Information Sheet RS 2
Seismic Wave Method for Ground Investigation
Department of Earth Sciences, The University of Hong Kong. //www.hku.hk/earthsci
Theory
Earthquakes or artificial shocks induce ground vibrations that propagate from the source in all
directions. Such ground motions can be recorded with high-sensitivity receivers, called geophones.
The velocity of the wave depends on the material. A relatively stronger material is generally
associated with a greater wave velocity. The seismic method aims to determine the ground velocity
by recording the travel time of the seismic waves.
Equipment & Procedure
The HKU seismograph system for in-situ seismic wave determination consists of
A multi-channel seismograph, with built-in internal computer
Geophones with metal disc or spike base
Cables for connecting geophones to seismograph
Sledgehammer with trigger connected to seismograph, metal plate
12-V battery power source
Multi-channel seismograph
Geophones are placed on the ground along the direction of the seismic
wave propagation. A geophone cable can connect up to 24 geophones
to the seismograph. The distance of each geophone from the source is
carefully noted. Ground vibrations are incurred by striking a
sledgehammer against a metal plate. The seismic waves produced by
the impact travel from the source and are detected sequentially by the
geophones. The arrival times at the geophones are used for
determination of the wave velocities and, in turn, the ground property.
Similar methods are used in studies of natural earthquakes.
Applications
The seismic method is commonly used in Hong Kong to determine slope stability and for ground
investigation. Examples of application include
(1)
(2)
(3)
(4)
(5)
(6)
measuring of rock head depth (construction, piling),
studying seabed layers (reclamation, cross-harbour tunnel, bridge projects)
mapping water table (slope investigation, groundwater),
determining pile and soil nail length (construction, slope maintenance),
detecting underground caves (site formation, foundation)
monitoring ground vibration caused by blasting in quarries
Ground vibrations
geophones
Figure. Field layout of seismic survey