Steve Nelson, age 15, of Albuquerque, N.M., for his question:
WHAT IS GEOPHYSICS?
Geophysics is the branch of science that applies physical principles to the study of the earth. Geophysicists examine physical phenomena and their relationships within the earth, such as the earth's magnetic field, heat flow, the propagation of earthquake waves and the force of gravity.
The scope of geophysics also broadly includes outer space phenomena that influence the earth, even in subtle ways: the effects of the sun on the earths magnetic field and manifestations of cosmic radiation and the solar wind.
Geophysical methods for studying the solid earth are divided into geodesy, geothermometry, seismology and tectonophysics. Historically, geophysics became differentiated from geology as the applied aspects of geophysics evolved into distinct subdisciplines.
Embracing all fields devoted to the earths interior, solid earth physics involves studying the behavior of earth materials from the crust down to the core, particularly as they relate to the earths size and shape, gravity, magnetics and seismicity. The specialized field of geodesy is concerned with determining the earths size and shape.
Geomagnetism refers to the study of magnetic phenomena exhibited by the earth and its atmosphere. Generation of the magnetic field seems to be related to the motion of fluid, electrically conducing material within the earth, so that the planet acts as a self exacting dynamo.
The geography of seismic activity shows that the distribution of earthquakes is closely related to surface and crustal features such as ocean ridges and ocean trench island arcs.
Geophysical exploration, commonly called applied geophysics or geophysical prospecting, is conducted to locate economically significant accumulations of oil, natural gas and other minerals, including ground water. Geophysical investigations are also employed with engineering objectives in mind, such as predicting the behavior of earth materials.
Predicting the behavior of earth materials is necessary in relation to foundations for roads, railways, buildings, tunnels and nuclear power plants. Surveys are generally identified by the property being measured namely electrical, gravity, magnetic, seismic, thermal, or radioactive properties.
Used primarily in the search for oil, gas and base metals, electrical and electromagnetic surveys map variations in the conductivity or capacitance of rocks.
Gravity surveys measure density variations in local rock masses. Used mainly in petroleum exploration, these surveys are based on use of a device called a gravimeter. Gravity surveys are mane on land, at sea and down boreholes.
In ground magnetic surveys, variations in the earths magnetic field are measured at stations placed closely together.
Measurements of seismic wave travel time is one of the most common geophysical methods used in surveys.