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== Source of Ambient Noise ==
Data collection of ambient noise is the prior stage of subsurface mapping, which is important for further analysis and correlation. The seismic noise can be transmitted by either body wave ([[P-wave]], [[S-wave]]) or surface wave ([[Rayleigh wave]], [[Love wave]]). The seismic source can be further classified into active and passive, where ambient noise once received far less attention as other seismic source in relevant research. The usage of ambient noise field rises from 2001 when the seismologists try to retrieve the Green function from the diffused ambient noise wave field.<ref>{{cite journal |last1=Sager |first1=Korbinian |last2=Ermert |first2=Laura |last3=Boehm |first3=Christian |last4=Fichtner |first4=Andreas |title=Towards full waveform ambient noise inversion |journal=Geophysical Journal International |date=12 July 2021 |volume=212 |pages=566–590 |doi=10.1093/gji/ggx429}}</ref>
For conducting seismic research and exploration, active seismic sources would be created intentionally to record the velocity change of seismic waves. Some examples of tools creating active seismic source include hammer[note1], airgun, and even artificial explosion, which may create seismic waves with similar magnitude as large earthquakes. Other than the artificial seismic sources, passive ambient noise can also be recorded and analysed. Ambient noise refers to the background noise originated either from natural events or anthropogenic activities. The use of ambient noise on velocity structure modelling has received more attention, especially in seismically inactive regions.
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