Transition Zone 3D Seismic Surveys; Technical Perceptions to Overcome Seismic Processing Challenges

Abstract

Abstract Transition zones (TZ) are described as dynamic and complex environment especially in the Arabian Gulf region for seismic operations. Different source and receiver combinations are required to acquire the data on the changing topography of land, tidal flats and islands, shallow water, channeled and marine open water environments. This in turn gives rise to varying seismic signal propagation and significant near surface velocity changes which must be considered carefully in the processing. Due to large areas of very shallow water, less than 2m, and with the help of small tidal variation of up to 2.5m, it was necessary to utilize a modified small airgun array and vibrators at the same shot point location and also to record into the same hydrophone and geophone spread to match the signal from different sources. In this project, 3D TZ wide-azimuth seismic survey was acquired utilizing different source and receiver combinations, 3 types of receivers: 1) OBC dual sensors, 2) marsh phones and, 3) geophones alone. On the source side, we deployed three different airgun arrays (1120 cu.in, 800 cu.in and 320 cu.in) for marine part of which each array represents a particular water depth range whilst single source vibrator with extended frequency sweep for the land part. Each source was designed to accommodate specific requirements in Sabkha, coastal shorelines, shallow water and deeper marine environments. Of particular interest, due to the large tidal flats exposed at low tide and covered by sufficient water at high tide, it was coincidentally possible to shoot both airgun shots and vibrator sweeps at the same spread location. This occasion, so called co-located shot, is the first event for 3D shallow water seismic acquisition in Abu Dhabi, and hence it provided us unique opportunity to assess definitive matching of phase and amplitude between airgun and vibrator energy at the coincident locations. It also delivered us occasions to create superior bandwidth data which is useful to the understanding of differing source energy characteristics for the application of correct data matching processes. Several iterative schemes of noise reduction were required pre- and post-stack including; 3D data adaptive noise modeling and subtraction techniques; frequency median filtering; random noise attenuation; cross-spread processing, and 3D slant post-stack noise attenuation. Strong amplitude variability between the different sourcereceiver combinations was addressed primarily via gross scalars matching in addition to several iterations of surface consistent deconvolution and amplitude gain corrections. This eventually facilitated seamless amplitude merge between the different data types followed by 3D deconvolution to attenuate shallow water reverberations. This, with resilient post stack processing has shown the way to produce significantly improved seismic data for interpretation with good level of confidence.