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.

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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°.