How are ambient noise dispersion curves recorded in OBSs affected by the water?
Abstract
Recent years have witnessed a steady increase both in ambient noise tomography studies and in OBS deployments. A natural consequence has been the application of ambient noise tomography techniques to seismic data recorded on the ocean bottom. However, ambient noise dispersion curves recorded on the sea bottom look quite different from their land counterparts, particularly at short periods (< 12 sec), an effect which has been attributed to P waves propagating in the water layer above the sensors. Some authors have pragmatically circumvented the issue by using ocean-botton surface wave group and phase velocities dispersion curves only at longer periods. Others have duly carried out tomographic inversions at shorter periods by including the water layer in the inversion.
In this work, we present a systematic study of the influence of the water layer in the dispersion of surface waves recorded at ocean-bottom stations. For a given model of the crust, we start by computing synthetic dispersion curves for OBS sensors located under water columns of variable heights, from zero (equivalent to a land station) to 5 km (oceanic basin). The dispersion for Rayleigh-wave fundamental-mode group and phase velocities shows a substantial dependence on water depth. In particular, with a 5 km thick water column, the Rayleigh wave group velocities have a very low value of ~1 km/s at periods of 1s to 13 s. At ~13 s, the velocity increases sharply, converging to the typical land-like (no water) dispersion curve. The thicker the water column, the higher the periods until which the group velocities remain very low. On the other hand, Love group and phase velocities, which are not affected by P-waves in the water column, are not at all affected by the water layer, and thus contain the more valuable information on Earth structure. Additionally, we computed the sensitivity kernels for Vp and Vs to understand the effect of the water column into the dispersion curves. To investigate if the short-period range could be used for crustal studies, we also compute synthetic dispersion curves by fixing the water depth to 5 km and varying the Earth structure below the stations. We find that ocean-bottom Rayleigh wave dispersion curves are more sensitive to structure at very short periods (1-7 seconds). In the range 7 - 13 sec, all the dispersion curves collapse, showing little or no variation with structure. Finally, above 13 sec, Rayleigh waves increase and converge to their land-based counterparts, becoming again sensitive to structure. We address the effect of ocean bottom topography by computing synthetic dispersion curves for surface waves propagating between two stations located at different depths on the seafloor. Finally, we present observed ambient noise cross-correlations and dispersion curves along varied paths that display the effects described above: ocean-ocean (OBS-OBS), ocean-land (OBS-land) and island-island (land - ocean - land). Notably, the effect of the water layer is visible in all these paths. In particular, we use data recorded with three different arrays located in the Atlantic Ocean, namely the NEAREST array, in the Gulf of Cadiz, the DOCTAR array, north of the Gloria fault, and the CV-PLUME project, in the Cape Verde archipelago.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S23C0649P
- Keywords:
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- 7212 Earthquake ground motions and engineering seismology;
- SEISMOLOGY;
- 7219 Seismic monitoring and test-ban treaty verification;
- SEISMOLOGY;
- 7255 Surface waves and free oscillations;
- SEISMOLOGY;
- 7270 Tomography;
- SEISMOLOGY