Kamal Belaid

3D OBC seismic survey parameters optimization offshore Abu Dhabi

The flexibility of 3D ocean bottom cable (OBC) seismic survey design allows a variety of survey parameters and geometries to be chosen. From the infinite number of possible variations, we seek to optimize the trade-off between higher data quality and lower survey effort. To understand the relationship between data quality and survey effort, we acquired a pilot 3D OBC seismic survey with very high specifications, simulated several datasets by decimating the full data or reducing the survey effort, and then evaluated the quality of each dataset against the survey effort. We show that: (a) data quality is related to trace and offset vector tile (OVT) densities; (b) data quality is an asymptotic function of survey effort in general, eventually reaching a plateau with increasing survey effort; (c) a gradient of the fit curve at a certain survey effort can represent the quality efficiency, dQ/dE, where Q is data quality, and E is survey effort; and (d) the quality curve for a survey can be estimated using existing datasets in a field with the proposed equation for Q and dQ/dE.

Multi-azimuth processing and its applications to wide-azimuth OBC seismic data offshore Abu Dhabi

Wide-azimuth ocean bottom cable (OBC) seismic data have several advantages for imaging, such as high fold, long offsets, and a wide range of source-receiver azimuths. However, conventional wide-azimuth processing has some drawbacks because the full range of advantages is not exploited. In conventional wide-azimuth processing, azimuthal variations are usually ignored, whereas multi-azimuth processing treats each source-receiver azimuth individually. We performed a pilot multi-azimuth processing trial on wide-azimuth OBC seismic data offshore Abu Dhabi, United Arab Emirates to evaluate its benefits compared to conventional wide-azimuth processing, and discuss its applications for fault imaging and fracture characterization. We show that: (a) multi-azimuth processing achieves significantly higher signal-to-noise ratio, and effectively corrects the distortions in the subsurface image resulting from azimuthal variations; (b) mono-azimuth processing enhances imaging of existing subtle faults; and (c) azimuthal variations extracted from mono-azimuth processed data provide indications about fracture orientation, relative fracture intensity, and their spatial distributions. The results should encourage us to apply multi-azimuth processing to wide-azimuth OBC seismic data to improve focusing quality, image subtle faults, and gain insights into fracture characteristics.

Dealing with spatial sampling sparseness and irregularity in 3D OBC seismic data offshore Abu Dhabi

Due mainly to commercial and operational constraints, seismic data are often sparsely and irregularly sampled, leading to several challenges in processing of 3D OBC seismic data offshore Abu Dhabi. Conventional linear noise attenuation techniques based on multi-channel filters are not effective with Scholte waves as they are usually aliased with typical sampling intervals in 3D OBC seismic data (e.g., 25 m source and receiver point intervals), and sometimes scattered because of near-surface heterogeneity. To address them, we apply model-based surface wave attenuation, Surface Wave Analysis Modelling and Inversion (SWAMI), which enables an estimate of local near-surface properties by analysing dispersion curves. Thus, both direct and scattered Scholte waves are effectively modelled and attenuated without suffering from under-sampling. A data interpolation and regularization technique called Matching Pursuit Fourier Interpolation (MPFI) is then applied to enhance spatial sampling. MPFI employs an anti-aliasing capability so optimum data reconstruction can be performed for any frequency range. In addition to the regularization aspect, MPFI with a 5D implementation (4 spatial coordinates and time) is targeted to densify receiver line interval and extend source lines, which consequently enhances fold, offset and azimuth distributions of the data.The implementation of the two techniques successfully addresses processing challenges in sparsely and irregularly sampled OBC seismic data.

3D OBC Seismic Survey Productivity Enhancement with Satisfying Geophysical Requirements

Ocean Bottom Cable (OBC) seismic survey has several technical advantages over conventional towed-streamer technique as it enables the acquisition of wide azimuth and high fold datasets having long offsets and less spatial discontinuities. However, its usage is still limited because of requirement of relatively large operational efforts which likely results in more survey cost and duration. Consequently, optimization of OBC seismic survey productivity is essential to narrow the business gap with towed-streamer acquisition and enhance widespread applicability of OBC techniques. Due to the decoupled aspect of the source and receiver lines, OBC survey can inherently form numerous survey designs. Our study is conducted with the aim to find efficient OBC seismic survey designs that still satisfy established geophysical objectives among infinite selections. We introduce survey designing criteria on the basis of sampling of OVTs (Offset Vector Tile) which allows us to achieve desired offset and azimuth distributions in final imaging. Productivity analysis is then performed based on current equipment availability enabling a variety of survey designs and geometries which were not feasible previously. We also adapt several geometry options including two dual source-vessel operations: (1) Distanced Separated Simultaneous Shooting (DS3); and (2) Dual Source-Vessel Flip-Flop Shooting (DSVFFS). Applicability of dual source-vessel operations to OBC survey has not been well described unlike marine towed-streamer and land cases. Thus, we analyse the impact of dual source-vessel operations on OBC survey efficiency. Additionally, we discuss technical challenges resulting from the relationship between OBC survey designs and the resultant interference noise wave fields not generally associated with other acquisition techniques.

Application of Statistical Analysis on Vertically Variable Azimuthal Anisotropy: Case Study from a Carbonate Field Offshore Abu Dhabi, UAE

In the mature oil and gas fields offshore Abu Dhabi, reservoir properties required for static and dynamic models are relatively well-defined by a number of wells. On the other hand, seismic data are considered as a fundamental and primary source to determine and optimize well placements. Azimuthal velocity analysis using wide-azimuth 3D-2C OBC seismic data is performed on different formations where several drilling issues arose mainly due to two dominant tectonic forces in the region. The results reveal different magnitude and orientation of azimuthal anisotropy from overburden to reservoir level. Available FMI and DSI logs confirm consistency between seismic and borehole-driven azimuthal anisotropy. The analysis results are also in agreement with the regional geology and tectonic history. Azimuthal anisotropy analysis generally provides two types of information such as the orientation of anisotropy and the amount of anisotropy. The amount of anisotropy can be simply quantified while the information obtained from the azimuth data has some complexity as it is a periodic function. In this respect, a statistical model of the bipolar von Mises distribution is proposed to determine the preferred orientation of azimuthal anisotropy. The model also provides the concentration parameter that can quantify the degree of preferred dimensional orientation of azimuth data. Additionally, we show utilization of the azimuthal anisotropy analysis particularly on a non-fracture layer and its benefit to field development by the analysis of spatially varying mud weight prediction.