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As mentioned before, the data collection of ambient noise is the primary step of any seismic research. The most common tool for seismic data collection is the seismometer. Other example could be the geophones. There are also seismic stations or observatories authorized by different official bodies. For example, Hong Kong Observatory has set up several seismic stations in different locations in Hong Kong. The seismic waves are recorded by the seismometers and shown as seismographs. Semimoist and geophysicists can then identify the arrival time of different body waves and surface waves. The seismic waves usually arrive in the order of P-wave, S-wave, Rayleigh wave, and Love wave. Nevertheless, the analysis of ambient noise is more difficult than simply identify the waves above.
=== Ambient Noise
Compared the seismographs of ambient noise with those recording active seismic sources, or simply reviewed any seismographs, you can discover the 'thickness' of the seismic waves. Despite the occasional increase in amplitude due to the active seismic source, the entire seismic waves are also in certain amplitudes, showing that there are certain activities that also produce weak seismic waves. The ambient noise seismographs are required to correlate together such that the velocity map can be produced.
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==== Inversion ====
Inversion is one of the techniques used in ambient noise tomography. Simply speaking, inversion of a function refers to finding the original parameters that output the function itself. In ambient noise tomography, inversion of the cross-correlation function is an important step to obtain the subsurface velocity structure. The cross-correlated seismic waves can be inverted either linearly or non-linearly.
=== Subsurface structure correlation with velocity imaging ===
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