No abstract

No abstract

Summary Seabed acquisition is expected to deliver broadband seismic data by using a combination of pressure and particle motion or acceleration measurements to eliminate the free surface ghost. In this paper, we have investigated the frequency bandwidth delivered by different seabed acquisition systems, i.e. two component ocean bottom cable (2C-OBC), four component ocean bottom cable (4C-OBC) and four component ocean bottom node (4C-OBN) systems on the low frequency side, under different geological settings and acquisition environments. The study was carried out by analysing the effect of total system (source, receiver and recording instrument) response and frequency bandwidth for different types of seabed acquisition systems and sensors. The analysis shows that after optimal compensation for total system response, different seabed systems and sensors deliver comparable low freaquency signal in the band of 2–5 Hz given the variations in the local seabed conditions. Introduction The recent emergence of towed streamer based broadband seismic technologies poses an interesting question, "how broad really is the broadband data delivered by various seabed acquisition systems, especially at the low frequency end?" During the last few years, PETRONAS has acquired more than 1,200 sq. km. of 3D and 540 LKM of 2D seabed seismic data using 2C and 4C acquisition technologies. Majority of the data were acquired using OBC technology and different acquisition systems with a limited amount of data acquired using OBN technology. The main objectives of the study were to analyse:The effect of total system (source, receiver and recording instrument) response on the signal bandwidth and the effect of source-receiver notchesThe frequency bandwidth delivered by different seabed acquisition systems and the conventional towed streamer system. The effect of source volume on the bandwidth, if any, was not analyzed in this study. One of the main benefits of seabed acquisition is that it delivers broadband and wide azimuth seismic data. The high frequencies are important for improving the resolution, while the low frequencies provide the opportunity for better seismic inversion and well ties. The seabed acquisition improves the signal bandwidth by enhancing both high and low frequency ends of the spectrum. It uses co-located measurements of two vertical components (P and Z) which help to remove the free surface ghost (Fig 1) and improve the overall signal bandwidth.

Summary PETRONAS recently conducted the world’s first 3D broadband full azimuth marine seismic survey by an operator in East Malaysia. The survey seamlessly deployed two of the most cutting edge acquisition technologies concurrently i.e. the circular shooting technique for extending the azimuth sampling and the constant gradient slant cable towing technique to extend the bandwidth sampling. The legacy narrow azimuth 3D streamer data, acquired in the year 2001, suffers from poor imaging in the zone of interest. Though there were two successful wells with hydrocarbon discovery in the areas which have good seismic data quality, the area in between these wells is characterized by poor quality seismic data not amenable for interpretation and delineation of the reservoir. This poor data zone is understood to be caused by the inadequate illumination of the target horizon and low energy penetration through the monotonous shale in the overburden. Prior to the data acquisition, a survey design and modeling study was performed to select the optimum acquisition technique. A 3D earth model was created using the surface seismic and borehole data. 3D illumination ray tracing was performed at the target horizon using different acquisition geometries such as conventional narrow azimuth, multi azimuth and full azimuth circular shooting. A pilot broadband circular shooting survey was conducted over a subset of the study area based on the parameters derived from the survey design and modeling study. The time imaging processing result shows significant improvement in the seismic image at the target interval as well as the shallow level as compared to the legacy data. The extension of azimuth sampling and broader bandwidth enabled by the slant streamer circular shooting acquisition technique provides a superior image compared to the conventional narrow azimuth, flat streamer legacy data. These advanced acquisition techniques have the potential to offer solutions for other areas with similar imaging challenges in South-East Asia.

No abstract