Define geophysical exploration

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.