Geophysics is a scientific discipline that investigates and visualizes the physical structure and dynamic behavior of the Earth and other planetary bodies by integrating mathematics and geology within the framework of physical laws. Geophysical studies focus on the identification of subsurface resources (including groundwater, oil, natural gas, mineral deposits, and geothermal energy); the evaluation of earthquake hazards (such as strong ground motion, landslides, and liquefaction); the execution of ground investigations for engineering structures; the exploration of archaeological and cultural sites; and the assessment of environmental risks.

Geophysics encompasses various methods developed based on different physical parameters to study the rocks and materials that constitute the Earth:

Examples for application areas of geophysical methods:

Investigation of subsurface structures:
*   Investigation of the Earth's inner structure and Crust
*   Plate tectonics and earthquake research
*   Investigation and mapping of geological structures
*   Investigation of the karstic cavities
*   Investigation of the Earth's magnetic field

Natural sources and archeogeophysical research:
*   Hydrocarbon research (coal, petroleum, gas)
*   Investigation of ore deposits
*   Groundwater research
*   Geothermal source research
*   Investigation of archeological and cultural structures

Environmental geophysics:
*   Groundwater pollution investigation
*   Mapping of the salt water intrusion
*   Investigation of the hazards of the solid waste deposits

Engineering geophysics:
*   Engineering structures: investigation of ground, soil and foundation properties
*   Engineering structures: investigation of buildings and reinforcements
*   Urban transformation site research


What are the major techniques used in geophysical surveys? 

The definitions given below are cited from: "Sheriff, R.E., 2002. Encyclopedic Dictionary of Applied Geophysics, 4th Edition, Geophysical References, Vol.13, Society of Exploration Geophysicists."

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.  

Well-logging : 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.)