Print this pageAAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Ocean Bottom Seismic in the Arabic Gulf; We Need to Adapt the Acquisition Parameters to the Local Environment
(1) Research, Petroleum Institute, Abu Dhabi, United Arab Emirates.
(2) Geosciences, Texas A&M University, College Station, TX.
A summary of our analysis and modelling results from a study of a 2D 4-Component (4C) Ocean Bottom Seismic (OBS) data set acquired in the shallow-water environment typical of the Arabian Gulf is presented. Our study illustrates both the challenges and opportunities of application of 4C OBS in such environment.
The 2D 4C OBS data set was acquired with receiver spacing 25 meter in the Arabian Gulf in water depth of about 10 m and a hard bottom with P-wave velocity varying from 3 to 4.8 km/s. Because of the shallow water, the hard bottom, and relatively long seismic wavelengths, the problem of energy partition and P-S wave conversion at the water/rock interface may not be addressed adequately using classical plane wave theory. We use numerical full waveform elastic modelling to understand the influence of shallow-water wave interactions between the air/water/rock interfaces on 4C seismic data. Comparative analysis of field records, logs and synthetic data is then used to investigate and assess the quality of existing 4C OBS data and their potential.
The preliminary results of this comparison are:
- The quality of multi-component data is dictated by the geological conditions and follows the source physics and sediment physics. The 4C data could be quite reliable; i.e. the instrument response is basically good.
- The shallow water environment of the offshore U.A.E is unique and very different from other major offshore fields such as the N. Sea and the GOM. This results in strong P-S wave conversion at the water/sediment interface. It also results in an efficient equivalent shear wave source, i.e. this gives a better way to extract shear-wave information from 4C data which again has important implication for waveform-based seismic processing and inversion.
- Signal/Noise ratio seems low. This is partly due to inadequate acquisition design (aliasing), but also due to the inherent complexity of multi-component physics.
- C-wave at cap/reservoir interface is strong which is a good indication for reservoir description; i.e. bypassed hydrocarbons, permeability heterogeneity and resolution.
In sum we believe that 4C ocean bottom seismic is promising for the offshore U.A.E. fields, but the acquisition parameters need to be adjusted for the special environment. This implies smaller shot/receiver spacing and longer time delay between shots, which obviously will have implications on the acquisition cost.
Finally; more acquisition tests should be done.