A Step Change in Seismic Imaging - Using a Unique Ghost Free Source and Receiver System

Summary Until the invention of the dual-sensor streamer in 2007, towed streamer seismic acquisition and processing projects where hampered by the imposed sea-surface ghost reflections. There was always a trade off in terms of towing depth and frequency content as they are inherently linked to each other. Deeper tow meant more low frequency signal at the expense of the high frequencies. Since 2007 the seismic industry has taken on the challenge of delivering broadband signal – uncompromised by the sea-surface ghost reflections. This paper describes the unique ghost free acquisition system based on the dual-sensor streamer combined with a ghost free time and depth distributed source. The new system allows for true removal of the surface ghosts, leading to an acquisition platform that can be towed deeper for increased low frequency content and increased signal to noise ratio (S/N) without compromising the high frequency S/N. Source and receiver ghosts Because towed marine streamer seismic always operates beneath the sea-surface, any seismic wavefield traveling in the water column will see the water surface acting as a reflection surface with reflection coefficient close to -1. This means that when the source emits its primary energy, this pulse is also reflected up to the sea surface and an almost identical source wavefield trails its original one with opposite polarity but later in time, depending on the source depth. This is illustrated in Figure 1 where the direct source wavefield is annotated with (1) and the source ghost reflection is annotated with (2). This similar physical behavior will also happen on the receiver side, such that the two source events now become four. In an ideal world of seismic – we would want to eliminate both the sea-surface mirror effect on the source- and on the receiver- side.