Reverse Time Migration

Generating a large-scale structural image beneath simple salt bodies has essentially become a solved problem through a combination of wide-azimuth data, such as Wide Azimuth Towed Streamer (WATS) or Ocean Bottom Nodes (OBN), and the use of industry standard techniques like Reverse Time Migration (RTM). For example, we can use RTM to image the standard Sigsbee2A synthetic dataset, provided by the SMAART JV, with excellent results.

 

RTM-sum_agc-vel

Sigsbee RTM Image overlain on the Sigsbee2A velocity model. Courtesy of the SMAART JV

Waveseis offers a comprehensive RTM-based imaging solution.

Revolutionary Imaging Technology

Complexities in the salt significantly degrade the image when using standard techniques like RTM. A simple modification to the Sigsbee2A dataset demonstrates this, as below.

The Sigsbee-2a model modified with a high frequency perturbation to the top of the salt and a number of added sedimentary inclusions within the salt model. Courtesy of the SMAART JV

The Sigsbee-2a model modified with a high frequency perturbation to the top of the salt and a number of added sedimentary inclusions within the salt model. Courtesy of the SMAART JV

The update to the Sigsbee-2a model highlights the high frequency perturbation to the top of the salt and a number of added sedimentary inclusions within the salt model

The update to the Sigsbee-2a model highlights the high frequency perturbation to the top of the salt and a number of added sedimentary inclusions within the salt model

Waveseis has developed our Revolutionary Imaging Technology (RIT) to address these more problematic cases. RIT provides the highest available image fidelity even under these challenging conditions.

RTM image on the sub-salt portion of the model, using the data from the modified Sigsbee model

RTM image on the sub-salt portion of the model, using the data from the modified Sigsbee model

RIT image on the sub-salt portion of the model, using the data from the modified Sigsbee model.

RIT image on the sub-salt portion of the model, using the data from the modified Sigsbee model.

Anisotropic Imaging

Proper modeling of the way in which the speed of seismic energy propagation varies with the orientation of travel is essential to provide the best quality of results.

In the isotropic regime, we assume that propagation speed is independent of orientation. This is computationally the least demanding model. However, the Earth is anisotropic, and it is often necessary to take this into account to achieve the highest quality of results.

In simple settings, the assumption of transverse isotropy with a vertical axis of symmetry (VTI) is often used to help align the seismic image with the correct depth in the Earth, which improves the tie between the seismic interpretation and well control. VTI is also important to help align seismic events between extra-salt events, which can be affected by the anisotropic nature of the sediments, and sub-salt events that have travelled through the relatively isotropic salt.

Today transverse isotropy with a tilted axis of symmetry (TTI) is a widely used approximation for seismic wave-propagation. Making a TTI approximation can be important whenever large dips are involved. Failing to take TTI into account in these cases can result in significant movement of the flanks of large synclinal structures, which could have a large impact on oil-in-place calculations or well placements on those flanks. Another area in which TTI has a large impact is salt flank imaging and hence salt model building: failing to accurately account for TTI will result in the incorrect positioning of the salt flanks and hence an inaccurate salt model and a poor sub-salt image.

BP_tti_2007_rtm_agc

This is a TTI RTM image based on the BP 2007 TTI benchmark dataset created by Hemang Shah and is provided courtesy of BP Exploration Operation Company Limited (“BP”)

Waveseis offers all our products with isotropic, VTI or TTI two-way wave propagation to best meet your requirements.

Angle Gathers

Angle gathers are important for updating velocity models as well as for generating Amplitude Verses Angle (AVA) products that can be used to help distinguish fluid properties and hence reduce the risk of exploration prospects. Waveseis has developed RIT based image gathers with significantly higher signal-to-noise ratio that traditional RTM based gathers, providing the possibility of more reliable velocity model building and AVA analysis.

Angle gathers from the sub-salt portion of the Sigsbee2a model. Data and model courtesy of the SMAART JV

Angle gathers from the sub-salt portion of the Sigsbee2a model. Data and model courtesy of the SMAART JV