Originally, most geophysical applications were designed for subsurface exploration of one sort or another. Even today, most geophysicists are employed in the mining or oil & gas industries.
The reflection seismic method has been the tried and true standard for exploration in the oil & gas industry ever since its inception. In northern California, a renewed interest in exploration for natural gas has been driven by the increasing cost of energy.
In the mining industry, both electrical resistivity and induced polarization are common tools for regional exploration. For the mining of gold placer deposits, a combination of magnetics and refraction seismic is often used to locate the positions of buried channels which contain the black sands (and the gold).
Another useful application of geophysics is in groundwater exploration. In areas such as the Sierra foothills, groundwater flow is controlled by fractures in the bedrock, and therefore well placement is critical to the development of the groundwater resource. The electrical resistivity method has been used successfully for years to determine the optimum location for a high yield water well.