4D Processing Between Variable Depth and Conventional Streamer Data

Processing data with variable-depth streamer acquisition has recently become possible through a new advanced algorithm called joint deconvolution (Soubaras, 2010). In this particular acquisition, the receiver depth increases non-linearly with offset and this allows for a wide diversity of receiver ghosts to be recorded. This acquisition and associated processing dramatically increases the possible frequency bandwidth, on both low & high frequencies sides, from 2.5Hz to the source notch. This particular broadband technique will be referred to as BroadSeis in this paper. While most acquisitions in the future will certainly be realized with broadband techniques, the question of 4D matching between conventional and BroadSeis data must now be addressed during an intermediate period when the baseline data is a conventional acquisition. This paper considers this challenge and demonstrates that a good 4D response can be obtained Compared to conventional flat streamer data, processing variable depth streamer data implies a major change: the receiver ghosts are rigorously taken into account, whereas they cannot be removed from the wavelet in conventional flat streamer processing. The variable receiver depths of BroadSeis give asymmetrical ray paths which are taken into account by the imaging process and by the proper summation of the up-going and down-going wave fields in the joint deconvolution. Typical cross-equalization in a 4D process aims at solving issues related to differences in the vintages acquisition (acquisition related time- and amplitude differences) and positioning (4D binning). The case between BroadSeis and conventional data has one more problem to solve: the difference in cable profiles. This problem can be handled by joint de-ghosting and re-ghosting processes. In the following sections, we will discuss all these topics: wavelet processing, time de-striping, 4D binning, regularization, imaging and final matching. The dataset used for this comparison is a dual recording acquired by Shell in a highly structured deep offshore play. Data Overview While shooting a conventional 3D survey offshore Gabon, Shell acquired an additional 430 sq km swath of BroadSeis data to evaluate the uplift brought by the broadband image. The first comparison on PSTM data was generated in the end of 2011 and is shown in Figure 1. It shows the overall improvement typically achieved by BroadSeis in terms of enhanced spectral bandwidth. The acquisition geometry consists of 10 cables, 8000m long. The conventional streamers were towed at 11m depth with a source depth of 9m, while the BroadSeis configuration was towed between 11m and 50m with a source depth of 7m. No specific repeatability of source positions was requested when the vessel acquired these swaths, as shown by the azimuth maps of the two acquisitions (Figure 2). With these limitations in mind, both volumes were processed in a 4D sense in order to assess any impact of the variable streamer depth on the 4D signature, which should ideally be zero in the common bandwidth.