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 2 Most of the geological maps only show the superficial deposits and outcrops, which allows the public to discover the geological structures that are above the sea surface. For most of the geology related disciplines, such as geotechnical engineering, volcanology, geomorphology, civil engineering, exploration geology, however, understanding on the subsurface structures and to a small extent, lithology, is as important as those on surface. One of traditional method would be borehole drilling but it is invasive to the ground surface. Instead, geophysical survey is a non-invasive alternative for subsurface exploration and thus subsurface mapping.

Major geophysical survey techniques include electrical resistivity, gravity anomaly and seismic. Seismic survey is particularly reliable on the detection of subsurface rock structures and even hydrocarbons. Seismologists attempt to correlate the velocity anomaly with the geological structure after the collection and further analysis of the seismic signals. The collection of seismic signals is easier for regions with frequent earthquakes. For some seismic inactive regions like Korea Peninsula, however, the seismic signal analysis is more difficult. Instead of the correlation from strong seismic sources like earthquake, the potential of ambient noise on subsurface structure mapping is discovered in recent years, which will be introduced below.

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 seismic source

  2.1 Artificial Source
    2.2.1 Air gun
    2.2.2 Explosion
  2.2 Natural Noise
    2.2.1 Earthquake
    2.2.2 Ocean
    2.2.3 Urban

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

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.

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.

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

4. Factors affecting quality of resolution

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