|
|
| Line 1: |
Line 1: |
| '''Seismic tomography''' is a technique for imaging Earth’s sub-surface characteristics in an effort to understand deep geologic structure. Gathering ample compressional wave ([[P-wave]]) and shear wave ([[S-wave]]) travel time measurements allows us to compile 3D images of earth’s velocity structure.<ref>{{cite book|last=Nolet|first=G.|title=Seismic Tomography |year=1987|publisher=Reidel Publishing Company|pages=1–23}}</ref>
| | I am 37 years old and my name is Veda Taft. I life in Engelsdorf (Austria).<br><br>Feel free to visit my webpage [http://semem.ru/tag/dog%20training%20portland%20oregon/ dog training tips puppy] |
| | |
| ==Theory==
| |
| Tomography is solved as an inverse problem. First measurements are made of [[seismic]] waves passing through a material. The character of these measurements is then analyzed to make inferences on the material such waves have passed through (velocity, density, etc.).<ref>{{cite web|title=Principles of Seismic Tomography |url=http://www.landtech.org/pdf/pdf1.pdf |publisher=Landtech Enterprises SA|accessdate=4 March 2012}}</ref> The velocity of p and s waves depends on the [[rheology]] of the material that they travel through ([[density]] and [[Elasticity (physics)|elasticity]]). In short, variations in [[chemical composition]]s and thermal structure result in a change of velocity. Such waves can travel faster through relatively colder material.<ref>{{cite journal|last=Tanimoto|first=T|coauthors=T. Lay|title=Mantle dynamics and seismic tomography |journal=PNAS |year=2000 |volume=97|issue=23|pages=12409–12410|accessdate=4 March 2012|bibcode = 2000PNAS...9712409T |doi = 10.1073/pnas.210382197 |pmid=11035784 |pmc=34063}}</ref> It is observed that wave velocity increases with increasing depth from 2–8 km/s in the [[Crust (geology)|crust]] up to 13 km/s in the [[Mantle (geology)|mantle]]. Current research is based on this premise.
| |
| | |
| ==Process==
| |
| [[Seismometers]] record ground movements in the form of seismic waves resulting from earthquakes or controlled explosions. These instruments are sensitive enough to record ground movement with a displacement 1000 times smaller than the width of a human hair (3x10<sup>−8</sup> m). A compilation of arrival times, sometimes millions of data, allows seismologists to have a clearer picture of the location and size of sub-surface structures. The result is a 3D image of a slice through the earth that serves as a seismic velocity map. The image depicts where seismic waves were able to travel faster or slower based on the differing arrive times of the waves. This in turn warrants the interpretation of the thermal structure for the deep Earth.<ref name=iris>{{cite web|title=EarthScope Education and Outreach: Seismic Tomography|url=http://www.iris.edu/hq/files/programs/education_and_outreach/lessons_and_resources/docs/es_tomography.pdf|publisher=Incorporated Research Institutions for Seismology (IRIS)|accessdate=17 January 2013}}</ref>
| |
| | |
| ==Methods==
| |
| As stated above, seismic tomography is typically solved as an [[inverse problem]]. In order to estimate P-wave velocity and further simplify seismic tomography, four main methods have been devised:
| |
| | |
| *Refraction traveltime tomography: Computationally simple but only reliable for making a shallow low resolution velocity structure. Within this method the observed data are the first arrival time, denoted as t: <math>t = Ls</math>. The model parameters (s) are the slowness of the waves and L is the ray path matrix.<ref>{{cite journal |last=Taillandier |first=C. |coauthors=Deladerriere, N et al.|title=First arrival traveltime tomography: when simpler is better|journal=EAGE, Vienna|year=2011}}</ref>
| |
| | |
| *Finite-frequency traveltime tomography: in an effort to obtain a higher resolution that the above method, the effects of wave diffraction are taken into account. Volumetric sensitivity kernels take the place of raypaths. Overall this allows the travel time and amplitude anomalies to be frequency-dependent, aiding the goal to increase resolution.<ref>{{cite journal|last=Tian |first=Yue |coauthors=Montelli Raffaella, et al.|title=Computing traveltime and amplitude sensitivity kernels in finite-frequency tomography |journal=Princeton University|year=2007|pages=1–40}}</ref>
| |
| | |
| *Reflection traveltime tomography
| |
| | |
| *Waveform tomography : Uses seismic data to its fullest potential. Seismograms are the observed data and are controlled by the acoustic wave equation which serves as an approximation to the elastic wave propagation
| |
| | |
| ==Applications==
| |
| Seismologists can use tomography to infer geologic structures such as the [[subduction]] of [[tectonic plate]]s into the warmer [[Mantle (geology)|mantle]] or the rise of a relatively hot plume body. Volcanologists use tomography to understand the scale of [[magma chamber]]s beneath [[volcano]]es. The study has also allowed for the correlation of tomographic observations with fluid dynamics models by providing a view of the internal thermal structure of the mantle.<ref name=iris/>
| |
| | |
| [[Yellowstone]] Tomography study: Project EarthScope is currently installing 20 new seismometers every month across the US in an effort to obtain higher resolution tomographic images. This will advance our understanding of how North America has and will evolve with special attention placed on the Yellowstone hotspot associated with the [[Yellowstone supervolcano]].<ref name=iris/>
| |
| | |
| ==Limitations==
| |
| Seismic tomography provides only the current velocity anomalies and all prior velocity structures are unknown. It is also difficult to image slender structures due to the fact that long wavelengths are easier to recover.<ref>{{cite web|last=Dziewonski|first=A.M.|title=Global seismic tomography: What we really can say and what we make up |url=http://www.mantleplumes.org/ |accessdate=4 March 2012|year=2004}}</ref>
| |
| | |
| ==See also==
| |
| * [[seismology]]
| |
| * [[seismograph]]
| |
| * [[EarthScope]]
| |
| | |
| ==References==
| |
| {{Reflist}}
| |
| | |
| ==External Links==
| |
| | |
| [http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/7/SeismicTomography_Background.pdf EarthScope Education and Outreach: Seismic Tomography Background]. Incorporated Research Institutions for Seismology (IRIS). Retrieved 17 January 2013.
| |
| | |
| [http://www.iris.edu/hq/programs/education_and_outreach/animations/7 Tomography Animation]. Incorporated Research Institutions for Seismology (IRIS). Retrieved 17 January 2013. | |
| | |
| [[Category:Fields of seismology]]
| |
| [[Category:Geophysical imaging]]
| |
| [[Category:Structure of the Earth]]
| |
| [[Category:Inverse problems]]
| |
I am 37 years old and my name is Veda Taft. I life in Engelsdorf (Austria).
Feel free to visit my webpage dog training tips puppy