Abstract
Seismic surveys are generally designed to image deep reservoirs, which leaves the near-surface woefully under-sampled. This is particularly a challenge offshore Abu Dhabi, where a complex near-surface – with karstic collapses and meandering channels – contaminates the seismic image with strong footprints. To mitigate these effects, we use near-field hydrophone data, primarily designed to QC the airgun source, for near-surface imaging.
Near-field hydrophones (NFH) are positioned about a meter above each airgun and are designed to record the source near-field pressure. They immediately capture dysfunctional or out-of-spec guns, which alerts the recording crew. Yet, in a shallow water environment, they unintentionally record seismic reflections from the near-surface, which we will use for seismic imaging.
Streamer vessels usually use two source arrays, 50 meters apart, which shoot in a flip-flop mode. The active NFH refer to the recordings directly above the shooting guns, while the passive NFH refer to the recordings from the array that is not shooting. Because the passive NFH are less contaminated by the source near-field, they are typically the preferred choice for near-surface imaging.
Waters are too shallow in offshore Abu Dhabi to use streamer vessels. Instead, seismic surveys involve ocean-bottom cables (OBC) or nodes (OBN) and smaller airgun arrays. The shooting vessels can be single-source or dual-source. While a single source vessel has only active NFH, a dual source vessel has both active and passive NFH. However, even if a dual-source vessel is used, the 50 m distance between the shooting source array and the passive NFH is too large to capture the water-bottom reflection for water-depths shallower than 25 m. For these reasons, we propose to combine both measurements, using active NFH for the very shallow section and passive NFH for the deeper section.
We have applied this technique to a recent node survey acquired offshore Abu Dhabi. By combining the active and passive NFH, a very high-resolution shallow image was obtained, which allows the interpretation of geological layers just below the water bottom. Comparisons with high resolution 2D site survey images show good agreement. Given the NFH do not require any additional acquisition and are delivered as a byproduct of standard seismic surveys, we have demonstrated that proper use of NFH can provide high quality images for pre-site survey interpretation, which reduces the need for additional – and expensive – geotechnical surveys.
This is the first published use of combined active and passive NFH in Abu Dhabi shallow waters for the purpose of imaging. The resolution of the shallow formation images allows detailed interpretation not achievable using conventional seismic data. In the long term, this technique may reduce the need for additional site survey acquisitions.
