Interferometric salt-flank estimation

Abstract

Accurately determining the position of salt-sediment interfaces is important in oil and gas exploration, salt mining, and [Formula: see text] or oil and gas storage operations. We used seismic waves from a vertical seismic profile conducted above a salt dome in Texas to image a 2D vertical slice of the salt flank that was longer than can be obtained with traditional depth migration methods alone. This was possible by combining interferometric imaging methods with data from 100 3C receiver positions over a 1524 m (5000 ft) spread in a borehole close to the flank. Interferometry was used to construct virtual source gathers at the receiver positions within the salt body. This velocity-independent redatuming process effectively moved the sources from the surface to the borehole, removed complications of the overlying geology, and provided favorable illumination of the suspected flank position. Full-waveform elastic forward modeling helped us to understand and confirm the flank position. From the processed virtual source data, we created a 2D image in the source-receiver plane. In effect, we have created an imaging problem geometrically equivalent to that of a simple 2D surface seismic line rotated by 90°, with the requirement that we knew the velocity model only within the salt body. Interferometric imaging of the salt flank reduced the picking ambiguity and provided a direct depth image that was unaffected by overburden effects where the salt-flank location could be interpreted more confidently. Ultimately, we were able to image the salt flank over 1371 m (4500 ft) of depth with a lateral resolution in the order of 45–60 m (150–200 ft). The original data were obtained with more traditional imaging methods and cost considerations in mind, but the emergence of interferometric imaging methods suggested significant improvements to the acquisition strategy that would optimize the illumination of the subsurface.