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.

To overcome the lack of information on the most superficial part of the near surface obtained by the use of the First Arrivals (refracted waves), we have implemented an innovative combined workflow that use the information from the Surface Waves (Rayleigh waves) to complement the first-break measurements. As the Rayleigh waves propagates along the free surface interface, they carry significantly more detailed information of the near-surface characteristics which can be used to better constrain the first-break inversion. The first step of the workflow starts with the surface wave dispersion curve picking. As reliability of the results directly depends on the quality of that picking, data regularization are used to improve picks accuracy on both low and high frequency of the phase velocity/frequency spectra. A surface wave tomography process is then applied to convert the spatially irregular frequency-dependent picks into a regularized (x, y, frequency) Rayleigh wave’s velocity volume. Lastly a laterally constrained depth inversion is performed, delivering a 3D shear wave’s near-surface depth velocity model. The second step of this workflow uses this S-wave velocity model, which contains the near-surface details captured by the Surface-Waves, to constrain the refracted P-wave first-break tomography. A regional scaling, here a 1D VP/VS ratio estimated from knowledge over the area or from fast-track refraction first-break analysis, is required to convert the S-wave velocity into a P-wave model. The constrained tomography aims to scale the trend from the high-resolution S-waves velocity model by fitting it with the trend of the P-wave field derived from the first arrivals. This corresponds in a sense in inverting the VP/VS ratio in such a way to preserve the high-resolution feature scaptured by the surface waves while remaining consistent with P-wave information. The resulting near-surface velocity model looks more geologic, better respects P-wave travel times and can be used with more confidence to compute the primary statics solution than the conventional P-wave field only obtained from the first-arrivals tomography… Furthermore, this accurate update of VP/VS ratio can be used to estimate the Poison’s ratio. It can be used to better plan geotechnical survey in order to reduce shallow drilling hazards.

High fidelity seismic amplitude reconstruction through pre-stack migration is crucial for accurate elastic inversion. Despite a relatively flat geology of the Abu Dhabi region, accurate imaging is required for a stable elastic inversion. This can be challenging because the main reservoir Arab lies underneath the strongly anisotropic overburden of the Nahr Umr formation. In this case study, we show how we effectively addressed this challenge through PSDM. With PSDM imaging, we have overcome the challenges of complex ray paths passing through the strongly anisotropic Nahr Umr layer and the rapid lateral velocity variation in the Mishrif formation. Evidently, the success of PSDM relies strongly on the accuracy of the depth velocity model used. To achieve this we adopt different forms of tomographic inversion, for example, using 3D non-linear slope tomographic inversion, where velocity and anisotropy (Epsilon) models are jointly inverted. Additionally, short wavelength velocity variations caused by the Mishrif interval are resolved through structurally-constrained tomography (SCT). The superiority of PSDM imaging over PSTM in reconstructing AVA compliant seismic amplitudes is demonstrated on an ocean bottom survey from the transition zone offshore Abu Dhabi. Fast-track AVA elastic inversion is used to assess the benefit of PSDM imaging over PSTM. With a more stable Vp/Vs ratio and smaller inversion residual, PSDM imaging demonstrates a greater accuracy in reconstructing the pre-stack seismic amplitude and thus are more appropriate for estimating elastic reservoir properties. The value of PSDM imaging for better understanding of reservoir characteristic has been well demonstrated in this case study from the Abu Dhabi transition zone, thus optimizing the value of the acquired seismic data for asset development.