We have demonstrated that there is correlated energy at frequencies between
-km deep borehole stations and surface stations at the Valhall field. However the results are not typical of seismic interferometry. The signal is asymmetric, not centered at zero time lag, and does not originate from the borehole stations. We have argued that these observations are due to seismic energy interacting with the borehole acting a wave-guide. In this scenario, the asymmetry would be due to seismic energy travelling up and down the borehole. The apexes would be at acausal times because seismic energy needs to enter the borehole at the surface before reaching the stations at depth. A virtual source appears to emanate from the platform because correlated energy in the borehole must reach the surface at the platform before propagating outward. Because this proposed mechanism does not satisfy the conditions of seismic interferometry, we cannot interpret these cross-correlation results as inter-station Green's functions.
Correlation energy between surface and borehole stations at the Valhall field