R. is the
highest among all rock types. Estimates of Susceptibility
values give the range of 0.5 – 100.0 with an average of 25 (in units of 103
SI). [13,
33,
238].
Igneous Rocks Density
– the basic
quantity that predetermines the gravity properties of igneous rocks. On average,
I.R.D. values are higher than that of sedimentary rocks with
considerable overlap. Generally, lavas have lower values compared to igneous Intrusive
Rocks. Generalized table of the common I.R.D.
values is shown below (in g/cm3). [33,
238].
Rock Type
Range
Average
andesite
2.40 – 2.80
2.61
basalt
2.70 – 3.30
2.99
diabase
2.50 – 3.20
2.91
diorite
2.72 – 2.99
2.85
gabbro
2.70 – 3.50
3.03
granite
2.50 – 2.81
2.64
lava
2.80 – 3.00
2.90
peridotite
2.78 – 3.37
3.15
porphyry
2.60 – 2.89
2.70
Igneous Rocks Susceptibility
–
the basic quantity that predetermines the magnetic properties of igneous rocks. I.R.S.
values are much higher (sometimes, 50-100 times and more) than those of Metamorphic
Rocks and Sedimentary
Rocks. Generalized table of the common I.R.S.
values is shown below
(in units of 103 SI). [33,
238].
Rock Type
Range
Average
andesite
–
160.0
basalt
0.2 – 175.0
70.0
diabase
1.0 – 160.0
55.0
diorite
0.6 – 120.0
85.0
dolerite
1.0 – 35.0
17.0
gabbro
1.0 –90.0
70.0
granite
0.0 – 50.0
2.5
peridotite
90.0 – 200.0
150.0
porphyry
0.3 – 200.0
60.0
Average of main types 0.25 – 92.0 22.5
S.I.
susceptibility unit =
4p c.g.s. susceptibility unit. See also
Metamorphic Rocks Susceptibility
and Sedimentary Rocks Susceptibility.
IGRF
– see International
Geomagnetic Reference Field.
IGRF Correction
– a time and
space varying correction applied to the observed magnetic data to compensate for
the Earth’s main (core) magnetic field. Along with other magnetic field
corrections, subtracting IGRF component
from the measured magnetic field provides, in principle, the magnetic field of
the crust, i.e., local magnetic field, which is usually of exploration interest.
This correction is calculated using the formula that accounts for sensor’s
latitude, longitude and elevation, as well as year and day (sometimes also
time), of the survey observations. State-of-the-art Leveling
requires its application on the line (point-by-point) basis, i.e., it should be
subtracted from values measured along the survey lines with the correction value
corresponding to the time period of acquisition. IGRF
Correction is considered as a part of the leveling process and
often referred to as GFR Leveling
Correction. [25,
238].
See Total Magnetic Field,
Earth’s Magnetic Field Components,
and Crust.
Image Enhancement
– a general
term describing various processing methods used to highlight subtle trends and
feature areas in images of the gridded potential field data. See Artificial
Sun Illumination and Filtering. [223].
Improved Fourier Terrain Correction
–
a method that uses FFT-based algorithms for calculating the gravitational
attraction of a layer with irregular top surface for application in Terrain
Correction of marine gravity surveys in relatively shallow
waters, as well as in land gravity surveys on observation surfaces with
large-amplitude topographic variations. At each gravity station, the
gravitational attraction is divided into two components: a local contribution
from the rock material within a cylinder (R = 5-6 grid intervals) centered on
the station and a regional contribution from the rock material outside the
cylinder. The local contribution is calculated by direct integrating the gravity
effects of vertical prisms within a cylinder. The regional contribution is
calculated through a series of standard convolutions computed numerically by Fast
Fourier Transform. [179,
180].
Improved Source Parameter Imaging (iSPI™) Method
– an
automated grid-based method of estimating and imaging the location and depth to
the source of a magnetic anomaly using the first-order and second-order
instantaneous (“local”) wavenumbers of Analytic
Signal. The
first-order local wavenumber is defined as:
k1 = d {tan–1[(dM/dz)/(dM/dx)]}/dx,
where M = M(x,z) is
the magnitude of the anomalous magnetic field. The second-order local wavenumber
is defined as:
k2 = d {tan–1[(d2M/d2z)/(d2M/dzdx)}/dx
After some simplification, these local wavenumbers can be presented as:
k1 = (nk +
1)hk
/(h2k +
x2) and k2 = (nk +
2)hk /(h2k
+ x2),
where “nk” is SPI Structural
Index and “hk” is the depth to the top of a model assumed. The first-order
and second-order local wavenumbers are independent of the susceptibility
contrast, the source dip, as well as Inclination,
Declination
and the magnitude of the Earth’s magnetic field. They have an identical
functional form for all three 2-D models accepted in the iSPI™
method (thin sheet, contact, horizontal cylinder) and they are symmetric about X
= 0 taking their maximum value at this position which defines the source
location. The model-independent Local
Wavenumber is
calculated from the difference between the second-order and the first-order
local wavenumbers. The depth estimate (“local depth”) is the inverse of the
peak value of this difference (which is always equal to 1/hk at X = 0) and it is also independent of the accepted model. I.S.P.I.M.
assumes two-dimensional geology and negligible interference from nearby
sources. As all analytic-signal-related computations, this method is very
sensitive to Random Noise
and requires high-quality acquisition, leveling, gridding, and filtering of the
magnetic data. [226].
See also Source Parameter Imaging (SPI).
Impulse Response
– an
output of a convolution operator when the input is a unit impulse. I.R. of the filter operator is the basic characteristic to
describe its performance and resulting changes of the filtered data.
Inclination
– the dip
(i.e., a vertical angle between the vector of geomagnetic field and the
horizontal plane) of Geomagnetic Field:
I = arctan Bz /
(Bx2 + By2)½,
where “Bz”
is the vertical component of the geomagnetic field vector; “Bx”
and “By” are horizontal components of the geomagnetic field vector in
directions “x” and “y” respectively. I.
varies from –90º at the magnetic South Pole to 0º at
the magnetic Equator and +90º at the magnetic North Pole. I.
has the greatest effect on magnetic anomaly appearance: at both magnetic Poles,
a source with a positive magnetic susceptibility contrast will generate a
positive anomaly with small negative lobes (assuming that magnetization is
induced); at the magnetic Equator, the same source, i.e., volume of rock with
identical lateral magnetic susceptibility contrast, will generate a negative
anomaly with small positives lobes. At I.
= –45º in the southern hemisphere, this source will
generate an anomaly with the positive lobe to the north, i.e., toward the
Equator, from the source and the negative lobe to the south, i.e., toward the
South Pole. At I.
= +45º in the northern hemisphere, the same amplitude and wavelength anomaly
will be generated but in the reversed order: the positive lobe is on the south
side of the source toward the Equator, and the negative lobe is on the north
side toward the North Pole. [25,
215,
238].
See also Declination
and Magnetic Meridian.
Inclinometer
– a surveying
instrument used to measure terrain topography in the proximity to the gravity Station (usually in zones “B” and “C” of Hammer
Chart). Gyroscopic or pendulous inclinometers are used for
measuring Pitch
and Roll of the survey aircraft or ship. [223].
See Gyroscope.
Indirect Gravity Effect
– a gravity
reduction phenomena, which arises from the fact that Station elevations usually refer to Geoid (or sea
level), while International Gravity
Formula refers to Reference
Ellipsoid, and the two may not coincide.
Induced Magnetic Anomalies
–
crustal magnetic anomalies created by contrasting lateral concentrations of
iron-rich rocks magnetized by the present geomagnetic field. See Induced
Magnetization.
Induced Magnetization
– a
vector quantity that defines the rock magnetization in the direction of the
present external field (i.e., Earth’s magnetic field). I.M. magnitude
is directly proportional to the strength of that field and the capacity of a
rock substance to be magnetized, i.e., Susceptibility
value. I.M.
amounts to the lining up the originally random distributed magnetic
dipoles within Magnetic Material
of a given rock substance. By this reason, I.M.
is sometimes referred to as Polarization.
[25, 223, 238]. See also Remanent
Magnetization.
Induction – the process by which a magnetizable rock becomes magnetized by the Earth’s magnetic field. [223 ]. See Induced Magnetization and Remanent Magnetization.
In-Field Processing
– a
computer-aided Quality Control
procedure which is applied to the observed potential field data at a survey
location in order to identify and correct acquisition problems as well as check
the acceptance level of diurnal variations (in magnetic survey). See also Post-Flight
Quality Control.
Infinite Dike
–
see Tabular Body.
Infinite Slab
– one of the
basic geometrical shapes which is used for the model calculation of
gravity/magnetic effects. I.S.
is an infinite length horizontal slab of the thickness “L” which is
equivalent to the case of Vertical
Cylinder with its
radius being very large compared with its height. [54, 238].
Infinitely Long
–
a term used in general approximations and Modeling meaning so long that the effects of the ends are
negligible. [223
].
Inflection-Tangent-Intersection Method
– see Naudy
Method.
Initial Gridding
– a
pre-gridding procedure that uses interpolation or extrapolation methods to
calculate values for cells within grid gaps and for the edge regions of a grid.
See Grid Gap.
Inner Terrain Correction
– see Inner Zone
Terrain Correction.
Inner Zone Terrain Correction
–
a correction applied to the gravity data to compensate for the deviation of the
surface topography from flat Bouguer Slab approximation by using a Zone
Chart, where the
outermost radius equals 558 ft and the innermost radius equals 6.56 ft. See
Terrain Correction and Outer
Zone Terrain Correction.
Instantaneous Amplitude
– an amplitude
of Analytic Signal. Often I.A.
is referred to as Analytic Signal
Amplitude and Energy
Envelope. [164,
165,
166,
214,
226,
236,
242].
Instantaneous Frequency
– an
attribute of Analytic Signal.
For 2-D case, I.F.
is defined as the rate
of change of Instantaneous Phase
with respect to “x” direction:
I.F.
= d2(x)
/ dx
If the analytic signal is defined in terms of the horizontal and vertical
derivatives of the total field, I.F.
can be presented as
I.F.
= {dtan–1 [(dM/dz)/(dM/dx)]/dx} / 2B
,
where “M” is the magnitude of the potential field. I.F. is also referred to as Local Frequency.
[226,
236,
242].
See also Instantaneous Frequency Image and
Local Wavenumber.
Instantaneous Frequency Image
– an image of the gridded potential field data obtained from the
calculation of Instantaneous Frequency.
I.F.I. has
a high degree of variation and, hence, can effectively map character changes in
potential field data for areas where the original total field intensity images
have little apparent differences in terms of magnetic or gravity response. [94].
See also Instantaneous Phase
Image.
Instantaneous Phase
– a phase
of Analytic Signal. For 2-D case, I.P.
is defined as:
2
= tan–1 [m
*
(x)/m(x)],
where “m *
(x)” is the
imaginary component of the analytic signal and “m(x)”
is its real part, i.e., magnetic or gravity anomaly. I.P.
values can vary between –180º and +180º. As compared to Energy
Envelope, I.P.
calculations are less stable since they involve a division of real and
imaginary components instead of the sum of their squares. [165,
226,
236,
242].
See also Instantaneous Frequency
and Instantaneous Phase Image.
Instantaneous Phase Image
– an image of the gridded potential field data obtained from the
calculation of Instantaneous Phase.
I.P.I. has
the property of enhancing the continuity of the imaged features regardless of
their amplitude values. In contrast to Automatic
Gain Control
technique, I.P.I.
does not attenuate
small anomalies on the flanks of large anomalies, and it is less susceptible to
the enhancement of a random noise. Sometimes, I.P.I.
can provide resolution superior to the resolution obtained using the vertical
derivative imaging. [94].
See also Instantaneous Frequency Image.
Instrumentation Lag
– an airborne
magnetometer system parameter is determined during Lag
Test by analyzing two sets of data flown in opposite directions
over a known and well-defined magnetic anomaly such as, for example, a large
river bridge. I.L. is
defined as a time difference between location of selected anomaly from the
positional (GPS) data and from the magnetometer data. I.L.
can also be determined as one-half the time shift required to
match the corresponding anomalous responses from data flown in opposite
directions.
Intensity of Magnetization
–
see Magnetization.
Interference of Anomalies
–
a combined effect of sources located nearby or at close depth levels. The
residual potential field anomaly always represents the vector sum of the
interfering anomalies generated by individual sources. The observed potential
field anomaly represents the vector sum of the superposition of anomalies
generated by all subsurface sources. See also Superposition
of Anomalies.
International Geomagnetic Reference Field
(IGRF)
– a
time-varying magnetic field which represents the Earth’s core component (i.e.,
the Earth’s main magnetic field) of the observed magnetic data. IGRF is computed from data by the worldwide magnetic
observatories and orbiting satellite-mounted magnetometers. It is updated every
five years. Generally, after the magnetic survey has been completed, IGRF
is subtracted from the
observed data to obtain the crustal component of the magnetic field (i.e., the
Earth’s local magnetic field). The most precise way to correct for IGRF is to compute its values for each point in the survey, i.e.,
IGRF
(x, y, z, t), and subtract proper values on a point-by-point basis. If the
survey is completed within a short period of time (a few days), then computing IGRF as a grid
may be adequate. The IGRF
value can vary by 1-8 nT/month or more, depending on location. IGRF
is sometimes referred to as Normal
Magnetic Field. [25,
223,
238].
See also Earth’s Magnetic Field
Components and Definitive
Geomagnetic Reference Field (DGRF).
International
Gravity Formula (IGF)
– a
formula which defines the theoretical value of the Earth’s gravity field at
any point on Reference
Spheroid assuming
homogeneous Density
distribution.
The most recently published formula (1998) from the United States
National Imagery and Mapping Agency (NIMA) is based on the WGS84 reference
spheroid:
g =
978032.53359 (1 + 0.00193185265241 sin2
f
) /
(1 – 0.00669437999014 sin2 f
)0.5,
where
“f” is the
latitude in degrees and “g”
is gravity in mGals. The application of this formula requires Athmospheric Gravity Correction
because the WGS84 Earth’s Gravitational
Constant includes the
mass of the atmosphere. I.G.F. accounts for three major phenomena that impact gravity
measurements: 1) the Earth spins at different angular velocities at
different latitudes and, hence, produces different outward accelerations
resulting in gravity readings which differ from those obtained on a non-spinning
Earth; 2) Earth has an ellipsoidal shape (i.e., all points on the surface
are not equally distant from the center of the Earth’s mass); 3) Earth’s
ellipsoidal bulges contain rocks (i.e., uneven mass distribution, due to the
ellipsoidal shape as compared to a sphere, needs to be accounted for). Because
of all these phenomena, the gravity acceleration (i.e., the Earth’s gravity
field) measurement values vary considerably from about 978 000 mGal at the
Equator to about 983 000 mGal at the Poles (the gravitational
attraction/acceleration is the highest at Poles as they are closest to the
Earth’s center of mass). [34].
Older versions of the I.G.F.,
which may have been used in the reduction of
data sets, are:
1930: g = 978049.0
(1 + 0.0052884 sin2 f – 0.0000059 sin2 2f ) mGal
1967: g = 978031.846 (1 + 0.005278895 sin2
f + 0.000023462 sin4 f ) mGal
1987: g = 978032.68 (1 + 0.00193185138639
sin2 f ) / (1-0.00669437999013 sin2 f ) 0.5 mGal
International Gravity
Standarization Net
– the
world-wide network of gravity base stations to provide reference measurements of
the Earth’s gravity field, using the same type gravity meters. [248].
See Base Station.
Interpretive Filters
– a general
definition of various conventional and specialized filters applied to the
gridded or line-oriented data in order to enhance Target anomalies and provide data for subsequent interpretation.
See also Filtering and Gridding.
Interruption Zones
– the
gravity or magnetic field map features defined as imaginary lines along which
the observed or filtered anomalies are interrupted or terminated. I.Z.
are often found to be associated with faults, basement block
contacts and other geological lineaments. I.Z.
are also referred to
as Offset Zones.
Sometimes, I.Z. are associated with wrench fault systems. [6,
75,
257].
Interval Factor
– a gravimeter
measurement range parameter (constant) that is applied to convert gravimeter
readings from counter (dial) units within a pre-selected interval of Operating
Range to milligals. I.F.
is determined by the manufacturer during gravimeter calibration. See also Milligal
Constant and Milligal.
Intra-basement Fault
– see Magnetized
Intra-basement Fault.
Intra-basement Magnetic
Anomalies
–
magnetic signatures of the exploration interest generated by lateral contrasts
in magnetic susceptibilities within Basement.
Such contrasts are created, primarily, by contacts between different basement
blocks and deep intrusions. As a rule, I.M.A.
are represented by low-frequency and mid-frequency (long-wavelength and
mid-wavelength) components of the observed magnetic field. As the basement
becomes relatively shallower, the wavelength of I.M.A.
becomes smaller. See Intrusion, Magnetic Contact,
and Magma.
Intra-basement Magnetic
Anomaly Sources
– a model
approximation of magnetic sources in the upper part of Basement. There are three main types of I.M.A.S.: 1)
thin vertical sheet (sheet thickness is much less than the basement depth) or Thin
Dike; 2) thick vertical sheet (sheet thickness is about the
same as the basement depth) or Thick
Dike; 3) vertical
block with two magnetic contracts/interfaces (block width is more than the
basement depth). [215].
See also Suprabasement Magnetic Anomaly
Sources.
Intra-sedimentary Fault
– see Magnetized
Intra-sedimentary Fault.
Intra-sedimentary
Magnetic Anomalies
–
magnetic signatures of the exploration interest generated by lateral contrasts
in magnetic susceptibilities within the sedimentary section. Such contrasts are
created primarily by magnetite-bearing formations, magnetized faults and/or
localized fractured zones. In general, I.M.A.
are represented by relatively high-frequency (short-wavelength) components of
the observed magnetic field and have magnitudes which can be several orders
smaller than those of the anomalies generated by sources in Basement.
[1, 6, 51, 80, 191]. See Magnetized
Intra-sedimentary Fault.
Intrusion
– a)
emplacement of Igneous Rocks
in the pre-existing subsurface environment of Basement
and/or sedimentary
section as a result of the magmatic activity; b) emplacement of a diapiric
salt plug or other sedimentary material in the pre-existing environment of the
sedimentary section as a result of gravitational compaction and high-pressure
subsurface structural deformations. [13].
See Intrusive Rocks
and Magma.
Intrusive Rocks
– masses
of Igneous Rocks, or quite rarely Sedimentary
Rocks, formed as a
result of Intrusion.
Invariants
–
combinations of components of a measured quantity that are unchanging with
respect to particular directions. I.
of Gradient Vector
can be used to infer the structure of a source of the observed potential field
anomaly. See Full Tensor Gradient.
Inverse Filter
– a
residual-type Wavelength Filter that
is complement to Regional Filter. I.F. is commonly referred to as Residual
Wavelength Filter or High-Pass Filter.
Inverse Fourier
Transform
– a
mathematical operation that converts gridded potential field data from the
frequency (spectral) domain back to their original Space Domain.
See Fourier Transform and Spectral
Domain.
Inverse Hartley
Transform
– a
mathematical operation that converts the line potential field data from the
spectral domain back to their original space domain. See Hartley Transform.
Inverse Modeling
– a
technique that allows to compute 2-D or 3-D density or susceptibility geometric
model of the subsurface geological structure that match the observed gravity or
magnetic field. Because of the fundamental ambiguity of the relationship between
the potential field and its sources, the output model geometry is always a
non-unique solution unless it is constrained by independent geological and/or
geophysical information. I.M.
has the same conceptual meaning as Inversion and
sometimes it is referred to as Inversion
by Forward Modeling. [249]. See also Forward
Modeling.
Inverse Wavelet Transform
–
a mathematical operation that recovers the original signal from its
space-frequency representation and removes Coherent
Noise components which
have been identified during the “forward” wavelet transform. [65,
69].
See also Wavelet Transform.
Inversion
– a
methodology to obtain a Model derived
from the observed data in order to describe the subsurface structure that is
consistent with these data. Principal difficulty with I.
of potential field data is their inherent non-uniqueness (see Gauss
Theorem), which is
also defined as the fundamental ambiguity of the relationship between the
potential field and its sources. The non-uniqueness problem is commonly
addressed by applying inversion algorithms which restrict consideration of
possible solutions to parameters of one model or small set of pre-selected
models which were defined on the basis of available geological and geophysical
information. For example, seismic data can be used to estimate the limits of a
shape, depth and lateral extent of Salt
Dome and adjacent
subsurface structures. Well logs provide information on lithology that is used
for estimation of densities and susceptibilities of rocks. [31,
32,
86,
89,
144,
145, 223].
See Gravity-Velocity
Modeling.
Inversion by Forward Modeling
–
see Inverse Modeling.
Inverted Gravity Signature
–
an image of the gravity field obtained with the use of the Shaded
Relief technique, but,
contrary to the standard visualization, the anomalous gravity peaks are inverted
and shown as “valleys,” while the anomalous gravity troughs are shown as
“ridges.” This visualization trick is considered to be useful for
interpretation of the gravity data on continental margins where the regional
gravity Signature tends (as in the Gulf of Mexico) to inversely reflect the
interface between sedimentary cover and Basement.
[141]. See also Continental
Margin.
Ionosphere
– the outer
atmospheric zone surrounding the Earth at altitudes roughly between 50 km and
1500 km, where the interaction between Solar
Wind and the Earth’s
internal magnetic field, coupled with the Earth’s rotation and tidal effects,
generates electrical currents, which in turn produce magnetic fields, called Magnetic
Storm, with magnitudes of up to 1000 nT. [25].
Isoanomaly
–
a line which connects points of equal values on contour maps showing magnetic or
gravity anomalies. [223
].
Isoclinic Map
– a
contour map of the geomagnetic field that shows lines of the equal Inclination values over the Earth’s surface. See also Isodynamic
Map and Isogonic Map.
Isodynamic Map
– a
contour map that shows lines of the equal values of either the magnitude of the
Earth’s magnetic field, or its vertical or horizontal components respectively.
See also Isogonic Map
and Isoclinic Map. [25].
Isogal Map
– a
contour map of the gravity survey data which shows lines of equal values of the
measured or processed gravity anomaly, usually in milliGals (mGal). For example,
Bouguer anomaly contour map. See Gal.
Isogam Map
– an
older term describing a contour map of the magnetic survey data which shows
lines of equal values of the total intensity of the magnetic field in gammas.
See Gamma.
Isogonic Map
– a
contour map of the geomagnetic field that shows lines of the equal Declination values over the Earth’s surface. See also Isoclinic
Map and Isodynamic Map.
[25].
Isomagnetic Charts
– lines of
equal values of Inclination,
Declination,
magnitude, horizontal/vertical intensity, and other Earth’s magnetic field
parameters. When plotted on maps, I.C.
show the variations in Geomagnetic
Field over the
Earth’s surface. [54,
238].
Isomagnetic Maps
– contour maps
representing various elements of Geomagnetic
Field, such as
contours of the equal field intensity (total intensity, vertical intensity, or
horizontal intensity) or contours of equal Inclination
and Declination.
[25]. See Total
Geomagnetic Intensity,
Horizontal Geomagnetic Intensity and Vertical
Geomagnetic Intensity.
Isoporic Map
– a contour
map of Secular Variation where contours represent constant rates of change of
the total intensity of Geomagnetic Field,
either in nanoteslas (nT) per year or in degrees per year. [25].
Isostasy
– a
concept of the gravitational balance of regional blocks of the Earth’s crust
which is based on the assumption that these blocks are “floating” on a more
dense underlying layer of Asthenosphere or
compensated at depth by lateral mass excesses and deficiencies. [25,
223,
238].
See Isostatic Compensation, Airy
Hypothesis, and
Pratt Hypothesis.
Isostatic
Anomaly
– a
long-wavelength (low-frequency) anomaly of the gravity field at sea level after
applying standard corrections to the observed data:
I.A.
= “observed data” + “free-air correction” – “Bouguer
correction” +
+
“terrain correction + “isostatic correction” – “theoretical gravity
correction”
Positive I.A. indicates the absence of the regional isostatic compensation
(i.e., undercompensation), and negative anomaly suggests regional
overcompensation. Isostatic anomalies usually cover large areas, hundreds of
kilometers or more in extent, and they can be interpreted to identify major
tectonic and structural elements of the regional offshore petroleum systems such
as Oceanic
Crust and the
transitional crust boundaries, rift-related lineaments, outlines of the primary
depositional areas, etc. [25,
54, 223,
238].
See Isostasy, Isostatic
Correction and Isostatic Residual Anomaly.
Isostatic Compensation
– the
phenomena of adjustment of Lithosphere
to maintain the equilibrium (balance) among units of varying mass and density: the extra mass of carge topographic features is compensated
at depth by lateral mass deficiencies, whereas large topographic depressions are
matched at depth by lateral mass excesses.
The density deficiency in ocean waters is compensated (balanced) by an
excess of the rock mass densities under the ocean floor. [13
, 25
, 223
, 238
]. See also Airy Hypothesis.
Isostatic Correction
– a correction
applied to the observed gravity data to compensate for lateral density and/or
thickness variations between regional blocks of the Earth’s crust. I.C.
is based on a
hypothesis that the gravitational effects of the continental rock masses
extending above sea level are compensated by a deficiency of the density of
rocks beneath those masses, while the effect of a density deficiency in ocean
waters is compensated by an excess of density in the rock masses under the ocean
floor. I.C.
is based on the
isostatic model which is made from elevation data and water depth data using
zone charts. Geological structures of exploration interest are usually much
smaller in extent than these regional blocks and corresponding isostatic
anomalies. For this reason, I.C.
is seldom applied to the exploration gravity data. [13
, 53,
223, 238].
See also Isostatic Anomaly.
Isostatic Equilibrium
– see Isostasy.
Isostatic Residual Anomaly
– an anomaly that the most closely represents lateral
variations in density of the middle and upper Crust.
Generally, I.R.A. can be defined as a sum of the anomalous effects due to
variations in the crustal density and anomalous effects due to the deeper masses
that support these crustal variations. [25]. See also Isostatic
Anomaly and Isostatic
Correction.
Isothermal Remanent Magnetization (IRM)
– a type
of the remanent magnetization which originates
from the local short-time exposure to a high-intensity external magnetic field
and represents the residual component that is left after the removal of that
field. Lightning can produce I.R.M.
over relatively small areas. [33,
238].
See also Chemical R.M.,
Detrital R.M.,
Thermal R.M.
and Viscous Remanent Magnetization.
Iterative Fourier Equivalent Source Method
–
a potential field continuation method. In this method, an initial magnetization
or density distribution is assigned to a horizontal plane located just below the
level of measurements. The field of this magnetization/density distribution is
calculated on the observation surface, using the Fourier technique, and
subtracted from the observed field. The obtained residual field is converted
into a residual magnetization/density distribution and added to the initial
equivalent magnetization/ density distribution. The process is repeated until
the residual field becomes sufficiently small or until the process starts to
diverge. Convergence for the magnetic field is the fastest if the observed
magnetic data are first reduced to the magnetic Pole. [196,
250].
Iterative Procedure
– a
procedure applied to data by successive steps, each based on results obtained
from the preceding iteration. Cascaded
Filtering, Forward Modeling
and Inversion
are examples of I.P.
ITI
– see Inflection-Tangent-Intersection
Method.