Planning, Acquisition, and Processing of a 3D Circular-Shooting Seismic Survey

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper IPTC 13363, ’Planning, Acquisition, and Processing of the First Industrial 3D Circular-Shooting Seismic Survey,’ by M. Buia, E. Tozzi, and L. Mapelli, Eni E&P, prepared for the 2009 International Petroleum Technology Conference, Doha, Qatar, 7-9 December. Conventional offshore 3D acquisition is performed mainly with narrow-azimuth-streamer configurations, even in structurally complex areas. A full 3D circular-shooting seismic survey (coil) over the Tulip discovery in Indonesia during August and September 2008 acquired a full-azimuth data set with a single-vessel towed-streamer operation by sailing in circles. The circular geometry introduced several differences and a number of new challenges in the modeling, acquisition, and processing workflow. Introduction Tulip is a complex structure in the Bukat Block offshore Kalimantan, Indonesia. The water depth ranges from 350 to 1800 m. Several unfavorable geological conditions cause a very poor seismic response in the area. Methane hydrates represent the main problem. The hydrates layer in this area generates a very reflective sea bottom that causes up to seven multiple bounces. The presence of shallow free gas is suggested by an abrupt frequency-amplitude dimming below this bottom-simulating reflector. The well drilled on the structure indicates a seismic-quality factor of less than 70. Moreover, the rough geomorphology of the water bottom, characterized by canyons and irregularities, causes a nonhomogeneous behavior of the illumination in the subsurface and a complex 3D ray path for surface and internal multiples. Complex deep geology (steeply dipping thrust with two culminations) represents a third critical factor, posing severe structural-illumination doubts. Also, the low reflectivity of the target sequence often falls below the noise level. Therefore, vintage seismic in the area is of very poor quality. Circular shooting is an acquisition technique in which the vessel follows a circular preplot line. Circles are repeated in the x- and y-directions to increase fold, offset, and azimuth distribution. This method allows for full-azimuth acquisition by use of a single vessel shooting on a continuous turn. Line changes are short, on the order of minutes, resulting in high acquisition uptime and efficiency. The offset and azimuth distribution of circular shooting is excellent, as good as and sometimes better than the wide multiazimuth techniques.