Tim Bunting

Full Azimuth Circular Survey in Indonesia: Survey Design, Onboard Illumination QC and Preliminary Processing Results

Conventional offshore 3D acquisition is still being performed mainly with narrow azimuth streamer configurations, even in structurally complex areas. Attempt at breaking this paradigm have been recently made by the industry through the successful acquisition of some unconventional “Multi-Azimuth” (MAZ), “Wide-Azimith” (WAZ) and “Rich Azimuth” (RAZ) marine surveys. Eni Indonesia and WestenGeco conducted “the first of its kind” full 3D Circular Shooting survey (Coil) over the Tulip Discovery in Indonesia between August and September 2008. This paper presents design, onboard illumination QC and preliminary processing results of this new “Full Azimuth” (FAZ) seismic effort.

Synthetic Modeling of Seabottom Node Acquisition over the Libra Field

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:

Marine broadband case study offshore China

Introduction The effect of the sea-surface ghost on marine seismic datasets is well understood. The ghost (sea-surface reflection) constructively and destructively interferes with the primary wavefield and acts to band-limit the recorded seismic measurement. This band-limiting effect has been a challenge since the inception of the marine seismic method. As a consequence, surveys are designed to optimize imaging of specific targets. For example, to optimize imaging of shallow targets, it is general practice to tow sources and streamers shallow. This practice results in good shallow resolution because the high frequencies are preserved, but at the expense of aggressive attenuation of the low frequencies. Ideally, we want data that are rich in both high and low frequencies to image shallow and deep targets optimally, and for seismic attribute analysis. Figure 1 shows the effect of the receiver ghost on the amplitude response for 5 m and 23 m tow depths in the frequency domain. The 5 m tow depth response is good at high frequencies, but has heavy attenuation at low frequencies, whereas the deep-tow response is good at low frequencies but has notches within the general seismic bandwidth. In recent years, two specific tow-depth combination solutions have been deployed, over/under and sparse-under, to solve for the band-limiting effects of the sea-surface ghost. Both configurations use streamers that are towed at different depths below the sea surface. The over/under technique (e.g., Hill et al., 2006) has been around for at least 50 years, but has gained wider acceptance over the last five years, since streamer steering technologies have been developed. Streamer steering minimizes the lateral separation of the two streamers, which can result from different water currents at the different tow depths, and is important because over/under wavefield separation techniques rely on the two streamers measuring the upgoing and downgoing wavefields at the same horizontal position. With the over/under technique, both streamers are towed deep, generally between 15 m and 30 m. The specific depths are chosen to ensure that there are no overlapping ghost notches in the bandwidth of interest. The measurements from the two streamers are combined post-acquisition using, for example, a de-phase and weighted-sum wavefield separation technique (Posthumus, 1993). Sparse-under is a more recent development. Initially described by Kragh et al. (2009), the technique complements a measurement from streamers towed at conventional depths (5–8 m) with measurements from a smaller number of deep streamers (15–30 m). Energy from the Abstract Dual tow-depth acquisition configurations are used to mitigate the band-limiting effect of the sea-surface ghost in marine streamer surveys. Here we report on the analysis of a 2D broadband marine seismic project acquired offshore China. The deployment configuration included over/under sources and three streamers towed at different depths (5, 17, and 23 m). This configuration allowed not only for analysis of dual tow depth as a method for increasing bandwidth, but also for evaluation of two distinct dual tow-depth combinations, over/under and sparse-under. The project provided the first opportunity to make this comparison. Evaluation of reflection images, filter panels, amplitude spectra, and signal/noise separations demonstrates that both techniques are effective at compensating for the filtering effect of the ghost response, but also suggest that the improved results from each dual tow-depth technique are very comparable. Given the operational advantages of the sparse-under technique, due to the reduced streamer requirements, we conclude that the sparse-under technique is an attractive option for 3D deployments.

Seismic imaging of the fault that caused the great Indian Ocean earthquake of 26 December 2004, and the resulting catastrophic tsunami

WesternGeco and Schlumberger, as part of the Sumatran Andaman Great Earthquake Research (SAGER) team, contributed to the effort to image the Sumatran seismogenic zone that ruptured on 26 December 2004. Resulting reflection images allow interpretation of the rupture point down to 40 km of depth, providing arguably the best seismic reflection images of a deep subduction zone to date. Early images, generated on the vessel, showed that the subducting mechanism (oceanic crust plus Moho) can be seen down to 12 s two-way traveltime (TWTT). Additional interpretation has shown seismic reflections as deep as 18 s TWTT (50–60 km of depth). This is a rare example of a complete subduction zone system being directly imaged with reflection seismic technology to over 40 km of depth.