Synthetic Modeling of Seabottom Node Acquisition over the Libra Field

Abstract

This paper describes a synthetic seismic modeling effort over the Libra field conducted by Schlumberger, in collaboration with geophysicists from the Joint Project Team (JPT) of the Libra consortium, to optimize acquisition parameters for the planned new seismic acquisition. The effort included the modeling of node and towed streamer geometries and generation of multiple seismic images for comparison purposes, including regular and mirror migrations, images of fulland downsampled node geometries, and comparisons against images of narrow-azimuth streamer geometries. Based on the results of this study, the acquisition template was finalized and used in the invitation to tender for the acquisition project. Introduction The Libra field is located in the presalt province of the BMS 11 block of the Santos basin, offshore Brazil (see Figure 1). The field was discovered in 2010 and has generated global interest because of its size and volume estimates. If estimations are correct the field will be one of the largest finds to date and the largest find since the Cantarell discovery in 1976. Production rights were awarded to a consortium of five companies in October 2013 (Petrobras, Shell, Total, CNPC, and CNOOC). As part of the exploration and development plans the consortium proposed to acquire new 3D surface seismic over the full field (approximately 1,500 km 2 ). Figure 1. Libra field location. In early 2014, a synthetic modeling study was initiated to evaluate a sparse-node seismic measurement over the field. The goals of the study were threefold: 1. Analyze and quantify the expected uplift of node geometries over the existing narrow-azimuth towed streamer measurement. 2. Understand the optimum sampling parameters of a node geometry including node separation, source separation, and maximum offset. 3. Compare upward (conventional) with downward (mirror) migration and the impact of each migration on the shallower and deeper areas seismic quality. Method An acoustic model (compressional velocity (Vp) and density) was created by Petrobras (the consortium’s operator) and provided to Schlumberger for the purposes of the synthetic seismic modeling study (Figure 2). Using this model, pressure and vertical particle velocity data were synthesized for a node geometry, and pressure-only data were synthesized for a narrow-azimuth towed streamer geometry. The full 2-way wave equation was used to extrapolate the seismic wavefield through the model. For both datasets, the maximum frequency modeled was 31 Hz. The node data were modeled with a slightly longer record length to allow for mirror migration. For the node data, to benefit from the efficiencies associated with the sparse receiver sampling, reciprocity was assumed, i.e. shooting from the sparsely sampled seabed receivers to the densely populated surface sources. Node geometry:  400 m × 400 m receiver grid  25 m × 25 m source grid  10,000 m × 10,000 m maximum offset Streamer geometry: