Geophysics

Geophysics is the study of Earth's physical character, including the solid planet, the atmosphere, and bodies of water. Geophysical investigations, therefore, often draw upon information and techniques developed in scientific disciplines such as physics, geology, and astronomy. Major areas of modern geophysical research include seismology, volcanology and geothermal studies, tectonics, geomagnetism, geodesy, hydrology, oceanography, atmospheric sciences, planetary science, and mineral physics.

Geophysics has many practical applications. Some seismologists, for example, help to explore for new petroleum reservoirs, monitor nuclear weapon testing by other countries, and better understand the structure and stratigraphy of important aquifers. Others provide the information necessary to design earthquake-resistant buildings and determine the risk posed by future earthquakes. Physical oceanographers monitor changes in ocean temperature that give rise to El Niño and La Niña phenomena, resulting in better long-term weather forecasts, and atmospheric physicists study the conditions that can give rise to lightning strikes. Hydrologists study the flow of surface water and groundwater, including the conditions that are likely to produce destructive floods.

Aristotle (384–322 B.C.) performed some of the first known geophysical investigations and published his findings in a work entitled Meteorologica. That work addressed such modern topics as weather, earthquakes, the oceans, tides, the stars, and meteors. By the first century B.C., Chinese investigators had developed a simple device for recording earthquakes and their points of origin. However, little additional progress was made in the field of geophysics until the fifteenth century A.D., when Leonardo da Vinci (1452–1519) took up the study of gravitational attraction and wave propagation.

The 300 years following da Vinci's death were marked by steady advances in the understanding of geophysical phenomena such as magnetism, gravity, and earthquakes. Most of these investigations were concerned only with what could be observed with the senses. But starting in the nineteenth century, scientists began to develop much more sophisticated techniques for geophysical observation.

Seismology is a branch of geophysics that draws on the physics of wave propagation to study earthquakes and determine the physical characteristics of Earth's interior. Seismic wave velocity is proportional to rock density. Therefore, seismologists can infer the composition and structure of Earth's interior by calculating the velocity of seismic waves from distant earthquakes. Seismic tomography uses computer analysis of seismic wave velocities to visualize structures within Earth and produce images that are much the same as medical CAT scans of the human body.

Seismologists can also generate seismic waves by using explosions or vibrating devices. Specialized seismometers known as geophones record the arrival of artificially created seismic waves, some of which are reflected back to Earth's surface when they reach boundaries between different rock types. The reflected waves can help to identify areas likely to contain undiscovered petroleum reserves and locate faults that may generate future earthquakes. Like earthquake seismologists, exploration seismologists can use three-dimensional data collection and computer processing techniques to produce detailed images of rocks that are otherwise inaccessible to humans.

Several types of energy produced by Earth's interior vary from place to place. This variation, called a potential field, is a characteristic of magnetism, gravity, temperature, and electrical conductivity. Geophysicists measure potential fields to learn about the distribution, composition, and physical state of rocks beneath Earth's surface. Because the gravitational field varies with changes in Earth's density from place to place, for example, geophysicists can use gravity measurements to map variations in rock composition and locate faults. Studies of variations in Earth's magnetic field through time are fundamental to our understanding of plate tectonics.

Experimental geophysicists attempt to reproduce the heat and pressure present in Earth's interior to determine how rocks and minerals behave under extreme conditions. Diamond anvils, for example, can generate pressures in the laboratory that equal or exceed any in Earth's interior.