What are seismic surveys?

Seismic reflection is a technique for mapping rock layers underneath the ground. Use of this technique together with surface mapping, provides geoscientists with the necessary tools to construct 3D geological maps. It was initially used in petroleum exploration, where an accurate knowledge of the rock layers is critical to successful exploration. In recent years, the technique is being used more and more in mineral exploration as the search for deeper ore bodies continues.

The technique gives a two- or three-dimensional picture of the structure of layers of sedimentary rock at depth by generating an elastic shock wave at the surface, analogous to a sound wave in fluids, and recording reflections from the rock layers by means of a linear array of detectors. The reflections are the result of differences in the acoustic impedance of rock layers (see figure). The waves were originally generated by using explosives, but in recent years mechanical vibrators have been used. Mechanical vibrators give a sharper signal with better depth penetration.

A relatively recent development in the application of seismic reflection profiling is its use on land to examine entire sections of continental crust from 35 to 60km deep, rather than upper crustal sedimentary basins only a few kilometres deep. Used for this purpose, seismic reflection profiling has revealed unexpected details of structures throughout the continental crust, producing better images of the subsurface than any other technique, and has generated new concepts of the relationships between crustal blocks and rock units within them.

Establishing the shape of faults by tracing them through the Earth's crust is particularly important in determining their role in the tectonic evolution and their potential to act as pathways for mineralising fluids to create ore bodies.