Geophysics Geophysical surveys are performed when information about the Earth's subsurface is desired but direct sampling is either impractical or ill-advised. Objectives could be at a whole-earth scale, or within the top few metres of the subsurface, or anywhere in between.
Energy is input to the earth and responses are recorded at the surface, in the air, or in boreholes.
Resulting data contain information about the Earth because the behavior of input energy is controlled by the distribution of physical properties such as density, electrical conductivity, acoustic impedance, and others. Earlier methods of interpreting geophysical data involved plotting the data in various forms, perhaps after applying some filtering operations. Resulting images were used to estimate the possible locations and quantities of buried materials, and to help choose locations for more invasive techniques such as drilling. Other approaches to geophysical interpretation have involved characterizing the Earth by a few prisms or layers and using numerical procedures to find geometrical and physical properties of these simplified earth models. All interpretation methods must deal both with severe nonuniqueness and with imperfect data.