Many of us in our middle-school life had the opportunity to read George Orwell’s dystopian novel “Nineteen Eighty-Four” (“1984”). The political novel placed good against evil and contained futuristic societal concepts that in 1949 had no link to future realities. Looking back at concepts and terms introduced in “1984” like Big Brother, “thoughtcrime” and “2+2=5,” we would have never guessed the occurrences that actually exist today.

The book has many parallels to subsalt imaging and our scientific challenges with “salt” being the Big Brother of seismic. In the early days of 2-D acquisition and processing, salt was the bane of many a geophysicist’s existence. The high reflectivity of the sediment-salt boundaries attenuated or dispersed acoustic energy to the point where nothing below was imaged. Over time the industry gained knowledge and experience with imaging simple salt structures like diapirs. Their symmetry and the general concept of imaging steeply dipping flanks along the sediment-salt boundary was generally a step change in the industry.

As technology advanced in data acquisition, improved processing algorithms and accelerated computer technology, individually and together the result was continuous improvement in imaging complex (allochthonous) salt, including overhangs, welds and subsalt “varietals.” In essence, the industry continued its role as the Winston Smith of “1984” to face the challenges presented by the salt and rebel against its position to defeat its efforts for the greater good.

Today for many geophysicists the unimaginable has become reality, akin to Orwell’s writing “1984” in 1949. In 2015 the ability to image complex allochthonous salt has never been better, and the Big Brother (salt) of yesterday has been defeated. Most interpreters with 30-plus years’ experience would probably attest that they have come a long way from interpreting analogue 2-D data with limited offsets and imaging processes that were largely manual. Today the rate of change of technology advances represents significant step changes in subsalt illumination.

Technology to the rescue
The primary factors in maximizing subsalt illumination results are twofold: a) acquiring optimal data and b) applying the latest technology advancements. Acquiring optimal datasets has primarily been driven by acquisition technology. Longer solid-state streamers, larger streamer arrays and simultaneous sources all permit new datasets that contain larger offsets and multiple azimuths.

Data acquisition configurations like multiwide azimuth or full azimuth now provide a step change in the volume and quality of input data. Combined with imaging technology, vast improvements in allochthonous salt province subsalt illumination have been realized.

So what leads to the next step change in data acquisition? This can be driven by the more critical question—how do we illuminate below the complexity of allochthonous salt overlying autochthonous salt (two prevalent salt layers) or high-velocity carbonate layers like the Norphlet play?

If we look to the Gulf of Mexico (GoM) as an analogue, considerable drilling success has emerged in subsalt provinces for Lower Miocene reservoirs under the allochthonous salt. More recently, exploration has moved to deeper Jurassic targets (Norphlet) that represent the next stage of exploration. These and other reservoirs generally exist below high-velocity carbonate layers and unexplored potential below the autochthonous salt. The latest initiatives benefit from the ability to forward-model complex scenarios and establish built-forpurpose solutions. Figure 1 shows examples of forward modeling to drive optimal acquisition configurations.

Given the complex ray paths initiated along the salt sediment interfaces, the value of acquiring data from all azimuths becomes obvious. However, forward modeling gives companies the ability to optimize configurations to maximize their understanding. With the application of continuous recording, simultaneous source recording, the large tow capacities of vessels and an endless degree of configurations, forward modeling allows new programs to be designed and optimized to solve the salt (Big Brother), which is only limited by costs and associated risks.

FIGURE 1. Figure 1a (top) shows a synthetic seismic and amplitude map (forward model), and 1b (bottom) is a 2-D finite difference modeling of varying streaming cable lengths showing uplift to 16 km (10 miles). (Source: TGS)

Subsalt imaging
With data acquisition accomplished, the second stage is to use these data to create accurate subsurface images. During the past 20 years the advancements in imaging algorithms combined with compute power have led to a number of significant step changes in the imaging world. For subsalt imaging the largest step change has been the commercial application of reverse time migration (RTM). Figure 2 shows a comparison of a Kirchhoff migration to a typical RTM.

FIGURE 2. Figure 2a (top) is a Kirchhoff migration from the Sophies Resolve survey, and 2b (bottom) is an RTM migration of the same data volume showing subsalt image uplift. (Source: TGS)

Primary observations are the improvement in subsalt imaging in the RTM example (not to mention the run time improvement using RTM algorithms). Although as a general rule RTM delivers lower temporal (frequency) resolution, the improved structural interpretation is the primary driver.

Imaging below the autochthonous salt still remains challenging. What are the latest tools to solve the challenges? The components to the solution have not changed: quality input data, signal processing algorithms and raw computational power. Optimization comes through data-driven solutions. With today’s knowledge of salt configurations from existing data combined with known well data, the ability to leverage this information should drive optimal solutions.

One of the most promising technologies in recent years is the application of least squares RTM, which incorporates both the forward model and data-driven approaches to improved imaging. Figure 3 shows an example from the GoM comparing an original RTM to a least squares RTM. The significant image uplift from increased spatial (lateral) and temporal (frequency) resolution represents a breakthrough for identifying new reserves and reducing exploration risk.

The results speak for themselves. The challenge now becomes commercializing this application. Even with application through GPUs, run times for such advanced applications are often in the order of six to 10 times that of a conventional RTM.

FIGURE 3. Figure 3a (top) is a conventional RTM of the Sophies Resolve survey, while 3b (bottom) is a least squares RTM of Sophies Resolve. (Source: TGS)

Additional uplift
Additional incremental imaging changes in subsalt illumination continue to come from advances in velocity analysis and salt sediment interface predictions along with the number of iterations applied to the data. Applications such as image-guided tomography and dip image scans cascaded sequentially provide additional uplift to final images. Even with today’s technology
it still remains that complex imaging is data-driven. However, distinguishing signal from noise (signal processing) still remains an art that requires a skilled operator to optimize results.

The good news is that the industry continues to challenge Big Brother, and today much that was thought not possible has become a reality. Explorationists continue to advance their understandings and evolve their technologies to deliver improved subsalt images. The bad news is that step changes become harder to define and progress. Commitment to defeat Big Brother remains a priority for subsalt explorers as they search for deeper exploration targets and greater understanding of basin-forming architecture. The excitement over what the future holds for subsalt prospectivity around the globe remains immense.

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