Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, spacecraft, guided missiles, motor vehicles, weather formations, terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object. Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed. Radar was developed secretly for military use by several nations in the period before and during World War II. A key development was the cavity magnetron in the UK, which allowed the creation of small systems with sub-meter resolution; the term RADAR was coined in 1940 by the United States Navy as an acronym for RAdio Detection And Ranging The term radar has since entered English and other languages as a common noun, losing all capitalization.
The modern uses of radar are diverse, including air and terrestrial traffic control, radar astronomy, air-defense systems, antimissile systems, marine radars to locate landmarks and other ships, aircraft anticollision systems, ocean surveillance systems, outer space surveillance and rendezvous systems, meteorological precipitation monitoring and flight control systems, guided missile target locating systems, ground-penetrating radar for geological observations, range-controlled radar for public health surveillance. High tech radar systems are associated with digital signal processing, machine learning and are capable of extracting useful information from high noise levels. Radar is a key technology that the self-driving systems are designed to use, along with sonar and other sensors. Other systems similar to radar make use of other parts of the electromagnetic spectrum. One example is "lidar". With the emergence of driverless vehicles, Radar is expected to assist the automated platform to monitor its environment, thus preventing unwanted incidents.
As early as 1886, German physicist Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895, Alexander Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, developed an apparatus using a coherer tube for detecting distant lightning strikes; the next year, he added a spark-gap transmitter. In 1897, while testing this equipment for communicating between two ships in the Baltic Sea, he took note of an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon might be used for detecting objects, but he did nothing more with this observation; the German inventor Christian Hülsmeyer was the first to use radio waves to detect "the presence of distant metallic objects". In 1904, he demonstrated the feasibility of detecting a ship in dense fog, but not its distance from the transmitter, he obtained a patent for his detection device in April 1904 and a patent for a related amendment for estimating the distance to the ship.
He got a British patent on September 23, 1904 for a full radar system, that he called a telemobiloscope. It operated on a 50 cm wavelength and the pulsed radar signal was created via a spark-gap, his system used the classic antenna setup of horn antenna with parabolic reflector and was presented to German military officials in practical tests in Cologne and Rotterdam harbour but was rejected. In 1915, Robert Watson-Watt used radio technology to provide advance warning to airmen and during the 1920s went on to lead the U. K. research establishment to make many advances using radio techniques, including the probing of the ionosphere and the detection of lightning at long distances. Through his lightning experiments, Watson-Watt became an expert on the use of radio direction finding before turning his inquiry to shortwave transmission. Requiring a suitable receiver for such studies, he told the "new boy" Arnold Frederic Wilkins to conduct an extensive review of available shortwave units. Wilkins would select a General Post Office model after noting its manual's description of a "fading" effect when aircraft flew overhead.
Across the Atlantic in 1922, after placing a transmitter and receiver on opposite sides of the Potomac River, U. S. Navy researchers A. Hoyt Taylor and Leo C. Young discovered that ships passing through the beam path caused the received signal to fade in and out. Taylor submitted a report, suggesting that this phenomenon might be used to detect the presence of ships in low visibility, but the Navy did not continue the work. Eight years Lawrence A. Hyland at the Naval Research Laboratory observed similar fading effects from passing aircraft. Before the Second World War, researchers in the United Kingdom, Germany, Japan, the Netherlands, the Soviet Union, the United States, independently and in great secrecy, developed technologies that led to the modern version of radar. Australia, New Zealand, South Africa followed prewar Great Britain's radar development, Hungary generated its radar technology during the war. In France in 1934, following systematic studies on the split-anode magnetron, the research branch of the Compagnie Générale de Télégraphie Sans Fil headed by Maurice Ponte with Henri Gutton, Sylvain Berline and M. Hugon, began developing an obstacle-locatin
A land mine is an explosive device concealed under or on the ground and designed to destroy or disable enemy targets, ranging from combatants to vehicles and tanks, as they pass over or near it. Such a device is detonated automatically by way of pressure when a target steps on it or drives over it, although other detonation mechanisms are sometimes used. A land mine may cause damage by direct blast effect, by fragments that are thrown by the blast, or by both; the name originates from the ancient practice of military mining, where tunnels were dug under enemy fortifications or troop formations. These killing tunnels were at first collapsed to destroy targets located above, but they were filled with explosives and detonated in order to cause greater devastation. Nowadays, in common parlance, "land mine" refers to devices manufactured as anti-personnel or anti-vehicle weapons. Though some types of improvised explosive devices are mistakenly classified as land mines, the term land mine is reserved for manufactured devices designed to be used by recognized military services, whereas IED is used for makeshift "devices placed or fabricated in an improvised manner incorporating explosive material, lethal, incendiary, pyrotechnic materials or chemicals designed to destroy, distract or harass.
They may incorporate military stores, but are devised from non-military components". The use of land mines is controversial because of their potential as indiscriminate weapons, they can remain dangerous many years after a conflict has ended, harming the economy. 78 countries are contaminated with land mines and 15,000–20,000 people are killed every year while countless more are maimed. 80% of land mine casualties are civilian, with children as the most affected age group. Most killings occur in times of peace. With pressure from a number of campaign groups organised through the International Campaign to Ban Landmines, a global movement to prohibit their use led to the 1997 Convention on the Prohibition of the Use, Stockpiling and Transfer of Anti-Personnel Mines and on their Destruction known as the Ottawa Treaty. To date, 164 nations have signed the treaty. Land mines were designed for two main uses: To create defensive tactical barriers, channelling attacking forces into predetermined fire zones or slowing an invading force's progress to allow reinforcements to arrive.
To act as passive area-denial weapons. Land mines are used in large quantities for this first purpose, thus their widespread use in the demilitarized zones of flashpoints such as Cyprus and Korea; as of 2013, the only governments that still laid land mines were Myanmar in its internal conflict, Syria in its civil war. Land mines continue to kill or injure at least 4,300 people every year decades after the ends of the conflicts for which they were placed. Jiao Yu in the preface to his Huolongjing Quanzhi, written in 1412 AD, claimed that in the third century, the chancellor Zhuge Liang of the Shu Han state had used not only "fire weapons" but land mines in the Battle of Hulugu Valley against the forces of Sima Yi and his son Sima Zhao of the rival Cao Wei state; this claim is dubious, as gunpowder warfare did not develop in China until the advent of the flamethrower in the 10th century, while the land mine was not seen in China until the late 13th century. Explosive land mines were used in 1277 by the Chinese during the Song dynasty against an assault of the Mongols, who were besieging a city in southern China.
The invention of this detonated "enormous bomb" was credited to one Lou Qianxia of the 13th century. The famous 14th-century Chinese text of the Huolongjing, the first to describe hollow cast iron cannonball shells filled with gunpowder, was the first to describe the invention of the land mine in greater detail than references found in texts written beforehand; this mid 14th century work compiled during the late Yuan dynasty and early Ming dynasty stated that mines were made of cast iron and were spherical in shape, filled with either "magic gunpowder", "poison gunpowder", or "blinding and burning gunpowder", any one of these compositions being suitable for use. The wad of the mine was made of hard wood, carrying three different fuses in case of defective connection to the touch hole. In those days, the Chinese relied upon command signals and timed calculation of enemy movements into the minefield, since a long fuse had to be ignited by hand from the ambushers in a somewhat far-off location lying in wait.
The Huolongjing describes land mines that were set off by enemy movement, called the'ground-thunder explosive camp', one of the'self-trespassing' types, as the text says: These mines are installed at frontier gates and passes. Pieces of bamboo are sawn into sections nine feet in length, all septa in the bamboo being removed, save only the last. Boiling oil is next left there for some time before being removed; the fuse starts from the bottom, is compressed into it to form an explosive mine. The gunpowder fills up eight-tenths of the tube, while lead or iron pellets take up the rest of the space. A trench five feet in depth is dug; the fuse is connected to a firing device. The Huolongjing describes the trigger device used for this as a "steel wheel", which directed sparks
Limestone is a carbonate sedimentary rock, composed of the skeletal fragments of marine organisms such as coral and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. A related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones. About 10% of sedimentary rocks are limestones; the solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock. Limestone has numerous uses: as a building material, an essential component of concrete, as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as foraminifera; these organisms secrete shells made of aragonite or calcite, leave these shells behind when they die. Other carbonate grains composing limestones are ooids, peloids and extraclasts. Limestone contains variable amounts of silica in the form of chert or siliceous skeletal fragment, varying amounts of clay and sand carried in by rivers; some limestones do not consist of grains, are formed by the chemical precipitation of calcite or aragonite, i.e. travertine. Secondary calcite may be deposited by supersaturated meteoric waters; this produces speleothems, such as stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular appearance; the primary source of the calcite in limestone is most marine organisms. Some of these organisms can construct mounds of rock building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone does not form in deeper waters.
Limestones may form in lacustrine and evaporite depositional environments. Calcite can be dissolved or precipitated by groundwater, depending on several factors, including the water temperature, pH, dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility, in which it becomes less soluble in water as the temperature increases. Impurities will cause limestones to exhibit different colors with weathered surfaces. Limestone may be crystalline, granular, or massive, depending on the method of formation. Crystals of calcite, dolomite or barite may line small cavities in the rock; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures. Travertine is a banded, compact variety of limestone formed along streams where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution supersaturated with the chemical constituents of calcite.
Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells. During regional metamorphism that occurs during the mountain building process, limestone recrystallizes into marble. Limestone is a parent material of Mollisol soil group. Two major classification schemes, the Folk and the Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks. Based on composition, there are three main components: allochems and cement; the Folk system uses two-part names. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample; the Dunham scheme focuses on depositional textures. Each name is based upon the texture of the grains. Robert J. Dunham published his system for limestone in 1962.
Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are for rock families, his efforts deal with the question of whether or not the grains were in mutual contact, therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of the rock; the Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright, it adds some diagenetic patterns and can be summarized as follows: See: Carbonate platform About 10% of all sedimentary rocks are limestones. Limestone is soluble in acid, therefore forms many erosional landforms; these include limestone pavements, pot holes, cenotes and gorges. Such erosion landscapes are known
Time Team is a British television programme that aired on Channel 4 from 16 January 1994 to 7 September 2014. Created by television producer Tim Taylor and presented by actor Tony Robinson, each episode featured a team of specialists carrying out an archaeological dig over a period of three days, with Robinson explaining the process in lay terms; the specialists changed throughout the programme's run, although it included professional archaeologists such as Mick Aston, Carenza Lewis, Francis Pryor and Phil Harding. The sites excavated ranged in date from the Palaeolithic to the Second World War. In October 2012, Channel 4 announced that the final series would be broadcast in 2013. Series 20 was screened from January–March 2013 and nine specials were screened between May 2013 and September 2014. At the start of the programme, Tony Robinson explains, in an opening "piece to camera", the reasons for the team's visit to the site and during the dig, he enthusiastically encourages the archaeologists to explain their decisions and conclusions.
He tries to ensure. The site is suggested by a member of the viewing public. Time Team uncover as much as they can of the history of the site in three days. Excavations are not just carried out to entertain viewers. Robinson claims that the archaeologists involved with Time Team have published more scientific papers on excavations carried out in the programme than all British university archaeology departments over the same period and that by 2013, the programme had become the biggest funder of field archaeology in the country. A team of archaeologists led by Mick Aston or Francis Pryor, including field archaeologist Phil Harding, congregate at a site in Britain; the original Time Team line-up from 1994 has altered over the years. Historian Robin Bush was a regular in the first nine series, having been involved with the programme through his long friendship with Aston. Architectural historian Beric Morley featured in ten episodes between 1995 and 2002. In 2005, Carenza Lewis left to pursue other interests.
She was replaced by Anglo-Saxon specialist. The regular team included: Stewart Ainsworth, landscape investigator. Guy de la Bédoyère has been present for Roman digs, as well as those involving the Second World War such as D-Day and aircraft. Architectural historian Jonathan Foyle has appeared in episodes relating to excavations of country estates. Paul Blinkhorn, Mark Corney and Jackie McKinley have appeared from time to time. Mick ‘the dig’ Worthington, an excavator in the early series returned as a dendrochronologist, whereupon he was dubbed'Mick the twig'. Margaret Cox assisted with forensic archaeology between 1998 and 2005. Other specialists who appeared from time to time include historian Bettany Hughes, archaeologist Gustav Milne, East of England specialist Ben Robinson and David S. Neal, expert on Roman mosaics. Local historians joined in when appropriate. In February 2012, it was announced; the disputed changes included hiring anthropologist Mary-Ann Ochota as a co-presenter, dispensing with other archaeologists and what he thought were plans to "cut down the informative stuff about the archaeology".
"The time had come to leave. I never made any money out of it. I feel really angry about it." He told British Archaeology magazine. Time Team producer Tim Taylor released a statement in response to the news reports saying "His concerns are of great importance to me. We have addressed some of them" and that "you’ve not heard the last of Mick on Time Team". More recent regular team members included archaeologist Neil Holbrook, Roman coins specialist Philippa Walton, historian Sam Newton. Younger members of Time Team who made regular appearances include: Jenni Butterworth. Time Team developed from an earlier Channel 4 programme, Time Signs, first broadcast in 1991. Produced by Taylor, Time Signs had featured Harding, who went on to appear on Time Team. Following that show's cancellation, Taylor went on to develop a more attractive format, producing the idea for Time Team, which Channel 4 picked up, broadcasting the first series in 1994. Time Team has had many companion shows during its run, including Time Team Extra, History Hunters and Time Team Digs, whilst several spin-off books have been published.
The programme features special episodes documentaries on history or archaeology and live episodes. Time Team America, a US version of the programme, was broadcast on PBS in 2009 and co-produced by Oregon Public Broadcasting and Videotext/C4i; the programme has been exported to 35 other countries. Time Team was commissioned by Channel 4 Television and made in partnership between VideoText Communications Ltd and Picturehouse Television Co. Ltd. Formed Wildfire Television was involved in the production of The Big Roman Dig and The Big Royal Dig, it was produced by the show's originator, with Robinson as associate producer. On 13 September 2007, during the filming of a jousting reenactment for a special episode of Time Team, a sp
Geophysical survey (archaeology)
In archaeology, geophysical survey is ground-based physical sensing techniques used for archaeological imaging or mapping. Remote sensing and marine surveys are used in archaeology, but are considered separate disciplines. Other terms, such as "geophysical prospection" and "archaeological geophysics" are synonymous. Geophysical survey is used to create maps of subsurface archaeological features. Features are the non-portable part of the archaeological record, whether standing structures or traces of human activities left in the soil. Geophysical instruments can detect buried features when their physical properties contrast measurably with their surroundings. In some cases individual artifacts metal, may be detected as well. Readings taken in a systematic pattern become. Survey results can be used to guide excavation and to give archaeologists insight into the patterning of non-excavated parts of the site. Unlike other archaeological methods, geophysical survey is neither destructive. For this reason, it is used where preservation is the goal, to avoid disturbance of culturally sensitive sites such as cemeteries.
Although geophysical survey has been used in the past with intermittent success, good results are likely when it is applied appropriately. It is most useful when it is used in a well-integrated research design where interpretations can be tested and refined. Interpretation requires a knowledge both of the archaeological record, of the way it is expressed geophysically. Appropriate instrumentation, survey design, data processing are essential for success, must be adapted to the unique geology and archaeological record of each site. In the field, control of data quality and spatial accuracy are critical. Geophysical methods used in archaeology are adapted from those used in mineral exploration and geology. Archaeological mapping presents unique challenges, which have spurred a separate development of methods and equipment. In general, geological applications are concerned with detecting large structures as as possible. In contrast, most archaeological sites are near the surface within the top meter of earth.
Instruments are configured to limit the depth of response to better resolve the near-surface phenomena that are to be of interest. Another challenge is to detect subtle and very small features – which may be as ephemeral as organic staining from decayed wooden posts - and distinguish them from rocks and other natural “clutter.” To accomplish this requires not only sensitivity, but high density of data points at least one and sometimes dozens of readings per square meter. Most applied to archaeology are magnetometers, electrical resistance meters, ground-penetrating radar and electromagnetic conductivity meters; these methods can resolve many types of archaeological features, are capable of high sample density surveys of large areas, of operating under a wide range of conditions. While common metal detectors are geophysical sensors, they are not capable of generating high-resolution imagery. Other established and emerging technologies are finding use in archaeological applications. Electrical resistance meters can be thought of as similar to the Ohmmeters used to test electrical circuits.
In most systems, metal probes are inserted into the ground to obtain a reading of the local electrical resistance. A variety of probe configurations are used, most having four probes mounted on a rigid frame. Capacitively coupled systems that do not require direct physical contact with the soil have been developed. Archaeological features can be mapped when they are of higher or lower resistivity than their surroundings. A stone foundation might impede the flow of electricity, while the organic deposits within a midden might conduct electricity more than surrounding soils. Although used in archaeology for planview mapping, resistance methods have a limited ability to discriminate depth and create vertical profiles. Electromagnetic conductivity instruments have a response, comparable to that of resistance meters. Underground archaeological features are detected by creating a magnetic field underground by applying an electric current that has a known frequency and magnitude through a sending coil.
The currents spur a secondary current in underground conductors, picked up by a receiving coil. Changes in the underground conductivity can indicate buried features. Although EM conductivity instruments are less sensitive than resistance meters to the same phenomena, they do have a number of unique properties. One advantage is that they do not require direct contact with the ground, can be used in conditions unfavorable to resistance meters. Another advantage is greater speed than resistance instruments. Unlike resistance instruments, conductivity meters respond to metal; this can be a disadvantage when the metal is extraneous to the archaeological record, but can be useful when the metal is of archaeological interest. Some EM conductivity instruments are capable of measuring magnetic susceptibility, a property, becoming important in archaeological studies. Magnetometers used in geophysical survey may use a single sensor to measure the total magnetic field strength, or may use two spatially separated sensors to measure the gradient of the magnetic field.
In most archaeological applications the latter configuration is preferred because it provides better
Geophysics is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, the use of quantitative methods for their analysis. The term geophysics sometimes refers only to the geological applications: Earth's shape. However, modern geophysics organizations use a broader definition that includes the water cycle including snow and ice. 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 precession of the equinox. In the 20th century, geophysical methods were developed for remote exploration of the solid Earth and the ocean, 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, assess sites for environmental remediation. Geophysics is a interdisciplinary subject, geophysicists contribute to every area of the Earth sciences. To provide a clearer idea of what constitutes geophysics, this section describes phenomena that are studied in physics and how they relate to the Earth and its surroundings; the gravitational pull of the Moon and Sun give 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. The surface gravitational field provides information on the dynamics of tectonic plates; the geopotential surface called. The geoid would be the global mean sea level if the oceans were in equilibrium and could be extended through the continents; the Earth is cooling, 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 the primordial heat and radioactivity, although there are contributions from phase transitions. Heat is 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, it is a potential source of geothermal energy.
Seismic waves are vibrations that travel along its surface. The entire Earth can 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 mantle convection. Recording of seismic waves from controlled sources provide 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, can lead to better estimates of earthquake risk and improvements in earthquake engineering. Although we 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 atmosphere has a net positive charge due to bombardment by cosmic rays. 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 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 man-made or natural disturbances. Telluric currents flow in the oceans, they have two causes: electromagnetic induction by the time-varying, external-origin geomagnetic field and motion of conducting bodies across the Earth's per
Tomography is imaging by sections or sectioning, through the use of any kind of penetrating wave. The method is used in radiology, biology, atmospheric science, oceanography, plasma physics, materials science, quantum information, other areas of science; the word tomography is derived from Ancient Greek τόμος tomos, "slice, section" and γράφω graphō, "to write". A device used in tomography is called a tomograph. In many cases, the production of these images is based on the mathematical procedure tomographic reconstruction, such as X-ray computed tomography technically being produced from multiple projectional radiographs. Many different reconstruction algorithms exist. Most algorithms fall into one of two categories: filtered back projection and iterative reconstruction; these procedures give inexact results: they represent a compromise between accuracy and computation time required. FBP demands fewer computational resources, while IR produces fewer artifacts at a higher computing cost. Although MRI and ultrasound are transmission methods, they do not require movement of the transmitter to acquire data from different directions.
In MRI, both projections and higher spatial harmonics are sampled by applying spatially-varying magnetic fields. On the other hand, since ultrasound uses time-of-flight to spatially encode the received signal, it is not a tomographic method and does not require multiple acquisitions at all; some recent advances rely on using integrated physical phenomena, e.g. X-rays for both CT and angiography, combined CT/MRI and combined CT/PET. Discrete tomography and Geometric tomography, on the other hand, are research areas that deal with the reconstruction of objects that are discrete or homogeneous, they are concerned with reconstruction methods, as such they are not restricted to any of the particular tomography methods listed above. A new technique called synchrotron X-ray tomographic microscopy allows for detailed three-dimensional scanning of fossils; the construction of third-generation synchrotron sources combined with the tremendous improvement of detector technology, data storage and processing capabilities since the 1990s has led to a boost of high-end synchrotron tomography in materials research with a wide range of different applications, e.g. the visualization and quantitative analysis of differently absorbing phases, cracks, precipitates or grains in a specimen.
Synchrotron radiation is created by accelerating free particles in high vacuum. By the laws of electrodynamics this acceleration leads to the emission of electromagnetic radiation. Linear particle acceleration is one possibility, but apart from the high electric fields one would need it is more practical to hold the charged particles on a closed trajectory in order to obtain a source of continuous radiation. Magnetic fields are used to force the particles onto the desired orbit and prevent them from flying in a straight line; the radial acceleration associated with the change of direction generates radiation. Volume rendering is a set of techniques used to display a 2D projection of a 3D discretely sampled data set a 3D scalar field. A typical 3D data set is a group of 2D slice images acquired, for example, by a CT, MRI, or MicroCT scanner; these are acquired in a regular pattern and have a regular number of image pixels in a regular pattern. This is an example of a regular volumetric grid, with each volume element, or voxel represented by a single value, obtained by sampling the immediate area surrounding the voxel.
To render a 2D projection of the 3D data set, one first needs to define a camera in space relative to the volume. One needs to define the opacity and color of every voxel; this is defined using an RGBA transfer function that defines the RGBA value for every possible voxel value. For example, a volume may be viewed by extracting isosurfaces from the volume and rendering them as polygonal meshes or by rendering the volume directly as a block of data; the marching cubes algorithm is a common technique for extracting an isosurface from volume data. Direct volume rendering is a computationally intensive task. Focal plane tomography was developed in the 1930s by the radiologist Alessandro Vallebona, proved useful in reducing the problem of superimposition of structures in projectional radiography. In a 1953 article in the medical journal Chest, B. Pollak of the Fort William Sanatorium described the use of another term for tomography. Focal plane tomography remained the conventional form of tomography until being replaced by computed tomography the late-1970s.
Focal plane tomography uses the fact that the focal plane appears sharper, while structures in other planes appear blurred. By moving an X-ray source and the film in opposite directions during the exposure, modifying the direction and extent of the movement, operators can select different focal planes which contain the structures of interest. Media related to Tomography at Wikimedia Commons Chemical imaging 3D reconstruction Discrete tomography Geometric tomography Geophysical imaging Industrial CT scanning Johann Radon Medical imaging MRI compared with CT Network tomography Nonogram, a type of puzzle based on a discrete model of tomography Radon transform Tomographic reconstruction Multiscale Tomography Voxels Image reconstruction algorithms for microtomography