Common geophysical techniques include:
Gravity Magnetic Method Radiometric Method Electrical/Induced Polarization (IP) Methods Electromagnetic (EM) Methods including Ground Penetrating Radar (GPR) Seismic Techniques
Gravity is used to measure density contrasts in the earth and can be applied to detecting massive sulfide ore bodies, regional mapping of geologic structures, and locating voids in the subsurface. The magnetic method measures changes in the earth's magnetic field caused by objects such as magnetite an iron. Magnetic methods are usually used to detect various types of ore bodies, regional mapping of geologic structures, and detection of buried metallic objects such as drums and underground storage tanks. The radiometric method measures gamma rays naturally given off by the earth. These methods are useful for mapping geology and alternation as well as detection of industrial contamination. Gravity, magnetic, and radiometric techniques generally can be performed much more quickly than the other methods and are therefore considerably cheaper. They do not provide as many details as the other methods however.
Electrical methods are used for detailed delineation of subsurface structures. This includes the mapping of the ground water table, faults, and ore bodies. Electrical methods drive current directly into the ground and measure the potential (voltage) difference at the receiver. By moving the current and potential electrodes to different locations, you create a map of the subsurface resistivity. The IP effect is very effective at mapping clay horizons and disseminated sulfide ore bodies. Numerous variations of the EM method exist which have been tailored to detect very specific targets. All EM methods use the principle of induction to map the subsurface; therefore no current is driven into the ground. This makes the EM methods cheaper and faster than electrical techniques but does not generally yield the same resolution. Ground penetrating radar is the newest EM method and is designed specifically for very high resolution mapping of the shallow subsurface. Seismic methods measure acoustic contrasts within the subsurface. They are best known for their application in oil exploration but can also effectively be used for geotechnical site characterization. Seismic methods are used to map the ground water table, lithologic contrasts between such units as overburden soil, slide zones, sandstone layers, shale layers, and hard rock such as granites and basalts. Seismic methods are commonly used prior to designing foundation supports for large buildings and bridges.
I have only briefly outlined the methods and a few of their applications. For any given project, the geophysicist must weigh the chance of success for each technique versus cost, terrain conditions, cultural noise contamination caused by sources such as powerlines, and safety issues. In some cases multiple surveys are required for the project. In some cases no geophysical technique will be effective. Many people fall into the trap of trying "a" geophysical technique to see if they can get the answers they want. In truth for geophysics to be effective, any survey must be carefully planned, executed, processed, and interpreted to solve the problem at hand.