🎓 Lesson 17
D5
Electrical Resistivity Imaging (ERI) for Clay Lens Detection
Electrical Resistivity Imaging (ERI) is a non-invasive method that sends small electrical currents into the ground and measures how much the subsurface resists the flow—helping us 'see' hidden clay layers without digging.
🎯 Learning Objectives
- ✓ Explain the physical basis of resistivity contrast between clay lenses and surrounding soils/rock
- ✓ Design an optimized Wenner-Schlumberger electrode array for detecting a target clay lens at 5–10 m depth
- ✓ Analyze inverted resistivity sections to identify and delineate clay lens geometry (thickness, lateral extent, top depth)
- ✓ Apply resolution criteria and sensitivity analysis to assess detection limits for clay lenses of varying thickness and saturation
📖 Why This Matters
Clay lenses pose serious geotechnical risks in mining and infrastructure projects—they can cause localized seepage, reduce slope stability, impede dewatering, and trigger unexpected pit wall failures. Traditional drilling alone may miss thin or laterally discontinuous clay layers due to sparse sampling. ERI bridges this gap: it provides continuous, cost-effective, non-invasive imaging of near-surface stratigraphy. In open-pit mine planning, detecting clay lenses early prevents costly re-designs, drainage failures, or foundation settlements—making ERI not just academic, but a frontline risk-mitigation tool.
📘 Core Principles
ERI builds on the fundamental relationship between bulk resistivity (ρ), material composition, pore-fluid salinity, and saturation. Clay minerals have high cation exchange capacity (CEC), attracting conductive ions in pore water—resulting in low resistivity (typically 1–50 Ω·m). In contrast, clean sands or fractured rock range from 100–10,000 Ω·m. The method uses multi-electrode systems to collect hundreds of apparent resistivity measurements across variable spacings (a), enabling vertical and lateral resolution. Forward modeling predicts expected responses; inversion (e.g., least-squares or smoothness-constrained) then iteratively adjusts a subsurface resistivity model until simulated and observed data match within tolerance. Resolution degrades with depth and is governed by array geometry, signal-to-noise ratio, and subsurface heterogeneity.
📐 Apparent Resistivity (Wenner Array)
The Wenner array is widely used in reconnaissance ERI surveys for its simplicity and good vertical resolution in uniform settings. Apparent resistivity quantifies the measured resistance scaled by electrode geometry—and serves as input to inversion. It assumes homogeneous half-space; deviations indicate layering or anomalies like clay lenses.
Wenner Apparent Resistivity
ρₐ = 2πaRCalculates apparent resistivity from measured resistance and electrode spacing for the Wenner array.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| ρₐ | Apparent resistivity | Ω·m | Measured resistivity assuming homogeneous half-space |
| a | Electrode spacing | m | Distance between adjacent electrodes in Wenner configuration |
| R | Measured resistance | Ω | Voltage/current ratio recorded by resistivity meter |
Typical Ranges:
Saturated clay lens: 1 – 50 Ω·m
Weathered granite: 300 – 2,000 Ω·m
Dry sand: 1,000 – 10,000 Ω·m
💡 Worked Example
Problem: A Wenner array with electrode spacing a = 2 m yields a measured resistance R = 48 Ω. Calculate apparent resistivity ρₐ.
1.
Step 1: Recall Wenner geometric factor K = 2πa
2.
Step 2: Apply ρₐ = K × R = 2π × 2 m × 48 Ω
3.
Step 3: Compute: ρₐ = 603.2 Ω·m (≈ 600 Ω·m)
Answer:
The apparent resistivity is 603 Ω·m, indicating a relatively resistive near-surface layer—suggesting absence of saturated clay at shallow depth under this measurement point.
🏗️ Real-World Application
At the Telfer Gold Mine (Western Australia), ERI was deployed prior to waste dump construction to map a suspected glacial till clay lens beneath alluvial cover. A 72-electrode Wenner-Schlumberger array (max a = 10 m) imaged a 1.5–3.0 m thick, laterally continuous clay unit at 4–7 m depth (ρ = 8–15 Ω·m), confirmed by three verification boreholes spaced 50 m apart. The ERI-derived geometry directly informed drain trench placement and liner design—avoiding $2.1M in unplanned excavation and liner overdesign. Inversion RMS misfit was <5%, and vertical resolution at target depth was ±0.8 m (per AGSO Geophysical Guidelines).