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

# Geophysical and geotechnical exploration

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

- Asked in: 2079 Asoj, 3 marks; 2073 Bhadra, 4 marks; 2075 Bhadra, 3 marks (asked 3 times)
- Video: [Seismic Geophysics Methods in Geological, Geotechnical and Civil Engineering Investigations](https://www.youtube.com/watch?v=c0B-XU8NESY) — Soil Mechanics and Engineering Geology
- 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-geotechnical-exploration
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4

## Question

Describe geophysical and geotechnical exploration (differentiate between them).

## Short answer

Geophysical exploration measures physical properties (seismic velocity, resistivity) indirectly from the surface; it is fast, cheap, covers large areas but needs borehole calibration. Geotechnical exploration directly samples and tests ground (boreholes, SPT) to obtain engineering properties, reliable but point-wise and costly.

## Answer

#### Geophysical exploration
It is the study of the sub-surface by measuring physical properties of the ground (velocity, resistivity, density, magnetism) from the surface, without excavation. It is an **indirect** method.

Methods:
- *Seismic refraction/reflection:* wave velocity gives layer depth and rock quality.
- *Electrical resistivity:* detects bed rock, groundwater, clay and cavities.
- *Gravity and magnetic methods:* large structures and faults.
- *Ground penetrating radar* and *electromagnetic* methods for shallow features.

Advantages: quick, cheap, covers large area, non-destructive. Limits: results are interpretation-based and need calibration.

#### Geotechnical exploration
It is the **direct** investigation of the ground by boring, test pits, trenches, sampling, and in-situ and laboratory tests, to obtain the engineering properties needed for design (bearing capacity, strength, compressibility, permeability).

Methods: boreholes with SPT, undisturbed sampling, core drilling with RQD, plate load test, vane shear, Lugeon test and laboratory tests.

#### Difference
| Point | Geophysical | Geotechnical |
|---|---|---|
| Nature | Indirect | Direct |
| Measures | Physical properties | Engineering properties |
| Cost and speed | Cheap, fast | Costly, slow |
| Area covered | Large | Point-wise |
| Accuracy | Interpretive, less precise | Reliable and precise |
| Use | Preliminary stage, interpolation | Detailed stage, design |

The two are used together: geophysics fills the gaps between boreholes, and borehole data calibrate the geophysics.

## Check yourself

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

1. Geophysical exploration is which type?
   A. Direct by boring
   B. Direct by test pits
   C. Indirect, measuring physical properties from the surface
   D. Laboratory testing only
   Answer: C. It does not excavate the ground. (Key point: Nature)

2. What does geotechnical exploration obtain?
   A. Magnetism of rock
   B. Satellite images
   C. Gravity anomalies
   D. Engineering properties through boring, sampling and tests
   Answer: D. It is direct and used in the detailed design stage. (Key point: Geotechnical)

3. Which is correct?
   A. Geophysical is cheap and fast over a large area; geotechnical is costly and point-wise
   B. Geophysical is precise and costly
   C. Geotechnical covers a large area cheaply
   D. Both are indirect
   Answer: A. Geophysical is interpretive; geotechnical is reliable. (Key point: Difference)

4. Why are the two used together?
   A. To avoid boreholes entirely
   B. Geophysics interpolates between boreholes and geotechnical data calibrates it
   C. To replace lab tests
   D. To increase cost only
   Answer: B. Geophysical results are not unique and need calibration. (Key point: Combined use)

## Related in Engineering Geology II

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

## Related in other subjects

- [Accessible and inaccessible exploration methods](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aaccessible-inaccessible-exploration) — Foundation Engineering, chapter 2