Introduction to March Focus: Salt and Subsalt Imaging in the Gulf of Mexico

Subsalt imaging is one of the most critical technologies driving exploration to farther and deeper frontier areas in the Gulf of Mexico. For almost every project about subsalt imaging in the Gulf of Mexico, the three most important issues always are:

• How to improve the salt model? In other words, how can we utilize our understanding of the geology and the data to design an appropriate workflow and to produce an accurate salt model timely?
• How to improve the velocity model? In other words, what tomography is best for producing the correct salt and sediment velocity model in a geologically complex area?
• How to attenuate subsalt noise to improve the signal/ noise ratio for a clear subsalt image?

The most popular topics about subsalt imaging – salt modeling, RTM COR tomography and noise removal – are the focus of this special issue of the CSEG RECORDER.

In the first article ("Salt Modeling Challenges and Strategies in the Mississippi Canyon, Gulf of Mexico") Quincy Zhang, Cristina Reta-Tang and Gary Rodriguez present the latest geological understanding of salt tectonics and technology of salt modeling in the Mississippi Canyon area. In this article, we introduce the concept of stacking hourglass-shaped salt, and a conventional salt modeling workflow for this type of salt. Then, we discuss salt-model challenges in some locally complex areas and a local salt scenario workflow to address the challenges. Finally, we present an optimized salt-modelling workflow which is the integration of the conventional salt modelling workflow and the local salt scenario workflow. From many projects, we have seen that with this optimal salt modeling workflow, we can greatly improve the accuracy of the resulting salt model, and can significantly reduce project turnaround time.

The second article ("Enhancing Salt and Subsalt Imaging in Deep Water, Gulf of Mexico, with Common Offset RTM Gathers"), by Jianshun Sheng, Seung Yoo, Cristina Reta-Tang and Gary Rodriguez, is focused on two of the latest processing technologies – Common Offset RTM (COR) gathers for dirty salt and subsalt velocity tomography update and Dual-Directional Image Stack (DDIS) with target image-based sum weighting to improve poorly illuminated subsalt areas. This article represents key results from Jianshun and his team’s pilot project which covers a key exploration area in the Mississippi Canyon, Gulf of Mexico, and utilizes two orthogonal WAZ surveys. As being presented in this article, with the technology of COR and DDIS, this project produces more accurate and significantly improved images without incurring the cost of acquiring new seismic data. The subsalt structures show substantial changes that may lead to new discoveries.

In the third article ("Post-Migration Coherent Noise Removal by Dip Decomposition to Enhance Subsalt Imaging in the Gulf of Mexico") Hongyan Li, Peter Winther, Carlos Espinoza, George Zhao and Neeraj Duhoon present one of the most advanced approaches for subsalt noise attenuation. In the Gulf of Mexico, the key to the success of subsalt imaging is not only related to model building, but also noise attenuation: especially coherent noise from surface multiple, interbed multiple, and converted waves, due to the strong impedance contrast of salt and the complexity of salt geometry. This article represents one of the results of Hongyan Li and her pilot team’s work to address coherent noise in both pre-migration and post-migration. Hongyan published a paper about dip decomposition at SEG in 2016, and now she is working on building a more turbo flow for post-imaging noise attenuation with the help of interpreted horizons and multiple model migration.

I sincerely appreciate the authors and co-authors’ effort to provide such excellent articles. I hope the articles by my friends and colleagues and myself will provide you an insight to the latest technologies of salt and subsalt imaging – one of the most critical challenges in the geophysical industry in Houston.