---
title: "Define geophysical exploration"
subject: "Engineering Geology II"
chapter: 4
key: "semester_4/engineering_geology_ii:geophysical-exploration"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/geophysical-exploration
site: BCT Engineering
---

# Define geophysical exploration

Engineering Geology II · Chapter 4: Engineering geology in site selection, investigation & construction/excavation

- Asked in: 2078 Chaitra, 3 marks; 2075 Baisakh, 3 marks; 2072 Asoj, 3 marks (asked 3 times)
- Video: [Unlocking Earth's Secrets: Geophysical Exploration methods : Seismic  Magnetic Gravity](https://www.youtube.com/watch?v=q-aq2wJFzWU) — Earth Science
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/geophysical-exploration
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4

## Question

Define and describe geophysical exploration.

## Short answer

Geophysical exploration indirectly investigates subsurface by measuring seismic velocity, resistivity, etc. from surface. Main methods: seismic refraction for rock depth, resistivity for groundwater and cavities, GPR for shallow features. It is fast, cheap, non-destructive but needs borehole calibration.

## Answer

**Geophysical exploration** is the investigation of sub-surface geology by measuring physical properties of the earth (seismic velocity, electrical resistivity, density, magnetic susceptibility, radar reflection) at the surface or in boreholes, and interpreting the readings to find the thickness, depth and nature of strata, bed rock, water and structures. It is an indirect and non-destructive method.

#### Main methods
1. **Seismic refraction:** a hammer or explosive creates waves; geophones record first arrival times; time-distance graph gives velocities and layer depths. Velocity shows rock quality (soil about 0.2-0.8 km/s; hard rock about 3-6 km/s).
2. **Electrical resistivity:** electrodes in a Wenner or Schlumberger array send current; apparent resistivity vs. spacing identifies water table, clay and bed rock.
3. **Gravity and magnetic methods:** variations detect cavities, faults, dykes.
4. **Ground penetrating radar (GPR):** shallow detail, cavities, buried objects.
5. **Borehole logging:** gamma, resistivity and sonic logs.

#### Uses in civil engineering
- Depth to bed rock for foundation, dam and bridge sites.
- Thickness of overburden and landslide slip surface.
- Groundwater exploration.
- Locating faults, cavities, weak zones along tunnel alignments.
- Rippability and quality of rock.

#### Advantages and limits
Fast, cheap, covers a large area; but results are not unique, and need calibration with boreholes.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Geophysical exploration is best described as
   A. Excavating test pits
   B. Measuring physical properties of the earth and interpreting them to find strata, bedrock, water and structures
   C. Drilling cores
   D. Testing concrete
   Answer: B. It is indirect and non-destructive. (Key point: Definition)

2. What does seismic refraction give?
   A. Density only
   B. Electrical resistance
   C. Wave velocities and layer depths, indicating rock quality
   D. Magnetic susceptibility
   Answer: C. Time-distance graphs of first arrivals give velocities and depths. (Key point: Seismic refraction)

3. What does electrical resistivity identify?
   A. Wave velocity
   B. Gravity
   C. Core recovery
   D. Water table, clay and bed rock
   Answer: D. Apparent resistivity vs spacing is interpreted with Wenner or Schlumberger arrays. (Key point: Resistivity)

## Related in Engineering Geology II

- [Geophysical and geotechnical exploration](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Ageophysical-geotechnical-exploration) — Engineering Geology II, chapter 4, asked 3×