Imaging internal multiples from subsalt VSP data — Examples of target-oriented interferometry

Seismic interferometry has become a technology of growing interest for imaging borehole seismic data. We demonstrate that interferometry of internal multiples can be used to image targets above a borehole receiver array. By internal multiples, we refer to all types of waves that scatter multiple times inside the model. These include, for instance, interbed, intrasalt, and water-bottom multiples as well as conversions among them. We use an interferometry technique that is based on representation theorems for perturbed media and targets the reconstruction of specific primary reflections from multiply reflected waves. In this interferometry approach, we rely on shot-domain wavenumber separation to select the directions of waves arriving at a given receiver. Using a numerical walkaway !WAW"VSPexperiment recorded by a subsalt borehole receiver array in the Sigsbee salt model, we use the interference of internal multiples to image the salt structure from below. In this numerical example, the interferometric image that uses internal multiples reconstructs the bottomand top-of-salt reflectors above the receiver array as well as the subsalt sediment structure between the array and the salt. Because of the limited source summation in this interferometry example, the interferometric images show artifact reflectors within the salt body. We apply this method to a field walkaway VSP from the Gulf of Mexico. With the field data, we demonstrate that the choice of shot-domain wavenumbers in the target-oriented interferometry procedure controls the wavenumbers in the output pseudoshot gathers. Target-oriented interferometric imaging from the 20-receiver array recovers the top-of-salt reflector that is consistent with surface seismic images. We present our results with both correlationbased and deconvolution-based interferometry. INTRODUCTION Most exploration seismic imaging is done from surface seismic records. In areas of high structural complexity !e.g., near salt bodies", borehole seismic data can yield detailed subsurface information that cannot be obtained from surface seismic data. Hornby et al. !2005" give an example in which walkaway !WAW" VSP data acquired in a subsalt receiver array were used to image sediments below salt that were invisible using surface seismic data. Hornby et al. !2005" use standard active-shot migration methods to image the VSP data. Grech et al. !2003" give another example that uses WAW VSP data to image geologic features in a complex compressional tectonic setting in which surface seismic was compromised. Seismic interferometry !Curtis et al., 2006; Schuster and Zhou, 2006" opens possibilities for innovative uses of borehole seismic data because it reconstructs waves that propagate between receivers as though one of them acted as a source. Hence, with interferometry, it is possible to reconstruct pseudoacquisition geometries that differ from the original physical experiments. Schuster et al. !2004" use the concept of interferometry to migrate free-surface reflections from reverse VSPdata. Bakulin and Calvert !2004, 2006" use their virtualsource method to image beneath a complex overburden from borehole sensors in a horizontal well with no knowledge of the overburden model parameters. Vasconcelos and Snieder !2008a; 2008b" and Vasconcelos et al. !2007" use drill-bit noise recordings with a deconvolution interferometry method to perform broadside imaging of the San Andreas Fault at Parkfield, California. In the context of saltflank imaging, Willis et al. !2006" present a numerical example that demonstrates that diving waves can be used for interferometric imaging of near-vertical salt reflectors. Xiao et al. !2006" present a model-based interferometric method to image-transmitted P-to-S waves that can be used for salt-flank imaging. Here, we use internal multiples in interferometry to reconstruct primary reflections. This type of interferometry is applicable, for example, to imaging of structures above a borehole receiver array, usManuscript received by the Editor 9August 2007; revised manuscript received 21 February 2008; published online 16 July 2008. Formerly at Colorado School of Mines, Department of Geophysics, Center for Wave Phenomena, Golden, Colorado, U.S.A; presently at ION Geophysical, GXT Imaging Solutions, Egham, Surrey, U. K. E-mail: [email protected]. Colorado School of Mines, Department of Geophysics, Center for Wave Phenomena, Golden, Colorado, U.S.A. E-mail: [email protected]. BPAmerica, Inc., Houston, Texas, U.S.A. E-mail: [email protected]. © 2008 Society of Exploration Geophysicists.All rights reserved. GEOPHYSICS, VOL. 73, NO. 4 !JULY-AUGUST 2008"; P. S157–S168, 11 FIGS., 3 TABLES. 10.1190/1.2944168