Applied Geophysics Overview

Applied Geophysics (GPS 314)

• Applied geophysics investigates the Earth's crust and near-surface to achieve practical and economic goals.
• It utilizes various physical theories and experimental techniques to determine Earth's properties.
• Geophysical techniques measure physical phenomena such as gravity, magnetism, elastic waves, electricity, and electromagnetic waves.
• These phenomena are sensitive to subsurface physical properties like density, magnetic susceptibility, seismic wave velocity, resistivity, conductance/inductance/permittivity.

Branches of Geophysics

• Solid Earth Geophysics: Studies the Earth's interior, from the surface to the core, using physics.
• Global Geophysics (Pure Geophysics): Examines the Earth as a whole or substantial parts of it.
• Engineering Geophysics: Applies geophysical methods to investigate subsurface materials and structures with significant engineering implications.
• Environmental Geophysics: Investigates near-surface physico-chemical phenomena with potential implications for local environmental management.

Applied Geophysics Applications

• Studies the crust's thickness for hydrocarbon exploration
• Investigates shallow structures for engineering site investigations
• Locates groundwater, minerals, narrow mine shafts, buried cavities, archeological remains, buried pipes and cables.

Geophysical Survey Environments

• Ground survey: On land
• Airborne or Aero-survey: In the air
• Marine survey: At sea
• Satellite-derived data: From space

Glacio Geophysics

• Utilizes radar and seismic reflection methods to study the ice surface and subglacial interface in polar regions.

Geophysical Methods

• Various methods measure different physical properties of the subsurface.
• Some methods are suited for shallow investigations, others for deep ones.
• Choice of method is based on the target and desired depth of investigation or resolution.

Mineral Exploration and Geophysics

• Mineral exploration is the process of finding economically viable ore deposits.
• A mineral deposit that holds enough minerals for profitable mining is termed an ore.
• Mineral prospecting is the physical search for valuable minerals.

Geophysical Methods and Physical Properties

• Geophysical methods measure various physical properties of subsurface rocks
• Examples include density, magnetic susceptibility, propagation velocity, resistivity, and electromagnetic wave reflectivity.
• These measurements reveal variations caused by geological features.

Planning a Geophysical Survey

• A clear prime objective is essential from the start.
• There are no ideal methods for all situations, techniques and interpretation methods need to be carefully chosen.
• Comprehensive survey planning, informed by geophysical methods and their principles, yields cost-effective and efficient surveys within constraints.
• Steps include defining survey objectives, site selection, selecting the most suitable geophysical methods, and data acquisition protocols.

Survey Limitations

• Cost is a key factor, influenced by the survey location, site access, survey scale and logistics, and required equipment.
• Geophysical surveys should be viewed as an investment in reducing costs and improving efficiency.
• The cost of a geophysical survey should be weighed against its potential to generate cost savings in a larger project.

Target Identification

• Geophysical methods find boundaries with significant differences in physical properties to find anomalies in subsurface targets.
• These anomalies point to targets like oil/gas structures, mineshafts, pipelines, etc.

Anomalies from Geophysical Surveys

• Anomalies are deviations from typical geophysical readings/measurements
• They represent features of interest such as geological structures, underground features, etc.

Choosing the Right Method

• The choice of geophysical method depends on the target or geologic feature sought.
• The method best suited for the geological features being prospected will maximize efficiency and minimize errors.

Background Values and Variations

• Background variations in physical properties (like resistivity, density, etc.) can obscure target anomalies.
• Understanding background variations is essential for accurate target detection and delineation.

Types of Noise in Geophysical Surveys

• Coherent noise (e.g., power lines) has predictable patterns and can frequently be removed through filtering procedures.
• Incoherent noise (e.g., random wind signals) is less predictable.
• Effective survey design and noise reduction techniques help get usable data and interpretations.

Noise Reduction Techniques

• Filtering: Removes or reduces noise based on frequency characteristics.
• Stacking: Averaging multiple measurements to reduce incoherent noise.
• Careful survey design: Selecting optimal survey locations and methods to minimize noise interference.

Qualitative Interpretation

• Preliminary interpretation often involves analyzing contour, density, or anomaly plots.
• Visual inspection of data helps find regional features.
• Data plotting and contouring of geophysical maps provide a visual picture, identifying regions with unusual characteristics.

Quantitative Interpretation

• Uses quantitative methods (inversion, numerical modeling, or neural network techniques).
• This converts and explores data into 2D models or 3D models of the survey area.
• The goal is to model or quantify geologic features, structures, or other subsurface parameters (e.g., depth, extent, etc.).

Electrical Method Applications

• Shallow ore prospecting (massive ore deposits).
• Geological mapping.
• Groundwater and engineering investigations.
• Environmental studies
• Hydrocarbon exploration.

Induced Polarization (IP)

• A current-driven phenomenon, offering an additional method in subsurface exploration.
• Useful for locating disseminated minerals, where other methods might not be effective.
• A technique based on the capacitive and electro-chemical nature of geological materials
• Measurements yield properties such as chargeability.

Factors Affecting Induced Polarization

• Mineral concentration
• Physical characteristics of the host rock
• Electrolyte types
• Presence of water (water conductivity/porosity)
• Current intensity
• Current frequency

IP Measurements

• Multiple measurement techniques are used, dependent on data needs, and may use more sophisticated multi-electrode and roll-along systems.
• The resulting data can be interpreted, identifying and evaluating subsurface targets and geological parameters (e.g., electrical resistivity).