Ambient seismic noise eikonal tomography for near-surface imaging at Valhall

Introduction

The permanent ocean-bottom array at the Valhall field in Norway provides an excellent source of passive seismic data to test what might be accomplished with seismic interferometry. The array, installed in 2003 (Kommedal et al., 2004), can record data for long periods in all weather conditions. The subsurface structure is well known, both from numerous wells and from seismic imaging. During periods without active seismic acquisition at Valhall, there is abundant passive energy in the data over a wide range of frequencies. Previous attempts to synthesize omnidirectional point sources at conventional seismic exploration frequencies (3-60 Hz) failed at Valhall (Artman, 2007; Landes et al., 2009), so passive data will not replace active data in the higher frequencies ranges there. However, at lower frequencies (0.18-1.75 Hz), omnidirectional virtual Scholte-wave sources have been successfully generated (Bussat and Kugler, 2009; Dellinger and Yu, 2009; Dellinger et al., 2010). We process three passive datasets from the Valhall permanent array in Norway, generate virtual Scholte-wave source gathers using seismic interferometry, and then use these sources to image the shallow structure using eikonal traveltime tomography (Lin et al., 2009). Finally, we compare our results to the P-wave velocity structure at Valhall as determined from active data (Sirgue et al., 2010).

Ambient seismic noise eikonal tomography for near-surface imaging at Valhall