Subsurface mapping by ambient noise tomography: Difference between revisions

Revision as of 16:08, 8 October 2023

Revision as of 04:14, 9 October 2023

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==== Natural Noise ====

Natural ambient noise refers to the background noise produced from the natural events. Our natural environment is not stationary but constantly changing every moment because the nature itself is continuously modified by the weather, tectonic movements and biogenic activities. They can also produce low frequency background noise that can be further analysed. Some of the most significant events are listed below.

Earthquake, as mentioned before, is one of the most remarkable events that can produce the most significant background noise. Earthquakes can be caused by the movement along two faults or plates. Seismic waves are released and penetrate through the soil and rock layers, causing the continents to shake. There are numerous earthquakes happened every day with different scales around the world. By recording the seismic waves produced from the earthquake, we can further understand the interior structure of the Earth.

The ocean wave is another possible natural ambient noise source. As mentioned before, the ocean wave can be produced by anthropogenic activities, such as commercial shipping and offshore engineering work. Besides, natural wind and marine animals can also induce weak ocean wave propagating through the ocean, which can be recorded by seismometers.

Draft 1

Geophysical survey would become more difficult for those seismic inactive regions if no artificial seismic source is provided. Compared with seismic source from earthquake, ambient noise is much weaker and has much lower frequency. Ambient noise was therefore much difficult to be detected in the past. Thanks to the technology advancement, geophysicists can now make use of ambient noise to generate velocity model such that correlation with subsurface structure is feasible.

Other than the natural ambient noise propagated through solid and liquid, ambient noise propagated via air can also be recorded if it is strong enough. One of the examples is typhoon. Typhoon is one of the natural hazards whose frequency and strength are magnified by global warming. The strong thermal current creates a low-pressure zone where the air can circulate around the centre. In such case, the vibration of the air is strong enough to be recorded by seismometers.

Ambient noise, one of the seismic waves, is the background noise produced by various natural and human activities. Ambient noise is originated from almost everywhere, though most of the relevant research is about the ambient noise from the ocean. Since the ambient noise is much weaker than earthquake source, it is mostly transmitted as surface waves, including Rayleigh wave and love wave. P- and S-wave transmission is also possible. There are three ranges of ambient noise which have different origin, which will be introduced below.

=== Variation of ambient noise ===

Nature 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.

Part 2: Seismic Velocity Structure Modelling 700-800 words

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1.1 Tools of collection

1.2 Available online source

2. Theory of ambientAmbient noise correlation

2.1 cross correlation

2.2 green's function

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3.2.1 low-velocity zone

3.2.2 high-velocity zone

4. Limitation

4.1 Factors affecting quality of resolution

== Seismic Velocity Structure Modelling ==

The collected ambient noise can be further analysed to produce velocity structure maps which are used to correlated with the possible subsurface structures. The processes involved are complex and require multiple mathematical calculations.

=== Data collection method ===

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 Correlation ===

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 sudden increase in amplitude due to the active seismic source, there are still

Part 3: Application of Subsurface Mapping 700-800 words