Summary With a shallow anhydrite layer, strong multiples and converted wave contamination, Southern Oman represents an outstanding challenge for land velocity model building and imaging. While acoustic land full-waveform inversion (FWI) has proved successful on new broadband datasets in Northern Oman, no successful application has been reported for Southern Oman. We show here that the challenge of acoustic FWI in South Oman can be overcome using a dedicated workflow combining Multi-Wave Inversion (MWI) and multi-Dimensional Optimal Transport FWI (multiD OT-FWI). The key component of the workflow is the very near surface characterization provided by surface wave dispersion curves, which allows delineation of the Rus layer in the initial FWI model. MultiD OT-FWI is then used to mitigate amplitude issues in the presence of short period multiples and reduce cycle skipping beyond the depth of penetration of diving waves.

Summary While there are very few applications of land Full Waveform Inversion (FWI) compared to marine, modern on-shore wide azimuth, dense, broadband acquisition designs offer an outstanding opportunity for FWI application. Several successful examples of acoustic land FWI application to such surveys have been published. The first applications showed the potential of the approach as an early stage velocity model building tool. Later publications demonstrated that acoustic FWI can be used as an efficient model building tool for high resolution velocity models (similar in quality to marine FWI) by investing more effort in dedicated pre-processing and using a two stage workflow based on data selection (Sedova et al., 2017). We illustrate a new case study based on this workflow. We also demonstrate the impact of optimal transport (OT) FWI on the resulting velocity model, which aims to mitigate cycle skipping. We demonstrate how it corrects several localized failures with conventional FWI and constructs a geologically consistent velocity model.

Shallow stratigraphy in Southern Oman is characterized by the presence of an anhydrite layer (RUS formation) causing a strong velocity inversion which makes seismic imaging particularly difficult. This known shallow sharp velocity inversion cannot be easily captured with methods relying on reflection or diving wave energy. We propose here to use multi-wave inversion using first breaks and dispersion curves of surface waves to provide near-surface high resolution velocity models in the shallow range depths (0-400m). The success of Multi-Wave Inversion strongly depends on the reliability of the surface wave velocity picking, which could be much more challenging compared to the conventional first break picking. Heavy preconditioning is often the solution to increase dispersion curves quality and to obtain a narrower velocity corridor. To improve reliability, we use K-means clustering, an unsupervised machine learning method in order to filter out the outliers as well as to define geologically dependent corridors. The unsupervised machine learning clustering helps to define more stable dispersion curves picking corridors for different areas, in order to extract better quality surface wave dispersion curves's, especially at low frequencies where their quality is low. The multi-wave inversion, fed with the optimized phase velocity picks, captures the shallow velocity inversion, which is impossible to recover with either first break tomography only or diving wave full waveform inversion only. The combination of two recently developed technologies allows us to characterize accurately the near surface for the first time in the South of Oman. The velocity inversion caused by the RUS formation is well captured and the velocity trend of the updated model follows correctly the checkshot trend down to 500m, confirming the reliability of the dispersion curves picks at a very low frequency. By incorporating this shallow inversion layer into the velocity model, the resulting seismic image is significantly improved and more interpretable. Geological features such as faults appear clearly and seismic layering in the tilted blocks is significantly improved with the multi-wave Inversion machine learning-guided workflow.

Summary In recent years there has been tremendous progress made on the seismic migration and velocity model-updating technology to achieve better resolution and higher positioning accuracy, thus providing more valuable information to reduce exploration risk. In a dataset from offshore North Africa, the velocity model building is challenging due to a highly faulted geological setting and the presence of gas hydrates in the shallow overburden. Furthermore the image resolution beneath these diffracting and absorptive bodies is poor if we do not compensate for the resulting attenuation. This case study illustrates how a combination of sophisticated velocity update technologies, such as full waveform inversion and the latest tomography developments, together with Q-prestack depth migration, can achieve high quality seismic imaging.

The carbonates in the pre-salt area of the Santos basin off-shore Brazil are good candidates for reservoirs of hydrocarbons. The presence of highly reflective stratified salt in this basin, combined with the focusing of energy due to the concave shape of these reflectors, causes relatively high amplitude inter-bed multiples to interfere with pre-salt reflectors. This multiple energy hampers the imaging and interpretation of these targets. We show that inter-bed multiple attenuation can be used to successfully remove the interference due to such multiples, thus improving the imaging of the pre-salt targets and facilitating improved interpretation. In this case, using the water-bottom as the only inter-bed multiple-generating reflector turned out to be sufficient to attenuate most of the multiple energy. The attenuation of the multiples was performed in the migrated domain.

The oil and gas exploration boom in offshore Brazil has driven the deployment of new acquisition strategies and processing technologies. The Campos basin has proven to be a challenging area for seismic imaging, because of its complex geology and deep pre-salt targets. By providing an improved low frequency response and a broader usable bandwidth, seismic data recorded using variable depth streamer acquisition appears to be an appropriate and promising solution for oil and gas exploration offshore Brazil. New acquisition technology can require optimisation of the processing workflow. We expose the need to take care with standard processing steps such as de-signature and demonstrate, comparing real images from streamer and variable depth streamer data, how new acquisition technology combined with advanced processing techniques is beneficial to pre-salt imaging in the Campos basin.

This paper presents a collaborative PETROBRAS-CGGVeritas project for optimizing the seismic image of Santos Basin pre-salt structures. For this study, 200 km2 of the Lula field (ex Tupi) was chosen as the pilot area. The target reservoir lies below 2000 m of water, then 1000 m of sediments and 2000 m of salt. Because the Top Of Salt presents a highly variable topology, we investigate how it’s geometry influences the illumination at the Base Of Salt level and the consequences on the pre-salt reservoir seismic image. The modeling of three narrow azimuth acquisitions showed highly spatially variable illumination at the base of salt level. By combining two existing datasets in a single processing sequence, we take advantage of having different azimuths of acquisition. Key processing methodologies were identified, such as cold water statics correction for compensating variations in the water column, high-definition tomography for building TTI velocity model optimized for both datasets and post-imaging data conditioning for improving the signal-to-noise ratio of the final stack image. This “bi-azimuth” project provides a comprehensive assessment of the potential benefits of multi-azimuth data that will improve pre-salt carbonate reservoir imaging.

Salt intrusions are known as being the main cause of velocity contrasts in Northern Germany. These salt accumulations from the Zechstein formation often mask reflections from the under laying target zones at circa 4.5 km depth. In addition, significant velocity contrasts may occur in the Cretaceous series. For this reason PreSDM has gained popularity over PreSTM in this area, thanks to its ability of better dealing with significant velocity contrasts in the overburden. Nevertheless, even using advanced migration algorithms a reliable interpretation of target areas is not always possible. The usual offset range used for velocity model building (VMB) and migration is generally limited to 3 km for land acquisitions, which does not allow to fully explain ray path distortions and which makes anisotropy estimation difficult. Furthermore, the nominal fold is usually low and the data is therefore generally very noisy. Specific acquisition schemes have been designed to record longer offsets data up to 17 km in order to successfully penetrate the high velocity discontinuities or to undershoot them altogether. One challenge that arises with such long offset data is how to correctly process and integrate them into regular offset data before migration. Illumination studies are frequently used to assess the potential of long offset data and their possible contribution to the final image, but such studies are limited by the quality of the available initial depth model. In our paper we use two different long offset datasets from the northern part of Germany to discuss the challenges of integrating them into the standard PreSDM methodology, using both Kirchhoff and Wave Equation imaging techniques. We will demonstrate the benefit of including LO data in achieving a much improved image quality.

B026 Application of Pre-stack Wave Equation Depth Migration in North West Europe Introduction 1 Travel-time based PreSDM methods are known to only image only part of the wavefront in the presence of high velocity layers or complex/rugose structure. For such environments in the Gulf of Mexico and more recently offshore Angola wave equation PreSDM has proven to be beneficial yielding significantly improved imaging. The technique has also been applied in the North Sea and North West Europe. We present and discuss case studies from the latter comparing and contrasting our observations with results from standard 3D Kirchhoff PreSDM. Method The

In recent years the search for hydrocarbon reservoirs has moved into deeper and more complex environments. In the Gulf of Mexico this has meant exploring below salt sheets, which distort the seismic raypaths and thus effectively mask the underlying geology. In northwest Europe subsalt plays face similar challenges and in the North Atlantic/Faroes imaging below basalt is a major issue.