Magnetic

Advances in
Multisensor Platforms
Greg Hodges
CGG Airborne
The All-in-one Geophysical Tool!
Full-spectrum Geophysics
Interpreting geophysics is like geology blindfolded –
by tapping the rock.
Consider: If you tapped the rock, scratched it,
sniffed it, bounced it in your hand….
… you might start to figure out what it is.
Combining geophysical measurements is far
superior to using each alone.
Mineral Deposits and Exploration Tools
Direct Detection:
•VMS
•Kimberlite
•Magmatic Seg’n
•Iron Formation
•MVT
•IOCG
•Porphyry
•SEDEX
•Lode Gold
•Metasomatic/Skarn
Mapping:
•Magnetic Susc.
•Conductivity
•Radioelements
•Density
•Chargeability
We want to collect them all!
Choosing the method(s): Susceptibility
Magnetic (mapping percent magnetite)
•Magnetic minerals in deposit: magnetite, pyrrhotite, ilmenite.
•Magnetic effects of host alteration: reduced, or enhanced.
•Detectable related structural features: faults, folds, contacts.
Geological mapping - Iron – Intrusives – Gold – Diamonds
Magnetic Methods
Magnetic: mapping percent magnetite
•Low cost survey or add-on to any other survey
•Great depth of exploration (poor depth sensitivity)
•Excellent mapping and structural interpretation aid.
Kidd Creek
What to look for:
Low-cost add-on
High Resolution (Low altitude)
>Towed from aircraft
Low noise
8 km
Diavik HFEM/Mag Examples
7200 Hz Resistivity
Magnetic Field
A154
A154
A418
A418
A2
A21
Data courtesy Diavik Diamond Mines
A2
A21
Detected
Doubtful
Choosing the method(s): Conductivity
Magnetic (mapping percent magnetite)
•Magnetic* minerals in deposit: magnetite, pyrrhotite, ilmenite.
•Magnetic effects of host alteration: reduced, or enhanced.
•Detectable related structural features: faults, folds, contacts.
Geological mapping - Iron – Intrusives – Gold – Diamonds
Electromagnetic (mapping electrical conductivity)
•Conductive mineral deposits: sulphides, oxides, clay-alteration
•Alteration: increase or decrease resistivity
•Structure: fault conductivity and resistivity mapping.
Base metals – Nickel – Porphyry – Iron – Diamonds
Electromagnetic Method
EM: mapping electrical conductivity
•Good target definition and discrimination
•Good depth and shape definition
•Sensitive to weak and strong conductors
•Map ALL the geology
900Hz Apparent Resistivity
Graphite
High power and sensitivity for depth
Multiple component: X, Y, Z
Broadband
B-Field (strong conductors)
On-Time (weak conductors)
Graphite
Adelaide / McFauld’s
X
Use ALL the components!
Z
Y
MULTIPULSE Sensitivity and deposits
TDEM
B-Field
MULTIPULSE
weathered Kimberlite
10 X
higher !
Adapted From Reeves and Bullock
VMS
IOCG
Porphyry
Iron Fmn
MVT
Sed Repl.
Vein
NiCuNiCu
SaskU
MULTIPULSE™
POP!
Time (milliseconds)
BANG!
BANG!
16 milliseconds
0.5 m distance
MULTIPULSE
1.8M Am2
800k Am2,
4ms half sine,
1st off-chnl at 50-150us (300Hz)
10-14 ms off time
Time (milliseconds)
65k Am2,
1ms pulse,
70us off-ramp,
1st chn ~10us (5000Hz)
1ms off-time
Apparent resistivity map: Sudbury
Halfsine Ch05
Halfsine Ch10
4000
ohm-m
Trapezoid Ch01
Better
Detail
Deep?
Weak,
Shallow
Strong
Too
Resistive
1km
Trapezoid Ch10
Better
Detail
Choosing the method(s): Density
Gravity Gradiometry (mapping density)
•Dense minerals in deposit: sulphides, oxides, heavy elements.
•Detectable related structural features: faults, folds, contacts.
•Density changes in host geology: intrusives, alteration
IOCG – Iron – Diamonds – VMS
FALCON AGG high resolution gravity
Gravity Gradiometry: mapping density
•High sensitivity (low contrast).
•Small targets
•Complementary to EM
No smoothing!
Low Turbulence effect
Good Terrain Map (DTM)
Ground gravimetry
Airborne gravimeter
FALCON AGG
Why use a gravity gradiometer?
Gravity
Accel’n
Accel’n
Target
Target
Difference cancels background gravity
and acceleration
M
Target Mass
Horizontal gradients de-coupled from gravity, acceleration!
Gravity
Gravity Wavelength and Amplitude
Gravity
Satellite
Gravity
Gradiometry
Mineral
Deposits
From Dransfield 2009
Iron Deposits : Republic, MI
Gdd Gravity
Gradient
Total Magnetic
Field
Reduced
mag
•Magnetite oxidized to
hematite decreases
magnetic, increases
density!
Increased
gravity
Ekati EM, Magnetic, Gravity Gradiometry
RESOLVE Inph
FALCON AGG
…no one method would have
identified all known pipes. But all
known pipes would have been
discovered by integrating all three
data sets.
Rajagopalan et al, 2007
RESOLVE Quad
Rajagopalan et al 2007
Magnetic CVG
Choosing the method(s): Radioelements
Gravity Gradiometry (mapping density)
•Dense minerals in deposit: sulphides, oxides, heavy elements.
•Detectable related structural features: faults, folds, contacts.
•Density changes in surrounding geology.
IOCG – Iron – Diamonds
Gamma Spectrometry (mapping K, U, Th at surface)
•Radioelements in deposit: uranium deposit; thorium with REEs.
•Effect of host alteration: Potassic alteration, uranium mobilization
Geological Mapping – Uranium – IOCG – REE
Regional Mapping: Morocco
Gamma-spectrometry: mapping surface K, U, Th content
•Excellent mapping add-on – geology discrimination
•Uranium and heavy-metal sedimentary deposits
•No depth of penetration
Apparent
Resistivity
High sensitivity
• large crystal volume
Add-on to other methods
Radiometric
Ternary
Total
Magnetic
Field
Radiometric Ternary: Morocco
Choosing the method(s): Chargeability
Gravity Gradiometry (mapping density)
•Dense minerals in deposit: sulphides, oxides, heavy elements.
•Detectable related structural features: faults, folds, contacts.
•Density changes in surrounding geology.
IOCG – Iron – Diamonds
Gamma Spectrometry (mapping K, U, Th at surface)
•Radioelements in deposit: uranium deposit; thorium with REEs.
•Effect of host alteration: Potassic alteration, uranium mobilization
Geological Mapping – Uranium – IOCG
IP Chargeability ([disseminated] metallic minerals and clays)
•Metallic minerals in deposit: sulphides, oxides
•Alteration: sericite, chlorite etc.
MVT – SEDEX – Gold – Porphyry
Chargeability from TDEM
IP Chargeability: mapping [disseminated] sulphides and clay minerals
•High sensitivity to weakly-conductive metal deposits
•New to Airborne surveys
•Extracted from CGG time-domain EM data
 Geotem Examples
High-powered TDEM transmitter
Low base frequency
Good calibration,
Good processing
Base Level
Offset
Data and studies by Richard Smith
Triple 9 Resources, Newfoundland
Datasets available from GRYPHON
Gravity
Topography
Magnetic
Gamma-Ray
Chargeability!
TDEM
X2!
Chargeability from EM
IP Magnetic
EM
Gamma
Gravity
Applications for Multiple Methods: Gold
Gamma
IP
IP
EM
EM
EM
Magnetic
Gravity
Deep EM
Deep EM
Gravity
Magnetic
From Corbett, 2007
Combined Methods Integrated Interpretation
Only stronger bodies shown
Obrigado!

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