oa Ocean Bottom Seismic in the Arabic Gulf; We Need to Adapt the Acquisition Parameters to the Local Environment

A summary of our analysis and modelling results from a study of a 2D 4-Component (4C) Ocean<br>Bottom Seismic (OBS) data set acquired in the shallow-water environment typical of the Arabian Gulf is<br>presented. Our study illustrates both the challenges and opportunities of application of 4C OBS in such environment.<br>The 2D 4C OBS data set was acquired with receiver spacing 25 meter in the Arabian Gulf in water<br>depth of about 10 m and a hard bottom with P-wave velocity varying from 3 to 4.8 km/s. Because of<br>the shallow water, the hard bottom, and relatively long seismic wavelengths, the problem of energy<br>partition and P-S wave conversion at the water/rock interface may not be addressed adequately using<br>classical plane wave theory. We use numerical full waveform elastic modelling to understand the<br>influence of shallow-water wave interactions between the air/water/rock interfaces on 4C seismic data.<br>Comparative analysis of field records, logs and synthetic data is then used to investigate and assess<br>the quality of existing 4C OBS data and their potential.<br>The preliminary results of this comparison are:<br>- The quality of multi-component data is dictated by the geological conditions and follows the source<br>physics and sediment physics. The 4C data could be quite reliable; i.e. the instrument response is<br>basically good.<br>- The shallow water environment of the offshore U.A.E is unique and very different from other major<br>offshore fields such as the N. Sea and the GOM. This results in strong P-S wave conversion at the<br>water/sediment interface. It also results in an efficient equivalent shear wave source, i.e. this gives a<br>better way to extract shear-wave information from 4C data which again has important implication for<br>waveform-based seismic processing and inversion.<br>- Signal/Noise ratio seems low. This is partly due to inadequate acquisition design (aliasing), but also<br>due to the inherent complexity of multi-component physics.<br>- C-wave at cap/reservoir interface is strong which is a good indication for reservoir description; i.e.<br>bypassed hydrocarbons, permeability heterogeneity and resolution.<br>In sum we believe that 4C ocean bottom seismic is promising for the offshore U.A.E. fields, but the<br>acquisition parameters need to be adjusted for the special environment. This implies smaller<br>shot/receiver spacing and longer time delay between shots, which obviously will have implications on<br>the acquisition cost.<br>Finally; more acquisition tests should be done.