Subsurface mapping by ambient noise tomography

Subsurface mapping by ambient noise tomography is the mapping underground geological structures under the assistance of seismic signals. Ambient noise, which is not associated with the earthquake, is the background seismic signals. Given that the ambient noises have low frequencies in general, the further classification of ambient noise include secondary microseisms, primary microseisms, and seismic hum, based on different range of frequencies. We can utilize the ambient noise data collected by seismometers to create images for the subsurface under the following processes. Since the ambient noise is considered as diffuse wavefield, we can correlate the filtered ambient noise data from a pair of seismic stations to find the velocities of seismic wavefields. A 2-dimensional or 3-dimensional velocity map, showing the spatial velocity difference of the subsurface, can thus be created for observing the geological structures. Subsurface mapping by ambient noise tomography can be applied in different fields, such as detecting the underground void space, monitoring landslides, and mapping the crustal and upper mantle structure.

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