They use explosives to create sound waves which reflect off bedrock. The time it takes for the shock wave to return to the surface indicates the depth of the bedrock. 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 influences.
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 the soil. Some are concerned with water supplies, irrigation, flood control, and soil erosion. 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.