In Overthrust Settings, Tie, Tie (2-D) Again

Contributors: Rob Vestrum

In the rough terrain of overthrust settings, 2-D seismic data continues to be a standard tool for subsurface mapping – and not only because of economic reasons. Two-D and 3-D seismic surveys are complementary in land environments, because each data type has its own strength and weakness.

Three-D seismic data gives us a three-dimensional image volume of the subsurface, with no out-of-plane energy problems or potential to miss structural details between 2-D profiles. With such limitations in 2-D seismic data, one might argue that a better exploration strategy would be to just shoot 3-D surveys and not bother with 2-D seismic data, which may be getting obsolete.

However, in land seismic acquisition with rough terrain and heavy vegetation, access restrictions make the logistics difficult and expensive to acquire 3-D seismic data with high density. Two-D surveys give us overall higher fold and much higher resolution – and the improved resolution in the shallow section helps us tie surface geology to the subsurface reflectors.

Where 2-D and 3-D data overlap, the 2-D lines can complement the 3-D interpretation with a higher-resolution perspective.

So, for scientific as well as economic reasons, 2-D seismic data will continue to be a mainstay in resource exploration in compressional and transpressional geologic settings.

One of the major pitfalls when interpreting 2-D seismic data is dealing with out-of-plane reflections, especially when trying to tie intersecting lines in structured areas.

Structural geologists and interpretation geophysicists can understand the problem of reflection event correlation across intersecting depth profiles and overcome the difficulty by considering the direction of propagation of seismic energy.

Tying 2-D Profiles in Structure

When processing seismic images in thrust-belt areas, it is rare that we are able to make a perfect tie between intersecting 2-D lines.

It is possible to manage the mistie in the time shifts and wavelet character differences between lines, but when we have dipping reflectors on our seismic data, the reflection energy will be coming from out of the 2-D plane of acquisition, resulting in a mistie in time that a simple static shift cannot repair.

\Figure 1 shows two intersecting depth-migrated lines over a thrusted structure in the foothills of the Andes. The left half of the figure shows the dip line. The dips in the overthrust range between 10 degrees and 30 degrees. The right side if the figure is the intersecting strike line.

Note that there is a reasonably good tie between the two lines below 3.5 kilometers depth, where there are relatively flat layers in the footwall. Above the fault (~3.3 kilometers depth at the intersection), the reflectors on the strike line do not line up with the reflectors on the dip line. The layers in the shallow section are dipping, so the reflectors on the strike line are imaged from out of the 2-D plane.

Since we illuminate the reflectors at angles near the bedding-plane normal, if one wanted to correlate these dipping reflectors, then one would need to align the sections along the bedding-normal direction.

Figure 2 shows the improvement in reflector alignment in the shallow section if we rotate the strike line 10 degrees counter-clockwise about the intersecting point at the surface. In this orientation, the correlation is along a direction normal to bedding on the dip line.

After the rotation (figure 2), the reflector alignment is significantly improved between dip and strike lines in the hanging wall. The footwall reflectors, which are more flat, do not tie as well in figure 2 as with the vertical tie in figure 1,because the normal-to-bedding direction of these layers is near vertical.

Even though the strike line imaged the subsurface reflector outside of its 2-D plane, we still can correlate the two lines by orienting the strike line in the direction normal to bedding.

There will still be challenges in creating a 3-D structure map, but at least one may tie the reflectors between lines to ensure consistent mapping over the entire area of 2-D coverage.


When tying 2-D lines in structure, one must not only consider possible differences in static shifts and the phase of the seismic wavelet between intersecting lines, but we also need to consider possible problems with out-of-plane energy.

In reasonably simple geometries with gentle dips, rotating the seismic section at the surface intersection point may simplify the problem of correlating reflectors between dip and strike lines.

Comments (0)


Geophysical Corner - Ritesh Kumar Sharma

Ritesh Kumar Sharma is with Arcis Seismic Solutions, Calgary, Canada.

Geophysical Corner

Geophysical Corner - Satinder Chopra
Satinder Chopra, award-winning chief geophysicist (reservoir), at Arcis Seismic Solutions, Calgary, Canada, and a past AAPG-SEG Joint Distinguished Lecturer began serving as the editor of the Geophysical Corner column in 2012.

Geophysical Corner - Rob Vestrum

Rob Vestrum is chief geophysicist at Thrust Belt Imaging, Calgary, Alberta, Canada.

Geophysical Corner

The Geophysical Corner is a regular column in the EXPLORER that features geophysical case studies, techniques and application to the petroleum industry.


Image Gallery

See Also: Book

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 21810 Book
Desktop /Portals/0/PackFlashItemImages/WebReady/book-s64-Heavy-oil-and-Oil-sand-Petroleum-Systems.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 4062 Book

See Also: Bulletin Article

In reservoir engineering, hydrodynamic properties can be estimated from downhole electrical data using heuristic models (e.g., Archie and Kozeny-Carman's equations) relating electrical conductivity to porosity and permeability. Although proven to be predictive for many sandstone reservoirs, the models mostly fail when applied to carbonate reservoirs that generally display extremely complex pore network structures.

In this article, we investigate the control of the three-dimensional (3-D) geometry and morphology of the pore network on the electrical and flow properties, comparing core-scale laboratory measurements and 3-D x-ray microtomography image analysis of samples from a Miocene reefal carbonate platform located in Mallorca (Spain).

The results show that micrometer- to centimeter-scale heterogeneities strongly influence the measured macroscopic physical parameters that are then used to evaluate the hydrodynamic properties of the rock, and therefore, existing models might not provide accurate descriptions because these heterogeneities occur at scales smaller than those of the integration volume of the borehole geophysical methods. However, associated with specific data processing, 3-D imagery techniques are a useful and probably unique mean to characterize the rock heterogeneity and, thus, the properties variability.

Desktop /Portals/0/PackFlashItemImages/WebReady/electrical-and-flow-properties-of-highly-heterogeneous-carbonate-rocks.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3251 Bulletin Article

See Also: Energy Policy Blog

As part of Senator Lisa Murkowski’s (R-AK) efforts to craft comprehensive energy legislation, the Senate Energy and Natural Resources Committee recently held a hearing to discuss 26 bills that would expand energy supplies. 
Desktop /Portals/0/PackFlashItemImages/WebReady/senate-energy-and-natural-resources-committee-mullsenergy-supply-legislation-hero.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 20391 Energy Policy Blog

See Also: Workshop

This three-day workshop aims to provide a forum for professionals from industry, academia and government agencies, who are actively involved in the study of Middle Eastern Source Rocks, to share their advances in source rock related fields, present their experiences and challenges, and demonstrate relevant technologies and solutions.

Desktop /Portals/0/PackFlashItemImages/WebReady/gtw2016me-source-rocks-hero.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 16518 Workshop