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Reflection seismology
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===Sources of noise=== [[File:Types of noise on a seismic record.png|thumb|Sources of noise on a seismic record. Top-left: air wave; top-right: head wave; bottom-left: surface wave; bottom-right: multiple.]] In addition to reflections off interfaces within the subsurface, there are a number of other seismic responses detected by receivers and are either unwanted or unneeded: ====Air wave==== The airwave travels directly from the source to the receiver and is an example of [[coherent noise]]. It is easily recognizable because it travels at a speed of 330 m/s, the [[speed of sound]] in air. ====Ground roll / Rayleigh wave / Scholte wave / Surface wave==== A [[Rayleigh wave]] typically propagates along a free surface of a solid, but the elastic constants and [[density]] of air are very low compared to those of rocks so the surface of the Earth is approximately a [[free surface]]. Low velocity, low frequency and high amplitude Rayleigh waves are frequently present on a seismic record and can obscure signal, degrading overall data quality. They are known within the industry as ‘Ground Roll’ and are an example of coherent noise that can be attenuated with a carefully designed seismic survey.<ref>{{cite web | website = [[Schlumberger]] Oilfield Glossary | title = Ground Roll | url = http://www.glossary.oilfield.slb.com/Display.cfm?Term=ground%20roll | access-date = 8 September 2013 | archive-date = 31 May 2012 | archive-url = https://web.archive.org/web/20120531161347/http://www.glossary.oilfield.slb.com/Display.cfm?Term=ground%20roll | url-status = dead }}</ref> The [[Scholte wave]] is similar to ground roll but occurs at the sea-floor (fluid/solid interface) and it can possibly obscure and mask deep reflections in marine seismic records.<ref>{{cite arXiv|eprint=1306.4383|last1=Zheng|first1=Yingcai|title=Scholte waves generated by seafloor topography|last2=Fang|first2=Xinding|last3=Liu|first3=Jing|last4=Fehler|first4=Michael C.|year=2013|class=physics.geo-ph}}</ref> The velocity of these waves varies with wavelength, so they are said to be dispersive and the shape of the wavetrain varies with distance.<ref>Dobrin, M. B., 1951, Dispersion in seismic surface waves, Geophysics, 16, 63–80.</ref> ====Refraction / Head wave / Conical wave==== A head wave refracts at an interface, travelling along it, within the lower medium and produces oscillatory motion parallel to the interface. This motion causes a disturbance in the upper medium that is detected on the surface.<ref name="SheriffGeldart" /> The same phenomenon is utilised in [[seismic refraction]]. ====Multiple reflection==== An event on the seismic record that has incurred more than one reflection is called a ''multiple''. Multiples can be either short-path (peg-leg) or long-path, depending upon whether they interfere with primary reflections or not.<ref>{{cite web | website = [[Schlumberger]] Oifield Glossary | title = Multiples Reflection | url = http://www.glossary.oilfield.slb.com/Display.cfm?Term=multiple%20reflection | access-date = 8 September 2013 | archive-date = 2 June 2012 | archive-url = https://web.archive.org/web/20120602102742/http://www.glossary.oilfield.slb.com/Display.cfm?Term=multiple%20reflection | url-status = dead }}</ref><ref>{{cite journal | last = Pendrel | first = J. | title = Seismic Inversion—A Critical Tool in Reservoir Characterization | journal = Scandinavian Oil-Gas Magazine | issue= 5/6 | year = 2006 | pages = 19–22}}</ref> Multiples from the bottom of a body of water and the air-water interface are common in marine seismic data, and are suppressed by [[seismic processing]]. ====Cultural noise==== Cultural noise includes noise from weather effects, planes, helicopters, electrical pylons, and ships (in the case of marine surveys), all of which can be detected by the receivers. ====Electromagnetic noise==== Particularly important in urban environments (i.e. power lines), it is hardly removable. Some particular sensor as microelectromechanical systems (MEMs) are used to decrease these interference when in such environments.<ref>{{Cite journal|last1=Malehmir|first1=Alireza|last2=Zhang|first2=Fengjiao|last3=Dehghannejad|first3=Mahdieh|last4=Lundberg|first4=Emil|last5=Döse|first5=Christin|last6=Friberg|first6=Olof|last7=Brodic|first7=Bojan|last8=Place|first8=Joachim|last9=Svensson|first9=Mats|last10=Möller|first10=Henrik|date=2015-11-01|title=Planning of urban underground infrastructure using a broadband seismic landstreamer — Tomography results and uncertainty quantifications from a case study in southwestern Sweden|url=https://library.seg.org/doi/10.1190/geo2015-0052.1|journal=Geophysics|volume=80|issue=6|pages=B177–B192|doi=10.1190/geo2015-0052.1|bibcode=2015Geop...80B.177M|issn=0016-8033|url-access=subscription}}</ref>
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