Low-frequency ambient ocean-bottom node surface-wave seismology: A Gulf of Mexico case history

Long-time marine seismic recordings are becoming more common with the increased use of ocean-bottom nodes (OBNs), which can measure ambient seismic energy at frequencies lower than the typical minimum values in active-source compressed air -gun surveys. Interferometric processing on long-time ambient multicomponent data allows for the extraction of low-frequency (sub-2.0 Hz) responses in virtual source gathers (VSGs). Using 40 days of continuous OBN recordings acquired on a large dense array during a field experiment in the Gulf of Mexico, we find that sub-2.0 Hz surface-wave energy in the computed VSGs is strongly coherent and exhibits an identifiable spatially varying character. In particular, after rotating the data components from a Cartesian geographic into a polar wave-vector reference frame, we find that radial VSGs (i.e., oriented along the vector connect-ing the virtual source and receiver) clearly indicate that surface -wave propagation is influenced by salt bodies as identified in a colocated active-source survey situated at a minimum of 0.7 km depth below the seafloor, an observation consistent with calculated 0.25-0.50 Hz surface-wave sensitivity kernels. This suggests that low-frequency ambient OBN surface-wave seismol-ogy could be important for estimating the long-wavelength elastic material properties (particularly S-wave velocity) and identifying the lateral boundaries of salt bodies without any prior knowledge of subsurface geology.