🌍Geophysics Unit 13 – Geophysical Field Methods and Instrumentation

Key Concepts and Principles

• Geophysical field methods involve measuring physical properties of the Earth's subsurface to infer its structure, composition, and processes
• Principles of physics (gravity, magnetism, seismology, electrical conductivity) are applied to investigate the subsurface
• Geophysical surveys are non-invasive and provide indirect measurements of subsurface properties
• Data collected from geophysical surveys require processing, analysis, and interpretation to derive meaningful insights
• Geophysical methods are used in various applications (mineral exploration, environmental studies, engineering projects, archaeological investigations)
• Integration of multiple geophysical methods enhances the understanding of subsurface features and reduces ambiguity in interpretation
• Geophysical field methods complement other geological and geotechnical investigations to provide a comprehensive understanding of the subsurface

Geophysical Survey Methods

• Gravity surveys measure variations in the Earth's gravitational field to identify subsurface density contrasts (mineral deposits, geologic structures)
• Magnetic surveys detect variations in the Earth's magnetic field caused by magnetic minerals in rocks and sediments
  • Magnetic anomalies can indicate the presence of ore bodies, igneous intrusions, or buried objects
• Seismic surveys use artificially generated seismic waves to image subsurface layers and structures
  • Reflection seismology is commonly used in oil and gas exploration to map sedimentary layers and hydrocarbon traps
  • Refraction seismology is used to determine the depth and velocity of subsurface layers
• Electrical and electromagnetic (EM) surveys measure the electrical conductivity and resistivity of subsurface materials
  • Electrical resistivity tomography (ERT) creates 2D or 3D images of subsurface resistivity distribution
  • Ground-penetrating radar (GPR) uses high-frequency EM waves to image shallow subsurface features (buried utilities, archaeological remains)
• Radiometric surveys measure the natural radioactivity of rocks and soils to map lithology and alteration zones
• Borehole geophysical logging provides in-situ measurements of physical properties along a drilled borehole (density, porosity, electrical conductivity)

Instrumentation and Equipment

• Gravimeters are highly sensitive instruments used to measure the Earth's gravitational field
  • Relative gravimeters measure the difference in gravity between two points
  • Absolute gravimeters measure the absolute value of gravity at a specific location
• Magnetometers measure the strength and direction of the Earth's magnetic field
  • Proton precession magnetometers use the precession of protons in a magnetic field to determine the field strength
  • Fluxgate magnetometers measure the magnetic field along three orthogonal axes
• Seismographs record the ground motion caused by seismic waves
  • Geophones convert ground motion into electrical signals
  • Seismic sources (explosives, vibroseis trucks, weight drops) generate seismic waves
• Resistivity meters measure the electrical resistance of subsurface materials by injecting current and measuring the resulting voltage
• GPR systems consist of a transmitter and receiver antenna that emit and detect EM waves
• Gamma-ray spectrometers measure the energy spectrum of gamma radiation emitted by radioactive isotopes in rocks and soils
• Borehole logging tools (gamma-ray, density, neutron, resistivity) are lowered into a drilled borehole to measure physical properties along its length

Data Acquisition Techniques

• Survey design involves selecting the appropriate geophysical method, survey parameters, and data acquisition geometry based on the project objectives and site conditions
• Gravity data acquisition requires accurate positioning and elevation measurements at each survey point
  • Corrections for tidal effects, instrument drift, and terrain are applied to the raw gravity measurements
• Magnetic data acquisition involves measuring the total magnetic field intensity at regular intervals along survey lines
  • Diurnal variations in the Earth's magnetic field are monitored and corrected for using a base station magnetometer
• Seismic data acquisition involves deploying an array of geophones and recording the seismic waves generated by a controlled source
  • Common midpoint (CMP) and common depth point (CDP) techniques are used to improve the signal-to-noise ratio and subsurface coverage
• Electrical resistivity data acquisition involves injecting current into the ground through electrodes and measuring the resulting voltage at other electrode pairs
  • Different electrode configurations (Wenner, Schlumberger, dipole-dipole) are used depending on the desired depth of investigation and resolution
• GPR data acquisition involves moving the antenna along survey lines and recording the reflected EM waves at regular intervals
  • The antenna frequency and sampling rate are selected based on the desired depth of penetration and resolution
• Radiometric data acquisition involves measuring the gamma-ray energy spectrum at regular intervals along survey lines
  • The detector crystal size and counting time are selected based on the desired sensitivity and spatial resolution
• Borehole logging data acquisition involves lowering the logging tools into the borehole and recording the measurements at regular depth intervals
  • The logging speed and sampling rate are selected based on the desired vertical resolution and log quality

Field Procedures and Safety

• Proper survey planning and logistics are essential for efficient and safe field operations
  • Site access, permits, and landowner permissions must be obtained before the survey
  • Equipment and personnel transportation, accommodation, and supplies must be arranged
• Health and safety considerations are paramount in geophysical field work
  • Hazard identification and risk assessment (HIRA) should be conducted before the survey
  • Personal protective equipment (PPE) must be worn as required (hard hats, safety boots, high-visibility vests)
  • Emergency response plans and communication protocols must be established
• Quality control (QC) procedures ensure the accuracy and reliability of the acquired data
  • Instrument calibration and testing should be performed before and during the survey
  • Repeat measurements and tie lines are used to assess data consistency and repeatability
• Environmental considerations include minimizing the impact of field activities on the natural environment and local communities
  • Proper waste management, site restoration, and cultural heritage protection measures should be implemented
• Data management involves organizing, backing up, and documenting the acquired data in the field
  • Field notes, metadata, and data files should be systematically recorded and stored
• Communication and coordination among field crew members, supervisors, and clients are essential for successful field operations
  • Regular progress updates, problem-solving, and decision-making should be facilitated through meetings and reports

Data Processing and Analysis

• Data processing involves converting the raw field data into a format suitable for analysis and interpretation
  • Data format conversion, merging, and quality control checks are performed
  • Corrections and filters are applied to remove noise, artifacts, and unwanted signals
• Gravity data processing includes applying corrections for tidal effects, instrument drift, elevation, and terrain
  • The Bouguer anomaly is calculated by removing the effect of topography and regional trends
• Magnetic data processing includes diurnal correction, leveling, and removal of the Earth's background magnetic field
  • Reduction to the pole (RTP) and vertical derivative filters are applied to enhance anomalies and delineate boundaries
• Seismic data processing involves a series of steps to enhance the signal-to-noise ratio and image subsurface reflectors
  • Deconvolution, velocity analysis, stacking, and migration are common processing techniques
• Electrical resistivity data processing involves inverting the apparent resistivity measurements to obtain a subsurface resistivity model
  • 2D and 3D inversion algorithms are used to create resistivity cross-sections and volumes
• GPR data processing involves applying gain, filtering, and migration to enhance the reflections and remove artifacts
  • Time-to-depth conversion is performed using the EM wave velocity in the subsurface
• Radiometric data processing involves calibrating the gamma-ray spectra and converting the counts to elemental concentrations (K, U, Th)
  • Statistical analysis and gridding are used to create radiometric maps and identify anomalies
• Borehole logging data processing involves depth matching, calibration, and correction of the log measurements
  • Log interpretation techniques (crossplotting, overlay analysis) are used to derive petrophysical properties (porosity, permeability, lithology)

Interpretation and Modeling

• Interpretation involves deriving geological meaning from the processed geophysical data
  • Anomalies, patterns, and trends in the data are identified and related to subsurface features and processes
  • Integration with other geological, geochemical, and geotechnical data is essential for a comprehensive interpretation
• Gravity data interpretation involves identifying density contrasts and estimating the depth, shape, and size of subsurface structures
  • Forward modeling and inversion techniques are used to create density models that fit the observed gravity anomalies
• Magnetic data interpretation involves identifying magnetic anomalies and estimating the depth, geometry, and magnetic properties of the source
  • Euler deconvolution, spectral analysis, and inversion techniques are used to create magnetic susceptibility models
• Seismic data interpretation involves identifying and mapping subsurface reflectors, faults, and stratigraphic features
  • Structural and stratigraphic interpretation techniques are used to create geologic cross-sections and maps
• Electrical resistivity data interpretation involves identifying resistivity anomalies and estimating the depth, thickness, and lateral extent of subsurface layers
  • Geological interpretation of the resistivity models is based on the known resistivity values of different rock types and fluids
• GPR data interpretation involves identifying and mapping subsurface reflectors, such as bedding planes, fractures, and buried objects
  • The depth and geometry of the reflectors are estimated based on the EM wave velocity and two-way travel time
• Radiometric data interpretation involves identifying areas of high or low radioactivity and relating them to lithology, alteration, or mineralization
  • Ternary maps (K, U, Th) and ratio maps are used to highlight specific geologic features and processes
• Borehole logging data interpretation involves identifying lithologic boundaries, porosity zones, and fluid contacts based on the log responses
  • Petrophysical models are created by integrating multiple log measurements and calibrating with core data

Applications and Case Studies

• Mineral exploration: Geophysical methods are widely used to detect and delineate mineral deposits (base metals, precious metals, industrial minerals)
  • Gravity, magnetic, and electrical methods are commonly used to identify anomalies associated with ore bodies
  • Seismic and borehole logging methods are used to map the subsurface structure and guide drilling programs
• Hydrocarbon exploration: Seismic reflection surveys are the primary tool for oil and gas exploration
  • 2D and 3D seismic surveys are used to map sedimentary basins, identify structural and stratigraphic traps, and guide well placement
  • Borehole logging is used to evaluate the reservoir properties and hydrocarbon potential of the target formations
• Groundwater investigations: Electrical resistivity, seismic refraction, and GPR surveys are used to map aquifers, estimate groundwater resources, and identify potential contamination sources
  • Borehole logging is used to characterize the aquifer properties and monitor groundwater levels and quality
• Geotechnical and engineering applications: Geophysical methods are used to investigate the subsurface conditions for construction projects (buildings, dams, tunnels, roads)
  • Seismic refraction, electrical resistivity, and GPR surveys are used to map the bedrock depth, soil layers, and geologic hazards (faults, cavities, landslides)
  • Borehole logging is used to obtain geotechnical parameters (rock strength, fractures, in-situ stress) for design and stability analysis
• Environmental studies: Geophysical methods are used to assess and monitor environmental conditions and remediation efforts
  • Electrical resistivity and GPR surveys are used to detect and map contaminant plumes, buried waste, and leaking underground storage tanks
  • Borehole logging is used to monitor the effectiveness of remediation measures and assess the natural attenuation of contaminants
• Archaeological investigations: Geophysical methods are used to detect and map buried archaeological features and artifacts
  • GPR, magnetic, and electrical resistivity surveys are commonly used to identify foundations, walls, graves, and other cultural remains
  • Integration with archaeological excavations and historical records is essential for the interpretation and preservation of the sites
• Volcanic and geothermal studies: Geophysical methods are used to investigate the internal structure and dynamics of volcanoes and geothermal systems
  • Seismic, gravity, and magnetic surveys are used to map the subsurface magma chambers, hydrothermal systems, and geologic structures
  • Borehole logging and temperature measurements are used to assess the geothermal gradient and reservoir properties