Geophysics (Or: how do I tell my mother what subject I am going to study)
As the name implies, geophysics involves the application of physical theories and measurements to discover the properties of the Earth. A geophysicist must acquire skills in physics and mathematics as well as learn the geological processes on Earth and its environment in space. This knowledge is combined to reduce complex phenomena in the real world to mathematical or physical models that further our understanding of Earth's physical characteristics and behavior. The discipline dates to antiquity, mainly as a scientific approach to earthquake prediction (a problem still unsolved), but major progress began in the late 1500's with initial work in such areas as magnetism and gravity. Tremendous improvements in instrumentation in the early years of the 20th century generated rapid progress in geophysics and ultimately led, in the 1960's, to the theory of plate tectonics.
The two great divisions of geophysics conventionally are labelled as "exploration geophysics" and "global geophysics" or "solid Earth geophysics". In global geophysics, we find studies of earthquakes, the main magnetic field, physical oceanography, studies of the Earth's thermal state and meteorology (amongst others, of course). In exploration geophysics we find the same physical studies applied, usually on a smaller scale, to the search for resources such as oil, gas, minerals, water and building stone. Most practicing geophysicists work in exploration geophysics, which is an applied research science of high significance to modern industrial societies. Generally, geophysicists will be part of a team with other Earth scientists (geologists and engineers) working on exploration problems.
Geophysics has increased dramatically man's ability to exploit natural resources. Human senses cannot quantify, or even detect many physical phenomena (e.g., magnetism). Humans cannot detect variations in the Earth's gravitation field of one part per million, but modern gravity meters can (in fact, to 0.02 parts per million or better). Seismology, the primary method of petroleum exploration, requires exact timing and recording of very low-amplitude vibrators, vibrations (or shaking) that is far below that which a human would sense.
The following definitions are from Robert E. Sheriff's "Encyclopedic Dictionary of Exploration Geophysics":
- The study of the Earth by quantitative physical method, especially by seismic reflection and refraction, gravity, magnetic, electrical, electromagnetic, and radioactivity methods.
- The application of physical principles to studies of the Earth. Includes the branches of (a) seismology (earthquakes and elastic waves); (b) geothermometry (heating of the earth, heat flow, volcanology, and hot springs); (c) hydrology (ground and surface water, sometimes including glaciology); (d) physical oceanography; (e) meteorology; (f) gravity and geodesy (the earth's gravitational field and the size and form of the earth); (g) atmospheric electricity and terrestrial magnetism (including ionosphere, Van Allen belts, telluric currents, etc.); (h) tectonophysics (geological processes in the earth); and (i) exploration and engineering geophysics. Geochronology (the dating of earth history) and geocosmogony (the origin of the earth) are sometimes added to the foregoing list.
- Often refers to solid-earth geophysics only, thus excluding (c), (d), (e), and portions of other subjects from the above list.
- Exploration geophysics is the use of seismic, gravity, magnetic, electrical, electromagnetic, etc., methods in the search for oil, gas, minerals, water, etc., with the objective of economic exploitation.
- One who studies the physical properties of the Earth or applies physical measurements to geologic problems; a specialist in geophysics.
Besides of the major topics (global and exploration geophysics), i.e., the study of the global Earth or the search for hydrocarbon reservoirs, there are a lot of other (smaller) topics. Among these are, for instance, environmental management, hydrogeological work, site investigation, groundwater study, contaminant study, catastrophe limit, and others.
Let's have a closer look at some disciplines:
- One of the most important disciplines is seismics. A seismic source (vibrator, explosive, air gun, etc.) generates elastic or acoustic waves which travel through the subsurface and reflect and refract at discontinuities within the Earth due to changes in rock properties (density, seismic velocity, etc.). Some of the waves travel back to the measurement surface where they are registered with special devices (geophones or hydrophones). The traveltimes of these waves allows to infer the structures of the subsurface while the amplitudes of the waves can lead to quantitative analyses of the rock properties. Dependent on the wave types that are used for the inversion, we have to distinguish between reflection and refraction seismics. Reflection seismics is frequently used in the search for hydrocarbon reservoirs and other natural resources. The depths that can be explored with seismics are limited to a couple of kilometers.
- The discipline of seismology is related to seismics but uses Earthquakes as natural sources to explore the structures and properties of the entire Earth. The field of seismology is broad and ranges from fundamental research about the Earth and Earthquakes to applied topics like Earthquake early warning or the compilation of construction standards to minimize human casualties.
- Ground penetrating radar is based on the same principles as reflection seismics. However, it uses electromagnetic waves that travel through the Earth. These waves are influenced by pores and water etc. and their speed changes compared to the speed in vacuum (speed of light) which allows to infer information about the subsurface. Due to the high frequencies and absorption, the depths that can be explored with GPR are limited to a couple of meters but allow a very detailed analysis.
- The discipline of gravimetry deals with the gravity field of the Earth and uses supersensitive instruments to record changes in gravitational force which are caused by density anomalies in the subsurface. One the one hand, gravimetric methods can be used to detect small-scale local anomalies like salt domes. On the other hand, it also allows to study the form and structure of the entire Earth.
- Geomagnetics deals with the Earth's magnetic field and the magnetic susceptibility of materials. An induced magnetic field will interfere with the Earth's magnetic field and detection of such anomalies allows to localise magnetic objects in the subsurface. Geomagnetics also deals with general research about, for instance, the process that generates the Earth's magnetic field.
- The discipline of geoelectricity tries to determine the resistivity of rocks and other material. Usually, direct current or low-frequency alternating current is used to study near-surface soil. Since electric and magnetic fields depend on each other, it's sometimes advantageous to combine both disciplines which leads to magnetotellurics.
- Geothermal studies become increasingly important. In this discipline, scientists explore the Earth's temperature field and geothermal processes within the Earth. Since energy flows continuously from the interior of the Earth to its surface, it can be used as a source of regenerative energy.
- Many other disciplines are closely related to geophyscis, e.g. geology, meteorology, geodesy, or oceanography. Furthermore, there exist many overlaps between geophysics and some fields of physics.