Subsurface mapping by ambient noise tomography: Difference between revisions

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To evaluate whether the collected ambient noise source can be further analysed, consider if there are any regular variations or patterns of certain ambient noise source.<ref name="tonyking" /> Referring to the urban noise source, it may experience a daily variation, where the human activities are conducted mostly in daytime and reduced in nighttime. The ambient noise should thus increase in the daytime while reducing at night. Apart from the temporal variation, the spatial variation can also matter. For example, the commercial shipping is usually concentrated on certain routes. The corresponding amplitude of ambient noise should also decrease when moving away from the shipping routes.<ref name="tonyking" /> Nevertheless, it is still difficult to distinguish the ambient noise sources.

== Seismic Velocityvelocity Structurestructure Modellingmodelling ==

Seismic velocity structure modelling is the modelling technique showing the velocity difference of seismic waves across areas. The modelling process involves some steps, including [[cross-correlation]], green’s[[Green's function]], and [[Seismic inversion|inversion]]. The usage of ambient noise as source of seismic velocity structure modelling rises from 2001 when the seismologists trytried to correlate the diffused ambient noise wave fields'<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>. Velocity structure modelling are complex and require multiple mathematical calculations.

=== Pre-processing of Ambientambient Noisenoise Datadata ===

The pre-processing of ambient noise data refers to the filtering of the raw data before proceeding to further analysis ([[cross-correlation]], [[inversion]]……). Raw seismic data can be collected by either [[geophone]]s, [[seismometers]], or from authorized official bodies. There are also public ambinetambient noise source maps available in recent years.<ref name="tonyking4" />. Since the seismometers collect all ambient seismic signals from all directions, the produced seismic waveforms may not reflect the actual background seismic vibrations. Instead, they often contain some occasional seismic signals from earthquakes and other instruments, which is unnecessary in general and thus required to be removed.<ref>{{cite journal |last1=da Silva |first1=Cicero Costa |last2=Poveda |first2=Esteban |last3=Dantas |first3=Renato Ramos da Silva |last4=Julia |first4=Jordi |title=Ambient Noise Tomography with Short-Period Stations: Case Study in the Borborema Province |journal=Pure and Applied Geophysics |date=2021-04-22 |volume=178 |issue=5 |pages=1709–1730 |doi=10.1007/s00024-021-02718-x |bibcode=2021PApGe.178.1709D |s2cid=233330462 |url=https://doi.org/10.1007/s00024-021-02718-x}}</ref>.

==== Cross-correlation of ambient noise ====

[[File:Simplest case of cross correlation.jpg|thumb|Figure 1: Simplest situation of ambient noise cross correlation|upright=1.5]]

[[File:Distribution of ambient noise source amended.jpg|thumb|Figure 2: Resulting Green's functions in different distribution of ambient noise sources|upright=1.5]]

Ambient noise [[cross correlation]] is the process of finding the receiving time lag of ambient noise sources between two nearby stations. Figure 1 illustrates the simplest case of ambient noise cross-correlation. For a pair of receivers (or seismometers/ or seismic stations) at different locations, the ambient noise signals would be received at a different time, assumed that they travel at the same velocity at the subsurface. The products of cross-correlation of those signals are new seismic waveforms, namely [[Green’sGreen's function]]. Regarding the case with multiple ambient noise sources, the shape of the cross correlation function depends on whether the ambient noise sources are evenly distributed across a certain area. For the most ideal situation where the ambient noise signals are distributed evenly across all directions, the Green’s function would be highly symmetrical (see Figure 2).

==== Inversion ====

Inversion is one of the techniques used in ambient noise tomography. Inversion of the Green’s function is used to retrieve the subsurface properties of the Earth, where seismic velocity is one of the important quantities. It is a crucial step in ambient noise tomography. Inversion in seismic analysis can be treated as finding the original factors of the subsurface that induce the current transmission of the ambient noise signals. Inversion of the Green’s function is conducted linearly in the early ambient noise tomography studies with the assumption that the velocity variation is small.<ref>{{cite journal |last1=Perez |first1=Ivan Cabrera |last2=D' Auria |first2=Luca |last3=Soubestre |first3=Jean |last4=Barrancos |first4=Jose |last5=Padilla |first5=German D. |last6=Perez |first6=Nemesio M. |title=A nonlinear multiscale inversion approach for ambient noise tomography |journal=Geophysical Journal International |date=2021 |volume=225 |issue=2 |pages=1158–1173 |doi=10.1093/gji/ggaa574 |url=https://doi.org/10.1093/gji/ggaa574}}</ref>. [[Forward model]], as the essential process of inversion, is used to estimate the closest quantities of the earth subsurface properties. The cross-correlated seismic waves can be inverted either linearly or non-linearly.

=== Linkage of geological structure with velocity zone ===

Before interpreting the velocity zone, it is necessary to understand how the seismic velocity varies. In general, P and S wave travel faster in high density medium. Only P wave can travel through any medium while S wave can only travel through solidsolids. Therefore, a low velocity zone can refer toindicate some vacuum space in the subsurface layer, such as void space and faults. Conversely, thea high velocity zone may refer to the [[lithology]] with closely packed rocks, such as igneous rock. To correlate the velocity zone with geological structure, it is necessary to consider the size and shape of the velocity zones, and more importantly, the resolution of the subsurface velocity image. The resolution of the image can affect the scale of the subsurface we can interpret. Sometimes, fieldwork is also needed in order to better correlate the velocity map.{{cn}}