All modern 3-D interpretation systems and many well path planning systems use 3-D visualization as the base display for the interaction required to conduct the work at hand. However, tools and techniques for “picking” geologic features in the data have advanced very slowly since the advent of interactive interpretation systems in the 1980s.

To meet the demands in today’s industry for interpreting more data in more detail in less time, technologies have been developed to move 3-D interpretation from tools that support viewing the data in 3-D and interpreting it in 2-D to tools that enable data interpretation in 3-D – “True Volume” interpretation.

True Volume interpretation enables direct “all at once” interpretation of complete 3-D surfaces representing not just horizons and faults but also salt bodies, channels, fans, and other geologic surfaces. The result is an interpretation that is much more accurate and complete since all of the data in the 3-D volume representing a given structural or stratigraphic feature are used simultaneously to obtain the result. It also takes substantially less time required than traditional methods. This style of interpretation is supported by a combination of new workflows, new processes, and new interpretation tools to enable the interpretation of complete 3-D surfaces in the data volume.

Advances in structural interpretation

The order in which structural elements are interpreted depends on the geology that is represented in the data and upon the interpreter’s understanding of geological relationships. In all cases, the workflow begins with the identification of the type of geologic feature of interest followed by processing to create an attribute volume that optimizes the imaging and visualization of that type of feature in the volume. The imaging step is followed by a 3-D surface interpretation step that employs technology that can range from 3-D autotracking to direct 3-D surface definition and editing.

The green oval in this time slice from the seismic volume contains an indication of a fluvial channel that intersects the time slice. Most of the channel cannot be seen because it crosses seven fault blocks as it crosses the survey. (Images courtesy of TerraSpark)

Since certain structures can impact the accuracy of interpretation of other features in a volume, the workflow initially focuses on the boundaries of salt bodies (if present), as these truncate faults and horizons. Voxel processing is applied to the seismic volume to isolate the imaging of the salt boundaries. The interpreter quickly creates an initial 3-D surface inside the salt. This initial surface is then “inflated” or expanded interactively until it stops on the imaged salt boundary.

Fault surfaces are imaged primarily as discontinuities within the seismic data. Automated fault extraction (AFE) uses a discontinuity or coherence volume as input and creates a fault probability volume. Faults are imaged in the fault probability volume with sufficient resolution and signal-to-noise level to support automatic interpretation of the fault surfaces. The AFE workflow begins by creating a discontinuity volume (in this case an edge stack volume) from the seismic data. AFE is then used to create a fault probability volume from the edge stack volume. The faults in the resulting volume are imaged with sufficient resolution and signal-to-noise ratio to allow the interpreter to either automatically extract major faults from the volume or selectively autotrack faults from the volume.

The final step in the structural workflow is horizon identification. The end result of structural interpretation is that along with the seismic volume, several attribute volumes and a structural interpretation exist.

Advances in stratigraphic interpretation

The initial interpretation of the structural features in a volume is necessary to enable imaging and interpretation of stratigraphic features and depositional systems in the data. The problem with trying to recognize and interpret depositional systems in the original seismic volume is that the structural effects on the seismic data make the depositional features quite difficult to recognize and interpret. Figure 1 shows a time slice from the seismic volume with a segment of a 0.6-mile (1-km) wide channel apparent on the time slice. Although the channel extends across the entire volume, it is difficult to interpret and put the pieces back together because it extends across seven fault blocks that are shifted and rotated with respect to one another. This can be remedied with domain transformation.

A strata slice from a co-rendered volume of four attributes – instantaneous amplitude, frequency, phase, and edge stack – illuminates the sharp edges of the channels and the variation in the channel fill.

Domain transformation

The concept of domain transformation is, at its heart, quite simple. First, the data are interpreted for structural elements. It is the presence of the structure in the data that has distorted and displaced the original depositional systems. These structural elements are then processed to remove noise. Horizons are trimmed and sealed to faults and salt surfaces, closing the structural interpretation. Next, the distortions caused by the structural elements are removed from the volume, producing a “stratal volume” where every horizontal slice represents a paleo-depositional surface.

Once calculated, the domain transformation can be used to transform any co-located seismic or attribute volume into the stratal domain. To illustrate this, a transform was used to create stratal volumes of seismic amplitude and three attributes – instantaneous amplitude, frequency, and phase. An edge stack process (an attribute in the same class as coherence) was applied to the stratal volume of seismic amplitude to create a stratal volume imaging the boundaries of fluvial channels. Figure 2 is a single slice from four co-rendered stratal volumes, where:

  • Instantaneous amplitude is rendered in blue intensity;
  • Instantaneous frequency is rendered in green intensity;
  • Instantaneous phase is rendered in red intensity; and
  • Edge stack controls the lighting on the surface.

In Figure 2, the edges of fluvial channels are crisply imaged as dark lineaments, and the combination of co-rendered attributes highlights the variability in the channel fill.

This image shows the combined result of interpreting the structure and the depositional systems. The 3-D boundaries of the channels were interpreted in the strata volume and then inverse-transformed back to the structural domain, putting all of the structural effects back in the interpreted channel boundaries so they are properly located in the original seismic data. Inverse-transformed channels are shown between the top and bottom horizons of the five horizons used to redefine the transform.

Interpreting depo-systems

The resultant stratal volume clearly images any depositional systems present in the volume. Channels and fan complexes that were not previously visible in the faulted and dipping seismic volume become immediately obvious and can be easily captured in the stratal volume by the same surface imaging and interpretation processes used to define salt bodies. The interpreted boundaries of these dipping channels are then transformed back into the structural domain, restoring the depositional features to their proper location in the seismic amplitude volume (Figure 3).

Less time, better product

True Volume interpretation delivers fully integrated 3-D seismic visualization and interpretation at every step of the interpretation workflow. Enabling direct 3-D surface interpretation of all structural elements helps geoscientists work better and more efficiently. Through domain transformation, True Volume enables much more rapid and thorough recognition and interpretation of depositional systems and other stratigraphic features. As a result, it substantially reduces the cycle time of interpretation while improving the accuracy and detail of the final interpretation. All of the data volume is used to create complete 3-D surfaces that most accurately represent all of the geology represented in the 3-D seismic volume.

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