Correlation energy between surface and borehole stations at the Valhall field

Introduction

Traditional seismic surveys for subsurface imaging deploy a collection of receivers and a controlled source. These surveys are often expensive to deploy and require much effort to oversee. Passive seismic interferometry does not require a controlled source. Cross-correlating recordings of ambient seismic noise at two receivers turns one of the receivers into a virtual source and extracts the time it takes the energy from that virtual source to reach the other receiver of interest. This travel-time information can then be used for purposes such as velocity modeling.

Tests of the practicality of passive seismic interferometry for these purposes has been ongoing with ambient seismic noise data provided by the permanent ocean-bottom cable (OBC) array at the Valhall field. Artman (2007) and Landes et al. (2009) showed that virtual omnidirectional point sources could not be captured at frequencies typical of seismic exploration ($3$ -$60$  Hz). Later, de Ridder and Dellinger (2011) showed that virtual low-frequency ($0.35$ -$1.75$  Hz), omnidirectional Scholte waves along the ocean floor could be generated from seismic noise. They were able to use the Scholte-wave travel-time information for tomographic imaging of structures in the near-surface (0 -$150$  m).

In previous studies of passive seismic interferometry for seismic exploration at the Valhall field (de Ridder and Dellinger, 2011; Artman, 2007; Bussat and Kugler, 2009) correlations were performed between stations on the ocean floor. In this study we correlate ambient seismic noise recorded over the same time period by the OBC array and by five downhole stations at nearly $2$  km depth. Our goal is to test whether Green's functions can be extracted between the borehole stations and the surface stations. In this report, we present the results for low frequencies ($0.175$ -$1.75$  Hz).

Correlation energy between surface and borehole stations at the Valhall field