Abstract

Mapping of shallow (depth ranging from ~ 400- 600m) highly fractured carbonate formation is a challenging task owing to a number of reasons: Firstly, the occurrence of a shallow carbonate layer induces velocity inversions which are not reliably estimated in time domain seismic imaging. Secondly, the fractures and vugs present in these layers cause lateral velocity variations which further complicate seismic wave path and thus seismic imaging. With an intention to map these carbonate layers in a geologically conformable manner, this paper highlights a novel experiment that was designed and executed to find out an ultra-high density 3D survey using vibroseis source to image these layers adequately. On the basis of this 2D ultra high-density (UHD) data, we plan to carry out a 3D UHD survey. This experimental 2D was acquired using a single vibroseis source having a receiver spacing of 5m and a source spacing of 10m. The same 2D profile was acquired multiple times with different number of sweeps and peak forces in order to assess the effects of each of these parameters on imaging of these carbonate layers. In this study, we have processed the datasets of a total of four overlapping 2D profiles (covering the same subsurface region) having different acquisition parameters, keeping the processing sequence identical, in order to optimize the parameters and to obtain the best possible image. This has been used to finalize the parameters, mainly vibroseis nos. of sweep and peak ground force, to acquire the UHD 3D survey to map the subtle variations in the carbonate layer. This was felt necessary as vibroseis data acquired in the desert region is fraught with significant wind noise and can be best attenuated by increasing the nos. of sweeps. However, it significantly increases per shot acquisition time. Moreover, increasing the vibroseis’ peak ground force not only increases energy penetration but also increases the ground roll amplitudes in a shot gather. Therefore, an optimum balance is necessary before deciding on these two parameters.