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Geophysics 101 ½
Garth Kirkham, B.Sc., P.Geoph., P.Geol.Geoscience BC
Vancouver, British ColumbiaSergio Espinosa, Ph.D.
Geophysics, GIS, and DatabasesTerrane Metals CorporationVancouver, British Columbia
© Sergio Espinosa, Terrane Metal Corporation
(3)Source of Energy
X
(4)Measurement of the
Geophysical Response
Surfaceof the Earth
(1) Covered Ore Body with- lithological characteristics, - alteration, and - mineralization.
CoverLithology
Bed and Host Rock Lithology
(2) Body with following geophysical characteristics:
Z: Depth (geometrical value)
V: Volume (geometrical value)
P.P.: Rock Physical Parameter (expressing lithology, alteration, and mineralization)
Conceptual Geological Body
Source of Energy
• Natural
… Techniques
• Man-made
Passive …
Active …
Magnetics
Radiometrics
Electro-Magnetics: Magneto-Tellurics
Seismic
Radiometrics
Electro-Magnetics: FDEM (Dighem, Csamt), TDEM (Protem, Geotem, Vtem)
Gravity
Resistivity / IP
GR = ----------------------------Volume * RPP
Depth (1…2…3)
Measured Geophysical Response(anomaly)
Rock Physical Property
Geometrical Parameter
© Sergio Espinosa, Terrane Metal Corporation
Geophysical Parameter Relation
TechniqueMass Density GravityMagnetic Susceptibility MagneticsResistivity IPChargeability IPConductivity ElectromagneticsElastic Wave Velocity SeismicElastic Wave Attenuation Seismic Radioactive Isotope Content Radiometrics
Geophysical Techniques
Physical UnitMass Density (kg/m3)
Magnetic Susceptibility (adim)
Resistivity (Ωm)
Chargeability (mV/V)
Conductivity (S/m)
Elastic Wave Velocity (km/s)
Elastic Wave Attenuation (db/m)
Radioactive Isotope Content (%, ppm)
Rock Physical Properties
Geophysical ParameterGravity Earth Gravity Field Magnetics Earth Magnetic InductionInduced Polarization (IP) Ground Voltage Electromagnetics (EM) Induced Field Seismics Ground Particle Velocity Radiometrics Radio-Activity
Geophysical Response
Physical Unit
Earth Gravity Field (mGal)
Earth Magnetic Induction (nT)
Ground Voltage (volts)
Electromagnetic Induced Field (nT/s)
Ground Particle Velocity (m/s)
Radio-Activity (Curie, CPS)
Geophysical Response
InstrumentationEarth Gravity Field Gravity-MeterEarth Magnetic Induction MagnetometerGround Voltage Electrodes / VoltmeterElectromagnetic Induced Field EM Receiver Ground Particle Velocity SeismometerRadio-Activity Spectrometer
Geophysical Instrumentation
•Depends on what we are looking for?•Purpose – Regional vs. Property vs. Deposit Scale•Cost $$$•Time and Logistical Considerations
Advantages and Disadvantages
GR = --------------------------Volume * RPP
Depth (1…2…3)
© Sergio Espinosa, Terrane Metal Corporation
Concept of Gradients
δ/δSδ/δS
Gradients are spatial derivatives ( δ/δS)
Horizontal, vertical, and total derivatives.
Gradient depends on body geometry !!!
Block of Airborne Geophysical VTEM Survey in the Quesnel Terrane
Gravity on Magnetics
Canadian Aeromagnetic Data Base(Natural Resources Canada)
(Belt Scale)
Block of Airborne Geophysical VTEM Survey in the Quesnel Terrane
Gravity on Magnetics
Canadian Aeromagnetic Data Base(Natural Resources Canada)
(Belt Scale)
Block of Airborne Geophysical VTEM Survey in the Quesnel Terrane
Outline of Quesnel Terrane
Geology of the Quesnel Terrane
Lithology
B.C. Geological Survey(www.mapplace.ca)
(Belt Scale)
Intrusives
Metamorphic Rocks
Sediments
Volcanics
Porphyry Occurrences in the Quesnel Terrane
Gravity on Magnetics(Belt Scale)Canadian Aeromagnetic Data Base(Natural Resources Canada)
Mt Milligan
Chuchi
SwanTakla-Rainbow
Tam
Lorraine
Rondah
Misty
QR
Mt PolleyGibraltar
Boss Mountain
Airborne Geophysical VTEM Survey
in the Quesnel Terrane
Geoscience BC – Quest Project(www.geosciencebc.com)
Data
• EW flight lines; • 4 km line spacing; • gridding with 800 m cell size.
Results
* Very good spatial resolution along profiles or EW flight lines (X);
* Very good depth penetration (Z) and spatial resolution along the EM sounding response;
* Just be careful when extrapolating across flight lines (Y).
x
y
?
Digital Elevation Model
Geoscience BC – Quest Project(www.geosciencebc.com)
Airborne Geophysical VTEM Survey
in the Quesnel Terrane
Total Magnetic Field Intensity
Geoscience BC – Quest Project(www.geosciencebc.com)
Airborne Geophysical VTEM Survey
in the Quesnel Terrane
Airborne Geophysical VTEM Survey in the Quesnel Terrane
Geoscience BC – Quest Project(www.geosciencebc.com)
Answer
New geophysical information along profiles.
Question
So, what is the advantage of flying VTEM?
EM responses are associated with conductivity distributions in the sub-surface.
Those conductivity (or resistivity) distributions might be associated with mineralization in the sub-surface or with other geological sources, such as the sedimentary covers.
Airborne Geophysical VTEM Survey
in the Quesnel Terrane
Amplitude of 682ms channel
Geoscience BC – Quest Project(www.geosciencebc.com)
Remarks
• A survey with 4km line spacing means gridding with 800m cell size.
• So, by gridding, resolution along profiles is lost, but a regional overview of that
data can be achieved.
Airborne Geophysical VTEM Survey
in the Quesnel Terrane
Geoscience BC – Quest Project(www.geosciencebc.com)
Log Amplitude of 682ms channel
Remarks
• A survey with 4km line spacing means gridding with 800m cell size.
• So, by gridding, resolution along profiles is lost, but a regional overview of that data
can be achieved.
Airgrav
Bouguer GravityPreliminary Image
VTEM Response of Mt Milligan
Geoscience BC – Quest Project(www.geosciencebc.com)
Mt Milligan
spatial windowon a property scale
Mt Milligan Deposit
Mt Milligan Claims
Spatial Window on a Property Scale
Calc-alkaline Volcanics(Upper Triassic)
Sedimentary Rocks
(Triassic Limestones)
Sedimentary Rocks (Triassic Limestones)
Volcano-Sedimentary
Rocks(Triassic)
Monzonite(Early Jurassic)
Paragneiss(Upper Cretaceous-Miocene)
Paragneiss(Upper Cretaceous-
Miocene)
Sedimentary Rocks(Eocene-
Oligocene)
Diorite(Early Jurassic)
Diorite(Early Jurassic)
Granite(Cretaceous)
Volcano-Sedimentary Rocks
(Triassic)
In the Background: Regional Magnetics (source: GSC) with 275m grid cell size
mapping sediments
strong conductor in the metamorphic
very strong conductor
conductor
EM Response
#1315:89
35 km
EM anomaly
mag anomaly in the sediments
conductor with mag anomaly
Magnetic Signal
Mt Milligan orientation: VTEM inversion example
1) Zoom SRTM Topography and Geology with MINFILE Cu Occurrences
2) SRTM Topography with 10x ExaggerationZoom with MINEFILE Cu Occurrences
3) SRTM Topography and GeologyZoom Bouguer Gravity & MINEFILE Cu Occurrences
NEXT CHALLENGES
ProcessingIntegration InterpretationData MiningIdentifying GapsCollecting more geoscience information to fill these gaps
ITERATIVE PROCESS
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