Seismic experts have turned their attention to quantitatively distinguishing rock from fluid effects using seismic information acquired on the seabed
Obtaining geophysical information on the seafloor is far from new to StatoilHydro.
Indeed, several years have elapsed since we caused a paradigm shift in the offshore seismic industry by pioneering the acquisition of seabed seismic data (SUMIC).
It was first successfully employed to ‘see through’ gas clouds obscuring details of the Tommeliten field in the North Sea.
Thereafter, StatoilHydro researchers have used seabed seismic data to distinguish quantitatively between seismic effects generated by the rocks and those caused by the fluids (hydrocarbons and water).
Many of the results have been taken into operational use.
The method
Conventional seismic surveys are done by towing sensors on streamers behind a survey vessel.
The hydrophones record compressional sound wave information (pressure or P-waves) as they are bounced back from rock interfaces.
Principle of seabed seismic acquisition.
Unfortunately, P-waves are not only influenced by rock types but also by porosity and fluids, which makes it difficult to discriminate between them.
However, by deploying geophones in addition to hydrophones on the seabed, it is possible to acquire information on shear waves as well (S-waves).
Unlike their P-wave counterparts, S-waves are almost insensitive to a rock’s fluid content, whether hydrocarbons or water.
The success factor lies in the integration of P- and S-wave data. In this way, geophysicists can vastly improve their ability to distinguish between rock and fluid property effects.
This makes a significant, predictive contribution to decreasing exploration risk and improving geophysical reservoir imaging and monitoring.
