🪨Intro to Geophysics Unit 7 – Potential Field Methods

Fundamentals of Potential Fields

• Potential fields encompass gravity and magnetic fields that vary in space due to subsurface variations in density or magnetic properties
• Governed by inverse square law where field strength decreases with the square of distance from the source
• Superposition principle allows for the summation of individual field contributions from multiple sources to determine the total field at any point
• Conservative fields characterized by the existence of a scalar potential function from which the field vector can be derived
• Laplace's equation $\nabla^2\phi = 0$ describes the behavior of potential fields in source-free regions
  • Solutions to Laplace's equation form the basis for many potential field interpretation techniques
• Gauss's theorem relates the flux of a potential field through a closed surface to the total mass or charge enclosed within that surface
• Green's functions provide a mathematical framework for calculating the potential field due to a distribution of sources

Gravity Method Basics

• Gravity method measures spatial variations in the Earth's gravitational field caused by lateral density contrasts in the subsurface
• Newton's law of universal gravitation $F = G\frac{m_1m_2}{r^2}$ describes the attractive force between two masses
  • $G$ is the gravitational constant $6.67 \times 10^{-11} m^3 kg^{-1} s^{-2}$
• Gravitational acceleration $g$ is the force per unit mass exerted by the Earth's gravitational field, typically ~9.8 m/s^2 at the surface
• Gravity anomalies represent deviations from a reference field (e.g., theoretical gravity) due to subsurface density variations
  • Positive anomalies indicate excess mass (higher density) while negative anomalies suggest mass deficiency (lower density)
• Free-air correction accounts for the decrease in gravity with elevation above the reference level (e.g., geoid or ellipsoid)
• Bouguer correction removes the effect of the mass between the observation point and the reference level, assuming a constant density (typically 2.67 g/cm^3 for the Bouguer slab)
• Terrain correction compensates for the gravitational effect of topography surrounding the observation point

Magnetic Method Principles

• Magnetic method measures spatial variations in the Earth's magnetic field caused by differences in the magnetic properties of subsurface rocks
• Earth's magnetic field approximated as a dipole field with field lines emanating from the south magnetic pole and converging at the north magnetic pole
• Magnetic field vector described by its magnitude and direction, with components:
  • Declination (angle between magnetic north and true north)
  • Inclination (angle between the field vector and the horizontal plane)
  • Total field intensity (scalar sum of the vector components)
• Magnetic susceptibility is a dimensionless proportionality constant that relates the induced magnetization of a material to the strength of the applied magnetic field
  • Diamagnetic materials (e.g., quartz, calcite) have negative susceptibility and are slightly repelled by a magnetic field
  • Paramagnetic materials (e.g., pyroxene, olivine) have positive susceptibility and are slightly attracted by a magnetic field
  • Ferromagnetic materials (e.g., magnetite, pyrrhotite) have high positive susceptibility and can retain permanent magnetization
• Koenigsberger ratio $Q$ quantifies the relative importance of remanent magnetization to induced magnetization in a rock
• Magnetic anomalies represent deviations from a reference field (e.g., IGRF) due to variations in subsurface magnetic properties
  • Positive anomalies indicate the presence of highly magnetic materials while negative anomalies suggest weakly magnetic or non-magnetic materials

Data Acquisition Techniques

• Gravity data acquired using gravimeters that measure the absolute or relative acceleration due to gravity
  • Absolute gravimeters (e.g., FG5) measure the actual value of gravitational acceleration using a free-falling test mass
  • Relative gravimeters (e.g., LaCoste & Romberg) measure the difference in gravity between stations using a spring-mass system
• Gravity surveys conducted on land, sea, or air, with station spacing dependent on the target size and depth
  • Land surveys often use GPS for precise positioning and elevation control
  • Marine surveys employ gravimeters mounted on stabilized platforms to compensate for ship motion
  • Airborne surveys use specialized gravimeters with high sampling rates and precise navigation systems
• Magnetic data acquired using magnetometers that measure the total field intensity or individual components of the Earth's magnetic field
  • Proton precession magnetometers measure the total field intensity by exploiting the magnetic resonance of protons in a hydrocarbon fluid
  • Fluxgate magnetometers measure the individual components of the magnetic field using a ferromagnetic core and sensing coils
  • Optically pumped magnetometers (e.g., cesium vapor) measure the total field intensity based on the Zeeman effect and atomic transitions
• Magnetic surveys conducted on land, sea, or air, with line spacing and sampling interval dependent on the target characteristics and survey objectives
  • Land surveys often use GPS for positioning and time synchronization
  • Marine surveys employ towed magnetometers or magnetometers mounted on autonomous underwater vehicles (AUVs)
  • Aeromagnetic surveys are the most common, providing rapid coverage of large areas with high sampling density

Processing and Filtering Methods

• Gravity data processing steps:
  • Drift correction to remove the effect of instrument drift over time
  • Tidal correction to account for the gravitational effect of the Sun and Moon
  • Free-air and Bouguer corrections to remove the effect of elevation and the mass between the station and the reference level
  • Terrain correction to compensate for the gravitational effect of topography
  • Latitude correction to account for the variation in gravity with latitude due to the Earth's rotation and equatorial bulge
• Magnetic data processing steps:
  • Diurnal correction to remove the effect of solar-induced variations in the Earth's magnetic field
  • Geomagnetic corrections to remove the effect of external magnetic fields (e.g., solar wind, ionospheric currents)
  • IGRF removal to isolate the crustal magnetic field by subtracting the global reference field
  • Leveling to minimize differences at line intersections and ensure consistency between adjacent survey lines
• Filtering techniques applied to enhance specific features or remove unwanted noise
  • Upward/downward continuation to simulate the field at different elevations
  • Wavelength filtering (e.g., low-pass, high-pass, band-pass) to isolate anomalies of different spatial scales
  • Directional filtering (e.g., strike filtering, derivative-based) to emphasize anomalies with specific orientations
  • Reduction to the pole (RTP) to remove the effect of magnetic inclination and simplify interpretation

Interpretation Strategies

• Qualitative interpretation involves visual analysis of potential field maps and profiles to identify anomalies, trends, and patterns related to geological structures or lithological variations
  • Anomaly shape, amplitude, and wavelength provide insights into the depth, geometry, and properties of the causative sources
  • Lineaments and trends can indicate faults, contacts, or folded structures
  • Comparison with geological maps, cross-sections, and other geophysical data aids in the interpretation
• Quantitative interpretation aims to estimate the depth, geometry, and physical properties of the sources causing the observed anomalies
  • Forward modeling involves creating a hypothetical subsurface model and comparing its predicted anomaly with the observed data
    • Iterative adjustment of the model parameters until a satisfactory fit is achieved
  • Inversion seeks to determine the subsurface distribution of physical properties that best explains the observed data
    • Automated algorithms (e.g., Euler deconvolution, Werner deconvolution) estimate source depth and geometry based on the anomaly characteristics
• Potential field data often integrated with other geophysical methods (e.g., seismic, electromagnetic) and geological information to constrain the interpretation and reduce ambiguity
  • Joint inversion of multiple datasets can provide a more comprehensive understanding of the subsurface

Applications in Exploration

• Mineral exploration:
  • Detection of high-density ore bodies (e.g., iron, chromite) using gravity methods
  • Mapping of magnetic minerals (e.g., magnetite, pyrrhotite) associated with mineralization using magnetic methods
  • Delineation of alteration zones, faults, and intrusive bodies that control mineralization
• Hydrocarbon exploration:
  • Mapping of basement topography and sedimentary basin geometry using gravity methods
  • Detection of igneous intrusions and volcanic rocks that may act as seals or reservoirs
  • Identification of structural traps (e.g., anticlines, faulted blocks) and salt structures using potential field data
• Geothermal exploration:
  • Mapping of high-density, magnetized intrusive bodies that may act as heat sources
  • Detection of faults and fracture zones that control fluid flow and reservoir permeability
• Groundwater exploration:
  • Mapping of bedrock topography and sedimentary aquifer geometry using gravity methods
  • Detection of clay-rich aquitards and low-density alluvial sediments
• Environmental and engineering studies:
  • Detection of subsurface voids, sinkholes, and cavities using microgravity surveys
  • Mapping of buried utilities, tanks, and infrastructure using magnetic methods
  • Assessment of soil and bedrock conditions for construction projects

Limitations and Challenges

• Non-uniqueness of potential field interpretation, as different subsurface models can produce similar anomalies
  • Ambiguity can be reduced by incorporating additional constraints from geology, boreholes, or other geophysical data
• Decreased resolution with depth, as the amplitude and wavelength of anomalies increase with distance from the source
  • Limiting factor for the detection of deep or small-scale features
• Interference from nearby sources, such as power lines, metal objects, or cultural noise, can contaminate the data and complicate interpretation
  • Careful survey planning and noise reduction techniques can help mitigate these effects
• Topographic effects can mask or distort anomalies, particularly in rugged terrain
  • Accurate terrain corrections and data processing are essential for reliable interpretation
• Variations in the physical properties of rocks (e.g., density, magnetic susceptibility) within a given lithology can lead to ambiguous or misleading anomalies
  • Collecting representative rock property measurements and understanding the geological context is crucial for accurate interpretation
• Remanent magnetization in rocks can produce anomalies that are difficult to distinguish from those caused by induced magnetization
  • Estimating the direction and intensity of remanent magnetization through rock magnetic studies or drilling can help constrain the interpretation
• Limited depth of investigation in airborne surveys due to the attenuation of the potential field signal with altitude
  • Complementary ground-based surveys or other geophysical methods may be necessary to image deeper targets