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Monday, October 14, 2024

Geological engineering

From Wikipedia, the free encyclopedia
Image of rock tunnel (background) and rockfall protection mesh a rock cliff face (foreground)
Example of infrastructure engineering (tunnel) and natural hazard engineering (rockfall protection), two subdisciplines of geological engineering

Geological engineering is a discipline of engineering concerned with the application of geological science and engineering principles to fields, such as civil engineering, mining, environmental engineering, and forestry, among others. The work of geological engineers often directs or supports the work of other engineering disciplines such as assessing the suitability of locations for civil engineering, environmental engineering, mining operations, and oil and gas projects by conducting geological, geoenvironmental, geophysical, and geotechnical studies. They are involved with impact studies for facilities and operations that affect surface and subsurface environments. The engineering design input and other recommendations made by geological engineers on these projects will often have a large impact on construction and operations. Geological engineers plan, design, and implement geotechnical, geological, geophysical, hydrogeological, and environmental data acquisition. This ranges from manual ground-based methods to deep drilling, to geochemical sampling, to advanced geophysical techniques and satellite surveying. Geological engineers are also concerned with the analysis of past and future ground behaviour, mapping at all scales, and ground characterization programs for specific engineering requirements. These analyses lead geological engineers to make recommendations and prepare reports which could have major effects on the foundations of construction, mining, and civil engineering projects. Some examples of projects include rock excavation, building foundation consolidation, pressure grouting, hydraulic channel erosion control, slope and fill stabilization, landslide risk assessment, groundwater monitoring, and assessment and remediation of contamination. In addition, geological engineers are included on design teams that develop solutions to surface hazards, groundwater remediation, underground and surface excavation projects, and resource management. Like mining engineers, geological engineers also conduct resource exploration campaigns, mine evaluation and feasibility assessments, and contribute to the ongoing efficiency, sustainability, and safety of active mining projects

History

While the term geological engineering was not coined until the 19th century, principles of geological engineering are demonstrated through millennia of human history.

Tunnel of Eupalinos aqueduct tunnel in Samos, Greece, which is a famous example of ancient tunnel and survey engineering.

Ancient engineering

One of the oldest examples of geological engineering principles is the Euphrates tunnel, which was constructed around 2180 B.C. – 2160 B.C... This, and other tunnels and qanats from around the same time were used by ancient civilizations such as Babylon and Persia for the purposes of irrigation. Another famous example where geological engineering principles were used in an ancient engineering project was the construction of the Eupalinos aqueduct tunnel in Ancient Greece. This was the first tunnel to be constructed inward from both ends using principles of geometry and trigonometry, marking a significant milestone for both civil engineering and geological engineering.

Geological engineering as a discipline

Although projects that applied geological engineering principles in their design and construction have been around for thousands of years, these were included within the civil engineering discipline for most of this time. Courses in geological engineering have been offered since the early 1900s; however, these remained specialized offerings until a large increase in demand arose in the mid-20th century. This demand was created by issues encountered from development of increasingly large and ambitious structures, human-generated waste, scarcity of mineral and energy resources, and anthropogenic climate change – all of which created the need for a more specialized field of engineering with professional engineers who were also experts in geological or Earth sciences.

Notable disasters that are attributed to the formal creation of the geological engineering discipline include dam failures in the United States and western Europe in the 1950s and 1960s. These most famously include the St Francis dam failure (1928), Malpasset dam failure (1959), and the Vajont dam failure (1963), where a lack of knowledge of geology resulted in almost 3,000 deaths between the latter two alone. The Malpasset dam failure is regarded as the largest civil engineering disaster of the 20th century in France and Vajont dam failure is still the deadliest landslide in European history.

Education

Post-secondary degrees in geological engineering are offered at various universities around the world but are concentrated primarily in North America. Geological engineers often obtain degrees that include courses in both geological or Earth sciences and engineering. To practice as a professional geological engineer, a bachelor's degree in a related discipline from an accredited institution is required. For certain positions, a Master’s or Doctorate degree in a related engineering discipline may be required. After obtaining these degrees, an individual who wishes to practice as a professional geological engineer must go through the process of becoming licensed by a professional association or regulatory body in their jurisdiction.

Canadian institutions

In Canada, 8 universities are accredited by Engineers Canada to offer undergraduate degrees in geological engineering. Many of these universities also offer graduate degree programs in geological engineering. These include:

American institutions

In the United States there are 13 geological engineering programs recognized by the Engineering Accreditation Commission (EAC) of the Accreditation Board for Engineering and Technology (ABET). These include:

Other institutions

Universities in other countries that hold accreditation to offer degree programs in geological engineering from the EAC by the ABET include:

Specializations

In geological engineering there are multiple subdisciplines which analyze different aspects of Earth sciences and apply them to a variety of engineering projects. The subdisciplines listed below are commonly taught at the undergraduate level, and each has overlap with disciplines external to geological engineering. However, a geological engineer who specializes in one of these subdisciplines throughout their education may still be licensed to work in any of the other subdisciplines.

Mountain lake showing surface water. Geoenvironmental engineers (subdiscipline of geological engineering) work on managing drinking water supplies and remediation of contaminated surface water and groundwater.

Geoenvironmental and hydrogeological engineering

Geoenvironmental engineering is the subdiscipline of geological engineering that focuses on preventing or mitigating the environmental effects of anthropogenic contaminants within soil and water. It solves these issues via the development of processes and infrastructure for the supply of clean water, waste disposal, and control of pollution of all kinds. The work of geoenvironmental engineers largely deals with investigating the migration, interaction, and result of contaminants; remediating contaminated sites; and protecting uncontaminated sites. Typical work of a geoenvironmental engineer includes:

  • The preparation, review, and update of environmental investigation reports,
  • The design of projects such as water reclamation facilities or groundwater monitoring wells which lead to the protection of the environment,
  • Conducting feasibility studies and economic analyses of environmental projects,
  • Obtaining and revising permits, plans, and standard procedures,
  • Providing technical expertise for environmental remediation projects which require legal actions,
  • The analysis of groundwater data for the purpose of quality-control checks,
  • The site investigation and monitoring of environmental remediation and sustainability projects to ensure compliance with environmental regulations, and
  • Advising corporations and government agencies regarding procedures for cleaning up contaminated sites.
A tunnel under construction by conventional excavation methods with a pilot tunnel through tunnel face and a drill jumbo positioned near the face. Rock engineers and geotechnical engineers (subdisciplines of geological engineering) are involved with the design and construction of underground excavations.

Mineral and energy resource exploration engineering

Mineral and energy resource exploration (commonly known as MinEx for short) is the subdiscipline of geological engineering that applies modern tools and concepts to the discovery and sustainable extraction of natural mineral and energy resources. A geological engineer who specializes in this field may work on several stages of mineral exploration and mining projects, including exploration and orebody delineation, mine production operations, mineral processing, and environmental impact and risk assessment programs for mine tailings and other mine waste. Like a mining engineer, mineral and energy resource exploration engineers may also be responsible for the design, finance, and management of mine sites.

A Ground Penetrating Radar (GPR) being used to conduct a geophysical survey. Geophysical engineers (subdiscipline of geological engineering) use multiple geophysical techniques to noninvasively investigate the Earth's subsurface at all scales and use the results in a variety of engineering projects.

Geophysical engineering (applied geophysics)

Geophysical engineering is the subdiscipline of geological engineering that applies geophysics principles to the design of engineering projects such as tunnels, dams, and mines or for the detection of subsurface geohazards, groundwater, and pollution. Geophysical investigations are undertaken from ground surface, in boreholes, or from space to analyze ground conditions, composition, and structure at all scales. Geophysical techniques apply a variety of physics principles such as seismicity, magnetism, gravity, and resistivity. This subdiscipline was created in the early 1990s as a result of an increased demand in more accurate subsurface information created by a rapidly increasing global population. Geophysical engineering and applied geophysics differ from traditional geophysics primarily by their need for marginal returns and optimized designs and practices as opposed to satisfying regulatory requirements at a minimum cost

Job responsibilities

Geological engineers are responsible for the planning, development, and coordination of site investigation and data acquisition programs for geological, geotechnical, geophysical, geoenvironmental, and hydrogeological studies. These studies are traditionally conducted for civil engineering, mining, petroleum, waste management, and regional development projects but are becoming increasingly focused on environmental and coastal engineering projects and on more specialized projects for long-term underground nuclear waste storage. Geological engineers are also responsible for analyzing and preparing recommendations and reports to improve construction of foundations for civil engineering projects such as rock and soil excavation, pressure grouting, and hydraulic channel erosion control. In addition, geological engineers analyze and prepare recommendations and reports on the settlement of buildings, stability of slopes and fills, and probable effects of landslides and earthquakes to support construction and civil engineering projects. They must design means to safely excavate and stabilize the surrounding rock or soil in underground excavations and surface construction, in addition to managing water flow from, and within these excavations.

Geological engineers also perform a primary role in all forms of underground infrastructure including tunnelling, mining, hydropower projects, shafts, deep repositories and caverns for power, storage, industrial activities, and recreation. Moreover, geological engineers design monitoring systems, analyze natural and induced ground response, and prepare recommendations and reports on the settlement of buildings, stability of slopes and fills, and the probable effects of natural disasters to support construction and civil engineering projects. In some jobs, geological engineers conduct theoretical and applied studies of groundwater flow and contamination to develop site specific solutions which treat the contaminants and allow for safe construction. Additionally, they design means to manage and protect surface and groundwater resources and remediation solutions in the event of contamination. If working on a mine site, geological engineers may be tasked with planning, development, coordination, and conducting theoretical and experimental studies in mining exploration, mine evaluation and feasibility studies relative to the mining industry. They conduct surveys and studies of ore deposits, ore reserve calculations, and contribute mineral resource expertise, geotechnical and geomechanical design and monitoring expertise and environmental management to a developing or ongoing mining operation. In a variety of projects, they may be expected to design and perform geophysical investigations from surface using boreholes or from space to analyze ground conditions, composition, and structure at all scales.

Professional associations and licensing

Professional Engineering Licenses may be issued through a municipal, provincial/state, or federal/national government organization, depending on the jurisdiction. The purpose of this licensing process is to ensure professional engineers possess the necessary technical knowledge, real-world experience, and basic understanding of the local legal system to practice engineering at a professional level. In Canada, the United States, Japan, South Korea, Bangladesh, and South Africa, the title of Professional Engineer is granted through licensure. In the United Kingdom, Ireland, India, and Zimbabwe the granted title is Chartered Engineer . In Australia, the granted title is Chartered Professional Engineer. Lastly, in the European Union, the granted title is European Engineer. All these titles have similar requirements for accreditation, including a recognized post-secondary degree and relevant work experience.

Canada

In Canada, Professional Engineer (P.Eng.) and Professional Geoscientist (P.Geo.) licenses are regulated by provincial professional bodies which have the groundwork for their legislation laid out by Engineers Canada and Geoscientists Canada. The provincial organizations are listed in the table below.

Regulatory body responsible for awarding licenses for professional engineering and geosciences in each province and territory of Canada
Province Regulatory Body
Alberta Association of Professional Engineers and Geoscientists of Alberta
British Columbia Association of Engineers and Geoscientists of British Columbia
Manitoba Engineers Geoscientists of Manitoba
New Brunswick Association of Professional Engineers and Geoscientists of New Brunswick
Newfoundland and Labrador Professional Engineers and Geoscientists of Newfoundland and Labrador
Northwest Territories Northwest Territories and Nunavut Association of Professional Engineers and Geoscientists
Nova Scotia Association of Professional Engineers of Nova Scotia
Nunavut Northwest Territories and Nunavut Association of Professional Engineers and Geoscientists
Ontario Professional Engineers Ontario
Prince Edward Island Association of Professional Engineers of Prince Edward Island
Quebec Ordre des ingénieurs du Québec
Saskatchewan Association of Professional Engineers and Geoscientists of Saskatchewan
Yukon Engineers of Yukon

United States

In the United States, all individuals seeking to become a Professional Engineer (P.E.) must attain their license through the Engineering Accreditation Commission (EAC) of the Accreditation Board for Engineering and Technology (ABET). Licenses to be a Certified Professional Geologist in the United States are issued and regulated by the American Institute of Professional Geologists (AIPG)

Professional Societies

Professional societies in geological engineering are not-for-profit organizations that seek to advance and promote the represented profession(s) and connect professionals using networking, regular conferences, meetings, and other events, as well as provide platforms to publish technical literature through forms of conference proceedings, books, technical standards, and suggested methods, and provide opportunities for professional development such as short courses, workshops, and technical tours. Some regional, national, and international professional societies relevant to geological engineers are listed here:

Distinction from engineering geology

Engineering geologists and geological engineers are both interested in the study of the Earth, its shifting movement, and alterations, and the interactions of human society and infrastructure with, on, and in Earth materials. Both disciplines require licenses from professional bodies in most jurisdictions to conduct related work. The primary difference between geological engineers and engineering geologists is that geological engineers are licensed professional engineers (and sometimes also professional geoscientists/geologists) with a combined understanding of Earth sciences and engineering principles, while engineering geologists are geological scientists whose work focusses on applications to engineering projects, and they may be licensed professional geoscientists/geologists, but not professional engineers. The following subsections provide more details on the differing responsibilities between engineering geologists and geological engineers.

Engineering geology

Engineering geologists are applied geological scientists who assess problems that might arise before, during, and after an engineering project. They are trained to be aware of potential problems like:

They use a variety of field and laboratory testing techniques to characterize ground materials that might affect the construction, the long-term safety, or environmental footprint of a project. Job responsibilities of an engineering geologist include:

  • collecting samples and surveys,
  • conducting lab tests on samples,
  • assessing in situ soil or rock conditions at many scales,
  • preparing reports based on testing and on-site observations for clients, and
  • creating geological models, maps, and sections.

Geological engineering

Geological engineers are engineers with extensive knowledge of geological or Earth sciences as well as engineering geology, engineering principles, and engineering design practices. These professionals are qualified to perform the role of or interact with engineering geologists. Their primary focus, however, is the use of engineering geology data, as well as engineering skills to:

In all these activities, the geological model, geological history, and environment, as well as measured engineering properties of relevant Earth materials are critical to engineering design and decision making.
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Geophysics

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Geophysics
false color image
Age of the sea floor. Much of the dating information comes from magnetic anomalies.
Computer simulation of the Earth's magnetic field in a period of normal polarity between reversals

Geophysics (/ˌdʒiːoʊˈfɪzɪks/) is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. Geophysicists, who usually study geophysics, physics, or one of the Earth sciences at the graduate level, complete investigations across a wide range of scientific disciplines. The term geophysics classically refers to solid earth applications: Earth's shape; its gravitational, magnetic fields, and electromagnetic fields ; its internal structure and composition; its dynamics and their surface expression in plate tectonics, the generation of magmas, volcanism and rock formation. However, modern geophysics organizations and pure scientists use a broader definition that includes the water cycle including snow and ice; fluid dynamics of the oceans and the atmosphere; electricity and magnetism in the ionosphere and magnetosphere and solar-terrestrial physics; and analogous problems associated with the Moon and other planets.

Although geophysics was only recognized as a separate discipline in the 19th century, its origins date back to ancient times. The first magnetic compasses were made from lodestones, while more modern magnetic compasses played an important role in the history of navigation. The first seismic instrument was built in 132 AD. Isaac Newton applied his theory of mechanics to the tides and the precession of the equinox; and instruments were developed to measure the Earth's shape, density and gravity field, as well as the components of the water cycle. In the 20th century, geophysical methods were developed for remote exploration of the solid Earth and the ocean, and geophysics played an essential role in the development of the theory of plate tectonics.

Geophysics is applied to societal needs, such as mineral resources, mitigation of natural hazards and environmental protection. In exploration geophysics, geophysical survey data are used to analyze potential petroleum reservoirs and mineral deposits, locate groundwater, find archaeological relics, determine the thickness of glaciers and soils, and assess sites for environmental remediation.

Physical phenomena

Geophysics is a highly interdisciplinary subject, and geophysicists contribute to every area of the Earth sciences, while some geophysicists conduct research in the planetary sciences. To provide a more clear idea on what constitutes geophysics, this section describes phenomena that are studied in physics and how they relate to the Earth and its surroundings. Geophysicists also investigate the physical processes and properties of the Earth, its fluid layers, and magnetic field along with the near-Earth environment in the Solar System, which includes other planetary bodies.

Gravity

Image of globe combining color with topography.
A map of deviations in gravity from a perfectly smooth, idealized Earth
Main article: Gravity of Earth
Further information: Physical geodesy and Gravimetry

The gravitational pull of the Moon and Sun gives rise to two high tides and two low tides every lunar day, or every 24 hours and 50 minutes. Therefore, there is a gap of 12 hours and 25 minutes between every high tide and between every low tide. Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases. Measurements of gravitational acceleration and gravitational potential at the Earth's surface and above it can be used to look for mineral deposits (see gravity anomaly and gravimetry). The surface gravitational field provides information on the dynamics of tectonic plates. The geopotential surface called the geoid is one definition of the shape of the Earth. The geoid would be the global mean sea level if the oceans were in equilibrium and could be extended through the continents (such as with very narrow canals).

Heat flow

Main article: Geothermal gradient
Pseudocolor image in vertical profile.
A model of thermal convection in the Earth's mantle. The thin red columns are mantle plumes.

The Earth is cooling, and the resulting heat flow generates the Earth's magnetic field through the geodynamo and plate tectonics through mantle convection. The main sources of heat are: primordial heat due to Earth's cooling and radioactivity in the planets upper crust. There is also some contributions from phase transitions. Heat is mostly carried to the surface by thermal convection, although there are two thermal boundary layers – the core–mantle boundary and the lithosphere – in which heat is transported by conduction. Some heat is carried up from the bottom of the mantle by mantle plumes. The heat flow at the Earth's surface is about 4.2 × 1013 W, and it is a potential source of geothermal energy.

Vibrations

Main article: Seismology
Deformed blocks with grids on surface.
Illustration of the deformations of a block by body waves and surface waves (see seismic wave)

Seismic waves are vibrations that travel through the Earth's interior or along its surface. The entire Earth can also oscillate in forms that are called normal modes or free oscillations of the Earth. Ground motions from waves or normal modes are measured using seismographs. If the waves come from a localized source such as an earthquake or explosion, measurements at more than one location can be used to locate the source. The locations of earthquakes provide information on plate tectonics and mantle convection.

Recording of seismic waves from controlled sources provides information on the region that the waves travel through. If the density or composition of the rock changes, waves are reflected. Reflections recorded using Reflection Seismology can provide a wealth of information on the structure of the earth up to several kilometers deep and are used to increase our understanding of the geology as well as to explore for oil and gas. Changes in the travel direction, called refraction, can be used to infer the deep structure of the Earth.

Earthquakes pose a risk to humans. Understanding their mechanisms, which depend on the type of earthquake (e.g., intraplate or deep focus), can lead to better estimates of earthquake risk and improvements in earthquake engineering.

Electricity

Although we mainly notice electricity during thunderstorms, there is always a downward electric field near the surface that averages 120 volts per meter. Relative to the solid Earth, the ionization of the planet's atmosphere is a result of the galactic cosmic rays penetrating it, which leaves it with a net positive charge. A current of about 1800 amperes flows in the global circuit. It flows downward from the ionosphere over most of the Earth and back upwards through thunderstorms. The flow is manifested by lightning below the clouds and sprites above.

A variety of electric methods are used in geophysical survey. Some measure spontaneous potential, a potential that arises in the ground because of human-made or natural disturbances. Telluric currents flow in Earth and the oceans. They have two causes: electromagnetic induction by the time-varying, external-origin geomagnetic field and motion of conducting bodies (such as seawater) across the Earth's permanent magnetic field. The distribution of telluric current density can be used to detect variations in electrical resistivity of underground structures. Geophysicists can also provide the electric current themselves (see induced polarization and electrical resistivity tomography).

Electromagnetic waves

Electromagnetic waves occur in the ionosphere and magnetosphere as well as in Earth's outer core. Dawn chorus is believed to be caused by high-energy electrons that get caught in the Van Allen radiation belt. Whistlers are produced by lightning strikes. Hiss may be generated by both. Electromagnetic waves may also be generated by earthquakes (see seismo-electromagnetics).

In the highly conductive liquid iron of the outer core, magnetic fields are generated by electric currents through electromagnetic induction. Alfvén waves are magnetohydrodynamic waves in the magnetosphere or the Earth's core. In the core, they probably have little observable effect on the Earth's magnetic field, but slower waves such as magnetic Rossby waves may be one source of geomagnetic secular variation.

Electromagnetic methods that are used for geophysical survey include transient electromagnetics, magnetotellurics, surface nuclear magnetic resonance and electromagnetic seabed logging.

Magnetism

Further information: Earth's magnetic field, Aeromagnetic survey, and Paleomagnetism

The Earth's magnetic field protects the Earth from the deadly solar wind and has long been used for navigation. It originates in the fluid motions of the outer core. The magnetic field in the upper atmosphere gives rise to the auroras.

Diagram with field lines, axes and magnet lines.
Earth's dipole axis (pink line) is tilted away from the rotational axis (blue line).

The Earth's field is roughly like a tilted dipole, but it changes over time (a phenomenon called geomagnetic secular variation). Mostly the geomagnetic pole stays near the geographic pole, but at random intervals averaging 440,000 to a million years or so, the polarity of the Earth's field reverses. These geomagnetic reversals, analyzed within a Geomagnetic Polarity Time Scale, contain 184 polarity intervals in the last 83 million years, with change in frequency over time, with the most recent brief complete reversal of the Laschamp event occurring 41,000 years ago during the last glacial period. Geologists observed geomagnetic reversal recorded in volcanic rocks, through magnetostratigraphy correlation (see natural remanent magnetization) and their signature can be seen as parallel linear magnetic anomaly stripes on the seafloor. These stripes provide quantitative information on seafloor spreading, a part of plate tectonics. They are the basis of magnetostratigraphy, which correlates magnetic reversals with other stratigraphies to construct geologic time scales. In addition, the magnetization in rocks can be used to measure the motion of continents.

Radioactivity

Further information: Radiometric dating
Diagram with compound balls representing nuclei and arrows.
Example of a radioactive decay chain (see Radiometric dating)

Radioactive decay accounts for about 80% of the Earth's internal heat, powering the geodynamo and plate tectonics. The main heat-producing isotopes are potassium-40, uranium-238, uranium-235, and thorium-232. Radioactive elements are used for radiometric dating, the primary method for establishing an absolute time scale in geochronology.

Unstable isotopes decay at predictable rates, and the decay rates of different isotopes cover several orders of magnitude, so radioactive decay can be used to accurately date both recent events and events in past geologic eras. Radiometric mapping using ground and airborne gamma spectrometry can be used to map the concentration and distribution of radioisotopes near the Earth's surface, which is useful for mapping lithology and alteration.

Fluid dynamics

Main article: Geophysical fluid dynamics

Fluid motions occur in the magnetosphere, atmosphere, ocean, mantle and core. Even the mantle, though it has an enormous viscosity, flows like a fluid over long time intervals. This flow is reflected in phenomena such as isostasy, post-glacial rebound and mantle plumes. The mantle flow drives plate tectonics and the flow in the Earth's core drives the geodynamo.

Geophysical fluid dynamics is a primary tool in physical oceanography and meteorology. The rotation of the Earth has profound effects on the Earth's fluid dynamics, often due to the Coriolis effect. In the atmosphere, it gives rise to large-scale patterns like Rossby waves and determines the basic circulation patterns of storms. In the ocean, they drive large-scale circulation patterns as well as Kelvin waves and Ekman spirals at the ocean surface. In the Earth's core, the circulation of the molten iron is structured by Taylor columns.

Waves and other phenomena in the magnetosphere can be modeled using magnetohydrodynamics.

Mineral physics

Main article: Mineral physics

The physical properties of minerals must be understood to infer the composition of the Earth's interior from seismology, the geothermal gradient and other sources of information. Mineral physicists study the elastic properties of minerals; their high-pressure phase diagrams, melting points and equations of state at high pressure; and the rheological properties of rocks, or their ability to flow. Deformation of rocks by creep make flow possible, although over short times the rocks are brittle. The viscosity of rocks is affected by temperature and pressure, and in turn, determines the rates at which tectonic plates move.

Water is a very complex substance and its unique properties are essential for life. Its physical properties shape the hydrosphere and are an essential part of the water cycle and climate. Its thermodynamic properties determine evaporation and the thermal gradient in the atmosphere. The many types of precipitation involve a complex mixture of processes such as coalescence, supercooling and supersaturation. Some precipitated water becomes groundwater, and groundwater flow includes phenomena such as percolation, while the conductivity of water makes electrical and electromagnetic methods useful for tracking groundwater flow. Physical properties of water such as salinity have a large effect on its motion in the oceans.

The many phases of ice form the cryosphere and come in forms like ice sheets, glaciers, sea ice, freshwater ice, snow, and frozen ground (or permafrost).

Regions of the Earth

Size and form of the Earth

Contrary to popular belief, the earth is not entirely spherical but instead generally exhibits an ellipsoid shape- which is a result of the centrifugal forces the planet generates due to its constant motion. These forces cause the planets diameter to bulge towards the Equator and results in the ellipsoid shape. Earth's shape is constantly changing, and different factors including glacial isostatic rebound (large ice sheets melting causing the Earth's crust to the rebound due to the release of the pressure), geological features such as mountains or ocean trenches, tectonic plate dynamics, and natural disasters can further distort the planet's shape.

Structure of the interior

Main article: Structure of Earth
Diagram with concentric shells and curved paths.
Seismic velocities and boundaries in the interior of the Earth sampled by seismic waves

Evidence from seismology, heat flow at the surface, and mineral physics is combined with the Earth's mass and moment of inertia to infer models of the Earth's interior – its composition, density, temperature, pressure. For example, the Earth's mean specific gravity (5.515) is far higher than the typical specific gravity of rocks at the surface (2.7–3.3), implying that the deeper material is denser. This is also implied by its low moment of inertia ( 0.33 M R2, compared to 0.4 M R2 for a sphere of constant density). However, some of the density increase is compression under the enormous pressures inside the Earth. The effect of pressure can be calculated using the Adams–Williamson equation. The conclusion is that pressure alone cannot account for the increase in density. Instead, we know that the Earth's core is composed of an alloy of iron and other minerals.

Reconstructions of seismic waves in the deep interior of the Earth show that there are no S-waves in the outer core. This indicates that the outer core is liquid, because liquids cannot support shear. The outer core is liquid, and the motion of this highly conductive fluid generates the Earth's field. Earth's inner core, however, is solid because of the enormous pressure.

Reconstruction of seismic reflections in the deep interior indicates some major discontinuities in seismic velocities that demarcate the major zones of the Earth: inner core, outer core, mantle, lithosphere and crust. The mantle itself is divided into the upper mantle, transition zone, lower mantle and D′′ layer. Between the crust and the mantle is the Mohorovičić discontinuity.

The seismic model of the Earth does not by itself determine the composition of the layers. For a complete model of the Earth, mineral physics is needed to interpret seismic velocities in terms of composition. The mineral properties are temperature-dependent, so the geotherm must also be determined. This requires physical theory for thermal conduction and convection and the heat contribution of radioactive elements. The main model for the radial structure of the interior of the Earth is the preliminary reference Earth model (PREM). Some parts of this model have been updated by recent findings in mineral physics (see post-perovskite) and supplemented by seismic tomography. The mantle is mainly composed of silicates, and the boundaries between layers of the mantle are consistent with phase transitions.

The mantle acts as a solid for seismic waves, but under high pressures and temperatures, it deforms so that over millions of years it acts like a liquid. This makes plate tectonics possible.

Magnetosphere

Main article: Magnetosphere
Diagram with colored surfaces and lines.
Schematic of Earth's magnetosphere. The solar wind flows from left to right.

If a planet's magnetic field is strong enough, its interaction with the solar wind forms a magnetosphere. Early space probes mapped out the gross dimensions of the Earth's magnetic field, which extends about 10 Earth radii towards the Sun. The solar wind, a stream of charged particles, streams out and around the terrestrial magnetic field, and continues behind the magnetic tail, hundreds of Earth radii downstream. Inside the magnetosphere, there are relatively dense regions of solar wind particles called the Van Allen radiation belts.

Methods

Geodesy

Main article: Geodesy

Geophysical measurements are generally at a particular time and place. Accurate measurements of position, along with earth deformation and gravity, are the province of geodesy. While geodesy and geophysics are separate fields, the two are so closely connected that many scientific organizations such as the American Geophysical Union, the Canadian Geophysical Union and the International Union of Geodesy and Geophysics encompass both.

Absolute positions are most frequently determined using the global positioning system (GPS). A three-dimensional position is calculated using messages from four or more visible satellites and referred to the 1980 Geodetic Reference System. An alternative, optical astronomy, combines astronomical coordinates and the local gravity vector to get geodetic coordinates. This method only provides the position in two coordinates and is more difficult to use than GPS. However, it is useful for measuring motions of the Earth such as nutation and Chandler wobble. Relative positions of two or more points can be determined using very-long-baseline interferometry.

Gravity measurements became part of geodesy because they were needed to related measurements at the surface of the Earth to the reference coordinate system. Gravity measurements on land can be made using gravimeters deployed either on the surface or in helicopter flyovers. Since the 1960s, the Earth's gravity field has been measured by analyzing the motion of satellites. Sea level can also be measured by satellites using radar altimetry, contributing to a more accurate geoid. In 2002, NASA launched the Gravity Recovery and Climate Experiment (GRACE), wherein two twin satellites map variations in Earth's gravity field by making measurements of the distance between the two satellites using GPS and a microwave ranging system. Gravity variations detected by GRACE include those caused by changes in ocean currents; runoff and ground water depletion; melting ice sheets and glaciers.

Satellites and space probes

Satellites in space have made it possible to collect data from not only the visible light region, but in other areas of the electromagnetic spectrum. The planets can be characterized by their force fields: gravity and their magnetic fields, which are studied through geophysics and space physics.

Measuring the changes in acceleration experienced by spacecraft as they orbit has allowed fine details of the gravity fields of the planets to be mapped. For example, in the 1970s, the gravity field disturbances above lunar maria were measured through lunar orbiters, which led to the discovery of concentrations of mass, mascons, beneath the Imbrium, Serenitatis, Crisium, Nectaris and Humorum basins.

Global positioning systems (GPS) and geographical information systems (GIS)

Further information: GIS

Since geophysics is concerned with the shape of the Earth, and by extension the mapping of features around and in the planet, geophysical measurements include high accuracy GPS measurements. These measurements are processed to increase their accuracy through differential GPS processing. Once the geophysical measurements have been processed and inverted, the interpreted results are plotted using GIS. Programs such as ArcGIS and Geosoft were built to meet these needs and include many geophysical functions that are built-in, such as upward continuation, and the calculation of the measurement derivative such as the first-vertical derivative. Many geophysics companies have designed in-house geophysics programs that pre-date ArcGIS and GeoSoft in order to meet the visualization requirements of a geophysical dataset.

Remote sensing

Main article: Remote sensing

Exploration geophysics is a branch of applied geophysics that involves the development and utilization of different seismic or electromagnetic methods which the aim of investigating different energy, mineral and water resources. This is done through the uses of various remote sensing platforms such as; satellites, aircraft, boats, drones, borehole sensing equipment and seismic receivers. These equipment are often used in conjunction with different geophysical methods such as magnetic, gravimetry, electromagnetic, radiometric, barometry methods in order to gather the data. The remote sensing platforms used in exploration geophysics are not perfect and need adjustments done on them in order to accurately account for the effects that the platform itself may have on the collected data. For example, when gathering aeromagnetic data (aircraft gathered magnetic data) using a conventional fixed-wing aircraft- the platform has to be adjusted to account for the electromagnetic currents that it may generate as it passes through Earth's magnetic field. There are also corrections related to changes in measured potential field intensity as the Earth rotates, as the Earth orbits the Sun, and as the moon orbits the Earth.

Signal processing

Main article: Signal processing

Geophysical measurements are often recorded as time-series with GPS location. Signal processing involves the correction of time-series data for unwanted noise or errors introduced by the measurement platform, such as aircraft vibrations in gravity data. It also involves the reduction of sources of noise, such as diurnal corrections in magnetic data. In seismic data, electromagnetic data, and gravity data, processing continues after error corrections to include computational geophysics which result in the final interpretation of the geophysical data into a geological interpretation of the geophysical measurements.

History

Main article: History of geophysics

Geophysics emerged as a separate discipline only in the 19th century, from the intersection of physical geography, geology, astronomy, meteorology, and physics. The first known use of the word geophysics was in German ("Geophysik") by Julius Fröbel in 1834. However, many geophysical phenomena – such as the Earth's magnetic field and earthquakes – have been investigated since the ancient era.

Ancient and classical eras

Picture of ornate urn-like device with spouts in the shape of dragons
Replica of Zhang Heng's seismoscope, possibly the first contribution to seismology

The magnetic compass existed in China back as far as the fourth century BC. It was used as much for feng shui as for navigation on land. It was not until good steel needles could be forged that compasses were used for navigation at sea; before that, they could not retain their magnetism long enough to be useful. The first mention of a compass in Europe was in 1190 AD.

In circa 240 BC, Eratosthenes of Cyrene deduced that the Earth was round and measured the circumference of Earth with great precision. He developed a system of latitude and longitude.

Perhaps the earliest contribution to seismology was the invention of a seismoscope by the prolific inventor Zhang Heng in 132 AD. This instrument was designed to drop a bronze ball from the mouth of a dragon into the mouth of a toad. By looking at which of eight toads had the ball, one could determine the direction of the earthquake. It was 1571 years before the first design for a seismoscope was published in Europe, by Jean de la Hautefeuille. It was never built.

Beginnings of modern science

The 17th century had major milestones that marked the beginning of modern science. In 1600, William Gilbert release a publication titled De Magnete (1600) where he conducted series of experiments on both natural magnets (called 'loadstones') and artificially magnetized iron. His experiments lead to observations involving a small compass needle (versorium) which replicated magnetic behaviours when subjected to a spherical magnet, along with it experiencing 'magnetic dips' when it was pivoted on a horizontal axis. HIs findings led to the deduction that compasses point north due to the Earth itself being a giant magnet.

In 1687 Isaac Newton published his work titled Principia which was pivotal in the development of modern scientific fields such as astronomy and physics. In it, Newton both laid the foundations for classical mechanics and gravitation, as well as explained different geophysical phenomena such as the precession of the equinox (the orbit of whole star patterns along an ecliptic axis. Newton's theory of gravity had gained so much success, that it resulted in changing the main objective of physics in that era to unravel natures fundamental forces, and their characterizations in laws.

The first seismometer, an instrument capable of keeping a continuous record of seismic activity, was built by James Forbes in 1844.
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