Subsurface mapping by ambient noise tomography

Title: Subsurface Mapping by Ambient Noise Tomography (Brief Introduction) 150-200 words Importance of Subsurface mapping 1. Understanding subsurface (lithology, structure...): important in several discipline: geotechnical engineering, volcanology, geomorphology, civil engineering, economic geology... 2. Drilling borehole: traditional method, but invasive 3. Geophysical survey: non-invasive method to image the subsurface

Ambient noise as alternative geophysical survey method 4. Example of geophysical survey technique: electrical resistivity, gravity anomaly, seismic 5. Geophysical survey is more convenient in seismic active region: more active seismic source for correlation 6. For less seismic region: Use of ambient noise: raising usage and thus attention

Draft 1 Ambient noise, one of the seismic waves, is the background noise produced by various natural and human activities. Recently scientists, especially seismologists, discover the potential of ambient noise on subsurface structure mapping. Borehole drilling is considered as the most ordinary method to understand the subsurface lithology, but it is invasive. Geophysical survey acts as an alternative non-invasive measure on the exploration of subsurface structure. Some of the main survey technique include electrical resistivity, gravity anomaly and seismic. Comparing with other geophysical survey technique, measuring ambient noise becomes a rising star. Subsurface mapping is an important skill in geological mapping because it can show the subsurface structure that may affect the stability of infrastructure. Moreover, we can better understand the structural, sedimentary and volcanic processes in a 3-dimensional manner.

Part 1: Source of Ambient Noise 700-800 words 1. Brief description of seismic source

  1.1 Most of ambient noise is originated from ocean
  1.2 Frequency range of ambient noise
  1.3 How do the ambient noise spread?

2. Nature of ambient noise: passive/background noise

  2.1 Active/Artificial Noise
  2.2 Passive/Natural Noise

3. Variation of Ambient Noise

  3.1 Cause of variation
  3.2 Periodic change of ambient noise

4. Limitation in collecting ambient noise

Draft 1 Subsurface mapping by geophysical survey can be conducted much easier in regions with frequent earthquake, that is, the regions along plate boundary or faults. When earthquake happens, the energy is released in all direction as body waves and thus surface waves. Geophysicists can collect the seismic data more easily since the seismic energy released from earthquake is much stronger. In contrast, geophysical survey would become more difficult for those seismic inactive regions if no artifical 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.

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.

Nature of Ambient Noise

Part 2: Seismic Velocity Structure Modelling 700-800 words 1. Data collection method

  1.1 Tools of collection
  1.2 Available online source

2. Theory of ambient noise correlation

  2.1 cross correlation
  2.2 green's function
  2.3 Inversion

3. Subsurface structure correlation with velocity imaging

  3.1 example of subsurface structure
  3.2 linkage of structure with velocity zone
     3.2.1 low-velocity zone
     3.2.2 high-velocity zone

Part 3: Application of Subsurface Mapping 700-800 words 1. Exploration of underground mines 2. Volcano Monitoring 3. Landslide Monitoring