Using Free Surface VSP Downgoing Multiples to Obtain Unconventional VSP Image Above the Well Trajectory - A Case Study in Offshore Vietnam

Abstract

Abstract The Vertical Seismic Profile (VSP) technique is routinely used to create seismic images near the wellbore; traditionally, only the up-going wavefield is used to image the subsurface below the receivers. The standard VSP technique does not provide seismic image information above the well trajectory. The objective of this study was to produce a seismic image above the top receiver depth up to the seafloor by using the downgoing multiples recorded in the VSP data for a more complete seismic-to-well tie. In VSP acquisition, the seismic source is positioned below Mean Sea Level and deployed above a downhole receiver. The source signal is recorded by a downhole receiver that is moved to cover a large number of depth levels in the well. The upgoing and downgoing arrivals are separated during processing; the up-going wavefield is used for subsurface illumination, whereas the downgoing wavefield and multiples are normally excluded from the processing. The standard VSP technique using the VSP upgoing wavefield gives a seismic image along the range of receiver depths and below the well trajectory. However, a VSP image can also be obtained from the downgoing multiple sequences in deep water. The processing of sea surface multiple is used mainly to obtain a VSP image of formations above the top receiver depth; such an image is unattainable with the standard VSP technique. Our results show that illumination coverage increases significantly when using multiples versus primaries. In addition, reflectors above the shallowest receiver can be imaged by multiples, including the seabed itself. This can be useful for shallow hazard identification for sidetracks or to avoid the expense of infill nodal seismic below the rig (Farmani et al., 2012). We also show that the image obtained from the VSP multiples can have good resolution comparable to the surface seismic image, with better signal to noise ratio especially for the shallower reflectors. The VSP data is true amplitude, zero-phase, and perfectly tied in depth and in time. In very deep water, the seafloor-sea surface multiple is completely separated in time from the air gun signature and is out of the time range of the conventional processing and interpretation steps. By using the deepwater surface-related VSP multiples and the mirror-imaging technique, the VSP image was extended successfully above the well trajectory upward to the seafloor and shows a good correlation with the surface seismic section (Marques et al., 2011). We present a case study using the sea surface downgoing multiples for this unconventional VSP imaging technique.