Ghost-free Imaging: Synchronized Multi-level Source & Variable-depth Streamer Acquisition

Abstract

Variable-depth streamer acquisition is a key solution for broadband marine seismic allowing for a drastic increase in the available frequency bandwidth, in both the low and high ends of the frequency spectrum, from 2.5 Hz to the source ghost notch. During acquisition, the direct and ghost arrivals at the receiver side creates a well-known interference pattern, which includes the receiver ghost notch, where frequency loss at the notch frequency depends directly on the receiver depths. As a consequence of this, a clear strong notch will always occur with a conventional flat-streamer acquisition. On the contrary, varying the receiver depths along a streamer allows recording a wide diversity of receiver ghosts or notches. Additionally, the sea-state noise level decreases as the cable is towed deeper: this technique thus benefits from towing solid streamers at what is generally considered as extreme depths (up to 50 meters), which allows recording superior signals to noise ratio at low frequencies. Rather than using a linear increase in streamer depth with offset (original slant streamer geometry), a custom profile is designed in order to provide the optimum receiver ghost diversity, particularly for shallow events, and can be tuned to provide the maximum possible bandwidth for a given geological setting and water depth.