A large‐offset 2‐D seismic survey for imaging imbricate structures in thrust belts

Abstract

Summary Turkish Petroleum Corp. conducted a multichannel largeoffset 2-D seismic survey near the town of Ergani, Southeast Turkey, in October, 2004. The objective is to image the complex, imbricate target structures in the Southeast Thrust Belt. The data were acquired using a common-spread recording geometry whereby the receiver spread was fixed for all shots. A total of 960 receiver groups was placed along a 23,975-m line traverse in the NNW-SSE dominant structural dip direction at a 25-m interval. A total of 145 shots was fired at a 250-m interval along the line traverse, beginning at a location outside the spread and 6 km away from the first receiver group in the SSE end of the line. The distance between the first and last shot locations is 35,975 m. Land seismic data acquisition with conventional spread length (3,000 m) and conventional processing in midpointoffset coordinates may fail to image complex imbricate structures associated with overthrust tectonics. Irregular topography associated with a rugged terrain, complexity of the near-surface that includes high-velocity layers and outcrops with significant lateral velocity variations, complexity of the overburden caused by allocthonous rocks, and the complexity of the target imbricate structures themselves, all pose challenges to exploration in thrust belts. The shot-domain analysis of the data from the largeoffset Ergani seismic survey based on common-spread recording geometry, on the other hand, has indeed unraveled the imbricate structures that can lead to significant discoveries in the Southeast Thrust Belt. We analyzed the Ergani large-offset data for earth modeling and imaging in depth. By a nonlinear first-arrival traveltime tomography, a velocity-depth model was estimated for the near-surface. Then, a subsurface velocitydepth model was estimated based on rms velocities derived from prestack time migration of shot gathers. Finally, prestack depth migration of shot gathers from a floating datum that is a close representation of the topography was performed to generate the subsurface image in depth.