1. The study of the Earth by quantitative physical methods, especially by seismic reflection and refraction, gravity, magnetic, electrical, electromagnetic, and radioactivity methods. 2. The application of physical principles to studies of the Earth. Includes the branches of (a) seismology (earthquakes and elastic waves); (b) geothermometry (heating of the Earth, heat flow, volcanology, and hot springs); (c) hydrology (ground and surface water, sometimes including glaciology); (d) physical oceanography; (e) meteorology; (f) gravity and geodesy (the Earth’s gravitational field and the size and form of the Earth); (g) atmospheric electricity and terrestrial magnetism (including ionosphere, Van Allen belts, telluric currents, etc.); (h) tectonophysics (geological processes in the Earth); and (i) exploration, engineering, and environmental geophysics. Geochronology (the dating of Earth history) and geocosmogony (the origin of the Earth) are sometimes added to the foregoing list. 3. Often refers to solid-earth geophysics only, thus excluding (c), (d), (e), and portions of other subjects from the above list. 4. Exploration geophysics is the use of seismic, gravity, magnetic, electrical, electromagnetic, etc., methods in the search for oil, gas, minerals, water, etc., with the objective of economic exploitation.