Imaging the geology subsalt and at the transition between extra-salt and subsalt has been a challenge at Mad Dog even with extensive seismic data coverage, including two WATS surveys and multiple NATS surveys. WATS acquisition and TTI velocity model processing generated major improvements in the image at Mad Dog. One of the observations of a previous TTI project is the presence of a strong orthorhombic anisotropic effect in a salt mini basin above the field. This finding led to the decision to reprocess the Mad Dog data with a tilted orthorhombic (TOR) velocity model. The main objective of this project is to build an orthorhombic velocity model with nine parameters compared to five with the TTI processing. The TOR anisotropic parameters are generated with the latest FWI and tomography techniques and take guidance from the stress field from a geomechanical model. The outcome of the project is very encouraging with results including better constructive imaging in crucial areas of the field, an incremental increase in signal-to-ratio everywhere and increased fault resolution. The TOR velocity model will be used to migrate a future ocean bottom nodes survey to address some of the remaining imaging challenges.
Presentation Date: Wednesday, October 17, 2018
Start Time: 8:30:00 AM
Location: 208A (Anaheim Convention Center)
Presentation Type: Oral
Summary Subsalt imaging at the Kepler field, located in deep water Gulf of Mexico, has proven to be challenging, largely because of suboptimal illumination and complicated ray bending caused by complex overburden salt. It is well understood that building an accurate velocity model is one of the prerequisites to obtain a good subsalt image. Multiazimuth seismic data sets not only help with improving subsalt illumination, they also benefit the velocity model building process by providing more data to constrain the updates. In this paper, we demonstrate the impact of multi-azimuth data and the Kepler salt geometry on subsalt illumination through a 3D ray-tracing and finite difference modeling illumination study. The integration of a top of salt (TOS) anomaly layer into the Kepler salt velocity model significantly improved the subsalt images and reduced base of salt depth uncertainties.
Summary 3D VSP data provides a unique opportunity to improve image resolution and fault definition in the vicinity of a well. However, the processing and imaging of VSP data requires special accommodations for its distinctive acquisition geometry. In this abstract, we demonstrate two key VSP pre-processing steps that greatly impacted the final image from the Mad Dog 3D VSP data, including XYZ vector field reorientation based on 3D elastic finitedifference modelling, and shot-to-shot directional designature using near field hydrophone data. We also discuss how utilizing the multiple energy - in addition to primary - extends our capability to image the shallow overburden. However, small gyroscopes often drift from their original positions, and inclinometers may increase the cost and weight of downhole receivers (Greenhalgh et al., 1995).
Vertical seismic profile (VSP) surveys rely on three-component (3C) geophones to acquire high-resolution data around target reservoirs. These 3C geophones are composed of three independent receivers, mounted orthogonally. When inside the borehole, the orientation of each 3C geophone is unknown. To enhance image stacking power from different receivers, it is necessary to reorient all the 3C VSP receivers to a common coordinate system.
We introduce a VSP coordinate reorientation workflow using elastic finite-difference modeling. The only condition required is an adequate knowledge of the overburden velocity. Since VSPs today are typically acquired to supplement existing surface seismic images, adequate velocity models already exist and this method can almost always be applied effectively. We conduct synthetic tests to demonstrate the robustness of our workflow with a variety of noise levels, velocity errors, and acquisition coverages. We also show a real data example from the deep water Gulf of Mexico (GoM).
Presentation Date: Monday, October 17, 2016
Start Time: 4:10:00 PM
Location: Lobby D/C
Presentation Type: POSTER
Subsalt imaging at the Thunder Horse Field in the Gulf of Mexico is challenging primarily because the salt canopy, overlying roughly 75% of the structure, greatly distorts subsalt illumination and causes imaging and resolution problems. Since the Thunder Horse discovery, advancements in seismic acquisition techniques and imaging technologies have significantly improved subsalt images. The latest successful application is from a tilted transverse isotropy (TTI) reverse time migration (RTM) project combining two wide azimuth towed streamer (WATS) data sets and three narrow azimuth towed streamer (NATS) data sets. The addition of an extra WATS data set and the application of the recent imaging technologies are key contributors to the dramatic structural image improvements with better defined three-way events and a higher signal-to-noise ratio (S/N).
The Thunder Horse Field has been producing since 2008 and is located in the south-central part of the Mississippi Canyon protraction area in the Gulf of Mexico. A large overlying allochthonous salt body causes rapid spatial and temporal changes in illumination and image quality, making interpretation difficult, especially near the steeply dipping three-way closure against the salt stock. During the course of discovery and development, BP has made continuous efforts to better understand and improve Thunder Horse’s subsalt image with new seismic data sets and more advanced imaging technologies (Pfau et al., 2002; Ray et al., 2002, 2005; Gherasim et al., 2012). The latest successful TTI RTM project with two WATS data sets and three NATS data sets is the continuation of this effort to improve Thunder Horse subsalt images.
This project aimed to improve the structural image in poorly illuminated areas and to maximize the usable vertical and horizontal resolution for well targeting and planning. The latest image shows a dramatic improvement over the previous TTI RTM image produced in the 2012 project for three reasons. First, the additional WATS data in the NE-SW direction illuminated some key areas that the NW-SE WATS and three NATS surveys did not. Second, the majority of the NATS traces were migrated rather than just used to infill missing traces in the NW-SE WATS shot gathers, as was done in 2012. Finally, more advanced imaging workflows and technologies were used to address specific problem areas in the data. Shot patch-based angle gather illumination weighting (AGILW) and input data selection technologies, which were applied in this project, effectively attenuate noise while preserving signal. Specular imaging using RTM dip gathers also helped enhance the S/N. We also discovered one of the reasons for frequency loss underneath the salt.
In the processing of marine streamer seismic data, free surface multiples are generally treated as noise and removed before migration. However, valuable information may be hidden in the multiple wavefield that can be beneficial to seismic processing and imaging. To discover its potential, we modify the conventional reverse time migration (RTM) algorithm to utilize both the primary wavefield and the multiple wavefield. We name our migration approach Reverse Time Multiple Migration (RTMM).
In this paper, we demonstrate that RTMM has a wider and more balanced source illumination power compared to conventional RTM. We then illustrate that RTMM can provide a precise water bottom horizon, which is critical for model-based demultiple techniques. We also show that RTMM can produce a complementary image to conventional RTM, where shallow sediment and portions of the salt structure are better imaged. In the end, we demonstrate a cost effective way to separate crosstalk noise from true signal. We divide RTMM output into azimuth and offset dependent sectors and apply a nonlinear stacking algorithm to attenuate crosstalk noise.
Moving from VTI (Vertical Transverse Isotropy) to TTI (Tilted Transverse Isotropy) was arguably the most important factor contributing to the success of the 2009 Mad Dog TTI reprocessing (Bowling et al., 2010). Incorporating data from a previously recorded NATS (narrow-azimuth towed-streamer) survey, oriented 66° from the Puma/Mad Dog WATS (Wide-Azimuth Towed-Streamer) sail direction, provided important additional azimuthal coverage, which allowed for a more robust estimation of the TTI model’s five components (Huang et al., 2008). A secondary benefit of the additional NATS data was the enhanced illumination it provided for both the salt delineation and the subsalt image. However, even with the imaging improvements in the 2009 TTI project, the subsalt image of some poorly illuminated areas remained unclear. Encouraged by the step-change improvement driven by TTI imaging and additional azimuthal coverage, BP initiated a Multi-WATS TTI reprocessing in 2011. In addition to the data sets used in the 2009 reprocessing, additional data sets - including the newly acquired WesternGeco Phase 14 and infill WATS, Vertical Seismic Profiles (VSPs) and new well data - were utilized to improve velocity model building and provide the best possible illumination for the subsalt reservoir. The project was very successful - a better TTI model was achieved and, together with new imaging technologies, the image of the reservoir structure was improved and better positioned. The almost full-azimuth (FAZ) surface data and advanced model building flow also exposed the limitations of TTI modeling. Tests conducted as part of the project suggest that emerging TOR (tilted orthorhombic) modeling may provide an even more accurate approximation of the Mad Dog velocity field.
The impedance contrast of a salt/sediment interface can generate significant wave mode conversions in seismic reflection data. The salt-related converted waves (C-waves) travel at different velocities than pure-mode compressional (P) waves and thus generate artifacts if migrated with P-wave velocities. These artifacts can often interfere with salt and subsalt interpretation. However, C-waves can actually provide additional information about a structure, especially in regions of low illumination; if migrated with the correct velocity, they may actually aid imaging and interpretation. We explore the possibility of correctly positioning high-amplitude C-waves from the base of salt reflection with a dual-leg 3D acoustic modeling method. Additionally, we study the artifact removal through pre-migration adaptive subtraction and post-migration Vector Offset Output (VOO) RTM stacking using Gulf of Mexico (GOM) data. To aid base of salt interpretation, we run dual-flood RTM using C-wave energies as supplements for base of salt imaging.