Continuous monitoring by ambient-seismic noise tomography

Discussion and conclusions

Previous passive seismic interferometry studies have used tomography to image the surface wave Green's functions for reservoir studies (Bussat and Kugler, 2009; de Ridder and Dellinger, 2011). Here we investigate the utilization of passive seismic interferometry as a permanent, continuous, nearly real-time monitoring system. The correlations stabilize faster for lower frequencies than for higher frequencies. We show that correlations of as little as 1 day of data between nearby receivers for frequencies below $1$  Hz converges to $~95\%$ of the correlation of 5 days of data. The group-velocity maps obtained from ambient-seismic noise tomography contain similar features in different frequency regimes. The smaller shallow channels are visible between $0.95 - 1.75$  Hz. The edge of the deeper channel is visible on the southeastern end of the array between $0.35 - 1.35$  Hz. A low-velocity zone crosses the array and is clearly distinguishable between $0.35 - 1.15$  Hz. The images are generally smoother than the P-wave velocity images from ambient-seismic noise tomography. This is partly because of the smoothing of the velocity map, but since the frequency of the ambient-seismic noise is also lower than that of active seismic surveying, the resolution cannot be expected to be as high. The very nature of the seismic waves used for the imaging is different. Surface waves propagate along the surface, and thus the depth resolution is provided only by their dispersive characteristics and cannot be expected to be as high as for P-waves. Nevertheless, we can achieve remarkable images of the top $240$  meters based on Scholte-wave velocities obtained by ambient-seismic noise correlations. These are of interest for continuous reservoir monitoring, especially for short time scale (days) changes in the shallow subsurface. The similarity of tomographic images obtained from independent day long cross-correlations or even two-and-a-half long cross-correlations needs further quantification.

Continuous monitoring by ambient-seismic noise tomography