Electrical Methods

Electrical resistivity spans more orders of magnitude than any other rock property, which makes current-based methods the workhorses of environmental geophysics. This module covers galvanic resistivity surveying (VES and ERT), the passive self-potential (SP) method, and induced polarization (IP).

Learning Objectives

Undergraduate Core: By the end of this module, you will be able to:

• Explain electrolytic conduction and the roles of saturation, salinity, porosity, clay, and temperature.
• Calculate geometric factors and distinguish true, apparent, and inverted resistivity.
• Compare Wenner, Schlumberger, and dipole–dipole survey behavior.
• Distinguish ERT, SP, and IP mechanisms and avoid treating any response as a unique hydrologic property.

Graduate Extension

Examine sensitivity functions, regularization, equivalence, Cole–Cole conventions, and uncertainty in petrophysical conversion from resistivity to water content.

Practice this module Teach with active-learning slides

Learning Path

Work through the apps in order. Each one opens full screen; the bar at its top shows this path and has Prev and Next buttons.

1. How Do Rocks Conduct Electricity? Interactive lecture

   Electrolytic and electronic conduction, and dielectric polarization: how charges move or polarize in earth materials.

2. ERT · Geometric Factor K Interactive lecture

   Geometric factors of the Wenner, Schlumberger, and dipole–dipole arrays, and why a larger K means a smaller measured voltage.

3. VES · 3-Layer Forward Model Interactive lecture

   Build layered models and generate vertical electrical sounding curves.

4. Apparent-Resistivity Pseudosection Builder Demo

   Place a conductive or resistive body in the subsurface, pick an array, and build the pseudosection measurement by measurement.

5. SP · Signal Mechanisms Interactive lecture

   Streaming, mineralization (redox), and thermoelectric sources of self-potential signals.

6. SP · Field Applications Activity

   Survey a sulfide ore body, a leaking dam, and a geothermal upflow with a virtual SP sensor.

7. IP · Signal Mechanisms Interactive lecture

   Membrane and electrode polarization at the pore scale, and the decaying voltage they leave when the current switches off.

8. IP · Cole-Cole Model Interactive lecture

   Explore how Cole-Cole parameters shape the complex-resistivity spectrum.

After the path: practice questions · classroom lab · data and notebooks.

Classroom Lab

🧰 Three-layer VES group investigation — divide into curve-type, suppression, and equivalence teams, then explain why apparent resistivity and electrode spacing are not a literal depth section.

Research Code: PyHydroGeophysX

From resistivity to water content

The petrophysics in this module (Archie's law, and its clay-corrected cousin the Waxman-Smits model) is exactly how field ERT becomes hydrology. PyHydroGeophysX, developed in Dr. Chen's group, implements these transforms together with full 2D and 3D ERT forward modeling and inversion, including time-lapse and structure-constrained inversion for watershed monitoring.

• Full ERT workflow: mesh, forward model, invert.
• Time-lapse ERT inversion: track moisture change over time.
• Structure-constrained inversion: sharpen boundaries using seismic structure.

Background reading: Archie (1942), Loke et al. (2013), and Binley & Slater (2020) on the References page.

Data and Notebooks

• 📊 Datasets live in the Data area.
• 🚀 Python exercises (including pyGIMLi-based forward modeling) are in Notebooks.