Seismologists study the earth's interior and its earth vibrations caused by earthquakes and manmade explosions. They explore for oil and minerals study underground detection of nuclear explosions, and provide information for use in constructing bridges, dams, and buildings. Geodesists study the size, shape, and gravitational field of the earth and other planets. Their principal tasks are mapping the earth's surface and explaining the variations found. With the aid of satellites, geodesists determine the positions, elevations, and distances between points on the earth, and measure the intensity and direction of gravitational attraction.
Hydrologists are concerned with the fluid earth.
They study the distribution, circulation, and physical properties of underground and surface waters. They also study rainfall and its rate of infiltration into soil. Geophysicists involved in the atmosphere investigate the earth's magnetic and electric fields and compare its outer atmosphere with those of other planets. Geomagneticians study the earth's magnetic field. Paleomagneticians learn about the last magnetic fields from rocks or lava flows.