Prospect hunting below Deccan basalt: imaging challenges and solutions

Thick Mesozoic sediments under the basalt cover of the Deccan traps along the northwest coast of India are considered to be potential targets for hydrocarbon exploration together with the Indus basin of Pakistan. Sub-basalt seismic imaging is difficult in these formations, as the Deccan traps are composed of multilayered lava flows. Seismic imaging issues associated with high velocity basalt include: 1. Multiples generated in interbedded units of basalt and between the top of the basalt and the sea floor. 2. Energy scattering from and absorption by heterogeneities. 3. Wave mode conversion at the top of the basalt. Radon-based analysis can solve some of the difficulties related to multiples. The use of low frequency sources and multicomponent technology are other methods commonly suggested to improve sub-basalt imaging. The Deccan basalt area is very heterogeneous and has not been mapped effectively by high resolution seismic. At present, only standard streamer data are available in this area. Model-based imaging together with other methods can improve the sub-basalt image and add confidence in the reservoir modelling. Imaging challenges in this setting are discussed using examples from the Kutch basin on the northwest coast of India, where the primary reservoir (Bhuj formation) is overlain by thick basalt.