Virtual source gathers and attenuation of free‐surface multiples using OBC data:implementation issues and a case study

Abstract

Virtual source imaging is a technique based on extracting the Green's function that characterizes wave propagation between two receivers by cross-correlating the wave-fields recorded by these receivers. We focus on implementation issues in generating a virtual source gather from a multi-component OBC data recorded at the Mars field. The implementation issues include choice of the receiver that acts as the virtual source and the number of sources over which the cross-correlated data is stacked. The pre-stack correlated data (correlation gather) is a useful diagnostic for quality control and for assessing the source locations that give a stationary phase contribution. By stacking over specific source locations, we restrict the direction of the incoming energy and generate virtual source gathers containing arrivals within a specified horizontal slowness interval. We compare the virtual source gather generated by using a small number of sources to the virtual source gather generated by using a larger source aperture for stacking. Artifacts due to the traces at the edges of the source aperture can be suppressed by applying a taper before stacking the correlation gather. Another artifact observed in virtual source gathers is due to side-lobes of the auto-correlation of the source-time function. We show the use of dualsensor summation to separate the up-and the down-going energy in the raw data and using that to generate virtual source gathers containing only the upgoing energy, hence attenuating the free-surface multiples.

The virtual‐source method applied to Mars field OBC data for time‐lapse monitoring

“…The simplest approach to generate a virtual source gather is to correlate the total wavefield at the virtual source with the total wavefield at the receivers (Mehta, et al, 2006). The images for the years 2004 and 2005 obtained by migrating virtual source data generated us- Table 1.…”

Virtual real source

“…In seismic interferometry the Green's function between any two receiver locations can be computed by cross-correlating the receiver recordings due to random sources in the medium (Lobkis and Weaver, 2001;Derode et al, 2003a;Derode et al, 2003b;Snieder, 2004;Wapenaar, 2004;Weaver and Lobkis, 2004;Curtis et al, 2006). This principle has been applied to exploration seismology to remove overburden problems (Calvert et al, 2004;Mehta et al, 2006) and in imaging the subsurface (Schuster et al, 2004;Vasconcelos and Snieder, 2006). I use seismic interferometry for extracting the Green's function between two receiver locations.…”

“…In reality, the receivers are not usually surrounded by sources. This incomplete source aperture can result in some spurious events (Mehta et al, 2006) which can be removed through some special processing (K. Mehta, personal communication, 2007).…”