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Abstract This paper describes the refraction statics processing based on tomographic inversion of a 503km2 subset of a modern marine 3D seismic dataset acquired in very shallow waters offshore Qatar. The objective of the seismic survey was imaging of the Mesozoic interval from 0.4 - 1.8 seconds two way travel time below sea level. Characteristic for the study area is the presence of local shoal bodies, often associated with coral reefs at sea bottom and in the near surface below sea bottom. These features can have a significant effect on the imaging of seismic data and therefore the prospectivity assessment of the exploration area as their typically high velocity introduces distortions in the timing of events, i.e. false structures might be generated or true structures suppressed. Compensating for the reef structures in the statics model results in a more accurate image of the subsurface. This was achieved by applying first-arrival travel time tomography to obtain the shallow velocity information needed to calculate refraction statics corrections. Refraction statics tomography uses the first break travel time picks of the seismic data to derive a velocity model of the near surface. This velocity model is then used to generate static shifts to correct the data to a final datum plane using a known replacement velocity, thereby removing the velocity variation caused by the sea bottom and near surface features. The tomographic inversion algorithm for land data was adapted to marine data by including a new option to freeze the water column velocity, which should be constant and not taken into account in the velocity updates. Refraction statics tomography is superior to conventional refraction statics because the inverted velocity model reveals the lateral and vertical velocity variations in the near surface. The dense shot and receiver spacing of this data set provided a large number of first break picks for the tomographic inversion process and resulted in a stable near surface velocity model. The computed static shifts corrected for some of the time shifts observed below the sea bottom features. The application of refraction statics tomography in this study provided an improved subsurface image compared to the original processing. Introduction The study area (Figure 1) is located offshore Qatar on the western flank of the Qatar Arch, a very prominent almost N-S oriented paleohigh. The morphology of the area is in general very flat without any pronounced structures or tectonic elements. Although the depositional environment was fairly stable throughout geological time, significant velocity variations in the overburden occur, either being related to facies/reservoir quality variation within the dominating carbonate rocks or as a result of subsurface features. Characteristic for the study area is the presence of local shoal bodies, often associated with coral reefs at sea bottom and in the near surface below sea bottom. These features can have a significant effect on the imaging of seismic data and therefore the prospectivity assessment of the exploration area as their typically high velocity introduce distortions in the timing of events, i.e. false structures might be generated or true structures suppressed. Compensating for the reef structures in the statics model results in a more accurate seismic image of the subsurface. The objective of this study was to improve the seismic data through the application of refraction statics tomography processing. |
