Obtaining a Seismic Image Above the Top VSP Level Using Sea Surface Multiple - A Case Study in Nam Con Son Basin, Offshore Vietnam

Abstract

Abstract The Vertical Seismic Profile (VSP) technique is routinely used to create seismic images near the wellbore and traditionally only the up-going wavefield is used to image the subsurface below the receivers. The standard VSP technique does not give any information of the seismic image above the well trajectory. The objective here is to produce a seismic image above the top receiver depth up to sea floor using the downgoing multiples recorded in the VSP for a more complete seismic to well-tie. A seismic source is positioned below Mean Sea Level and deployed near the wellhead. 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 while the down- going wavefield and multiples are normally excluded from the processing. The standard VSP technique using 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. The main application of processing the sea surface multiple is to obtain a VSP image of formations above the top receiver depth which is unattainable with standard VSP technique. Results show that by using multiples, illumination coverage increases significantly in comparison to using primaries. In addition, reflectors above the shallowest receiver can be imaged by multiples, including the sea bed itself. This can be useful for different purposes such as shallow hazard identification for sidetracks or to avoid the expense of in-fill nodal seismic below the rig. We also show that the image obtained from the VSP multiples can have good resolution comparing to the surface seismic data, 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. By using the sea surface related VSP multiples and the mirror imaging technique, the VSP image was extended successfully above the well trajectory up to the sea floor and shows a good correlation with the surface seismic. In this case study, we present some results of using the sea surface downgoing multiple for this unconventional VSP technique.