Editing Geophysics (section)

== Methods ==

===Geodesy===
{{Main|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.<ref name=NRC>{{harvnb|National Research Council (U.S.). Committee on Geodesy|1985}}</ref>

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 [[GRS 80|1980 Geodetic Reference System]]. An alternative, [[astro-geodetic|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]].<ref name=NRC/><ref>{{harvnb|Defense Mapping Agency|1984}}</ref><ref name=Torge>{{harvnb|Torge|2001}}</ref>

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 altimeter|radar altimetry]], contributing to a more accurate [[geoid]].<ref name=NRC/> In 2002, [[NASA]] launched the [[Gravity Recovery and Climate Experiment]] (GRACE), wherein two twin [[satellite]]s 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.<ref>{{harvnb|CSR|2011}}</ref>

===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 field]]s, 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 field]]s of the planets to be mapped. For example, in the 1970s, the gravity field disturbances above [[lunar maria]] were measured through [[Lunar Orbiter program|lunar orbiters]], which led to the discovery of concentrations of mass, [[mass concentration (astronomy)|mascons]], beneath the [[Mare Imbrium|Imbrium]], [[Mare Serenitatis|Serenitatis]], [[Mare Crisium|Crisium]], [[Mare Nectaris|Nectaris]] and [[Mare Humorum|Humorum]] basins.<ref>{{harvnb|Muller|Sjogren|1968}}</ref>

=== Global positioning systems (GPS) and geographical information systems (GIS) ===
{{further|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.<ref name="Telford" /><ref name=Reynolds>{{harvnb|Reynolds|2011}}</ref> 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|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.<ref>{{Cite web |title=Energy Geosciences |url=https://www.jsg.utexas.edu/research/themes/energy-geosciences/ |access-date=2024-02-18 |website=Jackson School of Geosciences |language=en}}</ref> This is done through the uses of various [[remote sensing]] platforms such as; [[satellite]]s, [[aircraft]], [[boat]]s, [[Unmanned aerial vehicle|drones]], [[borehole]] sensing equipment and [[Seismic source|seismic receivers]]. These equipment are often used in conjunction with different geophysical methods such as [[Magnetism|magnetic]], [[gravimetry]], [[Electromagnetism|electromagnetic]], [[Radiometry|radiometric]], [[Barometer|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 survey|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]].<ref name="Telford" /> 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.<ref name="Telford" /><ref name=Reynolds>{{harvnb|Reynolds|2011}}</ref>

=== Signal processing ===
{{Main|Signal processing}}
Geophysical measurements are often recorded as [[time-series]] with [[Satellite Navigation|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.<ref name="Telford"/><ref name=Reynolds>{{harvnb|Reynolds|2011}}</ref> 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<ref name="Telford" /><ref name=Reynolds>{{harvnb|Reynolds|2011}}</ref>