Subsurface mapping by ambient noise tomography: Difference between revisions

Revision as of 07:02, 11 October 2023

Revision as of 03:13, 12 October 2023

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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.

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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=== Variation of ambient noise ===

To evaluate whether the collected ambient noise source can be further analysed, we must carefully consider if there are any regular variations or patterns of certain ambient noise source. 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 daytime while reduce in nighttime. 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. Nevertheless, it is still difficult to distinguish the ambient noise sources.

== Seismic Velocity Structure Modelling ==