The first question you ask yourself when you ask yourself when you hear about geophysics is "what is geophysics?" actually geophysics is a very important and complementary science to Geology.

The geophysicist uses Physics and Mathematics to identify subsurface Earth structures, we know that different rock types and subsurface structures will surely have different physical parameters, so why don't we use these parameters to identify the structures without constructing boreholes, or drilling (i.e. less expensive).

A petroleum exploration project for example is meaningless without using geophysical tools because it is the only way to assure the presence of oil, and precisely identify it's depth.

Geophysical methods of exploration are divided into "seismic" methods, and "potential" methods. In seismic methods we generate  waves, which are shot into the ground surface by any source (explosives for example), these waves will be reflected back to the surface and detected by sensors called "geophones". the arrival time of these sound waves can indicate the type of lithology (rock constituents), number of geologic beds, and any different body that is present under the surface of investigation (i.e. every type of rock has a certain arrival time). Of course this help us to study the behavior of earthquakes & we have been able to deduce its strength ,damage & it's center. You can see a distribution of the world activity in Fig (1).

Fig (1)

Other geophysical methods are called potential methods, an easy to use and helpful tool of these methods is the "Ground Penetrating Radar (GPR)" in which electromagnetic (EM) waves are introduced to the subsurface by an antenna called the transmitter antenna, the underground bodies will affect the return of these waves to another antenna called the receiver antenna and though indicating their presence, figure (2.a) shows an example of a radar subsurface section we can see that the presence of underground tanks affected the passage of EM waves which appeared as concentric curves in the section. figure (2.b) shows the ground penetrating radar used.                 

Fig (2.a)                                  Fig (2.b)   

We can deal now with another example of geophysical methods which are "Magnetic" methods, these methods are based on the variation in magnetism of a buried structure from its surrounding in the subsurface, the apparatus used is called the "Magnetometer" which is a sensitive device to magnetic field produced vertically from the underground. Magnetic methods have been used to detect very precisely the buried archeological Pharonic " sun boats" in here in Egypt, the sun boats were having a different magnetism than their surroundings, so they were easily detected by a great increase in the magnetic field detected by the magnetometer.