Geophysics

Definition:Geophysics is the branch of geology concerned with the study of the physical characteristics of Earth. This includes the hydrosphere and the atmosphere. It also includes the physical processes acting upon, above, and within Earth and its relationship to the rest of the universe. Included in geophysics are the fields of seismology, the study of earthquakes; volcanology, the study of volcanoes; meteorology, the study of climate and weather; and oceanography, the study of the oceans. The study of seismology and volcanology can lead to the ability to better predict the occurrence of earthquakes and volcanic eruptions. The study of meteorology contributes to an understanding of climate change, along with enhanced capabilities in the forecasting of catastrophic weather events.

Basic Principles

Geophysics is the study of Earth using quantitative physical methods. It examines the physics of Earth and the environment on and surrounding the planet. To gain an understanding of these relationships, geophysicists study a number of phenomena that include gravitational fields, magnetic fields, the hydrological cycle, fluid dynamics at work in the oceans, seismology, the ionosphere and magnetosphere, and solar-terrestrial relations.

Humans have been studying phenomena such as the magnetic field, earthquakes, volcanoes, and violent weather for centuries. The study of geophysics as a formal discipline originated in the nineteenth century, during the time when science was being used as a tool to add to understanding in many disciplines; there were, however, discoveries that came before the 1800s. Notably, there was William Gilbert’s work De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (1600; On the Loadstone and Magnetic Bodies, and on That Great Magnet the Earth, 1893), in which he put forth the theory that compasses point north because Earth is magnetic. Sir Isaac Newton published his Philosophiæ NaturalisPrincipia Mathematica in 1687. In this work, Newton laid the groundwork for classical mechanics and gravitation. He also offered explanations for such geophysical phenomena as the tides.

The water cycle first came into discussion as a result of the work of notable scientists such as Vitruvius, Leonardo da Vinci, and Bernard Palissy. Several others studied rainfall averages and other related measures of precipitation. Daniel Bernoulli’s work with pressure and Henri Pitot’s work with the Pitot tube took place in the eighteenth century. The first textbook on oceanography was published in 1855.

The twentieth century was a time of rapid discovery in the field of geophysics. Plate-tectonic theory was developed, and significant discoveries about the ocean were made through the use of acoustic measurements of sea depth and large-scale computer simulations. During the International Geophysical Year (IGY), an international science collaboration that took place in 1957 and 1958, many advances were made regarding auroras, cosmic rays, gravity, oceanography, seismology, and other related areas.

With a demand for petroleum exploration in the 1920s, the principles of geophysics were put to practical, industrial use. As a result, mining and groundwater geophysics improved. An understanding of the instability of specific soil and site areas was also gained.

Core Concepts

Mineral Physics. Mineral physics is the study of the material properties of minerals, particularly at the extremely high temperature and pressures found at the center of Earth. While material properties have been under investigation for quite some time, the conditions within Earth’s crust have proven difficult to replicate until the past century. Massive hydraulic presses could deliver enormous pressures and, when paired with furnaces, could replicate a wide range of temperature pressure conditions. The advent of diamond anvil presses in the 1950s greatly simplified matters, as they could be made far smaller and efficient. Some models can even fit in cryostats, which are chambers designed for low-temperature measurements. The data gathered from these experiments allows researchers to better understand what conditions are like inside Earth and other celestial bodies. This field is also important in many other areas, such as when studying groundwater and the oceans.

Fluid Mechanics. Fluid mechanics is the study of flows of gas, liquid, and plasma and their interactions with force. It is very useful when studying ocean currents and cycles, as well as when examining weather. Fluid mechanics often yields patterns such as Rossby waves, which govern weather patterns in the atmosphere and are tied to thermocline waves in the oceans. The flow of the lithosphere can also be examined through fluid mechanics. In addition to the flows of the depths, Earth’s mantle also acts as a fluid over long time periods; when examined, it yields data on postglacial rebound and isostasy, the latter of which is the equilibrium that keeps the tectonic plates floating.

Heat Transfer.Heat transfer, also called heat flow, is a concept critical for understanding Earth. Heat flows often take the form of giant convection cycles, which govern the movements of the oceans, the weather, and even plate tectonics. These trends are essential for the functioning of the climate and environment, with oceanic currents being of particular interest. All of the water in the oceans circulates, not only between oceans, but also between the different layers of the ocean. Major upper-level currents include the Gulf Stream, the East Australian Current, and the Australian Circumpolar Current. Water is separated into layers by temperature differences; the boundary between the layers is called the thermocline. The whole system is referred to as thermohaline circulation, and it not only raises nutrients from the seabed (which is why polar seas have so much biodiversity) but also keeps the climate in balance. Given the recent awareness of climate shift, a better understanding of the importance and functioning of the thermohaline conveyor is vital. Historically, observation of all of these patterns, weather and oceanic, has been difficult. In the twenty-first century, satellite observation has greatly simplified data collection.

Seismology and Vibrations. Though seismographs have existed since second-century China, it was not until recently that the field of seismology came into its own. With the invention of better equipment and continuous recorders in the twentieth century, scientists have been able to map Earth’s core through seismological data. They can determine the densities of the layers of the inner earth by collecting seismic data from large earthquakes in a variety of locations and examining the geographical spread of the sites where vibrations can be detected, as the angles at which those vibrations travel change by refraction depending on Earth’s composition. Understanding the reaction of various materials to earthquakes also helps ensure safer building practices.

Electromagnetism. An understanding of the mechanics that govern Earth’s magnetic field is essential to the study of electromagnetism. The planet’s magnetic field, which protects it from cosmic rays that could harm life, reverses every few hundred thousand years. Scientists can date material such as the oceanic crust by looking for records of this reversal in stone. Electromagnetism is also used in analyzing lightning and other phenomena of electric discharge. This is possible because, due to cosmic rays, Earth’s atmosphere has a net positive charge, which makes its way down into Earth by various mechanisms and back up during lightning. It has been recently discovered that there is more to lightning than the discharge from cloud to ground; upper-atmosphere phenomena known as sprites and elves (from the acronym ELVES, for emissions of light and very low-frequency perturbations due to electromagnetic pulse sources) are also generated. Terrestrial electromagnetism can also generate interference in signaling equipment. By understanding Earth’s response to electromagnetism, researchers can use it to survey for useful materials. A common system that does this is called transient electromagnetics. The system induces a charge in Earth and measures the time it takes to disperse. This data gives an indication of the sort of materials that can be found there.

Radioactivity.Knowledge of radioactivity is useful in geophysics. It is important for understanding Earth’s heating system, as nearly 80 percent of Earth’s internal heating is thought to come from radioactive decay. Equally, because half-lives are predictable, they can be used to establish the age of rocks; this has allowed us to build a picture of Earth’s history.

Applications Past and Present

Construction.Knowledge of geophysics is useful in construction applications because it gives those involved a method for determining the stability of the land they are building on. It is important to understand the likelihood of seismic activity when selecting a project site, whether that project involves homes, commercial real estate, roads, or other structures. Geophysics is the field of science that gives those in the construction industry the tools they need to assess the effect of stress on the soil and rock involved. It is also possible for a very large project, such as a dam or reservoir, to introduce seismic activity where none was present before, or to intensify any such preexisting activity. This is called induced seismicity and has occurred several times when large dams have been built.

Another important application of geophysics within construction is safety. This can take the shape of building codes that are designed to make structures more earthquake resistant. Because sand liquefies when shaken, it is especially important in areas with significant seismic activity to have building codes that lessen the impact of that activity on structures. For instance, geophysics can be used to help determine the best building materials and methods for compacting Earth.

In fact, building codes actually emerged from earthquakes and state responses to them. Building codes and earthquake engineering can be traced back to the 1755 earthquake that nearly leveled Lisbon, the capital of the Portuguese Empire, and shocked Europe. On November 1 of that year, a massive earthquake struck in the ocean off the southwest coast of Portugal. Much of Lisbon was destroyed by the earthquake and accompanying tsunami. The king and his prime minister, the Marquess of Pombal, survived, and the king immediately tasked his minister with reconstruction. To this end, the prime minister sent surveys around the countryside that asked such questions as whether water levels in wells had fluctuated, if animals had behaved strangely, and how many and what types of buildings had been destroyed. This survey allowed the earthquake to be reconstructed with great detail.

Back in the ruins of Lisbon, the king and the prime minister decided to level the ruined sections of Lisbon and completely rebuild. The peasantry was pressed into service and strict discipline was kept. As a result, there were no mass famines or epidemics, and Lisbon was cleared within a few months. To keep the city safe from future earthquakes, several innovative ideas were implemented. Scale models of buildings were made and soldiers were ordered to march around them to test their safety. These designs made use of an interior wooden frame that would shake but not collapse, presaging modern techniques. To get everyone rehoused as quickly as possible, the buildings were designed to use prefabricated units, a new idea at the time. To prevent massive fires like those seen after the earthquake, fire walls were included in designs. An even greater challenge was found in reconstructing the district of Baixa. The district, by the water, had been built on unstable ground. To circumvent this problem, wooden poles were buried beneath the area so as to stabilize the soil. Similar techniques are used to this day in areas where soil liquefaction is a concern.

The Great Lisbon Earthquake also had a massive effect on European philosophy and science. People decided that the earthquake must have had a natural cause, and in the spirit of the age, they set forth to figure out what that reason was. This launched the scientific quest to study earthquakes as a natural phenomenon. The Lisbon earthquake thus initiated the scientific study of earthquakes and also marked the first policies designed to improve survival.

Geothermal Power.Geothermal power is an application of geophysics that results in the generation of power that can be used to power the electric grid. Iceland is a country that has successfully integrated geothermal power into its grid. The benefit of using geothermal power is that it is a renewable energy source that produces little to no pollution and has no significant environmental impact.

Mining. Geophysics is especially important to the mining industry, where workers are regularly sent thousands of feet beneath the ground. An understanding of seismic activity and the likelihood of such activity is vital to their safety, as well as to the structural integrity of the tunnels. The principles of geophysics are used both to determine the integrity of the tunnel structure and to help pinpoint the types of minerals being sought.

Mineral exploration has always been a time-consuming processing, historically requiring panning for samples, but with geophysical techniques, the process has become swifter and more precise. Often it begins by looking at the sort of rock present in an area and then applying ore-genesis theories to determine what is likely to be found there. Geophysicists can then use readings of local magnetic and gravitational anomalies to identify areas of interest. Satellite images in other spectra can further refine the areas of interest. An understanding of what makes up minerals, gleaned from geophysics, has allowed sampling efforts to progress, meaning that chemical samples can suggest likely ores. For example, arsenic and antimony tend to accompany gold, so if tree buds or soil samples have high concentrations of these, there may be gold deposits nearby. The mineral is then located, typically by drilling. Often this is done by drilling on a set grid in order to determine the size of the deposit. This process results in less time and money being wasted on the exploration of areas that do not contain the desired minerals.

Climatology. Geophysics is used by those studying weather patterns and climate over time. Scientists can use sea sediment to identify climate changes that occurred over long periods of time. Fossil records can also be used for this purpose. By examining historical patterns and satellite data and incorporating them into predictive models, scientists can predict climate cycles such as El Niño that have an impact on weather and agriculture. In the case of climate change, an understanding of these cycles is even more important, as knowing what happens as they change will greatly aid in mitigating their negative effects. Climatological data also aids in fields such as agriculture and construction. By using climate models, climatologists can help farmers plan what crops to grow; and in the far north, where much of the ground was once permafrost but is now melting, they can give builders information about what ground conditions to expect and take into account.

Oceanography. The study of oceanic currents leads to a greater understanding of their effect on the biosphere. It also leads to an understanding of the way nutrients and other materials circulate through the oceans. Deep-sea currents carry the water throughout the oceans and up into the polar seas. This causes massive blooms of algae that attract other organisms to come and feed. This is why life is abundant in the polar seas. These algal blooms are an essential part of the ecosystem and the health of myriad ocean species, including humpback whales, penguins, puffins, seals, and polar bears. It is feared that if the oceanic conveyor system were to shut down, it would lead to deep-sea anoxia and a disruption of the nutrient cycle, which in turn would lead to an excessive proliferation of algal blooms across the surface. This would cause the demise of most ocean life and cause hydrogen sulfide to build up in the oceans and eventually be released into the atmosphere, making the air unbreathable for land-based species. This is thought to have happened at the end of the Permian period, 250 million years ago. Such events are an extreme case, but an understanding of them is needed as, at present, oceanic anoxia is on the rise. Equally, knowledge of the flows of the seas aids in understanding how ecosystems come into being and are sustained. In addition to the scientific benefit, this also is useful for keeping fish populations sustained for fishing operations.

Social Context and Future Prospects

Geophysics will be critical to resolving the problem of peak oil. Oil is, on the human time scale, a finite and nonrenewable resource. Thus, there is a maximum that can be extracted both in total and at any one time. Peak oil is the point at which oil production is maximized; after that, oil production will shrink and prices will rise. It is currently estimated that most of the cheapest oil has already been found and that the world is approaching peak oil. Managing peak oil requires a combination of effective global action and new technologies to make most of the remaining resources.

Future prospects for the application of geophysics are boundless. Geothermal construction, increased hydroelectric power, the prospect of geoengineering, and someday perhaps even terraforming other worlds leaves much to look forward to.

Bibliography

Cormier, Vernon F., Michael I. Bergman, and Peter L. Olson. Earth's Core: Geophysics of a Planet's Deepest Interior. Elsevier, 2022.

Gadallah, Mamdouh R., and Ray L. Fisher. Exploration Geophysics. Berlin: Springer, 2008. Print.

Kirsch, Reinhard. Groundwater Geophysics: A Tool for Hydrogeology. 2d ed. Berlin: Springer, 2010. Print.

Lowrie, William. Fundamentals of Geophysics. 2d ed. Cambridge: Cambridge UP, 2011. Print.

Milsom, John, and Asger Eriksen. Field Geophysics. 4th ed. Chichester: Wiley, 2013. Print.

Reynolds, John M. An Introduction to Applied and Environmental Geophysics. 2d ed. Chichester: Wiley, 2011. Print.

Svetov, B. S. “Self-Consistent Problems of Geophysics: A Review.” Izvestiya Physics of the Solid Earth 51.6 (2015): 910+. Print.