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How ‘leaky waves’ from ambient noise can reveal what lies underneath earth’s surface

Nodal seismometers can be used to gather information about ambient seismic noise and the structure of earth’s interior. (Image by Yellowstone National Park, Public Domain).

Researchers based at China’s Southern University of Science and Technology have improved the way of studying the geologic structures beneath earth’s surface by using ambient noise, such as the constant hum of sounds made by people and natural processes, like mining activities or waves crashing on a beach.

In a paper published in the journal Geophysical Research Letters, the group led by Zhengbo Li explains that when an earthquake occurs, several types of waves radiate from the event’s source. P waves and S waves move through the interior, and Love and Rayleigh waves or normal modes move across the planet’s surface. The speeds at which these waves travel and how each type is scattered in the subsurface can reveal a wealth of different information about the underground structures.

With this in mind, the scientists focused on demonstrating the possibility of imaging the subsurface using not only normal modes produced by ambient noise but also another naturally occurring type of seismic wave called leaky or leaking modes. 

Leaky modes can occur when a seismic wave is ‘trapped,’ bouncing back and forth between two layers of rock. When energy escapes from this bouncing wave, it creates a separate wave referred to as the leaky mode. Compared with normal modes, which are more sensitive to S wave velocity structures underground, leaky modes are more sensitive to P wave velocity structures.

The authors looked at one month of ambient noise picked up by a dense array of seismometers called the Large-n Seismic Survey in Oklahoma (LASSO). Studying both normal and leaky modes extracted from the seismic data, they calculated the speeds at which the waves had moved through the subsurface. Those speeds, in turn, are specific to the properties of the rocks below, such as their density and elasticity.

Knowing such properties can reveal what underground structures are made of, which is key to identifying untapped deposits of natural resources.