Geoscience Reference
In-Depth Information
a)
b)
1
Figure 3.66
Examples of airborne TMI data that
include sources where remanent magnetism is
dominant. (a) Data from the central Yilgarn Craton,
Western Australia, acquired at a survey terrain
clearance of 40 m. The local geomagnetic
2
3
field has a
declination of 0° and an inclination of
66°. (b) Data
from the Wawa area, Ontario, Canada, acquired at a
survey terrain clearance of 70 m. The local
geomagnetic field has a declination of
-
8° and an
inclination of +74°. The survey line separation for
both areas is 200 m. Source (b): Ontario Geological
Survey © Queen
'
s Printer for Ontario 2014.
-
1
0
5
Kilometres
when the remanent magnetism is signi
cantly stronger than
the induced magnetism, i.e. a high Königsberger ratio (Q). In
certain circumstances a magnetic
anomaly identi
cation, apply equally to gravity and
magnetic data. Interpretation involves delineating linears,
which will be due to contacts and structures, and interpret-
ing mappable lithological units, with repeated checks made
of the evolving interpretation for geological credibility. It is
essential to be aware of the fact that potential field data are
inherently smooth and that responses due to variations in
density or magnetism extend beyond the edges of their
sources. The magnetic responses from two very geologic-
ally different areas are described in detail in
Sections 3.11.3
of some commonly encountered geological features are
presented.
When working with magnetic data, it is essential to
know the magnetic inclination and declination of the indu-
cing
field in the survey area in order to understand the
expected forms of the magnetic anomalies. This also assists
in the recognition of responses that are
'
'
may result from a
highly magnetic source. Lockhart et al.(
2004
) describe
kimberlites with minimal magnetic responses due to near-
cancelling induced and remanent magnetisms.
The TMI data from Western Australia in
Fig. 3.66a
shows the prominent responses of several dyke swarms.
The dyke labelled (1) has a dipolar response with the
positive component on the northern (equatorial) side,
consistent with the direction of induced magnetism in
the area. The dyke labelled (2) has a symmetrical negative
anomaly inconsistent with an induced magnetism, imply-
ing dominant remanent magnetism. The northerly
trending linear features of lower amplitude are caused by
shear zones. The data in
Fig. 3.66b
from Ontario show a
negative anomaly (3) consistent with a source dominated
by remanent magnetism. It is likely to be caused by gabbro
and diorite mapped in the area (D. Rainsford, pers comm.).
Intrusions with responses consistent with a dominant rem-
anent magnetism are described in
Section 3.11.4
.
The change in the direction of the resultant magnetism
due to remanence strongly affects the response in the same
way as changing the dip and strike of the source with
respect to the Earth
low
'”
for the area, and possibly attributable to the presence of
remanent magnetism. Many interpreters prefer to work
with data that has been reduced-to-pole (RTP) (see
Section
inclination is too low. Also, RTP can in some circumstance
introduce artefacts into the data.
'
out of character
is field. Dip is the most strongly affected
source parameter and is generally difficult to resolve for a
source carrying significant remanent magnetism.
'
3.10.2.1
Data selection and integration
The foundations of the interpretation of potential
field data
are the fundamental datasets of TMI and Bouguer or free-
air gravity. However, interpretation is invariably based on
more than one representation of the data. Vertical
derivatives (see
Section 3.7.4
)
resolve contacts, edges and
boundaries, and emphasise features in the near-surface; but
3.10.2
Analysis of gravity and magnetic maps
The guidelines described in
Section 2.10.2
for the analysis
of geophysical data in terms of geological mapping and
Search WWH ::
Custom Search