Super-sparse marine 3D: A game changer for seismic exploration

TotalEnergies SE and partners Shell and PetroSA recently completed the acquisition and processing of a large (10,000 km2) ultra-sparse (200 m between streamers) marine seismic acquisition survey off the west coast of South Africa in block 5/6/7 using a large PGS Titan Class Ramform vessel. The sparse design enabled fast acquisition and low survey cost and health, safety, and environment exposure. Advances in sparse processing enabled high-quality final seismic data consistent with the exploration objectives. In addition, DUG optimized the 4D regularization/interpolation parameters to approach the near offsets differently than the offsets with more complete coverage to help several processing steps. The survey was designed to be upgradable to a higher-resolution survey if needed via the addition of a custom regular infill pattern, either in its entirety or over targeted areas.

Attenuation of long period multiple using F-k filter and surface-related multiple elimination methods on 2D broadband seismic data from Morowali Waters, Sulawesi

Wide range frequency bandwidth on seismic data is a necessity due to its close relation to resolution and depth of target. High-frequency seismic waves provide high-resolution imaging that defines thin bed layers in shallow sediment, while low-frequency seismic waves can penetrate into deeper target depth. As a result of broadband seismic technology, its wide range of frequency bandwidth is a suitable geophysical exploration method in the oil and gas industry. A major obstacle that is frequently found in marine seismic data acquisition is the existence of multiples. Short period multiple and reverberation are commonly attenuated by the predictive deconvolution method on prestack data. Advanced methods are needed to suppress long period multiple in marine seismic data. The 2D broadband marine seismic data from deep Morowali Waters, Sulawesi, contains both short and long period multiples. The predictive deconvolution, which is applied to the processing sequences, successfully eliminates short period multiple on prestack data. The combination of F-k filter and Surface Related Multiple Elimination (SRME) methods are successful in attenuating long period multiple of the 2D broadband marine seismic data. The Prestack Time Migration section shows fine resolution of seismic images.

A dictionary learning method for seismic data compression

For a marine seismic survey, the recorded and processed data size can reach several terabytes. Storing seismic data sets is costly and transferring them between storage devices can be challenging. Dictionary learning has been shown to provide representations with a high level of sparsity. This method stores the shape of the redundant events once, and represents each occurrence of these events with a single sparse coefficient. Therefore, an efficient dictionary learning based compression workflow, which is specifically designed for seismic data, is developed here. This compression method differs from conventional compression methods in three respects: 1) the transform domain is not predefined but data-driven; 2) the redundancy in seismic data is fully exploited by learning small-sized dictionaries from local windows of the seismic shot gathers; 3) two modes are proposed depending on the geophysical application. Based on a test seismic data set, we demonstrate superior performance of the proposed workflow in terms of compression ratio for a wide range of signal-to-residual ratios, compared to standard seismic data methods, such as the zfp software or algorithms from the Seismic Unix package. Using a more realistic data set of marine seismic acquisition, we evaluate the capability of the proposed workflow to preserve the seismic signal for different applications. For applications such as near-real time transmission and long-term data storage, we observe insignificant signal leakage on a 2D line stack when the dictionary learning method reaches a compression ratio of 24.85. For other applications such as visual QC of shot gathers, our method preserves the visual aspect of the data even when a compression ratio of 95 is reached.

Calculating Multiple Scattering from a Time-Varying, Undulating Sea Surface Using a Full-Wavefield Multistage Algorithm

The sea surface interface between ocean and air is time varying and can be spatially rough as a result of wind, tides and currents; the shape of this interface changes over time considering the influence of wind, tides, etc. As a result, waves impinging on the sea surface are continuously scattered. In the case of marine seismic, the multiple scattered waves propagate downward into the underwater formation and result in complex seismic responses. To understand the structure of the responses, we propose a multistage algorithm for computing the scattered waves at the sea surface. Specifically, we first extrapolate the upgoing incident waves stepwise using the thin-slab approximation from the scattering theory based on the De Wolf approximation of the Lippmann–Schwinger equation. Then, we implement the air-water boundary condition at the sea surface. Finally, we use the irregular boundary processing technique to compute the time-varying undulating sea-surface scattered waves from different scattering stages. To overcome the angular limitation of the original parabolic approximation, we introduce a multi-directional parabolic approximation based on computational electromagnetics. Numerical tests show that the multistage algorithm presented here can accurately calculate the sea surface scattered waves and should be useful in investigating the structure of marine seismic responses.

Spatio-temporal evolution of the Christiana-Santorini-Kolumbo volcanic field, Aegean Sea

The Christiana-Santorini-Kolumbo volcanic field (CSKVF) in the Aegean Sea is one of the most active volcano-tectonic lineaments in Europe. Santorini has been an iconic site in volcanology and archaeology since the 19th century, and the onshore volcanic products of Santorini are one of the best-studied volcanic sequences worldwide. However, little is known about the chronology of volcanic activity of the adjacent submarine Kolumbo volcano, and even less is known about the Christiana volcanic island. In this study, we exploit a dense array of high-resolution marine seismic reflection profiles to link the marine stratigraphy to onshore volcanic sequences and present the first consistent chronological framework for the CSKVF, enabling a detailed reconstruction of the evolution of the volcanic rift system in time and space. We identify four main phases of volcanic activity, which initiated in the Pliocene with the formation of the Christiana volcano (phase 1). The formation of the current southwest-northeast–trending rift system (phase 2) was associated with the evolution of two distinct volcanic centers, the newly discovered Poseidon center and the early Kolumbo volcano. Phase 3 saw a period of widespread volcanic activity throughout the entire rift. The ongoing phase 4 is confined to the Santorini caldera and Kolumbo volcano. Our study highlights the fundamental tectonic control on magma emplacement and shows that the CSKVF evolved from a volcanic field with local centers that matured only recently to form the vast Santorini edifice.