🌍Geophysics Unit 10 – Geophysics in Resource Exploration

Fundamentals of Geophysics

• Geophysics involves the application of physics principles to study the Earth's interior and surface processes
• Utilizes various methods to measure and interpret physical properties of the Earth (seismic waves, gravity, magnetism, electrical conductivity)
• Plays a crucial role in resource exploration by helping to locate and characterize subsurface resources (oil, gas, minerals, groundwater)
• Contributes to understanding Earth's structure, composition, and dynamics
  • Provides insights into plate tectonics, earthquakes, and volcanic activity
• Interdisciplinary field that combines aspects of geology, physics, mathematics, and computer science
• Geophysical data is acquired through field surveys, remote sensing, and borehole measurements
• Interpretation of geophysical data requires knowledge of rock properties, subsurface conditions, and geological processes
• Advancements in technology and computational methods have greatly enhanced the capabilities of geophysical exploration

Geophysical Properties of Earth Materials

• Earth materials exhibit various physical properties that can be measured and analyzed using geophysical methods
• Density is a fundamental property that influences gravity measurements
  • Density variations in the subsurface can indicate the presence of different rock types or fluids
• Seismic velocity describes the speed at which seismic waves propagate through Earth materials
  • P-waves (primary or compressional waves) travel faster than S-waves (secondary or shear waves)
  • Seismic velocity depends on the elastic properties and density of the material
• Electrical conductivity and resistivity are important properties for electromagnetic methods
  • Conductivity is the ability of a material to conduct electric current, while resistivity is the opposite
  • Pore fluids and certain minerals (clays, sulfides) can significantly affect electrical properties
• Magnetic susceptibility measures the extent to which a material can be magnetized in the presence of an external magnetic field
  • Magnetic minerals (magnetite, pyrrhotite) have high magnetic susceptibility
• Thermal conductivity describes the ability of a material to conduct heat
  • Thermal properties are relevant for geothermal exploration and heat flow studies
• Radioactivity is the spontaneous emission of radiation from unstable atomic nuclei
  • Some Earth materials contain naturally occurring radioactive elements (uranium, thorium, potassium)
• Understanding the relationships between geophysical properties and rock types is essential for interpreting geophysical data

Seismic Methods in Resource Exploration

• Seismic methods are widely used in resource exploration to image subsurface structures and identify potential hydrocarbon or mineral deposits
• Seismic surveys involve generating and recording seismic waves that travel through the Earth
  • Seismic waves are typically generated using controlled sources (explosives, vibrators) or natural sources (earthquakes)
• Reflection seismology is the most common seismic method used in resource exploration
  • Seismic waves are reflected from interfaces between layers with different acoustic impedances
  • Reflected waves are recorded by receivers (geophones or hydrophones) at the surface
• Refraction seismology utilizes critically refracted waves that travel along layer interfaces
  • Refraction surveys are useful for determining the velocity structure of the subsurface
• Seismic data processing involves various steps to enhance the signal-to-noise ratio and generate interpretable seismic images
  • Common processing steps include filtering, deconvolution, velocity analysis, and migration
• Seismic interpretation aims to extract geological information from seismic images
  • Interpreters identify and map seismic reflectors, faults, and stratigraphic features
  • Seismic attributes (amplitude, frequency, phase) can provide additional insights into rock properties and fluid content
• 3D seismic surveys provide detailed three-dimensional images of the subsurface
  • 3D surveys have revolutionized resource exploration by improving spatial resolution and reducing uncertainty
• 4D seismic (time-lapse seismic) involves repeating 3D surveys over time to monitor changes in the subsurface
  • 4D seismic is used to monitor reservoir dynamics and optimize production strategies

Gravity and Magnetic Surveys

• Gravity and magnetic surveys measure variations in the Earth's gravitational and magnetic fields to infer subsurface structures and properties
• Gravity surveys measure the local acceleration due to gravity using gravimeters
  • Gravity anomalies arise from density variations in the subsurface
  • Positive gravity anomalies indicate the presence of denser rocks, while negative anomalies suggest less dense rocks
• Gravity data is corrected for various factors (elevation, latitude, terrain, tides) to isolate the gravitational effect of subsurface structures
• Gravity modeling involves creating density models that fit the observed gravity data
  • Models can range from simple geometrical shapes to complex 3D structures
• Magnetic surveys measure the local magnetic field intensity using magnetometers
  • Magnetic anomalies are caused by variations in the magnetic properties of rocks
  • Magnetic minerals (magnetite, pyrrhotite) are the primary sources of magnetic anomalies
• Magnetic data is corrected for diurnal variations, regional fields, and other effects to isolate the magnetic response of subsurface features
• Magnetic modeling involves creating models of the subsurface magnetic sources that reproduce the observed magnetic anomalies
• Gravity and magnetic data are often used in conjunction with other geophysical methods to constrain subsurface interpretations
• Airborne gravity and magnetic surveys allow for rapid coverage of large areas
  • Airborne surveys are particularly useful for regional reconnaissance and mapping

Electrical and Electromagnetic Techniques

• Electrical and electromagnetic (EM) methods measure the electrical properties of the subsurface to detect and characterize geological structures and resources
• Electrical resistivity surveys measure the apparent resistivity of the subsurface by injecting electric current and measuring the resulting voltage differences
  • Resistivity is affected by rock type, porosity, fluid content, and temperature
  • Resistivity surveys can be conducted using various electrode configurations (Wenner, Schlumberger, dipole-dipole)
• Induced polarization (IP) is an electrical method that measures the chargeability of the subsurface
  • IP is sensitive to the presence of disseminated metallic minerals and clay alteration
  • IP surveys are commonly used in mineral exploration to detect disseminated sulfide deposits
• Electromagnetic methods involve inducing and measuring EM fields in the subsurface
  • EM fields are generated by transmitting a primary EM field and measuring the secondary field produced by conductive bodies
• Frequency-domain EM (FDEM) methods use a fixed-frequency transmitter and receiver
  • FDEM is effective for mapping near-surface conductivity variations and detecting conductive targets
• Time-domain EM (TDEM) methods use a transient transmitter pulse and measure the decay of the secondary field over time
  • TDEM is useful for detecting deep conductive targets and resolving layered earth structures
• Magnetotellurics (MT) is a passive EM method that utilizes natural EM fields generated by solar wind and lightning activity
  • MT surveys measure the Earth's impedance tensor over a wide frequency range to determine the subsurface resistivity structure
• Ground penetrating radar (GPR) is a high-frequency EM method that uses short radar pulses to image shallow subsurface features
  • GPR is commonly used for near-surface investigations, such as mapping soil layers, bedrock topography, and buried objects

Remote Sensing in Geophysics

• Remote sensing techniques involve acquiring data about the Earth's surface and subsurface from a distance, typically using satellites or airborne platforms
• Satellite imagery provides a synoptic view of large areas and can be used for regional geological mapping and structural analysis
  • Multispectral and hyperspectral sensors capture data in multiple wavelength bands, allowing for the identification of different rock types and alteration zones
• Synthetic aperture radar (SAR) is an active remote sensing technique that uses microwave energy to image the Earth's surface
  • SAR is sensitive to surface roughness, moisture content, and topography
  • SAR interferometry (InSAR) can measure ground deformation caused by natural or anthropogenic processes (earthquakes, subsidence, oil and gas extraction)
• Light detection and ranging (LiDAR) is an active remote sensing technique that uses laser pulses to create high-resolution digital elevation models (DEMs)
  • LiDAR data can be used to map surface topography, fault scarps, and geomorphological features
• Thermal infrared (TIR) sensors measure the emitted infrared radiation from the Earth's surface
  • TIR data can be used to map surface temperature variations, which may indicate the presence of geothermal resources or hydrocarbon seeps
• Airborne geophysical surveys combine remote sensing with traditional geophysical methods
  • Airborne magnetic, gravity, and electromagnetic surveys provide high-resolution data over large areas
• Remote sensing data is often integrated with ground-based geophysical surveys and geological data to enhance the interpretation of subsurface features and resources
• Advances in satellite technology, data processing, and machine learning have greatly expanded the applications of remote sensing in geophysics

Data Processing and Interpretation

• Geophysical data processing involves a series of steps to convert raw data into meaningful information about the subsurface
• Quality control (QC) is an essential first step to ensure data integrity and identify any errors or artifacts
  • QC procedures may include data editing, filtering, and calibration
• Data reduction techniques are applied to compress large datasets and remove redundant or irrelevant information
  • Decimation, stacking, and averaging are common data reduction methods
• Signal enhancement techniques aim to improve the signal-to-noise ratio and highlight features of interest
  • Filtering, deconvolution, and migration are examples of signal enhancement techniques used in seismic data processing
• Data integration involves combining multiple geophysical datasets and other relevant information (geological, geochemical, well data) to create a comprehensive subsurface model
  • Data integration helps to reduce uncertainty and improve the reliability of interpretations
• Inversion is a mathematical process that estimates subsurface properties from geophysical data
  • Inversion algorithms seek to find a model that best fits the observed data while honoring prior information and constraints
• Interpretation of processed geophysical data requires a combination of technical expertise, geological knowledge, and creativity
  • Interpreters use various visualization tools (cross-sections, maps, 3D models) to analyze and communicate their findings
• Uncertainty analysis is an important aspect of geophysical interpretation
  • Quantifying and communicating the uncertainty associated with subsurface models helps decision-makers assess risks and make informed choices
• Collaborative interpretation involving geophysicists, geologists, and other experts leads to more robust and integrated subsurface characterization

Case Studies and Real-World Applications

• Geophysical methods have been successfully applied to a wide range of resource exploration and characterization projects worldwide
• Seismic exploration has been instrumental in discovering and delineating major oil and gas fields
  • 3D seismic surveys have revolutionized the oil and gas industry by providing detailed images of subsurface structures and stratigraphic traps (Gulf of Mexico, North Sea)
• Gravity and magnetic surveys have been used to map regional geological structures and identify potential mineral deposits
  • Airborne magnetic surveys have led to the discovery of large iron ore deposits (Pilbara region, Australia)
• Electrical and electromagnetic methods have been effective in detecting and characterizing mineral deposits, particularly in areas with conductive ore bodies
  • Induced polarization surveys have been used to explore for porphyry copper deposits (Andes Mountains, South America)
• Remote sensing techniques have been applied to map and monitor geothermal resources
  • Thermal infrared imaging has been used to identify surface temperature anomalies associated with geothermal systems (Yellowstone National Park, USA)
• Integrated geophysical surveys have been conducted to assess groundwater resources and map aquifer systems
  • Combination of electrical resistivity, seismic reflection, and borehole logging has been used to characterize coastal aquifers (Mediterranean region)
• Geophysical methods have played a crucial role in geotechnical investigations for infrastructure projects
  • Seismic refraction and electrical resistivity surveys have been used to assess site conditions for dam construction (Three Gorges Dam, China)
• Environmental geophysics has emerged as a key application area, focusing on subsurface contamination, waste disposal, and site remediation
  • Ground penetrating radar and electrical resistivity imaging have been used to map and monitor contaminant plumes (Love Canal, USA)
• Geophysical monitoring has become increasingly important for managing and optimizing resource production
  • 4D seismic surveys have been used to monitor reservoir changes and guide enhanced oil recovery strategies (North Sea)