Geophysical Methods Indirect Site Exploration

A geophysical exploration may be included in a site investigation for an important engineering project in order to provide subsurface information over a large area at reasonable cost. 

The sudden release of energy from the detonation of an explosive charge in the ground or the mechanical pounding of the surface generates shock waves that radiate out in a 

When seismic waves pass from one layer to another in the ground, some energy is reflected back towards the surface while the remainder is refracted. Thus, two methods of seismic surveying can be distinguished, that is, seismic reflection and seismic refraction. Measurement of the time taken from the generation of the shock waves until they are recorded by detector arrays forms the basis of the two methods. 

The seismic reflection method is the most extensively used of all geophysical techniques, its principal employment being in the oil industry. In this technique, the depth of investigation is large compared with the distance from the shot to detector array. This is to exciude refraction waves. Indeed, the method is able to record information from a large number of horizons down to depths of several thousands of metres. 

In seismic refraction, one ray approaches the interface between two rock types at a critical angie that means that, if the ray is passing from a low, Vo, to a high velocity V, layer, it will be refracted along the upper boundary of the latter layer (Fig. 7.23). After refraction, the pulse travels along the interface with veiocity V. The material at the boundary is subjected to 

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