What is Geophysics?

Geophysics is a branch of earth sciences investigating the physical processes and phenomena occurring in the earth by combining physics, mathematics and geology. Specifically, some of the areas geophysical engineering deals with are 1) Exploration of coal, oil, gas and geothermal energy resources as well as groundwater and mineral deposits, 2) Assessment of earthquake hazards such as strong ground shaking, landslides and liquefaction, 3) Investigation of subsurface for engineering structures, 4) Investigation of archeological sites and 5) Imaging of the subsurface for environmental hazards.

What are the major techniques used in geophysical surveys? 

Gravity : Measurements of the gravitational field at various locations over an area of interest. The objective in exploration work is to associate variations with differences in the distribution of densities and hence of rock types. Occasionally the whole gravitational field is measured or derivatives of the gravitational field, but usually the difference between the gravity field at two points is measured (as with a gravimeter). Gravity data usually are displayed as Bouguer or free-air anomaly maps [*].   

Magnetics : 
Measurements of the magnetic field or its components (such as the vertical component) at a series of different locations over an area of interest, usually with the objective of locating concentrations of magnetic materials, mapping geological structure, or of determining depth to basement. Differences from the normal field are attributed to the distributionof materials having different susceptibility [*].    
Seismology : The study of seismic waves, a branch of geophysics especially refers to studies of earthquakes or of seismic exploration for oil, gas, minerals, engineering information etc. [*].    

Seismic : 
A survey for mapping geologic structures by observation of seismic waves, especially by creating seismic waves with artificial sources and observing the arrival time of the waves reflected from acoustic-impedence contrasts or refracted through high velocity members [*].

Electric : 
Measurements made at or near the Earth's surface of natural or induced electric fields, the objective usually being to map mineral concentrations or for geologic or basement mapping [*].  

Electromagnetic : 
A method in which the magnetic and/or electric fields associated with artificially generated subsurface currents are measured. In general, EM methods are considered to be those in which the electric and magnetic fields in the Earth satisfy the diffusion equation (which ignores displacement currents) but not Laplace's equation (which ignores induction effects) nor the wave equation (which includes displacement currents. Natural field methods (such as Magnetotellurics) employ natural energy as the source; controlled source EM methods (such as loop-loop methods) require a man-made source [*].     

Palaeomagnetism :
 Study of natural remanent magnetization of rocks and other materials to determine the intensity and direction of the Earth's field at the time the materials were magnetized [*].

Radiometrics and Geothermics : A survey of the amount of electromagnetic radiation emitted, usually involving gamma-ray spectrometry is called radiometric survey and prospecting for high temperature water and/or steam close to the surface is called geothermal prospecting [*].  

 A survey to obtain a record of one or more physical measurements as a function of depth in a borehole [*]. (such as Radioactivity logging, SP-resistivity logging etc.)

Examples for application areas of geophysical methods:

Plate Tectonics and Earthquake investigations
Investigation of geological structures by using land and marine seismic methods
Defining the Earth's magnetic field at geological times
Investigation of natural resources
Environmental Geophysics
Archaeogeophysical investigations
Atmosphere and space investigations
Geothermal investigations

[*] Sheriff, R.E., 2002. Encyclopedic Dictionary of Applied Geophysics, 4th Edition, Geophysical References, 
      Vol.13, Society of Exploration Geophysicists.