Geophysics: measuring the Earth

Geophysics is the application of physics to investigations of the Earth. The study of the interior of the Earth is known as Solid Earth Geophysics, subdivided further into Global Geophysics or alternatively Pure Geophysics which is the study of the whole or substantial parts of the planet and Applied Geophysics which is concerned with investigating the Earth’s crust and near-surface to achieve a practical and, more often than not, an economic aim.
Applied Geophysics covers everything from experiments to determine the thickness of the crust to studies of shallow structures for engineering site investigations, exploring for groundwater and for minerals and other economic resources. To try to locate narrow mine shafts or other form of buried cavities, mapping archaeological remains, locating buried pipes and cables and more where in general the total depth of investigation is usually less than 100 m

Geophysical essentially is the measurements of the contrasts in the physical properties of material beneath the surface of the Earth and the attempt to deduce the nature and distribution of the materials responsible for these observations. …. Variations in elastic moduli and density cause seismic waves to travel at different speeds through different materials. …. Variations in the electrical conductivity of rocks and sediments produce varying values of apparent resistivity as the distance between measuring probes is increased or as the position of the probes are changed on the surface. … Density variations in the subsurface lead to variations in gravitational acceleration at surface instrument station and variations in magnetic susceptibilities produce measurable differences in the magnetic field at field observations sites. H.R. Burger et Al. Introduction to Applied Geophysics.