🪨Intro to Geophysics Unit 6 – Electrical & Electromagnetic Methods

Basic Principles of Electricity and Magnetism

• Electricity involves the flow of electric charge, typically carried by electrons in a conductor
• Electric current ($I$) is the rate of flow of electric charge, measured in amperes (A)
  • Conventional current flows from positive to negative, while electron flow is in the opposite direction
• Electric potential difference (voltage, $V$) is the work done per unit charge to move a positive test charge between two points, measured in volts (V)
• Ohm's law relates current, voltage, and resistance ($R$): $V = IR$
• Resistance is a material's opposition to the flow of electric current, measured in ohms ($\Omega$)
• Conductivity ($\sigma$) is the reciprocal of resistivity ($\rho$) and measures a material's ability to conduct electric current
• Magnetism is a force of attraction or repulsion between objects with magnetic properties
• Magnetic fields are represented by magnetic field lines, which point from the north pole to the south pole of a magnet
• Moving electric charges create magnetic fields, and changing magnetic fields induce electric currents (Faraday's law of induction)

Electrical Properties of Earth Materials

• Electrical conductivity and resistivity of Earth materials depend on various factors, including composition, porosity, fluid content, and temperature
• Minerals and rocks exhibit a wide range of electrical properties, from highly conductive (native metals, graphite) to highly resistive (quartz, dry limestone)
• Porosity and fluid content significantly influence the bulk electrical properties of sedimentary rocks
  • Pore fluids (water, hydrocarbons) can dramatically increase the conductivity of a rock
• Clay minerals tend to have higher conductivity due to their ability to absorb and conduct ions in pore fluids
• Temperature affects electrical properties, with higher temperatures generally leading to increased conductivity
• Electrical anisotropy occurs when electrical properties vary with direction, often due to layering or preferred mineral orientation
• Dielectric permittivity describes a material's ability to store electric charge and affects the propagation of electromagnetic waves
• Magnetic susceptibility is a measure of a material's ability to become magnetized in the presence of an external magnetic field

Electromagnetic Fields and Waves

• Electromagnetic (EM) fields consist of coupled electric and magnetic fields that oscillate perpendicular to each other and the direction of wave propagation
• EM waves are characterized by their frequency ($f$) and wavelength ($\lambda$), related by the speed of light ($c$): $c = f\lambda$
• The EM spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays, in order of increasing frequency and decreasing wavelength
• EM waves can propagate through vacuum and various media, with their velocity and attenuation dependent on the material properties
• In geophysics, low-frequency EM methods (radio and audio frequencies) are commonly used for subsurface investigations
• The skin depth is the depth at which the amplitude of an EM wave is attenuated to 1/e (about 37%) of its surface value and depends on the frequency and the material's electrical properties
• EM waves can be reflected, refracted, and diffracted at interfaces between materials with different electrical properties
• Polarization of EM waves refers to the orientation of the electric field vector, which can be linear, circular, or elliptical

Resistivity Methods

• Resistivity methods measure the electrical resistivity distribution of the subsurface by injecting current and measuring potential differences
• The apparent resistivity ($\rho_a$) is the resistivity of a homogeneous half-space that would give the same measured resistance for a given electrode configuration
  • Apparent resistivity is calculated from the measured resistance ($R$), the injected current ($I$), and a geometric factor ($K$) that depends on the electrode arrangement: $\rho_a = KR = K\frac{V}{I}$
• Common electrode arrays include Wenner, Schlumberger, dipole-dipole, and pole-pole, each with its own geometric factor and sensitivity pattern
• Vertical electrical sounding (VES) involves increasing the electrode spacing to investigate deeper layers, assuming a 1D (layered) resistivity structure
• Electrical resistivity tomography (ERT) uses multiple electrode positions and spacings to create a 2D or 3D resistivity model of the subsurface
• The depth of investigation depends on the electrode spacing and array type, with larger spacings probing greater depths
• Interpretation of resistivity data often involves inverse modeling to find a resistivity model that best fits the measured data
• Applications include groundwater exploration, mineral exploration, environmental site characterization, and geotechnical investigations

Induced Polarization (IP) Methods

• Induced polarization (IP) is an extension of resistivity methods that measures the capacitive properties of the subsurface
• IP effects are caused by the accumulation of ions at grain boundaries or the presence of metallic minerals, leading to a delayed voltage response
• The IP response is measured in the time domain (as a voltage decay) or the frequency domain (as a phase shift between the injected current and measured voltage)
• Chargeability ($M$) is a measure of the IP effect in the time domain, defined as the ratio of the secondary voltage ($V_s$) to the primary voltage ($V_p$) during current injection: $M = \frac{V_s}{V_p}$
• In the frequency domain, the IP effect is characterized by the phase angle ($\phi$) between the injected current and measured voltage, or by the percent frequency effect (PFE)
• IP responses are influenced by the type and concentration of polarizable minerals (e.g., sulfides, clays), as well as the pore structure and fluid content
• IP data are often presented as pseudosections or 2D/3D models of chargeability or phase angle
• Applications of IP include mineral exploration (especially for disseminated sulfides), environmental site characterization (e.g., contaminant plumes), and hydrogeological studies (e.g., clay content)

Electromagnetic (EM) Survey Techniques

• EM methods use time-varying magnetic fields to induce eddy currents in the subsurface, which generate secondary magnetic fields that are measured by receivers
• EM methods can be classified as frequency domain (FEM) or time domain (TEM), depending on whether the primary field is a continuous sinusoid or a pulsed waveform
• FEM techniques include ground conductivity meters (GCM), very low frequency (VLF), and controlled-source audio-frequency magnetotellurics (CSAMT)
  • GCM uses small coils to measure the apparent conductivity of the near-surface at frequencies around 10 kHz
  • VLF utilizes distant radio transmitters (15-30 kHz) as a source and measures the tilt angle or ellipticity of the polarization ellipse
  • CSAMT employs a controlled source over a range of frequencies (1 Hz to 10 kHz) to measure the Earth's impedance
• TEM methods include transient electromagnetics (TEM) and ground penetrating radar (GPR)
  • TEM systems use a transmitter loop to generate a pulsed primary field and measure the decay of the secondary field in the off-time
  • GPR uses high-frequency (10 MHz to 1 GHz) EM pulses and measures the travel times and amplitudes of reflections from subsurface interfaces
• Airborne EM (AEM) surveys, such as VTEM and SkyTEM, allow rapid coverage of large areas for regional mapping and exploration
• Interpretation of EM data involves forward and inverse modeling to estimate the subsurface conductivity structure
• Applications of EM methods include mineral and groundwater exploration, environmental site assessment, and geotechnical investigations

Data Acquisition and Instrumentation

• Electrical and EM data acquisition involves measuring voltages, currents, and magnetic fields using various sensors and instrumentation
• For resistivity and IP surveys, a transmitter injects a controlled current into the ground through electrodes, while a receiver measures the resulting voltages
  • Modern resistivity/IP systems use multi-channel receivers and multi-electrode arrays for efficient data collection
• EM surveys require a transmitter to generate a primary EM field and a receiver to measure the secondary fields
  • Transmitters can be current loops, grounded dipoles, or magnetic dipoles, depending on the method and target depth
  • Receivers typically use coils (induction coils or magnetometers) to measure the magnetic field components
• GPR systems consist of a transmitting antenna that emits high-frequency EM pulses and a receiving antenna that records the reflected signals
• Proper survey design is crucial for obtaining high-quality data and includes considerations such as station spacing, line orientation, and target depth
• Accurate positioning and orientation of sensors are essential, often achieved using GPS, total stations, or inertial navigation systems (INS)
• Quality control (QC) measures, such as repeatability tests and reciprocal measurements, help assess data reliability and identify potential issues
• Advances in instrumentation, such as wireless communication, real-time data processing, and integration with other geophysical methods, have improved data acquisition efficiency and quality

Processing and Interpretation of Electrical/EM Data

• Processing and interpretation of electrical and EM data aim to convert the measured data into meaningful information about the subsurface properties and structure
• Data processing steps typically include:
  • Data quality control and editing to remove noise, outliers, and inconsistent measurements
  • Geometric corrections to account for topography, line orientation, and sensor positions
  • Filtering and smoothing to enhance signal-to-noise ratio and remove unwanted frequencies or spatial wavelengths
  • Drift corrections for time-dependent variations in the measurements
  • Normalization and scaling to facilitate data comparison and interpretation
• Data visualization techniques, such as pseudosections, contour maps, and 3D rendering, help identify patterns and anomalies in the data
• Quantitative interpretation often involves forward and inverse modeling to estimate the subsurface properties that best fit the observed data
  • Forward modeling predicts the data response for a given subsurface model, while inverse modeling seeks to find the model that minimizes the misfit between predicted and observed data
• Inversion algorithms, such as least-squares, regularization, and stochastic methods, are used to solve the inverse problem and obtain the best-fit model
• Interpretation should consider the non-uniqueness and resolution limitations of the data, as well as the geological context and prior information
• Integration with other geophysical, geological, and borehole data can greatly improve the reliability and interpretability of the results

Applications in Geoscience and Engineering

• Electrical and EM methods have a wide range of applications in geoscience and engineering, providing valuable information about the subsurface properties and structure
• Mineral exploration: detecting and delineating ore bodies, such as sulfide deposits, through their contrasting electrical properties
  • IP methods are particularly useful for identifying disseminated sulfides, while EM methods can detect massive sulfides and conductive alteration zones
• Groundwater exploration and management: mapping aquifers, estimating aquifer properties (e.g., porosity, permeability), and identifying potential contamination sources
  • Resistivity and EM methods can delineate freshwater-saltwater interfaces, clay layers, and fracture zones that control groundwater flow
• Environmental site characterization: assessing contamination, monitoring remediation, and detecting buried objects or infrastructure
  • EM methods, such as GPR and GCM, are effective for mapping shallow contaminant plumes, buried tanks, and pipelines
• Geotechnical investigations: characterizing soil and rock properties, identifying geologic hazards, and planning infrastructure projects
  • Resistivity and EM methods can map subsurface layering, faults, and voids that may affect foundation stability or tunnel construction
• Geothermal exploration: identifying and characterizing geothermal reservoirs based on their electrical properties and fluid content
  • MT and CSAMT surveys can image deep conductive structures associated with geothermal systems
• Archaeology and cultural heritage: detecting and mapping buried artifacts, structures, and features of historical or cultural significance
  • GPR and resistivity methods are commonly used for non-invasive archaeological prospecting and site investigation
• Integration with other geophysical methods, such as seismic, gravity, and magnetic surveys, can provide a more comprehensive understanding of the subsurface and reduce interpretation ambiguities