Ramping-up Exploration Value
from Aeromagnetic Surveys
- More Geological Input Needed!
Dave Isles
Southern Geoscience Consultants
AIG Geoscientists Symposia
Bali
May 2013
EAST ASIA: Geology, Exploration Technologies and Mines
Acknowledgements
 Society of Exploration Geophysicists
 Shell Sponsorship
 Leigh Rankin
 Geoscience Australia & State Surveys
 Fugro Airborne Surveys
 Besra Gold Inc
(Harrison)
Mineral discovery
mystery & murder!
GEOLOGICAL INTERPRETATION of
AEROMAGNETIC DATA
a
NEW e-BOOK
by Dave Isles and Leigh Rankin
INTEGRATION
OBSERVATIONS
AEROMAGNETICS
GEOLOGY
Published by ASEG
and the Society of Exploration Geophysicists
Australian Society of
Exploration Geophysicists
Scheduled for
release in June
NEW AEROMAG WORKSHOP
AEROMAGNETICS, GEOLOGY AND EXPLORATION
a 5-day ‘field workshop’
presented by
Dave Isles, Leigh Rankin and Jack Hallberg
Kalgoorlie October 21-25 (Mon-Fri) 2013
Handout with workshop flyer
available here
What is meant by ‘exploration value’?
 ‘Achieving exploration goals’
• Robust assessment of AOI
• Definition & rating of target areas
• Decisions on exploration ‘action’
How often is ‘good value’ achieved
from an aeromagnetic survey?
- less than 50% ?
Why is this?
-not enough ‘effort’ in interpretation
Motherhood Statement
Integration of aeromag with geology
provides a basis for ‘reading’
exploration themes and models
into the interpretation.
Without this we cannot clearly identify
‘geologically anomalous’ features
and differentiate them from
‘routine’ geological features that may be
geophysically ‘anomalous’.
using aeromagnetics to find ore bodies
Epithermal Au
-here are 6 ‘tier-one’ examples!
Haematite Fe
Greenstone Au
NONE of the major ORE BODIES
in these images is a
‘STAND-OUT’ MAGNETIC ANOMALY
but all could be FOUND
with the HELP of AEROMAGNETICS!
Finding the major deposits in each of these frames
requires careful observation, integration and
application of geological and exploration themes
IOCG (U, REE)
BHT Ag, Pb (Zn)
Sed hosted Pb, Zn /Epig _ Cu
Ramping-up Exploration Value
from Aeromagnetic Surveys
- More Geological Input Needed!
TOPICS
 Interpretation Methodology–
Golden Dyke Study
 Mt Leyshon Goldfield study
 Bau Goldfield study (very briefly)
 Concluding Comments
Aeromagnetics- “what does it do?”
MAP GEOLOGY !!!!!!!
MOST geological environments reveal
critical exploration & mapping information
through the eye of the magnetometer
Uses of Aeromagnetics
• Mapping geology? .. what attributes?
– Lithology (‘rock type’)
– Structure (geometry)
– Alteration
– Metamorphism
– Mineralisation
All of these attributes may be seen in
aeromagnetics
Mapping geology… why/how is it possible?
– Magnetic minerals are present in (almost) all ‘rock types’
(0.0003% ‘magnetite’ is detectable)
– Magnetometers can measure tiny ‘geological’ signals
(0.05nT is achievable with care)
– Airborne platforms allow rapid & inexpensive coverage
(e.g 50km x 50km flown with 200 m line spacing- ~<15,000line kms- could be
flown and processed in 6 week time frame and cost ~<$250,000)
Aeromagnetics- how do we use it?
• Plan
• Fly & Compile
• Process & ‘Enhance’
‘INTERPRET’
Data Acquisition and Processing
• High quality, high resolution acquisition is ‘the norm’
• Data Processing methodologies are robust/adequate
Grants
Austirex
pic
 NO BIG PROBLEMS
Filtering and Image Enhancement
many choices ----> ???too many choices
Imaging by Andy Roberts
AND THERE ARE MANY MORE !!
Our capabilities and efforts in
Filtering and Image Enhancement
acquisition, processing & enhancement are
excellent!
• SELECTIONS
TMI
But what about INTERPRETATION?
— choose to suit the purpose
RTP
RTP 1VD/2VD
 KEEP IT SIMPLE
 MAINTAIN GEOLOGICAL FOCUS
 ‘TAILOR MAKE’ YOUR IMAGERY
RTP 2VDagc
ANSIG
Interpretation Methodology
• Gather and Digest all available GEOLOGY
• Careful OBSERVATION on Aeromagnetic Imagery
OBSERVATION LAYER
• Ample RECORDING of observations
(draft)
SOLID
GEOLOGY
LAYER
(and all other relevant data, e.g. spec, EM, grav, etc)
• INTEGRATE Geology
• INTERPRET - develop a working hypothesis
INTEGRATED INTERPRETATION LAYER
•
TEST hypothesis – e.g. map, sample & drill
•
REVIEW - field tests will create new information
feed this information back into interpretation
Interpretation Methodology mineral exploration example
GOLDEN DYKE AREA,
Pine Creek Inlier, NT
Australian aeromagnetic grid courtesy of GADDSAustralian Government Geoscience Portal – www.geoscience.gov.au
Interpretation Methodology – Golden Dyke
GEOLOGY
AEROMAGNETIC PROFILES
1:100,000 scale mapping by Geoscience Australia
(formerly BMR)
Data courtesy WGC, NTGS and Geoscience Australia
200m lines at 60m mtc
Total of 385 line kms in example area
Area is 7kms x 10kms
Interpretation Methodology – Golden Dyke
‘QUICK’ INTERPRETATION (hours)
FILLS THE GAPS
ADDS STRUCTURE/ CONTINUITY
LOOKS OK
+
CAN WE DO BETTER THAN THIS?
=
Interpretation Methodology – Golden Dyke
STEP 1 -
‘WORKHORSE’
IMAGES
TIME
Imaging by A Roberts
Southern Geoscience Consultants
CAREFUL OBSERVATIONS
Interpretation Methodology – Golden Dyke
STEP 1 – OBSERVATION LAYER
CAREFUL OBSERVATIONS
stratigraphic markers
Structural Detail
‘anomalies’
Aleration Zone
Interpretation Methodology – Golden Dyke
STEP 2 – DRAFT SOLID GEOLOGY
AEROMAG
OBSERVATIONS
+
OUTCROP MAPPING
TIME
(draft) SOLID GEOLOGY
Interpretation Methodology – Golden Dyke
STEP 3 – INTEGRATED INTERPRETATION
STRUCTURAL
INTERPRETATION
(draft) SOLID GEOLOGY
mineralisation
+
TIME
‘END POINT’ – EXPLORATION TARGETING
HIGH EXPLORATION VALUE
Detail in geology and aeromag
THOROUGH INTERPRETATION
integrated and
interpreted
(days)
LIMITED EXPLORATION VALUE
Important detail in aeromag
QUICK INTERPRETATION
has been
overlooked
(hours)
‘END POINT’ – EXPLORATION TARGETING
HIGH EXPLORATION VALUE
Detail in geology and aeromag
THOROUGH INTERPRETATION
integrated and
interpreted
(days)
LIMITED EXPLORATION VALUE
Important detail in aeromag
QUICK INTERPRETATION
has been
overlooked
(hours)
the essential ingredients
in ‘good-value’ interpretation are
GEOLOGICAL THINKING
and TIME
Charters Towers
Mt Leyshon
Interpretation Study
Ravenswood Batholith
North Queensland,
Australia
Charters Towers Region
Aeromagnetics
mainly 400m spacing- (GSQ/ Geoscience Australia)
Geology and Landuse
Google
Earth
5 km
GSQ
Geology
Detailed Aeromagnetic Data
(200m lines 070° 80m agl)
(‘multiclient’)
courtesy Fugro Airborne Surveys
‘WORKHORSE’ IMAGES
RTP 1VD
drape
5 km
RTP
2VD
RTP
drape
Interpretation Step ‘1a’ – Mag Observations -’breaks’
Interpretation Step ‘1b’ – Mag Observations - zonation & contacts
Interpretation Step 2 – (draft) Solid Geology
BAU Goldfield, East Malaysia
Aeromagnetic Study
Data courtesy BESRA Gold Inc
BAU Goldfield, Sarawak
Miocene dacite
porphyries
host limestone
Triassic
granodiorite
Data courtesy BESRA Gold Inc
shale / sst
BAU Goldfield, Sarawak
‘workhorse’
aeromagnetic
images
100m 045° 80m agl
‘Dighem’ Survey
Data courtesy BESRA Gold Inc
‘workhorse’
aeromagnetic
images
Without robust RTP
sensible (structural) interpretation
is not achievable
BAU Goldfield, Sarawak
‘workhorse’
aeromagnetic
images
Data courtesy BESRA Gold Inc
BAU Goldfield, Sarawak
Interpretation
step1
Observations
‘shallow’
Data courtesy BESRA Gold Inc
BAU Goldfield, Sarawak
‘shallow’
observations
provide
‘extra’ geology
Data courtesy BESRA Gold Inc
BAU Goldfield, Sarawak
In this area
there is a
clearly defined
‘deeper’ set of
magnetic rock ‘units’
Data courtesy BESRA Gold Inc
BAU Goldfield, Sarawak
applying a 1st VD
to the
spectral window
refines the definition
of deep geometry
Data courtesy BESRA Gold Inc
BAU Goldfield, Sarawak
Interpretation step ‘1b’
-a separate set
of observations
is needed
to describe
this deep geometry
Data courtesy BESRA Gold Inc
BAU Goldfield, Sarawak
the deep geometry
and structure
adds greatly to the
geological picture
Data courtesy BESRA Gold Inc
BAU Goldfield, Sarawak
Combining
shallow and deep
observations
adds complexity,
but it is
REAL GEOLOGICAL
complexity
Data courtesy BESRA Gold Inc
BAU Goldfield, Sarawak
Bau ‘arch’/ ‘anticline’
andesites, cored with
granodiorite/diorite
?and Miocene dacite?
Major fracture systems
and dykes
-age?
-relevance to Au?
Interpretation step2
 Integrate
geology & aeromag
(draft) Solid Geology
???
Broad fracture zone
with en-echelon intrusions
?Miocene dacites?
? ‘arc-normal’ fracture zone?
Data courtesy BESRA Gold Inc
Major fracture systems
and dykes
-age?
-relevance to Au?
BAU Goldfield, Sarawak
Bau ‘arch’/ ‘anticline’
andesites, cored with
granodiorite/diorite
?and Miocene dacite?
Interpretation step2
 Integrate
geology & aeromag
More time & geological input needed!
Realisation of exploration value?
Broad fracture zone
with en-echelon intrusions
?Miocene dacites?
? ‘arc-normal’ fracture zone?
Not yet-
but the signs are very encouraging
Data courtesy BESRA Gold Inc
(draft) Solid Geology
???
Ramping-up Exploration Value
from Aeromagnetic Surveys
- More Geological Input Needed!
Final Message….
• Your existing aeromag probably has new and
important exploration clues ………………
waiting to be ‘found’
• Take more time, think more geology
• Terima Kasih.. thank you for your attention!