Imaging on the Edge - Processing Challenges of a Sparsely Acquired 3D-2C OBC Survey, Offshore Malaysia

Abstract

Introduction Seismic data acquisition using ocean-bottom cable (OBC) methods, whilst capturing valuable azimuthal information, often presents challenges in the signal processing sequence for successful removal of unwanted noise and multiples-related energy. Herein, we present the results of methods that have enhanced the standard noise and multiple attenuation workflows. We present results on linear noise and shear-wave removal utilizing wavelet scale decompositions to complement tau-p transforms, receiver and source line interpolations using a "beyond-aliasing" approach for improved multiple model prediction and also subsequent imaging, and wavefield extrapolation to sea level plus adaptive subtraction for long-period multiples removal. Whilst it is always desirable to acquire data with uniform and dense coverage, acquiring 3D OBC wide-azimuth surveys can be challenging and expensive due to limits imposed by operational complexity and safety, time scheduling due to weather windows and environmental considerations, in-situ infrastructure and obstructions, and others. This typically results in compromises to surface acquisition grid sampling, i.e., large source and/or receiver line spacing. To mitigate the sparseness of OBC acquisition, an innovative and cost-effective staggered-grid source-receiver survey design was implemented, with the goal of improving spatial sampling without incurring the need for more equipment or increased operational complexity (Chandola et al., 2014). In this work, we show the effect of the surface grid on seismic processing and imaging and illustrate seismic data processing techniques to improve surface coverage further to overcome the acquisition limitations efficiently. Survey Geometry The survey geometry was oriented north-south for receiver lines, and east-west for source lines. The nominal spacing between receiver lines was 300 m and 250 m for source lines, whilst the receiver interval was 50 m and the source point interval was 25 m. Water depths within the survey area ranged from 8 m to 50 m (Figure 1). An innovative staggered grid source plus receiver layout was implemented within the development area. The goal was to increase spatial subsurface sampling from 12.5- x 25-m bins within the exploration areas, to 12.5 m x 12.5 m within the developmental areas without incurring additional acquisition cost. This staggered design was coupled with denser source line spacing in the developmental area to improve the coverage fold and near-offset distribution.