Geophysical Exploration

Geophysical methods are used in geotechnical investigations to evaluate a site’s behaviour in a seismic event. By measuring a soil’s shear wave velocity, the dynamic response of that soil can be estimated. A number of methods are used to determine a site’s shear wave velocity:

1. Crosshole method
2. Downhole method (with a seismic CPT or a substitute device)
3. Surface wave reflection or refraction
4. Suspension logging (also known as P-S logging)
5. Spectral analysis of surface waves (SASW)
6. Reflection microtremor (ReMi)

Methods of observing the soils below the surface, obtaining samples and determining physical properties of the soils and rock include test pits, trenching (particularly for locating faults and slide planes), borings and CPT or SPT. CPT allows continuous recording of soil changes with depth, whereas SPT records only major changes at discrete steps of 150 mm (6″); however, SPT allows soil sampling for laboratory testing.

A CPT is typically performed using an instrumented probe with a conical tip, pushed into the soil hydraulically. A basic CPT instrument reports tip resistance and frictional resistance along the friction sleeve, which is located just above the tip. CPT data have been correlated to soil properties. Sometimes, instruments other than the basic CPT probe are used.

Geophysical exploration is also sometimes used; geophysical techniques used for subsurface exploration include measurement of seismic waves (pressure, shear and Rayleigh waves), using surface wave methods and/or downhole methods, and electromagnetic surveys (magnetometer, resistivity and ground-penetrating radar).