Correlation energy between surface and borehole stations at the Valhall field

Correlation energy between surface and borehole stations at the Valhall field

Jason P. Chang and Sjoerd de Ridder

Abstract:

Ocean-bottom cables at the Valhall field have provided an abundance of passive seismic data for testing the potential of seismic interferometry. We cross-correlate recordings from surface stations and $2$ -km deep borehole stations. Results show correlated energy at frequencies between $0.175$  Hz and $1.75$  Hz. However, the signal we retrieved is not time-symmetric, as there are multiple arrivals at acausal correlation time lags compared to the one arrival visible at causal time lags. The apexes of the causal events are found at acausal time lags rather than at zero time lag. The virtual source is centered northwest of the borehole stations at the offshore platform. We conclude that these observations are due to the borehole acting as a wave-guide. Because this 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