Geophysics 101

Geophysics is a discipline within  earth science that concerns itself with the physics of the earth. Within geophysics there are different application areas, including whole earth geophysics, exploration geophysics and near surface geophysics. One common denominator in all these application areas is that for all of them we need to determine the physical properties of the earth.

In order to do this, geophysicists use a large number of different non invasive methods such as seismic methods, electrical methods, gravity and electromagnetic methods. These methods are based on the same physics and are  analogous to methods used in medical imaging.

There are several commonalities between all geophysical methods which are useful to know and keep in mind. These are

• Geophysics is noninvasive. We can take measurements from the surface (and in many cases from the air)
• Geophysical measurements are not point measurements: each geophysical measurement is sensitive to (and a function of) physical properties in a volume.
• We can process geophysical measurements (through a process known as inversion). This processing step is non unique so requires constraints
• Geophysics measures the physical properties of the subsurface. To the extent that the property that you are interested in changes these physical properties, geophysics could possibly detect it. However, there are things which can not be measured with geophysics. A good example are low level (ppm) concentrations of  common contaminants in water. As low levels of contaminants do not change the physical properties of the water (density, electrical conductivity) appreciably, it is physically impossible to detect these with geophysics. Note that we can detect the macroscopic effect of biogeochemical processes (see here for a review paper), but it is essential to understand the limitations of geophysics.

More detail on geophysics can be found in the links in this section and in the textbooks and links provided  here.