Geophysical exploration for civil engineering: which methods are most commonly adopted for subsurface investigation of foundations, alignment studies, and groundwater?

Introduction / Context:
Geophysical methods help infer subsurface conditions without extensive drilling. For most civil engineering tasks—like assessing depth to rock, rippability, cavities, and groundwater—some methods are more practical and widespread than others.

Given Data / Assumptions:

• Routine civil works: roads, bridges, buildings, dams.
• Need rapid coverage and interpretable contrasts (velocity or resistivity).
• Budget/time constraints compared to oil/mineral exploration.

Concept / Approach:
Seismic methods (mainly refraction in civil work) estimate shear/pressure wave velocities to map bedrock depth and rippability. Electrical resistivity profiles detect groundwater, clay layers, fractures, and contamination by mapping resistivity contrasts. Gravity and magnetic methods are important in regional geophysics but are less commonly adopted for standard civil site investigations due to lower resolution for shallow engineering targets.

Step-by-Step Solution:
Match common civil objectives: depth to bedrock, groundwater, weak zones.Select practical methods: seismic and resistivity meet these efficiently.Thus, “Both (a) and (b)” is correct.

Verification / Alternative check:
Case histories of highways, dams, and foundations frequently cite seismic refraction and resistivity surveys in the preliminary stages.

Why Other Options Are Wrong:

• Gravity/magnetic: valuable for regional mapping but less used for typical shallow engineering decisions.

Common Pitfalls:

• Assuming one method fits all; often a combination of seismic and resistivity provides complementary insight.

Final Answer:
Both (a) and (b)