Ambient OBN surface-wave seismology

What is it about?

A long-standing challenge in seismic exploration is generating and recording active-source low-frequency (i.e., below 2 Hz) signals. However, acquisition hardware such as ocean-bottom nodes (OBNs) constructed for longer-term deployments is able to record naturally occurring ambient low-frequency seismic energy that propagates through the subsurface. This ambient waveform energy, when stacked over a sufficiently long period of time, is potentially useful for subsurface imaging and inversion analyses. To demonstrate this assertion, we use continuous ambient wavefield data acquired over a 40-day period on a 2000-node OBN array located in the Gulf of Mexico. Using an ambient processing workflow for precorrelation long-time recordings that includes an interferometric calculation to compute virtual shot gathers (VSGs), weshow that at least two different low-frequency surface-wave modes became apparent. The key observation is that the observed surface-wave energy is sensitive to subsurface geologic structure. By overlaying wavefield propagation from individual radial-component VSGs on a velocity model known from active-source surveying, the depicted wavefield propagation clearly identifies salt body boundaries at depths corresponding to the frequency of the surface-wave energy.

Featured Image

Photo by Pawel Czerwinski on Unsplash

Photo by Pawel Czerwinski on Unsplash

Why is it important?

Due to the interferometric nature of the VSG calculation, each OBN location can be used as a VSG pseudosource location; thus, these wavefields potentially can be used to construct low-frequency shear-wave velocity models. These elastic model components may be valuable as initial models for full-waveform inversion, as well as for seismic imaging applications involving complex offshore geologic scenarios.