10.4 Environmental geophysics and site characterization

Geophysical Methods for Site Characterization

Environmental geophysics applies non-invasive survey techniques to map subsurface conditions at contaminated or development sites. These methods let you detect buried objects, track contaminant plumes, and characterize soil and rock properties without extensive drilling. The results feed directly into site conceptual models that guide remediation design and long-term monitoring.

Electrical Resistivity Methods

Electrical resistivity methods measure how strongly subsurface materials resist the flow of electric current. Resistivity values vary with lithology (clay vs. sand vs. bedrock), porosity, and the type of fluid filling pore spaces (fresh water, saline water, or contaminants). This makes resistivity a powerful tool for distinguishing geological layers and spotting anomalies.

Two key techniques fall under this heading:

• DC resistivity injects current into the ground through surface electrodes and measures the resulting potential difference. By varying electrode spacing and geometry, you build up a profile of how resistivity changes with depth and lateral position.
• Induced polarization (IP) measures the voltage decay after current injection is switched off. Materials like clay minerals or metallic sulfides (pyrite, galena) store and release charge slowly, producing a measurable IP response. This helps distinguish clay-rich zones or mineralized contamination from clean sand or gravel.

Ground-Penetrating Radar (GPR)

Ground-penetrating radar transmits high-frequency electromagnetic pulses into the ground and records reflections from subsurface interfaces. Reflections occur wherever there's a contrast in dielectric properties, which are controlled mainly by water content, lithology, and the presence of man-made objects (pipes, tanks, foundations).

Two trade-offs govern GPR performance:

• Higher antenna frequencies (e.g., 400–1600 MHz) give better resolution but shallower penetration.
• Lower frequencies (e.g., 25–200 MHz) penetrate deeper but resolve less detail.

Electrically conductive materials (clay-rich soils, saline groundwater) strongly attenuate GPR signals, limiting depth of investigation in those settings.

Other Methods

Several additional techniques see regular use in environmental site characterization:

• Seismic refraction and reflection determine bedrock depth, sedimentary layering, and water table position.
• Electromagnetic (EM) induction maps lateral variations in ground conductivity without requiring ground contact, making it efficient for large-area surveys.
• Magnetic surveys detect ferrous metallic objects like buried drums, steel tanks, or pipelines.

The choice of method depends on target depth, required resolution, site conditions (terrain, vegetation, cultural noise), and the physical property contrasts you're trying to exploit.

Interpreting Geophysical Data

Electrical Resistivity and IP Data Interpretation

Raw resistivity measurements are inverted to produce 2D or 3D models of the subsurface resistivity distribution. These models can reveal:

• Lithological boundaries (e.g., a clay aquitard overlying sand)
• Fracture zones (which often appear as low-resistivity linear features)
• Groundwater salinity variations (saltwater is far more conductive than freshwater)

IP data add another diagnostic layer. Elevated IP responses can indicate clay-rich horizons or contamination plumes containing polarizable materials such as hydrocarbons or dissolved heavy metals (lead, chromium). Combining resistivity and IP results helps you distinguish, for example, a clean clay layer from a contaminated sand unit that might have similar resistivity values.

GPR and Seismic Data Interpretation

GPR profiles display reflected wave travel times, which you convert to depth using the electromagnetic wave velocity of the subsurface materials (typically estimated from hyperbola fitting or common-midpoint surveys).

• Continuous reflectors delineate stratigraphic layers, the water table, or the tops of buried structures.
• Discrete hyperbolic reflections often indicate point targets like pipes or boulders.
• Signal attenuation (loss of reflections at depth) can itself be diagnostic, pointing to conductive materials such as clay or contaminated groundwater.

Seismic data reveal bedrock geometry, sedimentary layering, and the water table through velocity contrasts. They can also identify voids, fracture networks, and low-velocity zones caused by saturated unconsolidated fill.

Electromagnetic and Magnetic Data Interpretation

EM induction maps apparent conductivity across a site, highlighting conductive anomalies like leachate plumes or zones of saline intrusion. Magnetic surveys detect ferromagnetic objects (buried drums, steel pipes, rebar) as dipolar anomalies. Together, these methods are especially useful for rapid screening of large sites before deploying more detailed techniques like resistivity or GPR.

Integrating Geophysical Data for Site Assessment

Data Integration Techniques

No single data type tells the whole story. Effective site characterization combines three streams of information:

• Geophysical data provide spatially continuous images of subsurface properties (resistivity, velocity, dielectric constant).
• Geotechnical data (borehole logs, soil index properties, penetration tests) supply ground-truth constraints at discrete locations.
• Geochemical data (contaminant concentrations, pH, redox potential) characterize the nature and extent of contamination and help explain geophysical anomalies.

Bringing these together produces a site conceptual model that identifies contaminant sources (leaking tanks, surface spills), transport pathways (fractures, permeable sand layers), and receptors (drinking water wells, surface water bodies).

Common integration approaches include:

1. Co-located data comparison — overlaying resistivity profiles on borehole logs to calibrate geophysical interpretations.
2. Geostatistical methods — using techniques like kriging to interpolate between sparse borehole data, guided by the denser geophysical coverage.
3. Joint or coupled inversion — simultaneously inverting multiple geophysical datasets (e.g., resistivity and seismic) to produce models that satisfy both datasets and reduce ambiguity.

Applications in Site Assessment and Remediation Planning

The integrated site model directly supports practical decisions: where to place monitoring wells, how to orient extraction or injection wells, and where to install barriers or caps. By reducing uncertainty about subsurface conditions, geophysical integration cuts the number of boreholes needed and helps avoid costly surprises during construction.

Geophysics in Subsurface Monitoring and Remediation

Monitoring Subsurface Processes

Time-lapse geophysical surveys repeat measurements at the same locations over weeks, months, or years to track changes in subsurface conditions. The baseline survey establishes a reference, and subsequent surveys reveal how properties evolve.

Specific monitoring applications include:

• Resistivity and IP monitoring can track the migration of conductive contaminant plumes or detect geochemical changes caused by remediation activities like biostimulation or in-situ chemical oxidation.
• Repeat GPR surveys can detect changes in the saturation and spatial distribution of non-aqueous phase liquids (NAPLs) during pump-and-treat or surfactant flushing, and can verify the integrity of containment structures (slurry walls, engineered caps).
• Seismic monitoring can detect changes in mechanical properties, such as fracture development during hydraulic treatment or soil consolidation following stabilization.

Assessing Remediation Effectiveness

Time-lapse geophysical data serve two roles in evaluating remediation:

1. Model calibration and validation — Monitoring results can be compared against numerical flow-and-transport models to test predictions and refine parameters, improving forecasts of how the site will evolve.
2. Performance assessment — Comparing current surveys to the baseline lets you quantify progress toward cleanup goals. For example, increasing resistivity in a previously conductive plume zone suggests declining contaminant concentrations, while reduced GPR attenuation may indicate NAPL removal.

These comparisons help site managers decide whether to continue, modify, or conclude active remediation.