Utilizing Near-Field Hydrophones for Near-Surface Imaging Over Shallow Salt Structure Offshore Abu Dhabi

Abstract

Abstract The subsurface of offshore Abu Dhabi is characterized by shallow waters followed by a complex near-surface and shallow salt diapirs with associated faulting. We focus on the objective of imaging the near-surface above a salt structure in order to reduce drilling hazards and to assess if the faults reach the surface. Before acquiring a new high-resolution seismic site survey, an analysis of existing data focused on the near-field hydrophones (NFHs) from a recent ocean-bottom node (OBN) survey. Legacy data, which included an old marine streamer survey and a recent OBN survey, were not designed to image the near-surface and could not be used for shallow imaging. NFH data were acquired as part of the OBN survey, chiefly to QC the performance of the airgun source arrays. These data included near-surface reflections, and since the source grid for the OBN survey was dense enough (25 × 50 m), we considered their use for a high-resolution 3D image of the near-surface. One challenge was strong near-field source energy that overwhelmed the recorded seismic signals. We focus on a 100 km2 area located in the overlap zone of two zippers. Shot coverage overlap, combined with off-end shooting geometry, meant that the area was covered four times by the source carpet, which helped improve the signal-to-noise ratio. Aggressive noise attenuation was still required to attenuate the high-energy near-field source direct arrival in the active NFHs, which could only be used for the very shallow imaging. For the deeper image, we relied on the passive NFH recordings. Since NFHs are zero-offset (the hydrophones are located just above the guns), the velocity field had to be built using available wells. In the end, we obtained a high-resolution 3D image that met our objectives and helped confirm that the faults were not reaching the surface. This allowed us to forego the planned high-resolution site survey. Since the overlapping sources were shot four months apart, we also assessed the repeatability of these NFH surveys. The resulting 3D high-resolution images were better than those from 2D high-resolution site surveys and at a fraction of the cost since no new acquisition was needed.