Helmholtz Tomography of ambient noise surface wave data to estimate Scholte wave phase velocity at Valhall Life of the Field.

Aurelien Mordret, Nikolai M. Shapiro, Satish S. Singh, Philippe Roux, and Olav I. Barkved.

Thursday 11 April 2013, by briandx // Publications

Helmholtz Tomography of ambient noise surface wave data to estimate Scholte wave phase velocity at Valhall Life of the Field.

Aurelien Mordret, Nikolai M. Shapiro, Satish S. Singh, Philippe Roux, and Olav I. Barkved.

GEOPHYSICS, VOL. 78, NO. 2 (MARCH-APRIL 2013)

Abstract

We applied the ’Helmholtz tomography’ technique to 6.5 hours of continuous seismic noise
record dataset of the Valhall Life of Field network. This network, that has 2320 receivers,
allows us to perform a multi-frequency high-resolution ambient-noise Scholte wave phase
velocity tomography at Valhall.
First, we computed cross-correlations between all possible pairs of receivers to convert every stations into a virtual source recorded by all other
receivers. Our next step was to measure phase travel-times and spectral amplitudes at
different periods from cross-correlations between stations separated by distances between
2 and 6 wavelengths. This is done in a straightforward fashion in the Fourier domain.
Then, we interpolated these measurements onto a regular grid and computed local gradi-
ents of travel times and local Laplacians of the amplitude to infer local phase velocities
using a frequency dependent Eikonal equation. This procedure was repeated for all 2320
virtual sources and final phase velocities were estimated as statistical average from all these
measurements at each grid points. The resulting phase velocities for periods between 0.65
and 1.6 s demonstrate a significant dispersion with an increase of the phase velocities at
longer periods. Their lateral distribution is found in very good agreement with previous
ambient noise tomography done at Valhall as well as with a full waveform inversion P-wave
model computed from an active seismic dataset. We put efforts on assessing the spatial
resolution of our tomography with checkerboard tests and we discussed the influence of the
interpolation methods on the quality of our final models.

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