Description

The seismic refraction technique involves inducing a seismic impulse ('shot) in the ground, and recording the arrival times of the seismic energy which travels through ground, at a series of geophones (a 'spread') laid at the surface. By firing shots at both ends of each spread it is possible to calculate depths to rockhead, typically to an accuracy of between +/-10 and 20%.

The best type of seismic impulse is produce by very small explosive charges, typically of the order of 100 or 200 gms. These shots are fired in holes dug to around 1 - 1.5 metres depth; the shots generally produce very little or no appreciable noise. Hammers and other similar methods of producing a seismic impulse are not suitable except where the rockhead is close to the surface, typically within 5 metres, due to energy dissipation within the overburden.

Further small charges are fired at locations along each spread, to determine shallow seismic velocities.

A typical spread designed to locate rock at depths up to 50 metres would consist of 48 geophones at 5 metre intervals, resulting in a spread length of 235 metres.

The system is an excellent reconnaissance tool, well suited to areas where there is a clear velocity contrast between overburden and rock. It has a particular application in areas where rockhead profiles may be incorrectly inferred due to the presence of boulder beds.

In addition to rockhead profiles, the technique can be excellent at identifying areas of changing seismic velocity, for example in shatter zones around faulting, or in areas where weak horizons are at near-exposure due to a combination of faulting and tilting : for example in areas of coal measures.

Other uses :

• Rippability and Rock strength studies
• Buried Channel location
• Depth of weathering
• Elastic moduli values for soils and bedrock
  

Can be a very cost-effective tool for determining suitable locations for boreholes, e.g. at the beginning of a site investigation for dam sites etc.