Unlocking Subsurface Insights with Electrical Resistivity Surveys

Unlocking Subsurface Insights with Electrical Resistivity Surveys

Electrical resistivity surveys are a powerful, non-intrusive geophysical method for imaging underground conditions. By measuring how subsurface materials resist electrical current flow, these surveys provide essential data for environmental assessments, groundwater studies, geotechnical investigations, and mineral exploration (just to name a few applications).

How Electrical Resistivity Surveys Work

Electrical resistivity surveys rely on a basic principle: different subsurface materials conduct electricity differently. By injecting current into the ground through a pair of electrodes and measuring the resulting voltage at another pair, we can calculate the apparent resistivity of the subsurface. The term “apparent” is used because the result reflects an average over an unknown 3D volume of varying material—not a precise point measurement.

Advanced surveys (like the ones HydroGeo performs) use multi-electrode arrays connected to an automated resistivity meter. These systems can collect hundreds or thousands of data points in a single deployment, often in configurations like Wenner, Schlumberger, or Dipole-Dipole arrays depending on the resolution and depth objectives.

What Influences Resistivity?

• Fluid Saturation & Salinity – Water-saturated soils and rocks conduct electricity better than dry ones. The conductivity Unlocking Subsurface Insights with Electrical Resistivity Surveysincreases significantly when groundwater contains dissolved salts or contaminants. This makes resistivity a sensitive method for detecting both water and some types of pollution.
  

• Soil & Rock Type – Clay-rich soils tend to have low resistivity, while materials like dry sand, gravel, or bedrock (especially crystalline types like granite) show higher values.
  

• Porosity & Connectivity – The amount and connectivity of pore spaces in rock or soil affects how easily fluids—and therefore electrical current—can move.
  

• Temperature & Mineral Content – Conductive minerals (like graphite or sulfides) can skew results. Temperature can also subtly affect measurements.

Applications of Electrical Resistivity Surveys

Electrical resistivity methods are especially useful when you need spatial context for what’s underground—without poking dozens of expensive holes in the earth. HydroGeo has used resistivity imaging across a wide range of sites—from groundwater studies in fractured rock to shallow subsurface investigations at complex brownfield sites and landfills. Common applications include:

Environmental Studies

• Locating groundwater sources

• Mapping contaminant plumes for remediation efforts

• Monitoring landfill conditions and potential leachate migration

• Delineating the extent of buried waste in old, unmapped landfill cells

Geotechnical Investigations

• Identifying shallow bedrock, weathered zones, or structural weaknesses
  

• Detecting voids, karst features, and sinkhole-prone areas
  

• Supporting design of stormwater infiltration systems by mapping clay lenses or high-resistivity features

Groundwater and Water Resource Studies

• Characterizing fractured bedrock aquifers
  

• Guiding well placement in rural and agricultural areas
  

• Supporting drought resilience planning by mapping recharge zones
  

Mining & Natural Resource Exploration

• Mapping geological structures for mineral exploration

• Identifying underground water reservoirs

• Assessing potential excavation sites for resource extraction

Advantages of Electrical Resistivity Surveys

• Non-invasive: No drilling or excavation required
  

• Cost-effective: Rapid data collection across large areas
  

• Flexible: Works in open fields, many urban settings, forested sites, and along slopes
  

• Integratable: Combines well with EM, GPR, and soil boring data for cross-validation

Limitations & Considerations

While electrical resistivity surveys are highly effective, they do have some limitations:

• Depth Resolution: Deeper imaging requires wider electrode spacing, which can reduce detail. Shallow, high-resolution imaging and deep surveys are often a trade-off.
  

• Site Access & Surface Conditions: Thick vegetation, paved surfaces, or rocky terrain can limit electrode installation.
  

• Data Interpretation: Resistivity data requires geologic context for reliable interpretation. Without it, models can be misleading. (Which is why you want someone like us doing the interpretation.)

HydroGeo Environmental: Your Experts in Electrical Resistivity Surveys

At HydroGeo, we don’t just “run resistivity”—we integrate it into a broader understanding of site geology, soils, groundwater behavior, and environmental risks. We've performed resistivity imaging on vineyards, military bases, archaeological sites, construction sites, brownfields, and water supply projects across the US. We understand that every site is different—our team works closely with clients to design tailored surveys that answer specific questions about the subsurface.

Whether you’re siting a well, evaluating a potential contaminant source, mapping voids and shallow rock, or planning a new development, our experienced team ensures accurate results for informed decision-making.

Need a geophysical survey? Contact HydroGeo Environmental today to learn how electrical resistivity surveys can help with your next project.