Geology Reference
In-Depth Information
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Regional gradient
7.9.2 Geomagnetic correction
The magnetic equivalent of the latitude correction in
gravity surveying is the
geomagnetic correction
which re-
moves the effect of a geomagnetic reference field from
the survey data. The most rigorous method of geomag-
netic correction is the use of the IGRF (Section 7.4),
which expresses the undisturbed geomagnetic field
in terms of a large number of harmonics and includes
temporal terms to correct for secular variation. The
complexity of the IGRF requires the calculation of cor-
rections by computer. It must be realized, however, that
the IGRF is imperfect as the harmonics employed are
based on observations at relatively few, scattered, mag-
netic observatories.The IGRF is also predictive in that it
extrapolates forwards the spherical harmonics derived
from observatory data. Consequently, the IGRF in areas
remote from observatories can be substantially in error.
Over the area of a magnetic survey the geomagnetic
reference field may be approximated by a uniform gradi-
ent defined in terms of latitudinal and longitudinal gra-
dient components. For example, the geomagnetic field
over the British Isles is approximated by the following
gradient components: 2.13 nT km
-1
N; 0.26 nT km
-1
W; these vary with time. For any survey area the relevant
gradient values may be assessed from magnetic maps
covering a much larger region.
The appropriate regional gradients may also be ob-
tained by employing a single dipole approximation of the
Earth's field and using the well-known equations for the
magnetic field of a dipole to derive local field gradients:
0
Distance
-
Fig. 7.13
The removal of a regional gradient from a magnetic
field by trend analysis.The regional field is approximated by a
linear trend.
7.9.3 Elevation and terrain corrections
The vertical gradient of the geomagnetic field is only
some 0.03 nT m
-1
at the poles and -0.015 nT m
-1
at the
equator, so an
elevation correction
is not usually applied.
The influence of topography can be significant in
ground magnetic surveys but is not completely pre-
dictable as it depends upon the magnetic properties of
the topographic features.Therefore, in magnetic survey-
ing
terrain corrections
are rarely applied.
Having applied diurnal and geomagnetic corrections,
all remaining magnetic field variations should be caused
solely by spatial variations in the magnetic properties of
the subsurface and are referred to as magnetic anomalies.
2
M
R
M
R
m
p
m
p
0
0
Z
=
cos ,
H
=
sin
(7.12)
q
q
4
3
4
3
7.10 Interpretation of magnetic anomalies
∂
∂
Z
∂
∂
HZ
=-
2
H
,
=
(7.13)
2
q
q
7.10.1 Introduction
where
Z
and
H
are the vertical and horizontal field com-
ponents,
q
the colatitude in radians,
R
the radius of the
Earth,
M
the magnetic moment of the Earth and ∂
Z
/∂
q
and ∂
H
/∂
q
the rate of change of
Z
and
H
with
colatitude, respectively.
An alternative method of removing the regional gra-
dient over a relatively small survey area is by use of trend
analysis. A trend line (for profile data) or trend surface
(for areal data) is fitted to the observations using the least
squares criterion, and subsequently subtracted from the
observed data to leave the local anomalies as positive and
negative residuals (Fig. 7.13).
The interpretation of magnetic anomalies is similar in its
procedures and limitations to gravity interpretation as
both techniques utilize natural potential fields based on
inverse square laws of attraction.There are several differ-
ences, however, which increase the complexity of
magnetic interpretation.
Whereas the gravity anomaly of a causative body is
entirely positive or negative, depending on whether the
body is more or less dense than its surroundings, the
magnetic anomaly of a finite body invariably contains
positive and negative elements arising from the dipolar
nature of magnetism (Fig. 7.14). Moreover, whereas
