July 13-14, 2005
CSM Student Center

Workshop on Subsalt Imaging Problems:
Practical Problems Seeking Solutions

Index of Abstracts

KEYNOTE ADDRESS ~ Subsalt Imaging: Is the Salt Winning?

The standard approach to constructing a velocity model for sub-salt imaging is a top-down process, “flooding” the model with salt velocity beneath the tops of salt and “flooding” with sediment velocities beneath salt bases. Careful application of this approach may include iterations for overhangs as well as inter-salt and sub-salt velocity scans and updates.

However the degree of success of this approach, as measured by the quality of the sub-salt image, depends on the geometry of the salt and the acquisition as well as the geometry inherent in the migration method itself. We will present several sub-salt imaging examples, some of which are amenable to construction of the velocity model by the standard approach and some which are not.

Sub-salt imaging and interpretation suffer from the presence of salt-generated shadow zones, particularly beneath salt noses; steeply dipping base salt structures and where the base salt is too discordant with reflector dip. Therefore, it is legitimate to raise the following question: Where has the signal gone, and what could help to get it back?

Reasons for poor subsalt imaging may include inappropriate acquisition parameters, errors in salt geometry definition and imaging algorithm limitations. Another potential source of such imaging problems may be ascribed to geologic factors. In the present example from the deepwater GOM, subsalt imaging ranges from superb to poor over a short distance. Causes of this variation are currently not well understood.

Common Problems in Subsalt Imaging

While we have witnessed rapid improvement in subsalt imaging in recent years, major challenges remain. The following is a sample list: 1) interference from subsalt multiples; 2) lack of a serve-all migration algorithm; 3) variable salt velocities; 4) inadequate illumination; 5) long interpretation cycle time in difficult area; and 6) need for integrated regional interpretation. A brief discussion of each item follows.

Interference from subsalt multiple
Although multiples are ubiquitous in all seismic environments, the problem becomes more prevalent in the subsalt setting where: a) signal/noise is usually low, and b) strong impedance contrast at water bottom, top of salt and the base of salt are ideal (geologic settings) multiple generators.

Traditional demultiple schemes dealing either with periodicity or differential moveout become ineffective in subsalt settings due to two main reasons: a) complex ray path make periodicity harder to predict, and b) due to the salt focusing effect, subsalt reflections, if present, usually reside only in the near offsets making radon type of demultiple schemes ineffective.

Surface Related Multiple Elimination (SRME) is gaining increasing popularity as a viable demultiple tool. Caution remains, however, in utilizing SRME especially in the
2-D case in which ill-constrained adaptive subtraction, a necessary step for SRME, can do more harm than good by removing useful (and often scarcely preserved) signal in the subsalt region. Other well-known impediments to the effective usage of 3D SRME, other than vast computational cost, includes lack of near offsets, inadequate crossline aperture and sparse crossline shot and receiver sampling. There are documented studies in new acquisitions design with specifically 3D SRME in mind. One can only wait to see if the increased acquisition cost will be justified by the imaging results.

Lack of a serve-all migration algorithm
A variety of wave equation based migration (WEM) algorithms has provided viable alternatives to the ever popular Kirchhoff algorithm. WEM, however, has its own shortcomings such as failing to image steep dips and lack of high frequency. Difficult subsalt areas usually need more than one method to provide different looks. Even so, many times the picture is far from clear. While there were many incremental improvements made in the past years, some sort of algorithmic breakthrough is needed to bring subsalt imaging into focus. Is beam the answer? Are we far from the ultimate reverse-time?

Variable salt velocities and complicated internal structures
The usual assumption that salt bodies are one homogeneous constituent with a constant velocity has greatly simplified the process of model building. It is not uncommon, however, to find that a large salt mass consists of one or more smaller salt bodies sutured together exhibiting reflections at the suture boundaries. There are places where salt and sediment mixed together caused a “dirty salt” complication. It has been postulated as well that depth and orientation related salt velocity variations exist. Can tomography, which has been successful in mapping out velocities in the “out of the salt” region, be somehow extended to “inside the salt” area?

Inadequate illumination
When the subsalt fails to image and everything else has been tried, poor illumination is usually to blame. This is particularly true in places where rugous top of salt and/or base of salt are present or in narrow sediment basins. There are some industry examples of multi-azimuth acquisition, and efforts to re-acquire data with different configurations hoping to gain better illumination. Not much is known about the effectiveness of these recent efforts at this time.

Long interpretation cycle time in difficult area
While significant improvement in shortening the computational cycle time has been gained in recent years by boosting up the hardware and tuning up the software, the time needed to come up with a plausible model interpretation in difficult areas remains painfully long. With all the geologic and salt tectonic knowledge, it is still difficult, if not impossible, to interpret the invisible. Interpretation software providers have improved software efficiency greatly by taking the tedious chores such as interpolation and gridding away from interpreters who can then concentrate their effort in things that machine cannot easily accomplish such as incorporating well data, correlating reservoir intervals, and investigating trap and seal integrity among many others. Perhaps they can also try to model the invisible by testing different geologic concepts and geometric configurations. To do this, they need in their desktop some kind of quick turnaround tool that would produce a fresh image to validate or invalidate the new concept in a matter of days, if not hours.

Need for integrated regional interpretation
Understanding sand distribution, source timing, depth of burial, timing of trap formation and salt tectonics on a regional basis are all critical to the development of good prospects and drilling commercially successful wells. Exploration in subsalt trends is vastly complicated by the difficulties of unraveling the complex interaction of sediment deposition and salt movement. It is tougher still when subsalt imaging is poor, which seems to be the case in parts of most depth imaging projects.

Concluding remarks
While the theme of this workshop is to point out problems and to provide research direction, timing is of the essence. Historically, academic institutions, for one reason or another (lack of funding, data, interests, directions…), have not been in the forefront of developing useful tools in helping the oil industry solve difficult imaging problems in a timely fashion. The burden of advancing imaging technologies, which used to fall on the shoulders of major oil companies, is now disproportionately absorbed, willingly or unwillingly, by the service companies, who have their own share of resource and survival problems. Considering the price of oil today and the supply-demand equation in the near future, the timing is right to intensify the exploration of the extensive salt provinces of the oil-proven Gulf of Mexico and other similar areas in the world. The industry, however, cannot afford to wait five or ten years for answers. Technology, which is not brought to use in a timely fashion, will quickly become obsolete. With that note, we hope that this workshop will provide some useful guideline/direction for academia to come up with timely solutions.

Subsalt imaging is a challenge to the Oil Industry throughout the value chain. In exploration, complications due to salt (1) obscure the accurate definition of a trap, (2) blur the image making detailed stratigraphic and fault interpretation difficult, and (3) complicates our ability to make a geologic risk estimate. In preparing to drill, salt creates another set of issues. For example, (1) what is the pressure going to be as you exit salt? (2) will you encounter inclusions as you drill through salt? In development/production another set of issues are created by salt. Can you describe the details of the reservoir? Can you map all the potential baffles to the reservoir container? Can seismic attributes be used under salt to map reservoir and hydrocarbons? Can 4D be used on subsalt to more efficiently produce a discovery. Through cartoons and seismic displays, this paper will illustrate some of the issues industry faces when trying to explore and develop hydrocarbons under salt.

Located in the southern half of the Alaminos Canyon area, the Perdido Foldbelt consists of a series of NE-SW trending folds that extend south into Mexican waters and north beneath the salt canopy. Several recent discoveries along the Perdido Foldbelt have increased the exploration activities in the area. The remaining potential now lies in the subsalt portion of the trend.

The quality of subsalt images in the foldbelt ranges from fair in areas near the Sigsbee escarpment, where salt geometry is simple, to very poor in areas with complex and overlapping salt bodies. Complex salt bodies not only distort and disperse seismic energy, but they also increase the complexity of multiples. This problem also poses challenges in the salt interpretation for depth migration. In certain areas, the base of the salt is invisible even after the application of the latest imaging technology. Lack of illumination due to steep dips, complex salt/sediment interface, and abnormal pressure beneath the salt are potential culprits.

Great strides have been made in subsalt depth imaging including 3D tomography, 3D multiple attenuation and significant improvements in imaging algorithms. Nevertheless,

there are still many challenges. For example, exploration oil and gas traps have become more difficult to identify as we move from east to west across the Gulf of Mexico salt canopy system. Higher thermal heat flow and lower recent sedimentation rates cause greater salt mobility. This leads to more complicated coalescing salt bodies with deep keels, multiple salt sutures and higher rugosity on the top and base of salt.

The seismic data were migrated with Kirchhoff into common offset volumes and with CRAM into common reflection angle volumes. The WEM image was available only as a full stack. Key features of the formation geometry are that the underside of the salt overhang dips at around 45 degrees, while the sediments dip at shallow angles not exceeding 10 degrees.

All of the full stack subsalt images are unsatisfactory and typically include strong noises roughly parallel to the salt underside. These noises are thought to contain significant contributions from reverberations within the salt mass.

The Kirchhoff images show some selectivity in signal/noise with respect to offset, but are inferior to the CRAM images.

The CRAM images show strong dependence of signal/noise on the reflection angle. Under the salt the small reflection angle (less than 15 degrees) volumes contain much noise and cannot image reflectors at shallow dips because of the operation of Snell’s law at the lower salt boundary. The large reflection angle (30-45 degrees) volumes have good signal/noise down to about 8000 m depth and image the reflectors through reflected rays that enter the lower salt boundary at close to normal incidence.

Interpretation was greatly assisted by having several common reflection angle volumes towork with at the same time.

This paper presents sub-salt imaging problems that are a challenge for interpreters to resolve in order to help build velocity models that have sufficient integrity to improve the image and ultimately provide a realistic if not always reliable structural picture. The challenges are presented first, followed by issues that need to be addressed in the velocity modeling process.

Over the last decade, technology advances have significantly improved subsalt imaging capability. However, even with all the advances in imaging technology, there are many areas where obtaining an image subsalt is still very challenging and frequently not successful. An observation from numerous depth imaging projects is that salt geometry plays a significant role in determining the quality of images obtained of subsalt.

The technology being applied for prestack depth imaging has been evolving rapidly. Not too long ago, the geophysical interpreter could expect to have only a few volumes to work with on a depth imaging project. These volumes consisted primarily of a sediment flood, salt flood, and final migration. Now recent projects involve more than 40 volumes for the interpreter to both analyze and manage. These volumes include multiple floods for overhangs, offset/angle stack volumes, and the usage of two or more algorithms at many stages of the process. Having more images available is a double-edged sword that should lead to an improved answer but may also lead to more questions.

This presentation will compare and contrast the imaging of two subsalt prospects typical of the Gulf of Mexico. The first prospect is in an area yielding good subsalt reflectively and therefore a high confidence subsalt structure map. The second prospect is in a more difficult area giving more ambiguous subsalt results. It is clear by looking at even the time migrations (or raw field data) of these examples that all subsalt problems are not created equally. We will look into some potential factors causing the difference in image quality between the two examples. Some of the factors to be discussed are: multiple content of the data, demultiple algorithms, signal strength of subsalt primaries versus multiples, accuracy of the velocity model, relative complexity of the velocity model (salt geometries in particular), migration algorithms and acquisition geometries versus salt geometries.

Critical to sub-salt imaging is the accurate delineation of the salt and the sediments above it. With the large velocity contrast salt has with its neighboring sediments and the often presence of steep flanks, imaging around salt is sensitive to the velocity field. Building an accurate velocity model is hampered not just by the poor signal-to-noise ratio that results from the scattering of the wave energy at the salt-sediment interface and within the salt, but also by the unpredictable nature of the velocity variations in such environments that may not be resolved by the information present in surface seismic data. Moreover, with the wide range of dips that are often encountered, presence of multipathing of the wavefield, and large velocity contrasts, no single imaging algorithm may suffice in resolving all relevant features.

Here we focus on three problems related to the delineation of the salt body that are often encountered by the interpreter: 1) poor base salt reflections arising from poor illumination and a poor signal-to-noise ratio that may be encountered even if the velocity field is accurately known, 2) poor velocity definition in-and-around salt, which can cause poor imaging even though the target area may be well illuminated, and 3) the choice of the imaging algorithm in building the velocity field around salt and in defining the salt boundary in situations where the velocity field is accurately known and the acquisition geometry well illuminates the target.