'''Subsurface mapping by ambient noise [[tomography ]]''' is the mapping the underground geological structures under the assistantassistance of seismic signals. [[Ambient noise]], which is not associated with the [[earthquake]], is the background seismic signals .<ref>{{cite web |last1=Sleeman |first1=Reinoud |title=Ambient Earth noise and instrumental noise |url=https://www.knmi.nl/kennis-en-datacentrum/achtergrond/ambient-earth-noise-and-instrumental-noise#:~:text=Ambient%20Earth%20noise%2C%20or%20background |access-date=2023-11-01}}</ref> . Given that the ambient noises have low frequencies in general, the further classification of ambient noise include secondary [[microseisms]], primary microseisms, and seismic hum, based on different range of frequencies .<ref name="tonyking2">{{cite journal |last1=Nishida |first1=Kiwamu |title=Ambient seismic wave field |journal=Proceedings of the Japan Academy, Series B |date=2017-08-02 |volume=93 |issue=7 |pages= 423-448423–448 |doi=10.2183/pjab.93.026 |pmid=28769015 |pmc=5713174 |bibcode=2017PJAB...93..423N |url=https://doi.org/10.2183/pjab.93.026}}</ref> . We can utilize the ambient noise data collected by [[seismometers]] (or geophones) to create images for the subsurface under the following processes. The filtered ambient noise raw data Since the ambient noise is considered as diffuse wavefield, we can correlate the filtered ambient noise data from a pair of seismic stations (or seismometers) to find the [[velocities]] of seismic wavefields .<ref>{{cite journal |last1=Benson |first1=G. D. |last2=Ritzwoller |first2=M. H. |last3=Barmin |first3=M. P. |last4=Levshin |first4=A. L. |last5=Lin |first5=F. |last6=Moschetti |first6=M. P. |last7=Shapiro |first7=N. M. |last8=Yang |first8=Y. |title=Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements |journal=Geophysical Journal International |date=2007-06-01 |volume=169 |issue=3 |pages= 1239-12601239–1260 | url=https://doi -org.eproxy.lib.hku.hk/=10.1111/j.1365-246X.2007.03374.x |doi-access=free |arxiv=2007.03374 |bibcode=2007GeoJI.169.1239B |s2cid=229068738 }}</ref> . A 2-dimensional or 3-dimensional velocity map, showing the spatial velocity difference of the subsurface, can thus be created for observing the [[geological structuresstructure]] s. Subsurface mapping by ambient noise [[tomography ]] can be applied in different fields, such as detecting the underground void space ,<ref>{{cite journal |last1=Wang |first1=Yao |last2=Khorrami |first2=Mohammad |last3=Tran |first3=Khiem T. |last4=Horhota |first4=David |title=Application of ambient noise tomography for deep void detection |journal=Journal of Applied Geophysics |date= 2022-12-302023 |volume=209 |doi=10.1016/j.jappgeo.2022.104922 |bibcode=2023JAG...20904922W |s2cid=255338249 |url=https://doi.org/10.1016/j.jappgeo.2022.104922 |url-access=subscription }}</ref> , monitoring [[landslides]] ,<ref>{{cite journal |last1=Le Breton |first1=Mathieu |last2=Bontemps |first2=Noelie |last3=Guillemot |first3=Antoine |last4=Baillet |first4=Laurent |last5=Larose |first5=Eric |title=Landslide monitoring using seismic ambient noise correlation: challenges and applications |journal=Earth-Science Reviews |date=2021-01-28 |volume=216 |pages= 1-261–26 |doi=10.1016/j.earscirev.2021.103518 |bibcode=2021ESRv..21603518L |s2cid=234037825 |url=https://doi.org/10.1016/j.earscirev.2021.103518}}</ref> , and mapping the crustal and upper [[ Mantle (geology)|mantle]] structure .<ref>{{cite journal |last1=Yao |first1=Huajian |last2=Beghein |first2=Caroline |last3=Hilst |first3=Robert D. Van Der |title=Surface wave array tomography in SE Tibet from ambient seismic noise and two-station analysis - II. Crustal and upper-mantle structure |journal=Geophysical Journal International |date=2008-04-01 |volume=173 |issue=1 |pages= 205-219205–219 | url=https://doi -org.eproxy.lib.hku.hk/=10.1111/j.1365-246X.2007.03696.x |bibcode=2008GeoJI.173..205Y |s2cid=29548841 |doi-access=free }}</ref> .▼
= Subsurface Mapping by Ambient Noise Tomography =
▲Subsurface mapping by ambient noise [[tomography]] is the mapping the underground geological structures under the assistant of seismic signals. [[Ambient noise]], which is not associated with the [[earthquake]], is the background seismic signals<ref>{{cite web |last1=Sleeman |first1=Reinoud |title=Ambient Earth noise and instrumental noise |url=https://www.knmi.nl/kennis-en-datacentrum/achtergrond/ambient-earth-noise-and-instrumental-noise#:~:text=Ambient%20Earth%20noise%2C%20or%20background |access-date=2023-11-01}}</ref>. Given that the ambient noises have low frequencies in general, the further classification of ambient noise include secondary [[microseisms]], primary microseisms, and seismic hum, based on different range of frequencies<ref name="tonyking2">{{cite journal |last1=Nishida |first1=Kiwamu |title=Ambient seismic wave field |journal=Proceedings of the Japan Academy, Series B |date=2017-08-02 |volume=93 |issue=7 |pages=423-448 |url=https://doi.org/10.2183/pjab.93.026}}</ref>. We can utilize the ambient noise data collected by [[seismometers]] (or geophones) to create images for the subsurface under the following processes. The filtered ambient noise raw data Since the ambient noise is considered as diffuse wavefield, we can correlate the filtered ambient noise data from a pair of seismic stations (or seismometers) to find the [[velocities]] of seismic wavefields<ref>{{cite journal |last1=Benson |first1=G. D. |last2=Ritzwoller |first2=M. H. |last3=Barmin |first3=M. P. |last4=Levshin |first4=A. L. |last5=Lin |first5=F. |last6=Moschetti |first6=M. P. |last7=Shapiro |first7=N. M. |last8=Yang |first8=Y. |title=Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements |journal=Geophysical Journal International |date=2007-06-01 |volume=169 |issue=3 |pages=1239-1260 |url=https://doi-org.eproxy.lib.hku.hk/10.1111/j.1365-246X.2007.03374.x}}</ref>. A 2-dimensional or 3-dimensional velocity map, showing the spatial velocity difference of the subsurface, can thus be created for observing the [[geological structures]]. Subsurface mapping by ambient noise tomography can be applied in different fields, such as detecting the underground void space<ref>{{cite journal |last1=Wang |first1=Yao |last2=Khorrami |first2=Mohammad |last3=Tran |first3=Khiem T. |last4=Horhota |first4=David |title=Application of ambient noise tomography for deep void detection |journal=Journal of Applied Geophysics |date=2022-12-30 |volume=209 |url=https://doi.org/10.1016/j.jappgeo.2022.104922}}</ref>, monitoring [[landslides]]<ref>{{cite journal |last1=Le Breton |first1=Mathieu |last2=Bontemps |first2=Noelie |last3=Guillemot |first3=Antoine |last4=Baillet |first4=Laurent |last5=Larose |first5=Eric |title=Landslide monitoring using seismic ambient noise correlation: challenges and applications |journal=Earth-Science Reviews |date=2021-01-28 |volume=216 |pages=1-26 |url=https://doi.org/10.1016/j.earscirev.2021.103518}}</ref>, and mapping the crustal and upper [[mantle]] structure<ref>{{cite journal |last1=Yao |first1=Huajian |last2=Beghein |first2=Caroline |last3=Hilst |first3=Robert D. Van Der |title=Surface wave array tomography in SE Tibet from ambient seismic noise and two-station analysis - II. Crustal and upper-mantle structure |journal=Geophysical Journal International |date=2008-04-01 |volume=173 |issue=1 |pages=205-219 |url=https://doi-org.eproxy.lib.hku.hk/10.1111/j.1365-246X.2007.03696.x}}</ref>.
== CharacteristicCharacteristics of Ambientambient Noisenoise ==
Characteristic of ambient noise refers to several quantities that can distinguish different ambient noise, including origin, [[frequency]], property, and temporal variation.
=== Nature of ambient noise ===
Ambient noise, as the rising star of the seismic source for seismic research other than [[earthquake]], accounts for the naturally and anthropogenically produced seismic vibration of the background.<ref>{{cite journal |last1=Oakley |first1=David O. S. |last2=Forsythe |first2=Brandon |last3=Gu |first3=Xin |last4=Nyblade |first4=Andrew A. |last5=Brantley |first5=Susan L. |title=Seismic Ambient Noise Analyses Reveal Changing Temperature and Water Signals to 10s of Meters Depth in the Critical Zone |journal=Journal of Geophysical Research: Earth Surface |date=2021-01-13 |volume=126 |issue=2 |url=https://doi.org/=10.1029/2020JF005823 |bibcode=2021JGRF..12605823O |s2cid=234198739 |doi-access=free }}</ref>. This is different from the active seismic source created solely for seismic research or large seismic source from [[earthquake]]. [[Ocean]] is the most dominant natural origin of the ambient noise field.<ref name="tonyking4">{{cite journal |last1=Igel |first1=Jonas K. H. |last2=Bowden |first2=Daniel C. |last3=Fichtner |first3=Andreas |title=SANS: Publicly Available Daily Multi-Scale Seismic Ambient Noise Source Maps |journal=Journal of Geophysical Research: Solid Earth |date=2022-12-242023 |volume=128 |issue=1 |url=https://doi.org/=10.1029/2022JB025114 |bibcode=2023JGRB..12825114I |s2cid=255123230 |doi-access=free |hdl=20.500.11850/591152 |hdl-access=free }}</ref>.
Any seismic source is transmitted as either [[Body wave (seismology)|body waveswave]]s or [[surface waves]], where ambient noise is no exception. Summary of their properties are shown below.
{| class="wikitable"
2. Particle movement perpendicular to wave movement.
|-
| [[Rayleigh wave]] || Surface || 1. Include both longitudinal & transverse motions.
2. Amplitude decreases exponentially with increasing distance from surface.
|-
| [[Love wave]] || Surface || 1. Wave speed lower than P-wave & S-wave, but higher than Rayleigh wave.
2. Horizonal particle movement perpendicular to wave propagation.
|}
[[File:Overview Seismic Waves.jpg|thumb|Overview Seismic Waves|upright=2.5]]
The dominance of seismic wave transmission of ambient noise depends on several factors, while the research technique would determine the major type of seismic wave collected for ambient noise. For example, seismologists would often use spatial auto-correlation (SPAC) method which involve the collection and analysis of [[surface wave]].<ref>{{cite journal |last1=Nthaba |first1=Bokani |last2=Ikeda |first2=Tatsunori |last3=Nimiya |first3=Hiro |last4=Tsuji |first4=Takeshi |last5=Lio |first5=Yoshihisa |title=Ambient noise tomography for a high-resolution 3D S-wave velocity model of the Kinki Region, Southwestern Japan, using dense seismic array data |journal=Earth, Planets and Space |date=2022-06-20 |volume=74 |urlissue=https://1 |page=96 |doi.org/=10.1186/s40623-022-01654-x |bibcode=2022EP&S...74...96N |doi-access=free }}</ref>.
=== Frequency of ambient noise ===
Ambient noise is often known as [[microseism]], where ‘micro’ means very small,<ref>{{cite web |title=micro |url=https://dictionary.cambridge.org/dictionary/english/micro#google_vignette |website=Cambridge Dictionary |access-date=2023-11-08}}</ref>, and ‘seism’ is an alternative name for earthquake.<ref>{{cite web |title=seism |url=https://www.merriam-webster.com/dictionary/seism |publisher=Merriam-Webster |access-date=2023-11-05}}</ref>. It can be further classified based on their frequency ranges, namely hum, primary microseism and secondary microseism.<ref name="tonyking3">{{cite journal |last1=Tanimoto |first1=Toshiro |last2=Anderson |first2=Aaron |title=Seismic noise between 0.003 Hz and 1.0 Hz and its classification |journal=Progress in Earth and Planetary Science |date=2023-09-11 |volume=10 |urlissue=https://1 |page=56 |doi.org/=10.1186/s40645-023-00587-7 |bibcode=2023PEPS...10...56T |doi-access=free }}</ref>. The table below shows the comparison of frequency range between the microseisms, arranged from increasing order. Figure a also shows graph of the frequency range of microseisms.
{| class="wikitable"
! Microseisms !! Frequency Range (Hz)
|-
| Hum || 9*10^-3 -9×10<sup>−3</sup> 8*10^-2 8×10<sup>−2</sup>
|-
| Primary microseism || 9*10^-39×10<sup>−3</sup> - 8*10^-2 8×10<sup>−2</sup>
|-
| Secondary microseism || 3*10^-23×10<sup>−2</sup> - 1
|}
=== Origin of Ambientambient Noisenoise ===
Ambient noise can be further classified into two major categories based on the origins of the noise.
==== Anthropogenic ====
Anthropogenic ambient noise originateSoriginates from human activities. Considering the ocean ambient noise source as an example, there are noises that are created unintentionally by human activities, such as shipping and offshore engineering work.<ref name="tonyking">{{cite journal |last1=Hildebrand |first1=John A. |title=Anthropogenic and natural sources of ambient noise in the ocean |journal=Marine Ecology Progress Series |date=2023-12-03 |volume=395 |pages=5-205–20 |doi=10.3354/meps08353 |url=https://www.researchgate.net/publication/240809612_Anthropogenic_and_natural_sources_of_ambient_noise_in_the_ocean240809612|doi-access=free }}</ref>. During the shipping processactivity, [[mechanical waves]] can be driven up along the water surface and propagate through the ocean. Offshore [[Offshore engineering]] work can also produce surface waves. [[Engineering]] works include but are not limited to [[borehole]] drilling, [[Foundation (engineering)|foundation]] [[construction]] and [[geophysical surveyssurvey]]s.<ref name="tonyking" />. Shoreline [[Land reclamation|reclamation]] has been actively carried out by many countries to create more land for urban development. Those engineering works can thus also be carried out offshore. The processes of offshore drilling and exploration create continuous mechanical waves that can also propagate through the ocean.
In continental urban areas, there are more examples of human activities creating background noise. Other than engineering works, urban traffic is the major component of urban ambient noise.<ref name="tonyking" />. Although the mechanical waves of the continent are not as visible than those from the ocean, they can still be transmitted via the soil and rock layers. Cars travelling on the road can produce repeatable vibration on the road which can then be transmitted through the soil layers.
==== Natural Noisenoise ====
Natural ambient noise refers to the background noise produced from the natural events. OurThe natural environment is not stationary but constantly changing because nature itself is continuously modified by [[weather]], tectonic movements and biogenic activities.<ref name="tonyking" />. They can also produce low frequency background noise. Some of the most significant events are listed below.
[[Wind]] can induce weak ocean waves propagating through the ocean. The varying atmospheric [[pressure]] was hypothesized as the origin before but is inadequate to support the existence of all types of [[microseismsmicroseism]]s.<ref name="tonyking2" />. Instead, ocean waves are proposed as the alternative origin of natural ambient noise source. For example, the ocean swells interact with the sea coast to induce hum and primary microseisms, and the interaction of sea waves with opposite direction can produce secondary microseisms.<ref name="tonyking3" />.
=== Variation of ambient noise ===
To evaluate whether the collected ambient noise source can be further analysed, we must 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 reducereducing inat nighttimenight. 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|doi-access=free |hdl=20.500.11850/224948 |hdl-access=free }}</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 [[geophonesgeophone]]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=17091709–1730 |doi=10.1007/s00024-1730021-02718-x |bibcode=2021PApGe.178.1709D |s2cid=233330462 |url=https://doi.org/10.1007/s00024-021-02718-x|url-access=subscription }}</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=2020-07-212021 |volume=225 |pagesissue=1158-11732 |urlpages=https://1158–1173 |doi.org/=10.1093/gji/ggaa574 |doi-access=free }}</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 Geologicalgeological Structurestructure with Velocityvelocity Zonezone ===
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.{{citation needed|date=November 2023}}
==== Example of Subsurfacesubsurface Structurestructure ==== ▼Here are some examples of the subsurface structures and features, including botbut not limited to the following. ▼
▲==== Example of Subsurface Structure ====
* [[Fault (geology)|Fault]]
▲Here are some examples of the subsurface structures and features, including bot not limited to the following.
* [[Fold (geology)|Fold]]
== See also ==
3. *[[Seismic Interferometry]] ▼*[[Seismic inversion|Inversion]]
6. *[[Green 's function]] ▼{{Clear}}
==References==
2. [[Fault]]
{{Reflist}}
[[Category:Geophysical imaging]]
3. [[Fold]]
== External Link ==
2. [[Tomography]]
▲3. [[Seismic Interferometry]]
4. [[Cross-correlation]]
5. [[Inversion]]
| 