What lies beneath the surface controls everything — the stability of slopes, the flow of groundwater, the root zones of landslides, the foundations of structures. Geophysics gives us a way to see it without digging — and in Nepal's remote mountain terrain, that ability is not just convenient, it is often the only practical option.
Why Geophysics in Nepal?
Nepal's geological hazard landscape demands subsurface understanding at a scale and frequency that traditional drilling cannot provide. Boreholes are expensive, invasive, and provide point information only. In contrast, a single ERT survey profile can image subsurface conditions continuously along a 200 m traverse at a fraction of the cost — and can be deployed by a small team in terrain that no drilling rig could access.
For a country with Nepal's combination of acute geological hazards, limited technical resources, and vast areas of remote high-mountain terrain, non-invasive geophysical methods offer a compelling value proposition: more information, more affordably, in more places.
Electrical Resistivity Tomography (ERT)
ERT is my primary geophysical tool. It works by injecting electrical current into the ground between two current electrodes and measuring the resulting potential difference between two other electrodes — a measurement repeated thousands of times across an array of electrodes planted along the survey line. The apparent resistivity values measured are then mathematically inverted to produce a 2D cross-section of true resistivity variation with depth.
What Resistivity Tells Us
Electrical resistivity is sensitive to lithology, grain size, porosity, saturation, and pore fluid chemistry. This sensitivity is exactly what makes it valuable for geological investigation — a saturated clayey colluvium (5–30 Ω·m) looks completely different from the dry crystalline bedrock beneath it (>500 Ω·m), allowing the failure surface between them to be mapped with precision.
Electrode Arrays & Survey Design
Different electrode configurations (arrays) have different sensitivity patterns and resolution characteristics. For the shallow landslide applications that dominate my work, the Wenner-Schlumberger array offers an excellent compromise between horizontal resolution, depth penetration, and sensitivity to both lateral and vertical resistivity variations:
| Array Type | Depth Penetration | Resolution | Best Use |
|---|---|---|---|
| Wenner | Moderate | Good vertical | Layered geology |
| Wenner-Schlumberger | Good | Good both | Landslide / general |
| Dipole-Dipole | Shallow-moderate | Excellent horizontal | Lateral structures |
| Pole-Dipole | Deep | Moderate | Deep targets |
Data Processing & Inversion
Raw ERT data requires mathematical inversion to convert the measured apparent resistivity values into a model of true subsurface resistivity. I use RES2DINV (Geotomo Software) — the industry-standard package for 2D ERT inversion — applying a robust least-squares algorithm that minimises the effect of data outliers while producing geologically realistic subsurface models.
Quality control involves: removing obvious outlier data points (typically ±3 standard deviations from the mean for each measurement level), checking reciprocal measurements for repeatability, and evaluating the convergence of the inversion (target RMS error <10%). Sensitivity analysis is performed to assess confidence in features at the edges and base of the model section.
Integration with Geological Data
ERT results are most powerful when integrated with surface geological observations — lithological logs from road cuts and stream banks, structural measurements, and geotechnical data where available. The resistivity model provides the geometric framework; the geological data provides the physical interpretation. Neither is sufficient alone.
In my current Prithvi Highway work, this integration has been particularly productive: ERT profiles acquired across landslide features are interpreted directly in the context of geological cross-sections constructed from field mapping — allowing resistivity zones to be assigned specific lithological identities with high confidence, rather than relying on resistivity ranges alone.
Future Directions
I am interested in expanding my geophysical toolkit to include time-lapse ERT — acquiring repeated profiles at the same location over time to monitor seasonal saturation changes — and exploring the potential of ambient seismic noise tomography for regional-scale imaging of Nepal's shallow crustal structure. Both methods offer significant promise for hazard monitoring applications in the Himalayan region.