Broadband processing of West of Shetland data

Rob Telling, Nick Riddalls, Ahmad Azmi, Sergio Grion and R. Gareth Williams present broadband processing of 2D data in a configuration that enables demultiple algorithms, designed for processing conventional data, to be used as part of a standard prestack time sequence. The example dataset described here forms part of a 2D broadband well-tie survey conducted within the Shetland-Faroe basin. The water-bottom within the survey area is hard and varies in depth between 120 m and 1700 m. The deeper geology throughout is characterized by a prominent layering of Paleogene flood basalt, varying in thickness from a few hundred metres to over a kilometre. In terms of seismic response, the high impedance contrasts at the water-bottom and at the top of the basalt gives rise to prominent reflections and strong multiples. Additionally, severe attenuation takes place within the alternating layers of basalt and silt and clay stones which limits deeper penetration of energy. It is important for successful imaging to maintain good signal-to-noise ratio below this layer, particularly at the more penetrating low frequencies, and this survey is therefore well suited to a broadband acquisition and processing solution. The high signal-to-noise ratio offered by towing streamers deep below the source of wave noise is important for maximizing the processing bandwidth (Williams and Pollatos, 2012). A central feature of the processing of this dataset is the use of algorithms designed for processing conventional data, made possible by the adoption of a flat or, as used here, a very mildly slanted streamer configuration during acquisition. In contrast, where strongly slanted or curved profiles are used, algorithms require modification to account for the strong variability of the receiver ghost response with offset (Sablon et al., 2012), due to a breakdown of the stationary wavelet assumption that many processing algorithms require (e.g. semblance velocity analysis, SRME) or alternatively deghosting and re-datuming must be carried out at a much earlier stage in the processing sequence.