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AEGC 2019: From Data to Discovery – Perth, Australia 1
Distortion of the Magnetic Field at Paragon Bore, South Australia Clive Foss Blair McKenzie Laszlo Katona CSIRO Mineral Resources Tensor Research Pty Ltd Geological Survey of South Australia North Ryde, Sydney P.O. Box 5189 Adelaide, South Australia Greenwich, NSW,2065 Clive.foss@csiro.au blair.mckenzie@tensor-research.com.au Laz.katona@sa.gov.au
INTRODUCTION
Application of FFT filters to total magnetic intensity (TMI)
data is in most cases justified because TMI is consistently
directed across the region of analysis. However, in areas of
strong anomalies the TMI vector direction rotates locally and
application of FFT filters is invalidated. We present a magnetic
field study of the Paragon Bore area in South Australia, where
anomalies locally in excess of 18,000 nT, cause rotation of the
measured geomagnetic field by several tens of degrees. We
apply an approximate, iterative correction process (Clark, 2013)
to reduce the measured TMI to a vector-consistent TMI. This
process also supplies grids of the Cartesian (N, E, vertical)
components of the field, from which declination and inclination
can be mapped.
Unlike FFT filtering, 3D modelling of TMI data does not
require that the TMI vector is consistently directed because the
modelling algorithms compute the Cartesian components of the
field and calculation of TMI from those components provides
true TMI. In this study we invert the measured TMI data using
a model composed of ellipsoid bodies of homogeneous
magnetization. This is clearly a simplified representation of
what will be a much more complex and irregular distribution of
magnetization, but the use of ellipsoids supports computation
inclusive of self-demagnetization effects (within but not
between bodies). The model produced by the inversion is used
as an equivalent source to forward compute vector components
of the field for an alternative mapping of declination and
inclination.
Figure 1. Location of Paragon Bore in GCAS Block 2A.
PARAGON BORE GEOLOGY AND TMI
Paragon Bore is the highest amplitude magnetic feature within
GCAS Block 2A. The location is shown in Figure 1 and with
the GCAS Block 2A TMI image in Figure 2. SARIG (the South
Australian Resource Information Gateway
https://map.sarig.sa.gov.au/) reports 10 boreholes to basement
at Paragon Bore, as mapped in Figure 3. Cover thickness from
these borehole basement intersections varies from 150 to 188
metres. Four of the boreholes are over the western group of
anomalies (although only two directly test high amplitude
SUMMARY
Magnetic field anomalies measured by the Gawler Craton
Aeromagnetic Survey (GCAS) have revealed anomalies of
amplitude > 18,000 nT over Paragon Bore. The flying
height is 60 metres above ground and depth to basement is
150 metres below ground, so the causative basement
sources clearly have magnetizations of extreme intensity.
We apply an iterative processing of the GCAS TMI data to
a vector-consistent TMI. This also supplies vector
component grids which we downward continue to the
ground surface and then transform to declination and
inclination maps. We invert the measured TMI using a
model of multiple ellipsoids to enable inclusion of
substantial self-demagnetization effects. Vector
components forward computed from the inversion model
at ground level are also transformed to declination and
inclination maps which closely match those derived from
the filter transform. Deviations of declination and
inclination about the regional values are -15° to +21° and
-14° to +5° respectively.
High magnetic susceptibility values reported from
borehole intersections (up to 1.6 SI in 2 boreholes) are
mostly associated with banded iron formation (BIF) and
metasomatic magnetite-rich rocks. These values are about
1/3rd of the equivalent inversion model intersection
susceptibilities. We suggest that this apparent discrepancy
is due to self-demagnetization effects in the susceptibility
measurements and the presence of substantial (possibly
viscous) remanent magnetization.
Key words: Gawler Craton self-demagnetization
inclination declination.
Magnetic Field Distortion at Paragon Bore, SA Foss, McKenzie and Katona
AEGC 2019: From Data to Discovery – Perth, Australia 2
features), five are over the eastern anomalies, and one is in the
relatively magnetically flat central region. Lithologies
intersected in the eastern area include garnet amphibolite
gneisses, BIF and calc-silicates (McConachy, 1997). The
maximum reported magnetic susceptibility value is almost 1.0
SI for a BIF unit, but measurement procedures are not
described. Similar lithologies of altered gneiss, magnetite rich
metasomatic rocks and BIF are also reported for the western
area (Kary, 2004) with a maximum magnetic susceptibility of
1.4 SI. From another borehole designed to target the western
anomalies, Miller (1984) reports a broad intersection of BIF
with magnetic susceptibilities of over 1.0 SI and a peak value
of over 1.6 SI. No remanence measurements are reported from
these studies, but the coarse nature of the magnetite suggests
that it may carry a viscous remanent magnetization (and be
susceptible to being reset by drilling). A prominent banding
fabric reported for the BIFS might also impart an anisotropy of
magnetic susceptibility (AMS) which could be quite significant
in generation of the magnetic anomalies.
VECTOR-CONSISTENT TMI
For this study Blair McKenzie adapted the algorithm presented
in Appendix A of Clark (2013) to iteratively adjust measured
TMI that is inclusive of high amplitude anomalies and thereby
local rotations of the TMI vector, to a vector-consistent TMI,
which is a true potential field. This method is based on vector
relationships derived by Vestine and Davids (1945), Hughes
and Pondrom (1947) and Lourenço and Morrison (1973). The
iteration can be continued to any reasonable measure of
convergence. In this study we used 20 iterations, each using the
output of the previous step as the input field for the next step.
The difference between the derived vector-consistent TMI and
measured TMI is imaged in Figure 4. The maximum difference
of almost 2000 nT occurs in the eastern area over the highest
amplitude anomaly. As part of this processing vector
component grids are generated at each step. These component
grids at the final iteration can be used to generate declination
and inclination grids.
Figure 4. Difference between vector-consistent and
measured TMI, contour interval 200 nT.
INVERSION MODELLING
For a complex magnetic anomaly as imaged in Figure 2 there is
considerable overlap in the magnetic fields of adjacent
magnetizations and great uncertainty in resolving the true
distribution of magnetization. We constructed a model from 37
general triaxialellipsoids, each positioned beneath a local total
gradient anomaly with the intension that it would explain that
specific segment of the field. Ellipsoids are very versatile
bodies, and can adjust between near-spherical, plate-like and
pencil shaped bodies with full freedom of orientation. A further
advantage of ellipsoids for this study is that they accommodate
analytically computed self-demagnetization effects as required
for the extreme magnetizations generating these very high
amplitude anomalies.
Figure 6. Ellipsoid inversion model.
Figure 5 shows a subset of flight-line sections through the
inversion model, and Figure 6 shows the model in perspective
view. The ellipsoids flatten into predominantly steeply
plunging thin sheets of mostly east-west trend. This pattern is
consistent both with BIF units which have retained their
original sheet form, and with a metasomatic distribution of
magnetite controlled by fluid injection through steep fractures.
Some of the model bodies require extreme apparent
susceptibility values (values were capped at 10 SI). Larger, less
magnetic bodies do not match the data acceptably. The high
susceptibility values are in part due to allowance for self-
demagnetization effects, which then require even higher
susceptibility, with progressively greater self-demagnetization.
This is illustrated in Figure 7 which cross-plots apparent
susceptibility values for the ellipsoids including self-
demagnetization against susceptibility values for those same
ellipsoids again best-fitting the data, but without allowance for
self-demagnetization. Both inversions match the data using
assemblages of 37 ellipsoids, so there is not an exact one-to-one
correspondence between individual ellipsoid pairs. Also, the
magnetization directions are differently oriented between the
two models because of rotation of magnetization direction
associated with self-demagnetization. Nevertheless, Figure 7
clearly shows the expected increasing divergence between
susceptibility with and without self-demagnetization at higher
susceptibility values (except for the 2 highest values which are
constrained by the cap at 10 SI).
Figure 7. Ellipsoid susceptibility values from inversion
models with and without self-demagnetization effects.
Magnetic Field Distortion at Paragon Bore, SA Foss, McKenzie and Katona
AEGC 2019: From Data to Discovery – Perth, Australia 3
The susceptibilities incorporating self-demagnetization are the
more meaningful because they represent the true physical
process of induction, but their extreme values suggest part of
that magnetization is likely to be (viscous?) remanent
magnetization, which would be consistent with the reported
coarse nature of many of the magnetite rich rocks intersected in
the boreholes. Any co-directed remanent magnetization would
contribute towards the internal field and self-demagnetization,
but not be subject to self-demagnetization itself.
Magnetizations can therefore be produced by significantly
lower susceptibilities in combination with remanent
magnetization. TMI forward computed from the inversion
model is a close match to observed TMI shown in Figure 3. The
difference between measured and model computed TMI is
imaged in Figure 8. Differences are small (the standard
deviation of the difference grid is 173 nT, compared to 3363 nT
for the measured TMI grid) and those differences are localised
around the sharpest anomalies. These residual data misfits
could be reduced by increasing complexity of the model, but
this would not necessarily improve representation of subsurface
structure. Goodness of fit does not validate the model, but does
qualify it as an equivalent source from which we can generate
other magnetic field expressions.
Figure 8. Difference between model-computed and
measured TMI, contour interval 200 nT.
DECLINATION AND INCLINATION
ANOMALIES
From the inversion model we forward computed easting,
northing and vertical field components at the ground surface,
and from those components constructed magnetic declination
and inclination grids as shown in Figures 9 and 10. Declination
ranges by 35° from -9° to +26° about the regional value of +5°,
and inclination ranges by 19° from -56° to -75° about the
regional value of -60.8°. To the east of strong magnetization the
declination rotates clockwise, and to the west anti-clockwise.
To the south of strong magnetization inclination steepens, and
to the north it shallows.
Figure 11. Difference between model-computed and filter
derived magnetic field inclination.
Figure 12. Difference between model-computed and filter
derived magnetic field declination.
Declination and inclination derived from the inversion model
and from transform of measured TMI to vector-consistent TMI
provide very similar results, with identical broad patterns and
local differences of less than 2° as imaged in Figures 11 and 12.
The textures of these images are believed to reveal minor
artefacts of the iterative TMI transform filters, only evident
because differences between the angle estimates are close to the
noise level of the filter, particularly in flatter regions of the
field.
CONCLUSIONS
The 18,000 nT Paragon Bore magnetic anomaly is well
matched by a model of strongly flattened, east-west aligned
sub-vertical ellipsoids, possibly representing isoclinally folded
thin sheets of BIF, and/or of magnetite rich alteration about a
set of steep fractures. Inclusion of self-demagnetization effects
in inversion of the anomalies requires extreme magnetization
values, suggesting that the magnetization may be supplemented
by a broadly co-directed remanent magnetization, possibly of
viscous origin. From the inversion model, and from a filter
transform of measured to vector-consistent TMI, we predict
ground-level declination and inclination anomalies of up to 15°.
REFERENCES
Clark, D. A., 2013. New methods for interpretation of magnetic
vector and gradient tensor data: application to the Mount
Leyshon anomaly, Queensland, Australia: Exploration
Geophysics, 44, 114-127. doi:10.1071/EG12066.
Hughes, D.S., and Pondrom, W.L., 1947, Computation of
vertical magnetic anomalies from total field measurements:
Transactions American Geophysical Union, 28, 193-197.
Kary,G,, 2004, Hawks Nest EL2899, Annual Report for period
March 5 2003 to March 4 2004. Open File Envelope 9942.
Department for Manufacturing, Innovation, Trade, Resources
and Energy, South Australia, Adelaide.
Lourenço, J.S., and Morrison, H.F., 1973, Vector magnetic
anomalies derived from measurements of a single component
of the field: Geophysics, 38, 359-368.
McConachy,G.W., 1997, EL2212, Mabel Creek, Annual and
Final Reports for the period ending 21/11/96, Open File
Envelope 9169. Department for Manufacturing, Innovation,
Trade, Resources and Energy, South Australia, Adelaide.
Miller,G.C., 1984, EL633 and EL1021 Paragon Bore, Progress
and Final Reports for the period 27/5/80 to 2/2/84, Open File
Magnetic Field Distortion at Paragon Bore, SA Foss, McKenzie and Katona
AEGC 2019: From Data to Discovery – Perth, Australia 4
Envelope 3881. Department for Manufacturing, Innovation,
Trade, Resources and Energy, South Australia, Adelaide.
Vestine, E.H., and Davids, N., 1945, Analysis and
interpretation of geomagnetic anomalies: Terrestrial
Magnetism and Atmospheric Electricity, 50, 1-36.
Figure 2. GCAS Murloocoppie Area 2A Measured TMI.
Figure 3. Measured TMI, contour interval 1000 nT.
Figure 5. Example flight-line sections through the inversion model.
Figure 9. Model computed magnetic declination at ground level, contour interval 1°.
Figure 10. Model computed magnetic inclination at ground level, contour interval 1°.
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