Geophysics is an earth sciences that combines geology with physics. Geophysicists examine the Earth’s physical properties to better understand its structure and the forces that shape it. Geophysics is divided into two disciplines, whole-earth geophysics and exploration geophysics.
WHOLE-EARTH GEOPHYSICS
As the name suggests, whole-earth geophysics deals with the entire planet. Most whole-earth geophysicists are researchers and educators. These scientists examine physical properties of the Earth to describe its internal structure; properties such as gravity, magnetism, and seismic waves.
An important branch of whole-earth geophysics is seismology, the study of earthquakes. As earthquake shock waves travel through the earth, they are reflected and bent by the planet’s layers. By studying thousands of earthquake records, seismologists have determined the planet’s structure. Based on their study, we know that the earth has a solid inner core; a liquid outer core; a thick, rocky mantle; and a thin, brittle crust. All this information was determined without direct observation.
Whole-earth geophysicists also study forces within the earth, especially those driving plate tectonics. Tectonic theory depends upon geophysics to explain its mechanisms. For instance, the physical properties deep in the mantle are critical to convection cells, which move the plates on the surface. Geophysicists were also instrumental in early proofs of plate tectonic theory.
Other geophysicists use gravity measurements to model the structure of the crust. A local gravity high, for instance, can indicate something dense near the surface. Since different rocks also have different magnetic properties, measuring the earth’s magnetic field can also be used to identify buried metallic bodies. Gravity and magnetism are often combined to provide a more detailed study; a technique that lends itself well to surveys from low-flying aircraft, which can gather large amounts of data quickly.
Paleomagnetism is another type of whole-earth geophysics. Geophysicists use remanent magnetism of rocks to determine where on the earth’s surface they formed. Magnetic minerals like magnetite and ilmenite align themselves with earth’s magnetic field when deposited. After millions of years of continental drift, the difference between the current magnetic field and the original magnetic field is used to determine the latitude and longitude at which the rocks were formed. This information provides yet more clues about plate tectonics.
EXPLORATION GEOPHYSICS
Exploration geophysicists use the science to examine the uppermost few miles of the crust, seeking fossil fuels and strategic minerals. Most exploration geophysicists work in the petroleum industry, using seismic surveys to study the structure and composition of sedimentary rocks. Seismic surveys depend on acoustic properties of rocks, for they are complex records of sound waves reflected back to the surface by subsurface layers.
Unlike seismologists who study earthquake shock waves, exploration geophysicists create their own sound waves. Knowing the original sound frequency and distance to the source are critical to interpreting reflection seismic. Seismic surveys often use sound created by dynamite, vibrators on heavy trucks, or underwater air cannons for offshore surveys.
The signals are collected by a network of carefully-placed receivers, and digital records are processed to create the clearest image. Though once interpreted by hand on paper, geophysicists now interpret seismic using computer programs. Since seismic signals are indirect measurements, geophysicists depend on experience and training to interpret their data instead of direct observation.
Although petroleum geophysicists rarely use gravity and magnetic surveys except in extremely remote areas; the two techniques are still used in the minerals industry. Magnetic surveys are of especial importance to the exploration for metallic minerals.
Other physical properties of rocks and minerals, including natural radiation and spontaneous electrical potential, are widely used in the petroleum and water industries for indirect observation of rocks in wells. Such study, termed petrophysics, forms a branch between geophysics and engineering.
SUMMARY
Geophysicists use indirect methods to describe the earth and its natural processes. The discipline helps us understand what goes in deep within our restless planet; and to explore for water, hydrocarbons, and minerals in the top few thousand feet of its crust.
MORE INFORMATION: American Geophysical Union (whole-earth geophysics) and the Society of Exploration Geophysicists (exploration geophysics)
