Electrical Resistivity Imaging (ERI)

Electrical Resistivity Tomography: High-Resolution Subsurface Imaging

Electrical Resistivity Tomography (ERT) represents one of the most versatile and powerful tools in modern geophysical investigation, providing detailed cross-sectional images of subsurface conditions with exceptional resolution. This proven technique has become indispensable for environmental site characterization, groundwater studies, geotechnical investigations, and contamination mapping where understanding both lateral and vertical subsurface variability is critical to project success.

The Science Behind ERT

The ERT method operates on a fundamental principle of physics: different earth materials resist the flow of electrical current to varying degrees. By introducing a controlled direct current (DC) into the ground through a pair of electrodes and measuring the resulting voltage distribution at the surface through additional electrode pairs, we can map the electrical resistivity—or its inverse, conductivity—of subsurface materials. Resistivity is a bulk physical property that reflects the ease with which electrical current passes through a material, and this property varies dramatically across different geological conditions. Clean, dry sand or solid bedrock typically exhibits high resistivity, while water-saturated soils, clay-rich materials, or zones with elevated dissolved ion concentrations show much lower resistivity values. Structural features such as fractures, faults, and joint systems also create distinctive resistivity signatures that our experienced geophysicists can identify and interpret.

The measurements we collect provide detailed information about both the direction and magnitude of current flow in the subsurface. Through sophisticated mathematical processing and inversion algorithms, these individual measurements are transformed into comprehensive two-dimensional or three-dimensional images that reveal the spatial distribution of resistivity throughout the survey area. This imaging capability allows us to visualize subsurface conditions in ways that traditional point-sampling methods simply cannot achieve, providing our clients with a complete understanding of subsurface heterogeneity and structure.

Our Survey Methodology

Conducting an ERT survey requires both technical expertise and strategic planning to ensure optimal data quality and interpretability. We begin by deploying a linear array of electrodes across the area of interest, with each electrode connected to a multi-channel resistivity transmitter/receiver and intelligent electrode control system. The spacing between electrodes and the array configuration—whether Wenner, Schlumberger, dipole-dipole, or pole-dipole—is carefully selected based on your specific project objectives, the anticipated depth of investigation, and the local geological setting. Our extensive field experience allows us to adapt array geometries to accommodate challenging terrain, optimize resolution at critical depths, and maximize the information content of each survey.

In many applications, we strategically integrate electromagnetic (EM) surveys as a reconnaissance tool prior to ERT data collection. The EM method provides rapid, continuous profiling that helps us identify anomalous zones and optimize the placement of ERT survey lines where they will yield the most valuable information. This integrated approach ensures efficient use of project resources while maximizing subsurface coverage and data quality. The strength of ERT lies in its ability to map lateral subsurface features with excellent spatial resolution, producing cross-sectional images that clearly delineate geological layers, zones of varying saturation, contaminant plumes, and structural discontinuities. Unlike methods that provide only plan-view or point-specific information, ERT delivers true cross-sectional profiles that reveal how subsurface conditions vary both horizontally and with depth.

Applications and Interpretation

The interpretive power of ERT becomes particularly evident in environmental applications where defining the three-dimensional extent of contamination is essential for remediation planning and regulatory compliance. Through specialized analysis and integration with complementary data sources, our geophysicists can determine not only the lateral spread of inorganic contamination but also its vertical extent and concentration gradients. When we combine electromagnetic and resistivity data, the result is a comprehensive volumetric assessment of contamination that provides the quantitative information needed for cost-effective remediation design. Our interpretation goes beyond simply presenting resistivity values—we translate geophysical signatures into actionable geological and hydrogeological insights that directly support your decision-making process.

Whether you're characterizing a contaminated site, mapping groundwater resources, investigating foundation conditions, or delineating geological structures, our ERT services deliver the subsurface clarity you need with minimal invasiveness and cost-effectiveness that traditional drilling programs cannot match. Our commitment to quality data acquisition, rigorous processing, and expert interpretation ensures that you receive reliable, defensible results that support confident project planning and execution.