Eliminating the Ghost
The surface of the ocean acts like a perfect acoustic mirror. Every signal that is reflected back from the subsurface bounces off the sea-surface, creating echoes, or ghosts, that are visible on the seismic data
GeoStreamer® was designed to mitigate the effect of the ghost reflections coming from the perfectly reflecting sea surface. The ability to remove the ghost accurately allows us to tow the recording sensors deeper, further away from the noise generated by the ocean's surface.
Noise is reduced and signal increases with deeper tow, especially for low frequencies. Improved low-frequency content enhances the quality and accuracy of seismic inversion and reservoir characterization.
By using the complementary properties of two sensors, GeoStreamer enables the removal of the receiver ghost by combining pressure measurements and velocity measurements (shown below).
Pressure and velocity sensor measurements have complementary ghost functions and spectra which are used to de-ghost GeoStreamer data in the field
The measurements from the two sensors can be used to perform wavefield separation into up- and downgoing wavefields. As the figure below illustrates, the receiver ghost or downgoing wavefield carries the imprint of the shape of the reflecting sea surface, which is different for every shot. Accurately removing the time- and space-varying receiver ghost improves repeatability for 4D monitoring.
Shot gathers for pressure and velocity sensors. The lower panels show the separated up- and downgoing pressure wavefields. The downgoing wavefield (bottom right) represents all the receiver ghost reflections and shows the imprint of the reflecting sea surface.
Dual sensor and multi-component streamers benefit from improved overall signal-to-noise ratio (S/N) of the separated wavefields. The particle motion sensor is used except for the very low frequencies, typically below 15-20 Hz. Here the deep-towed high-quality pressure sensor is used to estimate the velocity sensor signal. This can be done with confidence since it is far below the frequency of the first notch of the pressure sensor.
The illustration below shows the pressure ghost function for two different receiver depths, 6 m and 15 m. Changing the depth alters the position of the interfering ghost notches. The figure illustrates that the ghost function attenuates the low frequencies much more at 6m (blue) than at 15 m (orange). Towing deeper enables us to record more low-frequency information.
Pressure ghost for different depths is illustrated above. Increasing the depth of the receivers moves the ghost notches caused by the sea surface reflections to lower frequencies. The 6 m ghost function (blue) attenuates the low frequencies far more than the 15 m ghost function (orange).
GeoStreamer dual-sensor wavefield separation allows
- Recording of high-quality data independent of the sea-state
- Longer operating windows in moderate to marginal weather
- Pre-stack AVO compliant (amplitude and phase preserved over full bandwidth)
- Data richer in low-frequency information due to deeper tow
- Improved subsurface velocity estimation (FWI)
- Improved imaging using separated wavefields (SWIMTM)
- Fully 4D compliant (full backward and forward compatibility)