**6. Conclusions**

Shear-wave tomography is performed using only 1.02-h ambient noise recorded by a permanent reservoir monitoring receiver array installed in the Norwegian North Sea. Passive tomography needs the wavefield to be diffuse and equipartitioned. However, the wavefield can usually be contaminated by various events in the real world. Time and frequency analysis indicates that the ambient noise used in the paper is contaminated by several high energy events. The beamforming results further clarify that these events are directional and should be suppressed before the noise is cross-correlated. An adapted eigenvalue-based filter is applied to accomplish the suppression and shows good effects both in beamforming and noise crosscorrelation results. The filter makes it possible to implement shear-wave tomography on the ambient noise with interference. Finally, an adaptive simplex simulated annealing algorithm (ASSA) is applied for estimating the three-dimensional subsurface shear-wave structure. The results indicate a four-layer structure with a generally increasing velocity as the depth goes deeper. The velocity values of the first three layers distribute relatively homogeneous, while the fourth layer shows a more complicated distribution, indicating a heterogeneous structure under 500 m. The comparison of the tomography result with other methods demonstrates that the tomography presented in this paper is reasonable and encouraging, although the ambient noise is very short and contaminated.

**Author Contributions:** Formal analysis, G.W., H.D., G.K., and J.S.; Funding acquisition, H.D. and J.S.; Methodology, G.W. and H.D.; Resources, G.W. and G.K.; Software, G.W. and H.D.; Writing—original draft, G.W.; Writing—review and editing, H.D., G.K., and J.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China, Grant Number [41605070].

**Acknowledgments:** The authors would like to acknowledge Grane license partners (Equinor ASA, Petoro AS, Vår Energi AS, and ConocoPhillips Skandinavia AS) for allowing to present this work. The views and opinions expressed in this paper are those of the authors and are not necessarily shared by the license partners; H.D. thanks the Norwegian Research Council and the industry partners of the GAMES consortium at NTNU for financial support (Grant No. 294404); We would like to give special thanks to section managing editor Bommie Xiong and unknown reviewers for their help and comments that significantly improved the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
