Extending the resolution of conventional deep seismic 3D surveys

Sammanfattning: The main objective of this master's thesis is to evaluate a new processing method for extracting seismic conventional 3D streamer deep target data for shallow subsurface imaging. Deep target acquisitions have greater potential than it's conventional use. The aim of these acquisitions is to discover natural resources, in particular oil and gas, at several kilometres of depth. Before drilling it is most important to map possible geohazards, especially shallow ones. Also government requirements, risks for human lives and facility failure oblige actions to map geohazards. A method on how to reprocess deep target data acquired in shallow water has been developed for improving imaging of the shallow subsurface. Shallow water in this case is about 75 meters, while shallow subsurface refer to down to approximately 750 meters. The new reprocessing sequence is based on performing interpolation, multiple reflection attenuation and statics correction with focus on the shallow subsurface. The main objective of the evaluation is to compare resulting image from the reprocessed 3D data with the initial image and an overlapping dedicated high resolution 2D acquisition image. The conclusion also includes a recommendation for if the high definition reprocessing sequence can aid a dedicated high resolution 2D survey, from the perspective of this case. First a synthetic data model was made to investigate minimum detectable layers. Within the model a stratied earth model was set up and model parameters varied: dominant frequency of source wavelet, target layer thickness and velocity. The outcome became the foundation for investigating the frequency content reflected in the shallow layers. Frequency content is important for the question if thin layers could be resolved. This was the basis for the next step to analyse the imaging result from the investigated reprocessing sequence. The new processing sequence contributes to a significant improvement to the resolution of the high denition shallow 3D processed image evaluated in this report, from close to the seabed and beyond the horizon of this report. The frequency content seems to be sufficient to be able to resolve layers for aiding interpretations of the shallow subsurface. One comparison between the imaging from the new processing sequence and imaging of dedicated high resolution 2D acquisition show that the 2D acquired data generates a better result from the seabed to about 200 to 300 milliseconds. Somewhere in that range of 200 to 300 milliseconds the imaging from the new processing sequence becomes the better alternative. Thus to reprocess deep target seismic 3D streamer data can, for this case, aid dedicated high resolution 2D acquisition both in setting up the survey as well as discover geohazards.