Imaging By Multiples: A Case study in the Carnarvon Basin

Abstract

Vertical Seismic Profiling (VSP) is renowned for its high resolution images of the subsurface. By and large, the images derived are beneath the well. Now a new technique allows imaging above the borehole by utilizing free surface multiples as a secondary source. A number of conditions need to be met for this technique to successfully meet its objectives. This paper presents a case study of data acquired recently in the Carnarvon basin and processed to derive an image above the well. The high-resolution, multiple-free VSP image allows verification of the shallow part of the subsurface. This information can be used to identify drilling hazards, faults and generally improve subsurface interpretation. The result can also be used to overcome the limitations of poor cementing which often causes casing ringing noise, which in itself is detrimental to VSP imaging. Subsequently, the size of the VSP image for this survey was increased by a factor of 2, thus greatly improving the value of acquisition.