Processes: Exploration Geophysics
Resources, Processes & Materials Engineering
Dr Sinem Yavuz
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Lecture Focus
Reproduced from “Materials and Man’s Needs”, National Academy of Sciences, Washington D.C., 1974.
Lecture Overview
What is geophysics?
Rock Properties
Geophysical Techniques Mineral and Water Resources Hydrocarbons Resources
Seismic Acquisition : Curtin Oval, May 2021
Reproduced from Desjardins, 2016, Visual Capitalist, retrieved Aug 2021 from https://www.visualcapitalist.com/extraordinary-raw-materials-iphone-6s/
Most materials we use originate from raw resources within or on the earth
Many of these resources are extracted from mines.
But… Where to look? Where do you look?
The Physical Properties of the Earth Where do you look?
• Velocity (P and S wave) • Density
• Magnetic Susceptibility
Electrical
• Conductivity
• Electrical Permittivity • Magnetic Permeability
Radioactivity
• U238 / Th232 / K40 • Spectral
And more…
• Chargeability
• Porosity
• Temperature
• Hydraulic Permeability • Tortuosity
Learning Outcome Check
Natural resources are discovered by utilising their properties— Name 5 properties that have been used for exploration resources.
The Earth’s composition changes with depth and distance. With it, the physical properties of the Earth changes as well.
Some resources have unique physical properties.
How do we measure these physical properties?
Lecture Focus
Geophysics
geo·phys·ics
(n.) a branch of natural science related to the study of Earth’s composition and structure.
1. Applies principles of physics to predict or measure the geological makeup of the subsurface
Physical Properties
Mineral Deposit • Conductivity
• Magnetic Susceptibility • Velocity
Hydrocarbon • Density
• Velocity
• Conductivity
Physical Property
We can estimate subsurface density by measuring the gravitational field over the earth
Physical Properties
We can measure subsurface magnetic susceptibility by measuring the earth’s Magnetic Field
Electromagnetics
Physical Property
We can estimate subsurface electrical conductivity by transmitting and measuring an electromagnetic field
Physical Properties
We can measure subsurface density and velocity by transmitting and measuring Sound Waves
How deep can we measure?
Depth of Investigation
Depth of Investigation: Airborne Electromagnetics
Airborne Electromagnetics
Depth of Investigation: Active Seismic
The depth of investigation is determined by the geophysical method. Methods such as ground penetrating RADAR can see centimetres into the earth— while methods such as seismic can see 10’s of kilometres deep.
Learning Outcome Check
The locations of valuable deposits can be detected by using the differences between their physical properties and that of their natural geological location. Explain how the following properties of material resources enable their detection:
• Velocity of sound waves
• Magnetic susceptibility
• Electrical conductivity
Explain in your own words, what is meant by seismic exploration? Over what range of depth is it applicable?
Explain in your own words, what is meant by electromagnetic exploration? Over what range of depth is it applicable?
What physical property should be measured to find a specific resource?
Mineral and Water Resources Introduction
Magnetite /Pyrrhotite
Hematite Pyrite
Sand / Shale / Salinity
Magnetic susceptibility Density Electrical Conductivity Velocity + Density
Electrical Conductivity
Magnetics Gravity Electromagnetic Seismic
Electromagnetic
Uranium Uraninite Radioactivity (U238 decay) Radiometrics (Scintillator)
Resource Mineralogical Rock Property Geophysical Parameter Method
Iron Magnetic Susceptibility Magnetics
Each resource is hosted in rock with distinct mineralization. These minerals have distinct physical properties….
We apply the specific geophysical method sensitive to the unique rock property of that resource.
How do we find mineral resources?
Magnetics: Recording the Magnetic Field
Recorded Vertical Secondary Magnetic Field
Magnetic Field Vector on Earth’s Surface
Positive Vertical Magnetic Field (Vectors pointing up)
Earth’s Magnetic Field
Description
The Earth’s magnetic field interacts with magnetically susceptible rocks.
Minerals with higher magnetic susceptibility, such as Maghemite and Magnetite, the greater the induced magnetic field.
The magnetic field is measured with a magnetometer. This is attached to an airplane or carried on-top of the ground. In this schematic the recorded magnetic field is denoted by the purple arrows.
Magnetically susceptable material
Magnetics Overview
Magnetic Anomaly Map of Australia
Modified from “Geoscience Australia, Data Compilations, Geophysics”, Retrieved, Jan 2020.
Propertydetected: MagneticSusceptibility
Magnetic Anomaly
Reproduced from, Meteoritical Society WA Craters Excursion 2012: Yallalie.
Magnetic intensity over the Yallalie Impact Crater
Magnetics: Porphyry Deposit Example
Description
This is Airborne Magnetic dataset was acquired using a magnetometer over the Elder Creek Porphyry deposit.
The outer ring of the “donut” contains Pyrrhotite, a highly magnetic susceptible material. This produces a magnetic high (shown in red).
Reproduced from Timberland resources, 2020, Elder Resources, retrieved from “https://timberlineresources.co/project/elder- creek-overview/”,Aug2020.
Porphyry Copper from UMass Lowell, from http://faculty.uml.edu/nelson_eby/89.308/Instructor%20 31 Pdfs/Chapter%2016%20-%20Economic%20Minerals.pdf
Learning Outcome Check
Explain what geophysical technique would you apply to find a shallow high- magnetic susceptibility gold deposit.
How do we find Water resources?
Dry Sand Sand saturated with water Sand saturated with salt water
Which rock conducts electricity more easily?
Electromagnetic Rock Property: Electrical Conductivity
Charges will flow in the presence of an electric field (E).
The amount of electric current flowing per unit area (J) is determined by the material’s electrical property (𝜎𝜎).
Current𝐉𝐉 Density Current per unit
cross sectional area J has SI units A/m2
E has SI units V/m
Electrical Conductivity
Measure of how easily current can flow through a medium
SI units mS/m (‘S’ siemens)
𝐴𝐴𝑖𝑖𝑖𝑖 ~5 × 10−12 𝑆𝑆𝑆𝑆𝑆𝑆 𝑊𝑊𝑆𝑆𝑊𝑊𝑆𝑆𝑖𝑖 2000 − 4000
𝐹𝐹𝑖𝑖𝑆𝑆𝐹𝐹𝐹 𝑊𝑊𝑆𝑆𝑊𝑊𝑆𝑆𝑖𝑖 < 5
An analogy can be made to Ohm’s Law𝜎𝜎
V=IR↔E=J 1
Measuring Conductivity in the Earth
0.1 Massive Sulfides
Resistivity (Ω•m)
Unweathered Rocks
Weathered Layer
Glacial Sediments
Sedimentary Rocks
Igneous Rocks
Gravels and sands
Metamorphic
Tills Sandstone
Salt Water
Lignite/Coal Water
Conglomerate
Dolomite, Limestone Permafrost
Sea Ice 100 10
Conductivity (mS/m)
Modified from Zonge, ., L. J. Hughes, and M. N. Nabighian. "Electromagnetic
methods in applied geophysics." Electromagnetic methods in applied geophysics (1991). 36
Viewing the Water System in terms of Electrical Conductivity
~4000 mS/m
Water flow
~1000 mS/m
~0.01 mS/m
Measuring Conductivity in the Earth
Transient Electromagnetics
Receiver records electromagnetic field
1. Generate a transient electromagnetic field in a large coil loop
2. The generated electromagnetic field diffuses into earth.
Aircraft / Helicopter with Transmitter Wire
3. The diffusing electromagnetic field interacts with
the earth. The more conductive the ground— the ° higher amplitude the electromagnetic field.
4. Record the EM field in a receiving coil and convert the EM field signal into an distribution of subsurface electrical conductivity
A time varying electromagnetic field causes current to flow in the earth.
Measuring Conductivity in the Earth
SkyTEM Rock quality survey NGI - Institutte, retrieved from https://www.youtube.com/watch?v=ARn18gOmHQo&ab_channel=NGI- NorgesGeotekniskeInstitutt
Clays (150+ mS/m)
Fresh water Aquifer (40 to 100 mS/m)
Dry Sands (<10 mS/m)
Airborne Electromagetic Imaging
Learning Outcome Check
What electrical property can be used to find subsurface water resources? Why is salt water more electrically conductive than fresh water?
What other property of water maybe useful for exploration?
Process Overview
What resource?
Which rock/geological setting? Which physical rock property? Which geophysical method (s)? Acquire/interpret
“Porphyry Deposits with Pyrrhotite and Pyrite”
“High magnetic susceptibility, density and velocity”
“Magnetics and Seismic” Drill / Mine
What are the energy resources?
Global Energy Resources
Retrieved from https://www.bp.com/en/global/corporate/energy- economics/energy-outlook/introduction/overview.html, Aug 2021
BP Energy Outlook, 2020
Hydrocarbon Resources Introduction: Geological Setting
Hydrocarbon Resources; natural gas and crude oil
Origin of Hydrocarbon Resources; prehistoric plants and animals In what settings can you find hydrocarbon;
Natural gas & Crude oil buried in sediments – oil and gas traps
How do you find hydrocarbon? The primary method is Seismic Exploration.
How do we do seismic exploration?
Measuring Acoustics in the Earth
Seismic Exploration
1. Generate sound waves
2. Sound waves propagates through the earth 3. Record the reflected sound waves
4. Image subsurface
Animation is reproduced from https://www.iris.edu/hq/inclass/animation/ reflection_seismology_seeing_below_the_ground
Animation: Acquisition
Modified from The American Petroleum Institute, retrieved from https://www.youtube.com/watch?v=FN8IAb0rG9A, Jan 2020.
Acquisition: Seismic Sources
• Explosives
• Weight Drop
• Vibrators
• Surface Orbital
Vibroseis Truck
Lake Monger, Water Research Curtin Exploration Geophysics, 2018.
Acquisition: Seismic Receivers
• Geophones
• Hydrophones
• DAS (Distributed Acoustic Sensing)
Active piezo polymer hydrophones, V Cable Retrieved from http://www.seismics.net/rental- tools/, Jan 2020.
Geophone, Curtin Exploration Geophysics, 2018
Optical fibre
Laser pulse propagating through the fibre
Acoustic field
Backscattered light returning to iDAS
DAS, Courtesy of Ltd., Jan 2020.
Learning Outcome Check
Describe how we acquire seismic ?
Which equipment do we use to generate sound waves?
Which equipment do we use to listen/record the reflected sound waves? What creates the reflected sound waves?
How does seismic work?
State of Matter
Solid Liquid Gas
Seismic Rock Property: Acoustic Impedance
Acoustic Impedance Z is the product of density and seismic velocity. The differences in acoustic impedances between rock layers determine the reflection coefficients (R) at normal incidence.
Seismic velocity is defined as the speed which a sound wave propagates through a medium
The density here refers to the density of the medium through which the sound waves propagate. Not the density of the target resource!
ρ has SI units g/cm3
The amount of mass per unit
𝐴𝐴𝑖𝑖𝑖𝑖 ~1.2 × 10−3 𝑊𝑊𝑆𝑆𝑊𝑊𝑆𝑆𝑖𝑖 1
𝑆𝑆𝑆𝑆𝑆𝑆𝑖𝑖𝑆𝑆𝑆𝑆𝑆𝑆𝑊𝑊𝑆𝑆𝑖𝑖𝑎𝑎 ~1.5 − 2.5 𝑀𝑀𝑆𝑆𝑊𝑊𝑆𝑆𝑆𝑆𝑀𝑀𝑖𝑖𝑀𝑀𝐹𝑖𝑖𝑀𝑀 ~2.5 − 3.2 𝐼𝐼𝐼𝐼𝑆𝑆𝑆𝑆𝑀𝑀𝐼𝐼𝐹𝐹 ~2.3 − 3.5
𝒗𝒗 has SI units m/s
A vector quantity of displacement per time and direction. It changes with stress, temperature, porosity, permeability, fluid saturation, crack density, etc
𝐴𝐴𝑖𝑖𝑖𝑖 330 𝑊𝑊𝑆𝑆𝑊𝑊𝑆𝑆𝑖𝑖 1500
𝐿𝐿𝑆𝑆𝑆𝑆𝑆𝑆𝐿𝐿𝑖𝑖𝐿𝐿𝐿𝐿 500 − 2000 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 500 − 2500
𝐿𝐿𝑖𝑖𝑆𝑆𝑆𝑆𝐹𝐹𝑊𝑊𝑀𝑀𝑆𝑆𝑆𝑆 4000 − 5500 𝐺𝐺𝑖𝑖𝑆𝑆𝑆𝑆𝑖𝑖𝑊𝑊𝑆𝑆 ~6000
Sound Wave Propagation
Compressional Waves (𝑣𝑣𝑝𝑝) (𝑣𝑣𝑠𝑠)
Retrieved from http://www.sjvgeology.org/oil/seismic_waves.html, Jan 2020.
Snell’s Law
k the incident wave
𝜃𝜃𝒓𝒓 angle of reflection
𝜃𝜃𝑖𝑖 the incident angle
ki reflected wave
𝜃𝜃 angleoftransmission 𝒕𝒕
kr transmittedwave t
K1μ 1𝜌𝜌 1 222
Homogeneous media physical properties such as ....
Velocity (𝑣𝑣𝑝𝑝 , 𝑣𝑣𝑠𝑠 ) and Density 𝜌𝜌
𝐹𝐹𝑖𝑖𝑆𝑆𝜃𝜃𝑖𝑖 = 𝐹𝐹𝑖𝑖𝑆𝑆𝜃𝜃𝑟𝑟 = 𝐹𝐹𝑖𝑖𝑆𝑆𝜃𝜃𝑡𝑡 𝑣𝑣𝑖𝑖 𝑣𝑣𝑟𝑟 𝑣𝑣𝑡𝑡
Snell’s Law is the relationship between the path taken by a sound wave in crossing the boundary between two different media.
Density Range of Rocks
Schön, 2015.
Velocity Range of Rocks
Schön, 2015.
Acoustic impedance vs Reflectivity
Seismic Boat
Layer 1 - Water
Vp = 1500 m s-1 ρ = 1000 kg/m3
Layer 2 - Sediments
Vp = 2500 m s-1 ρ = 2000 kg/m3
Acoustic impedance Reflectivity
R= Z2−Z1 / Z2+Z1
Retrieved from:
https://eesc.columbia.edu/cours es/ees/lithosphere/labs/sonar/s onar.html, April 2020
Geology to Seismic
Z=ρ𝒗𝒗 R𝑖𝑖 =Z𝑖𝑖+1−Z𝑖𝑖/Z𝑖𝑖+1+Z𝑖𝑖 S=W*R + Noise
Reproduced from CGG Veritas, Software, 2020.
How to plot seismic data?
Retrieved from: https://www.globalspec.com/reference/60325/203279/part-iv-color-plates, April 2020
‘ ’ Reflection
Big change in acoustic impedance (Z=𝝆𝝆𝒗𝒗)
Learning Outcome Check
Define acoustic impedance.
What property of the hydrocarbon resource is used in seismic exploration?
Why do sound waves travel at different velocities through rocks? Which type of wave has a higher velocity through a given rock?
In which types of rocks do we typically find our hydrocarbon resources?
What is Snell’s Law? Write it down and explain what the terms mean, in your
own words. How is it used in seismic exploration?
Use a seismic plot to identify the hydrocarbon resource.
What is the difference in between acoustic impedance and reflectivity?
How do we find hydrocarbon resources?
Fluid Properties – Oil and Gas Traps
Example Fluid Density and Compressional Wave Velocity (Gardner et al., 1974, Schön, 2015)
Density (g/cm3)
~1035-1370
Hydrocarbon Resources Introduction: Geological Setting
Crude Oil and Natural Gas Traps
Image retrieved from http://www.glyfac.buffalo.edu/mib /class/325/Lecture/07/0701SeismReflect.pdf, Mar 2021.
Image reproduced from https://energyeducation.ca /encyclopedia/Oil_and_gas_traps, Aug 2021.
Learning Outcome Check
Explain the following terms in your own words:
How do we identify rock layers on seismic data?
How do we identify faults on seismic data?
How do we identify oil and gas traps on seismic data?
How do we find mineral resources?
Grid Drilling
Map of the till sampling grid (from Ahtola et al. (2015) ) with plotted Li anomalies (in ppm) in the Rapasaaret target area, Kaustinen Li pegmatite province. Steiner, 2019
How do we find deep mineral resources?
The depth of cover for discoveries in Australia: 1950 – 2013
(MinEx Consulting, September 2014)
Direct Targeting of Mineral Resources
Acoustic properties of the most common crystalline rocks and ore mixtures (modified after Salisbury and Snyder 2007). P- wave velocity and density fields are displayed for common rocks on a Nafe-Drake curve (grey) with constant acoustic impedance iso-contours at a standard confining pressure of 200 MPa
Seismic: Copper Deposit
Description
This is Seismic imaging depth section shows the Semblana porphyry copper sulfide deposit (2011).
The mineralization is associated with pyrite which has high density and high velocity. This contrast in acoustic parameters compared with the surrounding rock produces a high amplitude reflection (see arrow).
Copper Mineralisation
Semblana copper sulfide mineralization, 2011 (Yavuz, 2015)
Learning Outcome Check
What is the main exploration technique for exploring mineral resources? What makes seismic favourable in searching for deep mineral targets?
How do we identify mineral targets on seismic data?
Geophysics – Not Just in Exploration but also in Monitoring!
Carbon Capture and Storage (CCS)
Retrieved from CO2CRC, http://www.co2crc.com.au/whats-ccs-2/, Jan 2020.
Lecture Summary
The Physical Properties of the Earth Where do you look?
• Velocity (P and S wave) • Density
• Magnetic Susceptibility
Electrical
• Conductivity
• Electrical Permittivity • Magnetic Permeability
Radioactivity
• U238 / Th232 / K40 • Spectral
And more...
• Chargeability
• Porosity
• Temperature
• Hydraulic Permeability • Tortuosity
Lecture Summary: Mineral and Water Resources
• Each mineral and hydrogeological target has a geological / mineralogical (Iron Ore, Gold, Copper, Base metals and Water) setting
• Each target may have a unique Rock Property distribution (i.e., conductivity and magnetic susceptibility)
• We apply methods including Magnetics (magnetic susceptivity), Electromagnetics (electrical conductivity), Seismic Exploration (velocity and density) and Gravity (density) to resolve subsurface rock properties
• In magnetics we acquire data with a magnetometer and in electromagnetics we acquire data with transmitting and receiving coils.
Lecture Summary: Hydrocarbon Resources
• Hydrocarbon Resources; natural gas, crude oil, and coal
• Hydrocarbon Resources are found in sediments – oil and gas traps, coal seams
• Main method is seismic exploration, and the main physical property is acoustic impedance (velocity and density)
• The physics behind seismic exploration; Snell’s Law and Wave Propagation
• How we acquire seismic data? Seismic Sources and receivers
• We can even explore deep mineral resources using seismic exploration.
Retrieved from https://www.petropedia.com/ seismic-mapping-technology-that-has-changed- the-oil-production-mapping/2/9874, Aug 2020.
Lecture Summary
Overall, mineral, water and hydrocarbon resources are discovered by imaging physical rock properties using geophysical techniques.
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