ERTH 4121 Gravity and Magnetic Exploration Session 4 · Brownfields Exploration Day 3 – Lecture 2...

ERTH 4121

Gravity and Magnetic Exploration

Session 4

Introduction to magnetics - 2

Lecture schedule (subject to change)

Minimum 10 x 3 hour lecture sessions: 1:30pm Tuesdays

Aug 2 : 1. Introduction to gravity method 1

9: 2. Introduction to gravity method 2

16: 3. Introduction to magnetics method 1

23: 4. Introduction to magnetics method 2

[1st assignment]

Sept 13 : 5. Gravity forward modelling

20: 6. Magnetics forward modelling

[Term break]

Oct 4: 7. Introduction to inversion 1

Oct 7?: 8. Introduction to inversion 2

Oct 11: 9. Gravity inversion

Oct 18: 10. Magnetics inversion

1st Assignment

ERTH4121: Gravity & Magnetic Exploration

Assignment #1: Gravity & magnetic principles

Distributed: 23rd August, 2011

Due: 1.30pm, 22nd September

Late submission: 20% deducted per day

VPmg inversion software

Download v6 from www.fullagargeophysics.com

Install password is topper

Send Physical Address (PA) to [email protected]

for licence key

Physical Address is displayed at a C:\ command prompt

when IPCONFIG\ALL is entered

If more than one PA, send the Ethernet PA

Documentation in C:\Program Files\Fullagar\VPmg

http://www.fullagargeophysics.com/

mailto:[email protected]

• Review

• Poisson‟s relation

• Magnetic response of a uniform layer

• Magnetic response of a sphere

• Self-demagnetisation

• Magnetic susceptibility of igneous rocks

• Effect of inclination and declination

• Depth rules

• Image processing examples

Topics – Magnetics Session 2

The Geomagnetic Elements

The Earth‟s magnetic

field is a vector in space

• declination, D – angle of

horizontal component, clockwise

from true north

• inclination, I – angle of

vector from horizontal, positive

down. Inclination is negative in

southern hemisphere.

Magnitude of B also known as ?

Most instruments measure ? outright.

Units and Conversion

Susceptibility k (SI) = ? k (cgs)

Magnetic induction, B 1 nanoTesla (SI) = ? gamma (cgs)

1 nT = 10-9 T

Magnetic field, H defined in ? (SI)

Magnetisation, M defined in ? (SI)

Survey

Techniques

• employ a base station

magnetometer

• at least three profiles

across target

• profiles normal to strike

• data spacing less than half the smallest

wavelength

• ground clearance as low as is safe

• try to ensure survey extent sufficient to define

regional context

Brownfields Exploration Day 3 – Practical 1 9

Survey Line Orientation • Airborne and ground magnetic data

are most commonly collected with lines

oriented perpendicular to the main strike

direction – difficult decision in areas with

multiple structures.

• When data is gridded, information is

averaged in the along-line direction and

interpolated in the cross line direction.

• In diamond exploration magnetic

surveys are usually oriented sub-parallel

to the local magnetic declination

irrespective of regional geology. This is

done to ensure that the dipolar nature of

discrete kimberlite anomalies is

recognised.

KEA300 Geophysical Mapping – Lecture 3 10

Line Spacing One of the major factors that affects the resolution of airborne

magnetic data is line spacing.

For 50m lines (10m

grid mesh )most of

the major geological

features are well

resolved

For 100m lines (20m

grid mesh ) the NW

striking dykes break

up into a series of

discrete bullseye

anomalies

For 200m lines (40m

grid mesh ) the two

dykes in the central

south of the image

appear as a single

anomaly

Brownfields Exploration Day 3 – Practical 1 11

Sampling interval • The spacing between measurements defines the

smallest feature that can be detected in a survey.

• According to the sampling theorem the shortest

wavelength that can be properly defined is twice

the sample spacing (Nyquist wavelength).

• However, to better define a complex anomaly a much smaller

sample spacing is necessary (generally < 0.25 of the anomaly width).

• Short wavelength features

that are inadequately sampled

are aliased and appear in the

dataset as longer wavelength

features. In magnetic surveys

aliasing occurs in the X-line

direction.

1m uniform sampling 10m uniform sampling

Data Reduction Diurnal Correction - subtract data from

base station magnetometer Tie Line Levelling - remove „corrugation‟

effects

Parallax correction –

remove „herring bone‟

effects Reduction to Pole –

converts the magnetic

field to appear as it

would at the north

magnetic pole

Poisson

Relation

Potential of a magnetised body

0

3

0

000 1

).(4

)( rdrr

rMrUV

where is the gradient with respect to the

interior position vector,

If the magnetisation is uniform, then

0

0r

0r

r

O

P

)( 0rM

0

3

0

0 1.

4)( rd

rrMrU

V

GmM

4

0

where denotes the directional derivative in the

direction of , and where is the gravitational

potential of the volume, with uniform density .

m /M

Magnetic response of an infinite

horizontal layer

For an infinite horizontal layer of uniform

density, the gravity response is

hGgz 2

According to Poisson’s Relation, the magnetic

potential is equal to the component of gravity

in the direction of magnetisation =>

where is the z-component of .

The magnetic induction is

Therefore, the magnetic induction from a uniformly

magnetised layer is ?

hGmUB z 2

zz gmU

zm m̂

Gravity response of a uniform sphere Let a denote the radius of the sphere and its density. Apply

Gauss’ Theorem to any concentric sphere, with radius R > a:

Invoke Poisson’s Equation:

where M is the total mass of the sphere. Hence, sphere’s

gravity is

But this is just the gravity due to a point mass (Newton’s Law).

In view of Poisson’s Relation, what are the likely implications

for the magnetic response of a uniform sphere?

2R

GMgr

r

SV

gRdsngdvg 24ˆ

VV

GMdvGdvg 44

Magnetic response of a uniform sphere

By Poisson’s Relation, magnetic potential is gravity in the

direction of magnetisation. Arbitrarily assume magnetisation in

z-direction. For a cylindrical coordinate system (z,r), the z-

component of gravity is

Invoking the Poisson Relation, the magnetic

potential is potential for the sphere is

where m is the magnetic moment of the sphere. But this is just

the magnetic potential of a z-oriented dipole!

2/3222cos

rz

zGM

R

GMg z

2/322

00

44 rz

zm

G

g

V

mU z

[magnetisation=moment/unit volume]

Self-demagnetisation

Consider an infinite horizontal layer of uniform

susceptibility in a magnetic field,

everywhere

0H

For the pillbox straddling

the upper boundary,

dsBndvBSV

ˆ0

0z

0 B

0H

0H

1H

z

In the limit as ,

Thus, inside the layer, the magnetic field orthogonal to the

layer boundaries is reduced:

k is the susceptibility

0ˆ1100

HHdsBnS

k

HHH

1

00

1

01

Normal B is

cts at bdry

0H

0H

1H

0H

1H

1H

Self-demagnetisation

Self-demagnetisation occurs inside all magnetised bodies.

However, for induced magnetisation, it is negligible if

susceptibility is weak, say k < 0.1

The effect of self-demagnetisation is to reduce the strength

of magnetisation orthogonal to the surfaces of the body.

The component of magnetisation

parallel to the surfaces is not

affected. Therefore, the net

magnetisation is rotated, to

become “more nearly parallel” to

the body surfaces.

Brownfields Exploration Day 3 – Lecture 2 19

Melt Composition • The initial iron content of a silicate melt provides a crude measure

of its potential to crystallise magnetite

granitic melt 2-3% FeO (felsic)

andesitic melt 3-7% FeO (intermediate)

basaltic melt 7-9% FeO (mafic)

ultrabasic melt 9-12% FeO (ultramafic)

• Unaltered ultrabasic rocks often have lower magnetic

susceptibilities than basic rocks despite their higher iron content.

• This is primarily due to the greater availability of Mg and the

tendency for the iron to be partitioned into paramagnetic Fe-Mg

silicates.


Crystallisation Environment • The physiochemical environment controls how the iron in the melt

is partitioned into oxide, sulphide and silicate phases

• three main factors influence this process:

1. silica saturation

2. sulphur availability

3. oxidation state

Silica Saturation Low silica rocks generally have higher magnetic susceptibilities than

high silica rocks with the same total iron content

high silica - Fe Fe, Fe-Mg Silicates

low silica - Fe Fe oxides


Sulphur and Oxygen In an igneous rock the stable iron-bearing phase will be determined

by the interplay of temperature, sulphur and oxygen availability.

low oxygen and high sulphur availability sulphide minerals

high temperature and high sulphur pyrrhotite

high oxygen and low sulphur oxide minerals


Oxygen Availability Oxidation state is the most important factor that influences the main

oxide and sulphide phases and hence magnetic susceptibility.

Oxidation state is commonly indicated by the ratio of Fe3+ to Fe2+

low oxidation ratios Fe, Fe-Mg Silicates

intermediate oxidation ratios magnetite + silicates

high oxidation ratios haematite + ilmenite + silicates


Oxygen Availability Most igneous and meta-igneous rock suites display bimodal magnetic

susceptibility distributions due primarily to differences in oxidation

state during crystallisation that control the partition of Fe into

ferrimagnetic and paramagnetic minerals.

Conventional lithological

nomenclature for igneous

rocks is not well suited to

magnetic mapping as it is

mainly based on major mineral

species whereas the magnetic

properties are controlled by

the minor mineral phases.

Ishihara‟s (1977) classification of granitoids into ilmenite series

and magnetite series suites based on the oxidation state of the

magma is a more appropriate scheme for magnetic mapping than

the I and S sub-divisions of Chappell and White.


Metamorphism & Deformation • Metamorphism can have dramatic effects on magnetic properties

but the details are complicated and the creation or destruction of

magnetite depends on the interplay of a number of parameters

including P, T, and the presence of fluids.

• Deformation in the absence of hydrothermal fluids tends to

increase the grainsize of magnetite, thus increasing the magnetic

susceptibility but reducing its capacity to retain remanence.

• The original Fe content and the oxidation state during deposition

and diagenesis have a major bearing on the development of

magnetite during metamorphism.

• The iron content of pelitic [fine grained] material is usually higher

than psammitic [sandstone] rocks and hence magnetite formation

during metamorphism tends to occur more commonly in metapelites

than metapsammites.

Interpretation

Qualitative e.g. interpretation of imagery

Semi-quantitative e.g. Rules of Thumb for

approximate estimates of

width or depth to the top of a

source; Euler deconvolution

Quantitative e.g. modelling & inversion

ERTH4121

Brownfields Exploration Day3 - Lecture 3 26

• The aim of qualitative magnetic interpretation is to extract

lithological, stratigraphic and structural information that can be used

to produce an improved geo-magnetic map.

Qualitative Interpretation

KEA230 Lecture G5

Inclination EW Dyke - i = -72 degrees

• The gravity anomaly of a vertical

symmetrical body is always

symmetrical and is not affected by

latitude since the gravity field is always

vertical.

• Variations in inclination (and

declination) of the Earth’s magnetic

field significantly affect the form of an

induced magnetic anomaly.

• Induced magnetic anomalies are

generally bipolar with the negative

anomaly component located on the

“polewards” side.

Gz

TMI

Top - 5m Bottom - 205m

KEA230 Lecture G5

Inclination

Gz

TMI


EW Dyke - i = -40 degrees

Gz

TMI


EW Dyke - i = -10 degrees

KEA230 Lecture G5

Inclination • The bipolar nature of the induced

magnetic anomaly of a body in an

inclined field can also be

understood in terms of the pole

distribution on the surface of the

body.

• In this case poles are distributed

on the sides of the body in addition

to the top and bottom surface.

• The total anomaly is the sum of

the anomalies due to the poles on

each of the four sides.

KEA230 Lecture G5

TMI - i=-72


Inclination

The magnetic anomaly at the

pole for a shallow body with

significant depth extent is

unipolar.

TMI - i=90


At Hobart’s magnetic inclination

the anomaly for the same body is

bipolar with a low on the

southern side.

KEA230 Lecture G5

Inclination

At the magnetic inclination of

northern Australia (~-40)

magnetic anomalies have

roughly equal positive and

negative components

At an inclination of -10 degrees

(Indonesia) the position of the

body is marked by the magnetic

low.

TMI - i=-40


TMI - i=-10


Shape as a function of latitude

60o N 30o N Equator

30o S 60o S South Pole

KEA230 Lecture G5

Inclination • Interpretation of magnetic

anomalies at low magnetic latitudes

is complicated by the displacement

of the positive component of the

anomaly away from the body

• This displacement increases as

the depth to the top of the body

increases.

• A mathematical transformation

process called reduction to the pole

can correct for this displacement and

is commonly used to produce

images for qualitative interpretation

TMI - i=-10


KEA230 Lecture G5

Declination TMI - i=-60 d=20W


TMI - i=-60 d=20E


• The anomaly low is always located on the magnetic pole side of the

body.

• Variations in declination hence affect the location of the low on most

maps which have grids directed close to true rather than magnetic north.

KEA230 Lecture G5

TMI - i=-72 d=14E


Remanence • Measured magnetic anomalies are

due to the vector sum of magnetisation

induced in the body by the ambient

magnetic field and remanent

magnetisation preserved in the rock.

• In most cases induction is the

dominant mode of magnetisation but in

some cases remanence is more

significant, producing anomalies with

unexpected form.

• It is generally not possible to accurately infer the direction and

magnitude of remanence from field measurements alone since the

geometry of the body is usually not known. Laboratory measurements are

normally required.

KEA230 Lecture G5

Depth Extent E-W striking dyke - i = 90 degrees

TMI

Gz


TMI

Gz


KEA230 Lecture G5

TMI


Gz


Depth Extent Prism - i = 90 degrees

The upper surface of the body

controls the shape of the

magnetic anomaly

The gravity anomaly is broader

due to the effect of mass at depth

KEA230 Lecture G5

TMI

Gz


Depth Extent E-W striking dyke - i = 90 degrees


TMI

Gz

KEA230 Lecture G5


TMI

Gz

Thin Sheet Thin sheet - i = 90 degrees

• The gravity anomaly reaches a

plateau near the centre of the sheet.

• If the sheet is extensive and uniform

then its only effect would only be a

shift in the gravity base level.

• Magnetic anomalies only occur

above the margins of the sheet. The

field near the centre of the sheet is

close to the base level.

• Small irregularities in the upper

surface of the sheet usually result in

high frequency anomalies.

KEA230 Lecture G5

TMI


Thin Sheet Thin Sheet - i = 90 degrees

Bipolar magnetic anomalies are

apparent around the edge of the

thin sheet.

The gravity anomaly in this case

more closely reflects the shape

of the causative body.

Gz



Estimating Depth • As the depth of a magnetic source increases, the anomaly amplitude

decreases and the width (or wavelength) increases.

• There are a number of “Rules of Thumb” that can be applied to

quickly approximate the depth to a magnetic source from simple

measurements of the shape of an anomaly.

X 1/2

The Half Width of the anomaly

x½ is the width of the anomaly at

half its maximum amplitude

(or half the width at half maximum

amplitude in some texts)

SS

The Straight Slope of the

anomaly SS is the horizontal

distance over which the gradient

of the anomaly is linear.

depth

depth

depth

Effects of depth on anomaly width


Profile Selection

• For detailed analysis, choose

profiles that pass through the

centre of the anomaly,

oriented perpendicular to the

strike of the anomaly.

• Regional trends must be

removed from the profile

before analysis.

• Profiles derived from grids

are OK for general analysis but

detailed modelling should be

checked against the original

located data if possible.


Depth

Rules

depth to top

depth to top


Depth

Rules

depth to centre

depth to top

KEA230 Lecture G5

TMI

Gz

Top - 5m Bottom - 205m Top - 20m Bottom - 220m

TMI

Gz

Depth to Top EW Dyke - i = 90 degrees

KEA230 Lecture G5

TMI

Gz

Top - 80m Bottom - 280m Top - 200m Bottom - 400m

TMI

Gz

Depth to Top EW Dyke - i = 90 degrees

~ 1/z3

~ 1/z2

KEA230 Lecture G5

Halfwidth • As source depth increases, so

does the width of the anomaly

• Halfwidth is defined as half

the width of the anomaly at half

its maximum amplitude

• Rules of thumb can be used

to estimate source depth from

halfwidth measurements.

• For example, for gravity

anomaly of a sphere, the depth

to centre is ~1.3 x the halfwidth

of the anomaly.

KEA230 Lecture G5

TMI

Gz


Dipping Dyke EW Dyke - i = 90 degrees

• A dipping geological feature will

produce both an asymmetrical

gravity and magnetic profile.

• The degree of asymmetry of the

profile reflects the dip of the body.

• The steeper gradient on the

anomaly occurs in the up dip

direction.


Estimating Dip • Qualitative estimation of body dip is most easily carried out on

reduced to the pole data since the additional effects of magnetic

inclination on profile shape have already been removed.

• The anomaly gradient is more gentle on the down-dip side due to

the presence of magnetic material in the subsurface.


Dip and Orientation


3D Model The following slides illustrate processing and enhancement of a

simulated areomagnetic dataset for a 3D geological model including a

variety of magnetic source geometries and magnetisations.

deep high k source

folded low k stratigraphy

vertical dips

early high k dykes

induced magnetisation

late high k dykes

reversed magnetisation

magnetite orebodies


Located Data After preliminary processing by the contractor, raw magnetic data is

provided in the form of an ASCII located data file in which coordinate

and other information is provided for each magnetic reading.

5827 1 220804 31456.50 1600 429979.9 5338600.4 159.2 83.6 48674.9

5828 1 220804 31456.75 1600 429990.0 5338599.9 158.8 83.5 48670.8

5829 1 220804 31457.00 1600 429999.9 5338600.4 158.5 83.3 48666.6

5930 1 220804 31487.25 1550 427999.8 5338550.5 158.2 83.1 48448.3

5921 1 220804 31487.50 1550 428010.0 5338549.5 157.9 82.9 48451.8

5922 1 220804 31487.75 1550 428019.5 5338550.0 157.6 82.7 48455.7

5923 1 220804 31488.00 1550 428029.5 5338550.3 157.3 82.5 48459.9

5924 1 220804 31488.25 1550 428040.1 5338549.8 156.9 82.3 48464.5

fid

uc

ial

flig

ht

da

te

tim

e (

s)

lin

e

ea

sti

ng

(m

)

no

rth

ing

(m

)

ele

va

tio

n (

m)

Ra

da

r a

lt.

(m)

TM

I (n

T)

The located data must first be processed to remove line artefacts and

other effects and then gridded to produce images and contour maps.


Flight Path & Stacked Profiles

The flight path plan is used to

visualise the geometry of the

survey. In this case there are

50m spaced E-W flight lines and

500m spaced N-S tie lines

Stacked Profiles allow

assessment of data quality and

signal characteristics and are

helpful in semi-quantitative

anomaly interpretation


Gridding Gridding involves interpolation of magnetic values from the original

flight lines onto a regular grid.

Spline gridding techniques are the best methods for interpolating line-

based data. Other algorithms may not correctly interpolate linear

features. The grid mesh size is usually somewhere between 1/3 and

1/6 of the flight line spacing.


Levelling Levelling is the process used to remove artefacts parallel to flight

lines due to heading errors, parallax errors and level shifts between

survey segments.

• Tie line levelling adjusts the flight line values to match the tie line

values at their intersection point.

• Micro-levelling makes final small-scale adjustments to the data to

remove residual errors.


Grid

Display

contours greyscale

pseudocolor pseudocolor + contours


Contrast Enhancement

Linear Contrast Stretch Histogram Equalisation


Inclination and Declination

• When the Earth‟s field

is vertical, induced

magnetic anomalies

have a simple unipolar

form

i=-90 i=-72 d=14

i=-45 d=3 i=-10 d=-20

•If the field is inclined,

anomalies from the

same geological body

have a more complex

form

• These variations in

anomaly form

significantly complicate

qualitative interpretation

Brownfields Exploration Day3 - Practical 2 60

Filters • Filters can be used to modify the value of a pixel in an image

based upon the variations in the surrounding pixels.

• Low Pass Filters smooth out local variations to reveal an

underlying trend

• High Pass Filters reveal local variations, by ignoring the

regional trends in the dataset

• Directional filters are used to highlight local features in an

image which trend in a particular direction.

• Filtering of potential field data can be conducted in either the

space domain or the spatial frequency domain. Some operations

such as continuation, vertical derivatives or reduction to the pole

are best conducted in the spatial frequency domain while others

are accurate and efficient in the space domain.


Filtering

x

Filter

Function

Space

Input

TM

I

Distance

FFT-1

Output

TM

I

Distance

Spatial Frequency

FFT

Frequency

Am

pli

tud

e

=

Am

pli

tud

e

Frequency

=

* Filter

Kernel

Convolution


Space-Domain Filtering • In the space domain filtering is achieved by convolution of the

image with a filter kernel.

• The value in the image under the centre of the kernel is replaced

by the sum of each of the elements of the kernel multiplied by their

corresponding image elements. The kernel is moved to all possible

positions across the image.

2 3 3 4 2 7

4 8 3 5 6 7

2 3 4 5 7 8

2 4 5 6 6 7

3 5 5 2 8 9

4 2 4 7 7 9

5 4 5 6 7 8

Input Image

3.6 4.2 4.3 5.7

3.9 4.8 5.2 6.3

3.7 4.3 5.3 6.4

3.8 4.4 5.6 6.8

4.1 4.4 5.7 7.0

Filtered Image

1 1 1

1 1 1 x 1/9

1 1 1

Filter Kernel


Reduction to the Pole Reduction to the pole (RTP) is a mathematical transformation that

converts induced magnetic data for any inclination or declination to

the anomaly pattern that would be observed at the pole (I = 90)

RTP RTP RTP

• The RTP transform is

routinely applied to

magnetic images for

qualitative

interpretation

• The RTP transform works

well for inclinations >10º

• RTP images should not be

used for quantitative

interpretation


Model Data – TMI no RTP In the TMI image for the 3D model the NW striking dykes are marked

by both positive and negative anomalies. The positive anomalies due

to the ore bodies occur a short distance north of their causative

bodies. The broad positive anomaly due to the deep source is

positioned well north of the body.

i=-47º d=8º


TMI

i=-47º d=8º

Model Data – RTP TMI

• After RTP transformation, the NW dyke anomalies are unipolar and

the orebody anomalies are translated to the south.

• The broad anomaly due to the deep source is now repositioned

directly over the source.

• The EW dyke anomalies are still somewhat bipolar as they reversely

magnetised

RTP

TMI

RTP


Upward Continuation • If a magnetic or gravity field is measured on a surface above the

causative bodies then it is possible to recalculate the field at a higher

elevation by a mathematical process called upward continuation.

i=-45

large

deep

body

small

shallow

body

similar

amplitudes

i=-45

Up 50m


Upward Continuation

RTP TMI up 50m

• The dykes are still

visible with upward

continuation of 50m

up 200m up 500m

• The dykes

disappear at 200m

and at 500m the

main effect is from

the deep body

• Upward

continuation

highlights large

deep sources and

attenuates the

effects of shallow

features.


Residual Grids

RTP TMI up 50m

-

residual

=

• One way to highlight the near surface features is to calculate a

residual grid by subtraction of the smoothed upward continuation

grid from the original TMI data.

• This process of removing a regional effect to highlight residual

features (regional-residual separation) is very commonly applied to

gravity data but can also be effective for magnetic datasets.


Downward Continuation

RTP TMI down 5m

down 10m

• It is also possible

to downward

continue a gravity or

magnetic dataset

onto a surface that

lies below the

original observation

surface.

down 15m

• This process

enhances the

contribution of

near-surface

features but

becomes unstable if

data is continued

down too far.


Horizontal Derivatives • One simple way to enhance the

high frequency component (small-

scale detail) of an image at the

expense of the large amplitude

features is to calculate a horizontal

derivative or difference image. TMI

1HD

peak 1HD over

maximum TMI

gradient

+

-

1HD zero at

TMI max

• For grids, horizontal

derivatives are

calculated by

convolution with a

filter kernel.

1 1 1

0 0 0

-1 -1 -1

N-S

gradient

(sunangle)

kernel RTP TMI N-S Gradient


Horizontal Derivatives

1 1 1

0 0 0

-1 -1 -1

N - S

1 1 0

1 0 -1

0 -1 -1

NW - SE

1 0 -1

1 0 -1

1 0 -1

E - W

0 1 1

1 0 1

-1 -1 0

NE - SW


Vertical Derivatives • Vertical derivatives can be

directly measured by placing two

magnetic sensors one above the

other and measuring the difference

between the two instruments. This

method is often used for shallow

investigations (archaeology).

• An alternative would be to repeat a survey with a single instrument at

two different heights but this is time-consuming and expensive.

• Fortunately, because magnetic and gravity fields can be upward

continued (Laplace‟s Equation), it is possible to calculate the vertical

derivative (or any derivative) from measurements on a single surface.

• Since the magnetic effects of small, near surface features decrease

more rapidly with increasing elevation than the effects of deep

sources, shallow features are enhanced by vertical derivatives

TMI

sensors

vertical

gradiometer


1st Vertical Derivative • Vertical derivatives have some

significant advantages when

compared to horizontal derivatives.

• The peak vertical derivative lies

directly over the TMI peak (zero

crossing for horizontal derivatives)

•The vertical derivative is not

directional and has the effect of

equally enhancing features in all

directions

• The first vertical derivative is

probably the most commonly used

enhancement of TMI data for

interpretation of near-surface

geological features.


Higher Order Derivatives • It is also possible to calculate

higher-order derivatives (derivatives

of derivatives) to further enhance

near-surface features but as the order

of the derivative increases, noise is

progressively enhanced. TMI

1VD

2VD

2nd vertical

derivative

E-W horizontal

derivative of 1st

vertical

derivative


Composite Images

The image of RTP TMI

shows the range of

amplitudes present in

the data but due to

the high dynamic

range of the data

does not show the

small-scale features

RTP TMI 1VD

High-pass filter

images (derivatives

etc) highlight the

small scale

features but do not

show the amplitude

range

RTP TMI + 1VD

Blending the TMI

and the high pass

images produces

an image that

shows anomaly

amplitude and also

highlights small-

scale features.


Automatic Gain Control • Automatic Gain Control AGC is

a technique commonly applied in

signal and seismic processing to

reduce the dynamic range of a

signal so that small features

appear as significant as major

features .

• The AGC process involves varying the signal amplification for the

central sample in a moving window based on the variation in

amplitude within the window.

• Window length is the main parameter that affects the results of

the AGC process and the “best” window length varies between

datasets due to the characteristics of the data.

• Small windows may over-enhance signal noise while very large

windows have little or no effect.

AGC

TMI


AGC

7x7 15x15

23x23 31x31

• AGC filters are non-

directional.

• Amplitude

information is lost as

all anomalies are

displayed with equal

prominence.

• AGC image show

ALL features in an

image but may appear

messy and confusing

due to the loss of

amplitude information


Multi-scale Edges (Worms) • The wavelet transform converts gravity (or reduced to the pole

magnetics) into a form where edges due to physical property

contrasts can be easily identified.

• The wavelet transform process is equivalent to taking the horizontal

derivatives of upward continuations of the gravity or magnetic data.

gravity

• The shape and

amplitude of edge

clusters in 3D is

diagnostic of the

original source

geometry.

• Edges are identified

from maxima and

minima for successive

upward continuations.


Multi-scale Edges (Worms)

Edges (worms) can

be displayed in

plan view. Small

scale edges

shallow features,

large scale deep

If edges at each

scale are plotted at

different elevations

their shape is

diagnostic of

contact dip

Examples of edges

for different

geological models


Multi-scale Edge Examples

Western Australia

(gravity)

Kambalda

(magnetics)


Euler Deconvolution • This is an automated technique that can be applied to either gravity

or magnetic data to directly estimate the subsurface position of

sources of a particular type with no subjective input.

• The output consists of clusters of source solutions in 3D

The source type is

specified by a “structural

index” (N)

N=0 – contact

N=1 – dyke or sill

N=2 – vertical pipe

N=3 – sphere or dipole


RTP TMI

Interpretation Guidelines

• Use a variety of enhancements

of the magnetic dataset, don‟t

base your interpretation on a

single image.

• Take particular care with

images that enhance features in a

specific direction such as

horizontal derivatives.

• Use reduced to the pole data

for qualitative interpretation

where possible, even in areas

with relatively steep magnetic

inclination

N-S Gradient

RTP TMI + 1VD

1. Magnetic Datasets


Interpretation Guidelines 2. Geological Data

• First compare the magnetic

imagery to the available

geological data (maps, drill data).

• Look for areas where magnetic

anomalies correspond to mapped

units.

• Look for magnetic variations

within mapped units that might

indicate geological variations.

• Look at the signatures of

mapped structural features

(faults, folds, dykes).


Interpretation Guidelines 3. Magnetic Units

• Mark thin stratigraphic or structural

units as “worms”

• Remember – the width of the anomaly

is always greater than the width of the

body.

• Mark unit boundaries at gradient

maxima.

• Indicate breaks in continuity that may

mark structures or alteration

• Identify and mark large units based

on shape, amplitude, contact

relationships and texture.

• Indicate texture within units using

form lines.


Interpretation Guidelines 4. Linear Structures

• Draw structures after magnetic

lithologies

• Look for breaks in continuity of

lithological features or sharp linear

boundaries that may mark faults.

• Faults may have no magnetic

expression or be zones of magnetite

creation or destruction.

• Linear magnetic anomalies that

cross-cut stratigraphy are mostly

dykes.

• Mark lineaments even if their origin

is not clear.


Interpretation Guidelines 5. Dip and Depth

• Estimate body dips from the

asymmetry of the magnetic profile

(class as: vertical, steep, intermediate,

shallow).

• Use anomaly form and wavelength to

differentiate near-surface from deep

features.

• Wide shallow features typically have

steep edges, deep features have more

gentle gradients.

• Interpret fold dip and plunge and

mark fold axes.

• Use “rules of thumb” or 2D/3D

modelling to more accurately estimate

depths.


Acknowledgement

Dr. M. Roach

Geophysical Mapping

Magnetic Data Processing

And Enhancement


References

Fullagar, P.K., Hughes, N.A., and Paine, J., 2000, Drilling-constrained

3D gravity interpretation: Exploration Geophysics, 31, 17-23.

Fullagar, P.K., Pears, G.A., Hutton, D., and Thompson, A., 2004, 3D

gravity and aeromagnetic inversion for MVT lead-zinc exploration at

Pillara, Western Australia: Exploration Geophysics, 35, 142-146.

Fullagar, P.K., and Pears, G.A., 2007, Towards geologically realistic

inversion: Exploration ‟07, Fifth Dicennial Conference on Exploration,

Toronto.

Fullagar, P.K., Pears, G.A., and McMonnies, B., 2008, Constrained

inversion of geological surfaces - pushing the boundaries: The

Leading Edge, 27, 98-105.


• Magnetic Susceptibility(k) is dimensionless and relates the intensity

of magnetisation (M) to the intensity of the inducing field (H) M=kH

• Magnetite is the most magnetic and most widespread magnetic

mineral magnetic maps show the 3D distribution of magnetite

• Magnetic susceptibility distributions for a single rock unit are

commonly lognormal use geometric rather than arithmetic mean

• Susceptibility distributions for broad rock classes are commonly

bimodal

• Melt composition and crystallisation environment control the

distribution of magnetite in igneous rocks.

•

Summary


• Aeromagnetic data can be acquired as drape or barometric surveys

• Located data is gridded to produce images and contour maps for

interpretation. Grid mesh size 1/3 to 1/6 of the flight line spacing

• Heading errors produce artefacts parallel to flight lines that are

removed by tie line levelling and microlevelling.

• The reduction to the pole transformation simplifies qualitative

interpretation by transforming magnetic anomalies to the form they

would have at the magnetic pole.

• Upward continuation recalculates the magnetic field to a new surface

above the original measurement surface, enhancing deep magnetic

sources at the expense of shallow features.

• Derivatives are simple but effective methods for enhancing the high

frequency (shallow) components of a magnetic image.

• Horizontal derivatives are directional and produce a zero crossing at

the peak of the TMI anomaly. Vertical derivatives are not directional

and have a peak above the TMI anomaly.

Summary

KEA230 Lecture G5

Review • The Earth’s magnetic field comprises a number of constituent fields

including the dipole and non-dipole fields, the crustal field and a variety of

fields of external origin.

• Magnetic exploration seeks to isolate the crustal field from the other

components.

• Magnetic measurements are made using magnetometers.

• Fluxgate magnetometers are vector instruments that record the component

of the field parallel to the axis of the coils within the instrument.

• Proton precession magnetometers measure the total magnetic intensity by

recording the period of precession of protons due to the Earth’s field in a

sensor after an initial period of polarisation.

• Optically pumped magnetometers employ quantum principles to record the

total magnetic intensity. They are more accurate and can sample much faster

than PPMs.


• The magnetic susceptibility of sedimentary rocks is usually low

• Magnetite may be either created or destroyed during the various

stages of regional and contact metamorphism.

• Thermoremanent magnetisation is the only form of remanent

magnetisation that is significant from an aeromagnetic perspective.

• TRM is most significant for fine grained igneous rocks and is seldom

preserved in coarse rocks such as granites

• In some cases the effects of TRM can be recognised in aeromagnetic

data from anomalies with unusual polarity, symmetry or extreme

amplitude.

Summary

KEA230 Lecture G5

Summary • Magnetic anomalies are typically bipolar due to the dipolar nature of

magnetic phenomena.

• Anomalies are symmetrical at the pole but become more asymmetrical

with decreasing magnetic inclination

• The width (halfwidth) of an anomaly provides a crude measure of the

depth to the magnetic source. Narrow anomalies are due to near-surface

features while broad anomalies result from bodies at depth.

• Magnetic fields decrease more rapidly with increasing depth than gravity

fields.

•The Earth‟s main magnetic field is strong and vertical at the poles

and weak and horizontal near the equator.

• Raw magnetic data is corrected to remove diurnal variations and

the IGRF field to reveal the pattern of crustal magnetisation

Date post:	10-Aug-2020
Category:	Documents
Upload:	others
View:	0 times
Download:	0 times

ERTH 4121 Gravity and Magnetic Exploration Session 4 · Brownfields Exploration Day 3 – Lecture 2...

Documents