Ambient noise tomography

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Ambient noise tomography is a way to get a subsurface image by using an ambient noise source. When we have multiple stations in an area, we can cross-correlate them to one another, resulting in an impulse wave. This impulse wave is called the Green's function. With multiple green's function covering a certain area, we can do a tomography to generate a subsurface image.

Seismic Ambient Noise

Seismic ambient noise is a seismic wave generated by human and environmental activities. Some of the seismic noise sources are Atmospheric activities, ocean waves etc. This waves propagated in every direction and occurred randomly. Even so, this type of wave brings information about the medium it went through(Sneider and Wapenaar, 2010).[1] This ambient noise is dominated by surface wave. With the low resolution of the surface wave in a deeper depth, this type of wave is not suitable for exploration purposes.

Seismic interferometry and Surface Wave Tomography

In order to process the Seismic ambient noise, Wapenaar (2010) come up with a way which was called Seismic Interferometry. Seismic Interferometry is a way to generate new seismic wave by cross-correlating two seismic recording data.[2]

When we have 2 seismic stations which were surrounded by seismic noise sources. Both stations record all the seismic noises and will generate a recorded seismic data. these recordings are recordings of seismic noises in multiple directions. Both station recordings and then were cross-correlated. which will generate a new seismic recording. The results of the correlation are available in many azimuths. Afterward, all of the azimuths will be summed and this will result in an impulse responds. This impulse respond was commonly known as green's function. With the source being dominated by surface waves, these Green's function will be dispersive. Because of that, we can do surface wave tomography. Surface wave tomography will results in a 2D velocity map in every period of time. this period of time will have a relationship with depth.

  1. Snieder, R. dan Wapenaar, K. (2010): Imaging with ambient noise, Physics Today, 63, 44-49.
  2. Wapenaar, K., Draganov, D., Snieder R., Campman, X., Verdel, A. (2010): Tutorial on seismic interferometry: Part 1 — Basic principles and applications, Geophysics, 75, 75A195–75A209