3-D Structural Interpretation:

Earth, Mind, and Machine

Edited by Bob Krantz, Carol Ormand and Brett Freeman


Three-dimensional geologic interpretation of surface and subsurface data requires integration and application of both geologic knowledge and spatial cognitive skills. Much surface geologic mapping still employs pen and paper techniques, but subsurface interpretation is usually accomplished using sophisticated visualization software. In both cases, successful interpreters use mental models that bridge internal and external forms of 3-D visualization to construct 3-D geologic interpretations. This AAPG Memoir 111 sets out to understand more about the convergence of geology, 3-D thinking, and software, which collectively provide the basis for truly effective interpretation strategies. It should appeal to all geologic interpreters, and especially those who investigate and teach interpretation skills.

  1. Page 1

    Geologists as a group have and use above-average spatial thinking skills to interpret and communicate complex geologic structures. Interpretation challenges, especially with petroleum industry subsurface targets, come from abundant but still ambiguous data volumes, challenging geologic forms, powerful but difficult-to-learn software, and under prepared staff. In June 2013, 70 participants met in Reno to discuss these and related issues and to explore how spatial cognitive science can help us better understand and develop geologic interpretation skills, software tools, and education strategies. Industry interpreters and trainers, academic structural geologists, software developers, and cognitive scientists brought complementary perspectives to three days of presentations, posters, and discussions, plus a field day with interactive interpretation modules. This Hedberg conference provided new shared insights to the interpretation process, ideas for improving skill development, and abundant opportunities for further collaboration.

  2. Page 7

    Characterizing spatial thinking and the development of spatial expertise is essential to understanding how to train geoscientists to succeed in both academia and industry. The Spatial Intelligence and Learning Center has supported an eight-year-long collaborative research program, which brings together disciplinary expertise in cognitive science and geology to characterize and develop spatial thinking in the geological sciences. To facilitate our understanding of science education and practice, we have characterized the spatial skills of geoscience discipline experts and the spatial thinking impediments experienced by students studying the geological sciences. In this chapter we review recent research on measuring and improving spatial thinking skills in the geosciences and on characterizing individual differences in spatial thinking, including the role of gender and age. We conclude with a discussion of important unanswered questions and some directions for future research. The research discussed here may help guide the development of best practices for spatial thinking training in both academic and industry settings.

  3. Page 25

    In teaching structural geology, I have found it useful to design cartoon metaphors intended to quicken the grasp of meaning of terms, processes, and concepts. Structural geology places strong emphasis on deducing three-dimensional (3-D) forms and how they evolve. Visual metaphors ease students from the ordinary and familiar to the more abstract and unfamiliar. Cartoons can convey understanding in seconds. Analysis of the 67 visual metaphors in Structural Geology of Rocks and Regions shows that they are classified into a hierarchy of types, from simple literal metaphors to higher-order kinematic and dynamic metaphors. Most of these occur in chapters that present fundamentals; these chapters contain the least descriptive, most abstract materials. The visual metaphors aid student learning in unfamiliar territory. The basis for how this works is illuminated by theories of certain cognitive scientists, linguists, and psychologists who increasingly emphasize that conceptual metaphors are fundamental modes of thought, not just modes of language. Our minds continuously integrate vital relations from diverse scenarios, and this is what allows us to see connections between the familiar and the unfamiliar. The most powerful metaphors are analogical comparisons of relationships between objects, not simply literal comparisons of attributes of objects. In this study, the kinematic metaphors appear to be the most effective in grounding 3-D visualization. They express incremental movement and change. When kinematics of development of 3-D forms is grasped, it becomes easier to visualize the complete 3-D forms, even when only small bits of the final forms can be observed.

  4. Page 53

    Computer-based learning tools are becoming more prevalent in classrooms from elementary school to higher education. The potential value of interactive learning tools is particularly high in geoscience education. Students can benefit from interactive tools that allow them to explore different processes in one-dimensional (1-D), two-dimensional (2-D), and three-dimensional (3-D) space. Traditionally, geoscience education has relied on laboratory exercises to provide students with the opportunity to explore dimensionality. In this chapter, we introduce Visible Geology, an innovative web-based application designed for geoscience education. Visible Geology enables visualization of geologic structures and processes through the use of interactive 3-D models. As Visible Geology has been designed from a student-centric perspective, it has resulted in a simple and intuitive interface, allowing students to creatively explore concepts. We present a case study of a large first year class at the University of British Columbia, and show the utility of Visible Geology in teaching geoscience concepts of relative dating and cross-cutting relationships. The ease of use of the software for this assignment, including automatic grading, made this tool practical for deployment in classrooms of any size. The outcome of this type of large-scale deployment is that students, who would normally not experience a lab exercise, gain exposure to 3-D thinking. The level of ownership and interactivity inherent in Visible Geology encourages engagement, leading learners to practice visualization and interpretation skills and discover geologic relationships.

  5. Page 65

    It has been well established that spatial thinking is important to success in the sciences, but differences exist in spatial thinking between different science fields. Previously Hegarty et al. (2010) investigated differences in self-reported spatial abilities in a variety of non-scientific and scientific fields, including the geosciences. Geoscientists had the highest self-report ratings for spatial abilities compared to all other disciplines. In the present study, expert structural geologists were evaluated on a battery of paper-and-pencil tests that measure domain-general spatial abilities (i.e., the Perspective Taking/Spatial Orientation Test and the Paper Folding Test), a domain-specific spatial skill (i.e., the Geologic Block Cross-Sectioning Test), and self-reported spatial skill (i.e., the Santa Barbara Sense of Direction Scale). Compared to undergraduate students, expert structural geologists scored significantly higher on tests of cross-sectioning (i.e., spatial reasoning about internal structures based on surface information) and spatial perspective taking (i.e., mental transformation of one’s perspective relative to spatial forms), and rated their environmental spatial ability (i.e., sense of direction) as higher, but they performed no different from undergraduates on a test of spatial visualization (i.e., the Paper Folding Test). Taken together, self-report questionnaires alongside psychometric tests can start to elucidate differences in spatial abilities among scientists and in the spatial thinking required by each field.

  6. Page 75

    Our notion of reality in seismic interpretation and structural geology usually follows a series of careful observations and ideas that eventually crystallize into a best-case model. In most other branches of science the strength or reality of such models, or hypotheses, is increased by the number of robust tests that either refine or fail to disprove the original idea. However, geological models in the hydrocarbon exploration and production sector differ because the starting point for testing a hypothesis is usually an interpretation of seismic data or other remote measurements, rather than the direct observation of an effect.

    The scientific method of prediction tested by observation is a key part of mapping three-dimensional (3-D) structures in the field and geological training. An analogous, rule-based approach also applies to the accurate creation of 3-D subsurface structural models. A defensible structural model must embody more than fault and horizon surfaces. It must also honor the rules of structural geology. Some simple rules are outlined in this chapter. These can be applied iteratively throughout the life of the seismic interpretation. Failure to honor structural rules leads to poor interpretations that may be compounded by a lack of appreciation of the importance of 3-D perspective. In this chapter, we also briefly explore the historical use and understanding of perspective.

    Those in the exploration and production industry need to think carefully about how to leverage the 3-D interpretation and modeling process. Most importantly, since it is managers who control the exploration and production workflow, they above all need to be informed about the advantages of using a structurally qualified 3-D model in future projects.

  7. Page 91

    Accurate three-dimensional (3-D) models of the deformed subsurface are foundational to successful oil and gas exploration and development, and modeling 3-D structural complexity in the subsurface requires specific skills and software. The gold standard in modern structural interpretation is the structural framework: an air-tight network of intersecting fault and horizon surfaces that completely describes the 3-D structural geometry of a given area, field, or prospect. The complete geometry of faulted reservoirs, petroleum traps, basin margins, and other deformed regions are more likely to be accurately captured in a structural framework because their construction workflow promotes sculpting mental models into physical products. Within exploration and production geologic settings, structural frameworks can be built only using volumetric interpretation software and workflows that allow for complete geometrical descriptions using incomplete sets of seismic, well, and other geologic data.

    The structural framework represents the highest possible interpretation achievement and relies on complementary skills in structural geology, spatial thinking, and digital tools. Although challenging to learn, and sometimes time-consuming to complete, there is a strong business case for dedicating resources to construct hi-fidelity frameworks. Technical and economic decisions rely on interpretation accuracy and confidence. Furthermore, nearly all advanced structural analyses, including fault-seal analysis, stress prediction, and fracture modeling require robust 3-D models at their foundation.

    Geologic maps or models that are not built within a 3-D environment commonly contain errors that misrepresent the degree or complexity of geologic deformation, or even the size and nature of an oil or gas prospect. The traditional structure map, and even the 2.5-D workstation workflow for mapping and understanding fault systems, for instance, is outdated and introduces uncertainty to a given interpretation because geometric validity of fault intersections, terminations, and others cannot be visually or graphically determined. In contrast, a 3-D framework provides a representation of structural geometries that is much easier to assess and edit. The examples provided here demonstrate the power and utility of structural frameworks in oil and gas exploration and development, and are testament to their role as the new standard in structural interpretation as the industry explores in increasingly challenging geological settings in the 21st century.

  8. Page 111

    A valid structural geologic interpretation should simultaneously honor available surface and subsurface data (e.g., well and seismic) to constrain structural geometry; ideally be restorable to an original unstrained condition – taking into account the possibility of three-dimensional (3-D) movement, volume loss, or volume gain; and incorporate structural styles known or expected for the mechanical stratigraphy and deformation conditions in the region. Incorporating what is known about the mechanical stratigraphy can provide crucial constraints on viable structural styles, for example, where faults are likely to cut across stratigraphy vs. where fault displacement is likely to be accommodated by alternative mechanisms (e.g., ductile flow or folding). Conversely, the structural style can often help to understand the mechanical stratigraphy, including the recognition of dominant competent or incompetent mechanical stratigraphic units. Using this approach provides the interpreter another set of constraints toward improving interpretations, testing hypotheses, and developing valid structural interpretations.

    Outcrop characterization provides insights into the influence of mechanical stratigraphy and structural position on seismic- and subseismic-scale deformation in the layers. Examples of extensional deformation in Cretaceous carbonate strata in central and west Texas illustrate the utility of considering how mechanical stratigraphy influences the development of different deformation styles, even where deformation conditions are otherwise similar.

  9. Page 121

    The 50 km (31 mi) long Hat Creek fault, located along the western margin of the Modoc Plateau in northern California, is a geometrically complex segmented normal fault that offsets Pleistocene lavas by at least 570 m (1870 ft) of cumulative throw. Three subparallel, ∼NNW-trending sets of scarps (Rim, Intermediate, and Recent) reflect a progressive westward migration of surface rupture locations that offset progressively younger Pleistocene volcanic deposits during a ∼1 Myr fault history. The 50 km (31 mi) long Rim scarp comprises predominantly right-stepping segments with a maximum throw of ∼370 m (1214 ft) in ∼925 ka lavas. The 17.5 km (10.9 mi) long Intermediate scarp occurs 0.4 to 3.5 km (0.2–2.2 mi) west of the Rim, comprising left-stepping segments with a maximum throw of ∼177 m (581 ft). The 30.5 km (19 mi) long Recent scarp occurs several tens of meters west of the bases of older scarps, and is composed of left-stepping segments with a maximum throw of 56 m (184 ft). The northernmost segment of the Recent scarp offsets 53.5 ± 2 ka basaltic lavas, whereas the remaining segments offset 24 ± 6 ka basalt flows that erupted into Hat Creek Valley, indicating a youthful scarp system. Vertical propagation of the fault through young lavas produced fault-trace monoclines with amplitudes of up to 30 m (98 ft). The monoclines are commonly breached along their upper hinges by a vertical, dilational fault scarp. Shaking associated with repeated earthquakes progressively broke down these monoclines, causing disaggregation or partial to complete collapse. Fracture patterns and fault segment geometries and linkages were used to deduce the kinematic and stress history. The oldest segments of the Rim and Intermediate systems suggest initial NE-SW to ENE-WSW extension. Later Rim, Intermediate, and Recent segments responded to E-W extension, consistent with the previously documented stress state of the Cascades backarc. Complexity in Intermediate and Recent fault segments near a small shield volcano (Cinder Butte) suggests spatial variability in the stress field caused by a currently dormant magmatic system. Evidence for recent dextral-oblique kinematics along the Recent scarp, implying a slightly WNW-ESE extension, may reflect the transfer of dextral shear into the system from the Walker Lane Belt in western Nevada. Our interpretations require ∼45° of clockwise rotation of the horizontal principal stresses in the vicinity of the Hat Creek fault over the past ∼1 Myr, implying that significant complexity can develop in segmented normal fault systems over relatively short periods of geologic time.

  10. Page 155

    Modern subsurface exploration uses a three-dimensional (3-D) approach to interpret faults and layer boundaries on seismic reflection data. Three-dimensional seismic reflection datasets comprise a continuous volume of reflection samples with a vertical and horizontal resolution, of typically <15 m (c.50 ft). Increasing computer power has allowed the development of interpretation software that allows for direct mapping within an on-screen 3-D representation of the data. The seismic data volume can be processed to emphasize reflections from continuous layers, or to emphasize the discontinuities (e.g., faults) affecting those layers. A key workflow is to build the interpretation of horizons (layer boundaries) and faults into a consistent framework model, where all the stratigraphic and structural surfaces fit together in a geologically realistic way. Such a model is the necessary starting point for structural geological studies and for assessing trap geometry and viability. The framework model also provides the starting geometry for building an engineering model for simulating fluid flow in hydrocarbon reservoirs.

  11. Page 173

    Sketching, particularly in field settings, is a common but powerful means of communication and visualization in the geosciences. Here, we investigate the range of sketch types and annotations made by expert geoscientists and non-geoscientists during a field trip to the Hat Creek fault zone (northern California) taken during the 2013 AAPG Hedberg Research Conference. Participants (N=42) included geologists and seismic interpreters employed in the oil and gas industry (n=20), geologists employed in academia (n=16), and non-geoscientist software developers and cognitive scientists (n=6). A total of 361 sketches of the normal fault system were collected during stops at three field modules. Sketches were thematically coded by sketch type (e.g., map, perspective landscape view, cross-section, three-dimensional [3-D] block diagram) and annotation type (e.g., fault symbols, reference locations, questions, labels). Overall, two-dimensional (2-D) perspective sketches and maps were the most common representation type, whereas 3-D block diagrams were rare. Statistical analysis of code counts suggests that the choice of sketch and annotation types is largely driven by characteristics of the field trip stop and/or the particular task required. Non-geoscientists more frequently produced perspective sketches from their actual viewpoint, but were less likely to annotate diagrams. As compared to industry peers, academic geoscientists were more likely to create related sets of sketches. Conversely, industry geoscientists were more likely to explain their thinking and provide alternate explanations. This work is a first step in exploring geoscientists’ sketching practices in the field, and may have implications for both undergraduate education and industry training.

  12. Page 191

    Interpretation of faulted reservoirs is hindered by an industry-wide lack of structural specialists, which in turn hinders the development of structurally proficient interpreters. This can have expensive consequences, including poor models of dynamic flow in reservoirs, erroneous calculations of reserves, and difficulties during well drilling. Focused training using paper maps, outcrop visits, and digital models of the same structures helps to introduce and reinforce concepts.

    The first component of the training is to provide participants with a set of two-dimensional seismic lines created from a geological model of a faulted reservoir. Participants must create a structure contour map containing faults that honor simple rules such as conservation of throw at fault intersections, identification of fault tips, consistent sense of offset and vergence along strike, and identification of fault relays. The second component is a visit to the outcrop from which the paper map was derived, providing the opportunity to discuss differences between faults in outcrop and faults as visible on seismic data. The final component provides participants with a digital model of the outcrop, giving them the opportunity to create a geologically valid interpretation that can be used for fault property prediction or reservoir model creation.

    This three-pronged training provides grounding in structural geology and lets interpreters know the rules that their fault framework models should obey. Applying these techniques during interpretation saves time by ensuring that “busts” are caught and fixed before they become institutionalized, and also closes the gap between the geophysicist/seismic interpreter and the geologist/static modeler.

  13. Page 219

    This chapter integrates concepts from cognitive science with disciplinary geoscience practice, to illustrate how different disciplines can collaborate on research and expand what is known in both fields. We consider the practice and goals of structural geology within an observation-prediction framework, adapted from the perception-action framework of Ulric Neisser. In this framework, the geologist has a conceptual model, about which she or he can reason about the world, and that forms the link between predictions and observations. The scientist engages in predictions based on a conceptual model and seeks out observations to confirm or revise this model. This approach is applied to how geoscientists engage in both geometric reasoning (in the subsurface; volumetric thinking) and kinematic reasoning. We then consider how the three principle types of structural geology analyses (geometric, kinematic, and dynamic) and empirical vs. theoretical approaches to solving problems interact with the observation-prediction framework. Finally, we outline how this observation-prediction cycle might be generalized to geoscience education and the practice of other sciences.

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