The Gunashli field is part of the world class Azeri-Chirag- Gunashli oil field development in the South Caspian Sea. The streamer-tow seismic image within the Gunashli area is highly degraded by mud volcanoes and related shallow gas in the overburden. Following the successful acquisition of the Azeri 3D 4C survey in 2002, a separate survey was acquired over the Gunashli area using a similar interlaced geometry (Bouska, 2004). The Azeri 4C survey was processed through time and depth migration and gave an improved image of the structure with reduced uncertainty (Johnston, 2004). For the Gunashli 4C survey, several innovative methods were incorporated in both the P and PS-wave processing, preserving both signal and bandwidth, and attenuating the more coherent noise. Some of the key stages and results from the time processing of the Gunashli PS data are discussed herein. In a companion paper, Crompton et al, 2005 discuss results from the depth processing.
The giant Azeri-Chirag-Gunashli (ACG) structure lies in 150m to 450m of water, approximately 100km offshore the Apsheron peninsula of Azerbaijan in the South Caspian Sea (Figure1). The structure is asymmetric with steep dips (>40°) on the north flank and gentler (25°) on the south flank (Bouska, 2004). The reservoir consists of nine laterally extensive, stacked Pliocene sandstone intervals in the Pereriv and overlying Balakhany formations (Lyon, 2004). Varying sizes of mud volcanoes penetrate the structure near the crest. The mud volcanoes are characterized by debris cones and outflows on the seabed fed by over-pressured shale from strata below the target reservoirs. The cause of the poor data zones seen on the streamer data have been postulated as a combination of a number of factors (Lyon, 2004):
· P-wave absorption/attenuation through distributed gas in the overburden sediments
· Disturbed/disrupted sediments in the vicinity of the mud volcano plume
· Backscattered shot-generated noise from nearsurface heterogeneities. Given the obstacles to P-wave propagation, a natural alternative is to acquire multicomponent data and make use of PS-wave imaging.
The Gunashli OBC survey was acquired with interlaced 2 x 6km multicomponent (4C) cables. The shot patch was oriented parallel to the cables and this formed the basic swath unit (Figure 2). These swaths were repeated to cover the desired survey area; receiver lines are spaced 720m apart along the length of the structure and interlaced over the crest to give an effective line spacing of 360m (Bouska, 2004). The shot coverage is a regular 75m x 75m grid. Unlike many OBC surveys that shoot in the in-line direction for efficiency and to mimic narrow-azimuth towed streamer surveys, here the shots and receivers were decoupled to give a wide-azimuth geometry. This geometry also gives additional fold and illumination on the complex crest of the structure.
The Azeri survey was acquired and processed as the first multicomponent survey in the area and discussed by Johnston (2004) and Bouska (2004). An advanced P-wave processing flow was developed during the processing of the Azeri survey, allowing the Gunashli P-wave processing to largely follow the same flow.
