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Environmental geophysics plays a crucial role in site characterization, helping us understand what's beneath our feet. By using methods like electrical resistivity and ground-penetrating radar, we can map subsurface structures, detect buried objects, and track contaminants.

Interpreting this data gives us a clearer picture of what's happening underground. By combining geophysical, geotechnical, and geochemical data, we can create comprehensive site models. These models are invaluable for planning remediation efforts and monitoring their progress over time.

Geophysical Methods for Site Characterization

Electrical Resistivity Methods

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  • Electrical resistivity methods measure the resistance of subsurface materials to the flow of electric current, providing information about lithology (e.g., clay, sand, bedrock), porosity, and fluid content (e.g., fresh water, saline water, contaminants)
    • The direct current (DC) resistivity method injects current into the ground through electrodes and measures the resulting potential difference to determine the resistivity distribution
    • The induced polarization (IP) method measures the voltage decay after current injection, indicating the presence of polarizable materials such as clay or metallic minerals (e.g., pyrite, galena)

Ground-Penetrating Radar (GPR)

  • Ground-penetrating radar (GPR) uses high-frequency electromagnetic waves to image subsurface structures and detect buried objects (e.g., pipes, tanks, foundations)
    • GPR reflections are generated by contrasts in dielectric properties, which are influenced by water content, lithology, and the presence of man-made objects
    • The depth of penetration and resolution of GPR depend on the antenna frequency and the electrical conductivity of the subsurface materials
  • Other geophysical methods used in environmental site characterization include seismic refraction and reflection (for bedrock depth and layering), electromagnetic induction (for mapping conductivity variations), and magnetic surveys (for detecting metallic objects)
  • The choice of geophysical methods depends on the target depth, resolution requirements, site conditions (e.g., terrain, vegetation, noise sources), and the physical properties of the materials of interest

Interpreting Geophysical Data

Electrical Resistivity and IP Data Interpretation

  • Electrical resistivity data can be inverted to create 2D or 3D models of the subsurface resistivity distribution, revealing lithological boundaries, fracture zones, and groundwater salinity variations
  • IP data can indicate the presence of clay layers or contamination plumes containing polarizable materials, such as organic pollutants (e.g., hydrocarbons) or heavy metals (e.g., lead, chromium)

GPR and Seismic Data Interpretation

  • GPR profiles show the travel times of reflected waves, which can be converted to depth using the velocity of the subsurface materials
    • GPR reflections can delineate stratigraphic layers, buried pipes, tanks, and other man-made objects
    • The attenuation of GPR signals can indicate the presence of electrically conductive materials, such as clay or contaminated groundwater
  • Seismic data can reveal the depth and geometry of bedrock, sedimentary layers, and the water table, as well as the presence of voids, fractures, and low-velocity zones (e.g., saturated sediments, unconsolidated fill)

Electromagnetic and Magnetic Data Interpretation

  • Electromagnetic induction and magnetic data can detect the presence of metallic objects, such as buried drums or steel pipes, and map the extent of conductive contamination plumes (e.g., leachate, saline intrusion)

Integrating Geophysical Data for Site Assessment

Data Integration Techniques

  • Geophysical data provide spatial information about subsurface structures and properties, while geotechnical data (e.g., borehole logs, soil properties) offer ground-truth constraints and detailed local information
  • Geochemical data (e.g., contaminant concentrations, redox conditions) help characterize the nature and extent of contamination and guide the interpretation of geophysical anomalies
  • Integration of geophysical, geotechnical, and geochemical data enables the development of a comprehensive site conceptual model, identifying potential contaminant sources (e.g., leaking tanks, spills), pathways (e.g., fractures, permeable layers), and receptors (e.g., wells, surface water bodies)
  • Data integration techniques include co-located data analysis (e.g., comparing resistivity and borehole logs), geostatistical methods (e.g., kriging), and coupled inversion of multiple geophysical datasets (e.g., joint inversion of resistivity and seismic data)

Applications in Site Assessment and Remediation Planning

  • The integrated site model supports the design of targeted sampling and monitoring programs, optimizing the placement of wells and remediation infrastructure (e.g., extraction wells, injection points, barriers)

Geophysics in Subsurface Monitoring and Remediation

Monitoring Subsurface Processes

  • Time-lapse geophysical surveys can monitor changes in subsurface properties and processes, such as the migration of contamination plumes or the progress of remediation efforts
  • Electrical resistivity and IP monitoring can track the movement of conductive or polarizable contaminants, as well as changes in subsurface geochemistry induced by remediation activities (e.g., bioremediation, chemical oxidation)
  • GPR surveys can detect changes in the saturation and distribution of non-aqueous phase liquids (NAPLs) during remediation, as well as monitor the integrity of containment systems (e.g., slurry walls, caps)
  • Seismic methods can monitor the evolution of subsurface mechanical properties, such as the development of fractures or the consolidation of treated soils

Assessing Remediation Effectiveness

  • Geophysical monitoring data can be used to calibrate and validate numerical models of subsurface flow, transport, and remediation processes, improving the predictive capabilities and supporting the optimization of remediation strategies
  • The effectiveness of remediation can be assessed by comparing geophysical monitoring results with baseline surveys and performance metrics, such as the reduction in contaminant mass or the achievement of target geophysical properties (e.g., increased resistivity, reduced GPR attenuation)


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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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