Applied Geophysics: Electric and Refraction Methods

Applied Geophysics (Electric, Refraction)

• This presentation covers applied geophysics focusing on electrical resistivity and refraction methods.
• These methods are used to investigate subsurface materials and map geological features.

Electrical Resistivity Techniques

• Geophysical resistivity techniques analyze the Earth's response to electrical current flow.
• An electrical current is passed through the ground, and potential differences are measured.
• This data reveals subsurface material impedance (ratio of potential to current).
• Apparent resistivity is a function of the measured impedance and electrode array geometry.
• Data is presented in 1-D soundings, profiles, or 2-D cross-sections to identify anomalous regions.

Factors Affecting Resistivity

• Water content significantly influences subsurface conductivity, and thus, resistivity.
• Increased water saturation, salinity, porosity (voids), and fractures decrease measured resistivity.
• Conversely, reduced water content and increased compaction of rocks increase resistivity.
• Air presence in voids increases subsurface resistivity.

Resistivity Measurements

• Measurements are depth-dependent based on electrode separation.
• Apparent resistivity values are averages over the current path length.
• Two-dimensional images (pseudosections) of apparent resistivity variations represent subsurface current flow paths.
• Computer modeling is helpful in interpreting geoelectric data to create accurate earth models.

Geophysical Methods

• Geophysical methods are categorized as active and passive.
• Passive methods use naturally occurring fields to measure Earth properties (e.g., self-potential (SP), magnetotellurics (MT), telluric, gravity, magnetic).
• Active methods introduce a signal into the Earth and measure the response (e.g., direct current (DC) resistivity, seismic refraction, induced polarization (IP), electromagnetic (EM), Mise-à-la-masse (MALM), ground penetrating radar (GPR)).

Electrical Method Instruments

• Several instruments are used for electrical methods, including Terrameter Signal Averaging System (SAS4000), the Syscal.
• Many other instruments have been and continue to be made by various businesses and countries around the world.

Electrical Method Applications

• Used for calculating underlying layer depths and thicknesses.
• Used to determine horizontal and vertical resistivity for subsurface layers.
• Locating archaeological remains and identifying cavities, voids.
• Used for engineering purposes, such as detecting dam leakage locations.
• Used for groundwater table determination and flow direction assessment.
• Used in petroleum exploration (well logging), often in conjunction with other logs (SP).
• Useful for mineral exploration, though interpretation is challenging below certain depths (1000-1500 ft).

Ohm's Law

• Ohm's Law (V = IR) relates voltage, current, and resistance.
• Resistivity, not resistance, is the intrinsic material property indicating the resistance to electric current flow. It is calculated based on the change of resistance divided by the cross-sectional area and length of the material.
• Resistivity is measured in ohm-meters (Ωm) or ohm-feet (Ωft).
• Resistivity is the reciprocal of conductivity.

Material Resistivity Values

• A table lists the resistivity range for different materials (e.g., air, pyrite, galena, water).
• Different Earth materials often overlap.

Clay Minerals and Resistivity

• Clay minerals, due to their ability to combine with water and ions, generally decrease resistivity.
• Clay minerals can absorb cations on the surface, or ionize and contribute to free ions in the soil solution.
• Other factors also impact resistivity, including geologic age, salinity, pore content, temperature, porosity, pressure and depth.

Field Considerations for DC Resistivity

• Ensure good electrode contact with the ground through wet conditions, salt, bentonite.
• Conduct surveys along straight lines wherever possible.
• Avoid cultural features (power lines, pipes, metal fences, pumps).

Sources of Noise

• Electrode polarization, telluric currents, and nearby conductors introduce noise that affects measurements.
• Copper or copper sulfate solutions can be used for electrodes to reduce polarization effects.
• Reversing current or employing a slowly varying AC current can mitigate telluric current effects.

Additional Factors Affecting Resistivity

• High resistivity near the surface makes current flow deeper challenging.
• Topography (hills / valleys) influences current flow patterns, with valleys concentrating current flow and hills dispersing it.
• Different resistivity may be present when measuring parallel versus perpendicular to bedding planes (electrical anisotropy).
• Examine carefully the various forms of instrumental noise and cultural features.

Electrode Configurations

• Wenner, Lee-Partitioning, and Schlumberger arrays are commonly used to measure apparent resistivity.
• Wenner is widely used in the western hemisphere and is sensitive to horizontal variations.
• Schlumberger is a common prospect method for resistivity.

Survey Design

• Vertical Electrical Sounding (VES) measures resistivity variation with depth at a single point.
• Horizontal Electrical Profiling (HEP) maps variations in resistivity along a line, typically aiming to resolve layers and near-surface features.

Additional Resistivity Applications

• Used to find bedrock depth, locate permafrost, and identify or map landfills.

Disadvantages of Wenner and Resistivity Methods

• Wenner array interpretations are limited to simple, horizontal layers.
• Resistivity interpretations can be ambiguous, requiring independent geophysical/geological control.
• Topography can mask deeper variations in rock property.
• Penetration depth is limited by the maximum electrical power and cable length.
• Depth accuracy may be less than with other methods (e.g., seismic, or drilling).

Refraction Instruments and Applications

• Geophones are used to detect seismic energy propagation in the ground (Refraction).
• Refraction methods provide rapid and valuable information beneath the surface for reconnaissance, engineering, and groundwater prospecting (e.g., hazardous waste assessments).

Seismic Refraction Principles

• Refraction principles rely on Snell's Law to trace refracted seismic waves to understand the interfaces between subsurface layers.
• Travel times of first arrivals are recorded and plotted against distance (Time-Distance curves) to calculate layer velocities and depth to layers.

Seismic Refraction Data Acquisition and Interpretation

• A source and multiple geophones are used to gather arrival data.
• Data is plotted on a travel time vs. distance graph, with arrival time on the vertical (y) axis.
• Slopes from the curves give velocities.
• Depth to the boundary / layers can be determined using velocity and distance-to-point-along-the-curve calculations.

Seismic Velocities of Rock Materials

• A table provides approximate P- and S-wave velocities for different rock materials and water.
• Velocities vary with factors such as mineral content, lithology, porosity, fluid saturation, and pore pressure.

Seismic Refraction Exercise (Procedure steps)

• Steps for data acquisition, such as picking first breaks, constructing travel-time versus distance plots, deriving layer velocities, and calculating layer depths, are laid out.
• Steps for creating stratigraphic columns are included.

This quiz covers applied geophysics, focusing on electrical resistivity and refraction methods for investigating subsurface materials. Learn how these techniques help map geological features by analyzing the Earth's response to electrical currents and the factors that affect resistivity. dont write question about laws