Geoscience Reference
In-Depth Information
S
N
S
N
a)
d)
150
50
5
Direction
of Earth's
field
Combined
100
0
1
Direction
of Earth's
field
50
-50
Combined
1
0
-100
5
-50
-150
-100
-250
b)
e)
20
6
5
16
Combined
Combined
4
12
3
1
8
2
1
4
1
5
0
0
5
Location (m)
Location (m)
c)
f)
0
1000
2000
0
1000
2000
0
0
1
1
200
2
3
4
200
2
3
4
r
= 0.1 g/cm
3
r
= 0.1 g/cm
3
400
400
k
= 0.1 SI
k
= 0.1 SI
5
5
600
600
Figure 3.65
Depth-dependent characteristics of the TMI and gravity responses of a vertical prism and a vertical step; both are magnetic and
have high density. In both cases the source is divided into five depth-portions. See text for details.
3.10.1.2
Effects of remanent magnetism
If an observed variation in TMI does not have the general
form of the anomaly, e.g. dipole with low to the north etc.,
expected for the local orientation of the geomagnetic field
direction different from that of the present-day Earth
to the sensor, makes the greatest contribution to the amp-
litude of the responses, with the contributions from the
deeper portions decreasing with depth. As expected, the
amplitude of the magnetic response decreases faster with
depth than the gravity response.
Figure 3.65
confirms the increase in wavelength of
responses from deeper sources. Note that the gravity
response has the greater wavelength, which increases more
rapidly with source depth than the magnetic response. Two
other important differences between gravity and magnetic
data are illustrated in the
s field.
The form of the magnetic response depends on the source
geometry and the strength and direction of its magnetism.
In principle a remanent magnetism can have any direction,
so there is clearly an in
'
figure. Firstly, magnetic anomal-
ies are more localised to their source than gravity anomal-
ies and, secondly, gravity data provide information to
greater depth than magnetic data (survey parameters and
noise notwithstanding).
In general, and importantly, the lesser rates of amplitude
decrease for gravity responses means there is more infor-
mation about the geology at depth in gravity data than
magnetic data. In terms of target detectability, it means
gravity surveys are usually better at detecting deeper targets
than magnetic surveys. In terms of mapping, the gravity
data will be more prone to interference between responses
from different depths than the magnetic data.
nite range of variations possible for
the resultant response. However, the following observations
about anomaly character provide some information about
the nature of the remanent magnetism carried by the source,
essential for interpretation and modelling.
Where the remanent magnetism
is direction is roughly the
same as that of the present-day Earth
'
s
field, the anomaly
amplitude will be stronger and the source will appear to have
higher susceptibility than it actually has. Where the reman-
ent magnetism is in roughly in the opposite direction, the
anomaly amplitude will be weaker and the source appears to
have lower susceptibility. The anomaly will have opposite
polarity (to that expected owing to induced magnetism) only
'
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