1
Donald S. Stone, 2013, Using freehand three-dimensional drawings to clarify and
verify subsurface structural interpretations, in C. Knight and J. Cuzella, eds.,
Application of structural methods to Rocky Mountain hydrocarbon exploration
and development: AAPG Studies in Geology 65, p. 1–13.
Using Freehand Three-dimensional
Drawings to Clarify and Verify Subsurface
Structural Interpretations
Donald S. Stone
Independent Consultant, 6178 S. Lakeview St., Littleton, Colorado, 80120, U.S.A. (e-mail: don@dsstone.com)
ABSTRACT
The generation of one or more three-dimensional (3-D), freehand drawings, based on integrated analysis of a two-dimensional (2-D) geologic database (e.g., borehole data, seismic
profiles, surface geology, etc.), is proposed here as a rewarding exercise in the development of
a final interpretation of subsurface geologic structures. A freehand 3-D drawing based on integration of 2-D interpretive structural contour maps (of at least two horizons) and structural
cross sections can clarify and verify the 3-D details of complex subsurface geologic structures,
check on the internal consistency of the interpretation, uncover untenable, interpretive, geologic configurations, and highlight possible obscure trap geometries. In some cases freehand
3-D drawings can aid in the visualization of impenetrable 3-D images produced by computer software programs. Isometric projection or linear perspective drawings are generally the
most useful kinds of 3-D renditions, but strict adherence to these disciplines is not a requirement in the generation of an initial 3-D sketch.
Generating a 3-D image using computer software is dominantly the functional domain of
the left hemisphere of the brain (left brain), whereas the generation of freehand 3-D drawings
is dominantly the functional domain of the right brain and requires penetrative visualization in
the conversion of 2-D data to 3-D imagery. The right brain excels in intuitive, creative, imaginative structural interpretation. Examples of freehand 3-D drawings of complex subsurface
and surface geologic structures, both self-generated and from literature, are presented along
with some auxiliary 3-D analog modeling methods.
INTRODUCTION
In analysis of complex subsurface structures that may
have potential as exploration targets, the ultimate
objective is a realistic and internally consistent three-­
dimensional (3-D) interpretation. The time-honored
methods of constructing structural contour maps and
structural cross sections based on the integration of surface geology, borehole control, and two-dimensional
(2-D) seismic data have served the oil and gas industry well and still constitute a fundamental exploration methodology. Three-dimensional seismic data
Copyright ©2013 by The American Association of Petroleum Geologists.
DOI:10.1306/13381686St653577
1
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2 Donald S. Stone
volumes are not usually available in new exploration
areas as the acquisition of 3-D seismic surveys is generally not an economically justifiable option in these
areas until an initial integrated interpretation of the
available one- and 2-D database has been completed.
Existing 2-D seismic information can usually be acquired through brokers at reasonable cost, and new,
critically located 2-D seismic profiles can augment
these other data.
Interpreters can benefit from the creation of one
or more freehand drawings that illustrate and clarify
the 3-D aspects of their structural interpretation and
check on the internal consistency and plausibility of
the final product. This exercise obviously requires
some skill in penetrative visualization based on the
integration of the various 2-D illustrations mentioned
above. While good evidence exists that anyone can
learn to draw well with some instruction and dedication (Thiel, 1973, or Edwards, 1999), spatial visualization skills are apparently not so easily acquired.
While Ormand et al. (2010) found that “in general,
students’ skills improve only slightly over one term,
in both introductory and advanced classes,” Kastens
et al. (2009) report that “recent studies show that performance on abstract and applied spatial tasks can be
enhanced through instruction and practice.”
KINDS OF 3-D DRAWINGS
For the geologic conditions and limitations referred
to in this chapter, 3-D isometric projections (Figure 1A)
or linear perspective (Figure 1B) drawings are the most
appropriate categories. Engineers, architects, and designers, on the other hand, generally demand mathematical precision in their 3-D renditions and may use
orthographic, axonometric (dimetric, trimetric, isometric), or two- and three-point perspective drawings.
But the detail associated with these techniques is not
generally required in the kind of drawings highlighted
here. (Details and tutorials for these 3-D drawing techniques can be found on the Internet.) I advocate experimenting with simple freehand sketches without
conscious geometric restriction as a good way to start.
I have generally followed this approach, although
most of my 3-D drawings fall under the general isometric category.
The freehand drawings in Figure 1 illustrate isometric and perspective drawings used in geology.
In isometric drawings all sets of lines theoretically
remain parallel, whereas in perspective drawings
A DICHOTOMOUS BRAIN
After Roger Sperry’s Nobel-Prize-winning (1981) research on the natural division of the cerebral cortex of
the brain into right and left hemispheres with different
functional attributes, psychologists, neuroscientists,
and cognitive scientists have continued to investigate
this intriguing and consequential discovery. Simply
stated, the left hemisphere (or left brain) reasons sequentially, excelling in specific details, logical analysis,
linguistics, and vertical integration, the kind of functionality routinely called upon in computer software
manipulation. Contrastingly, the right hemisphere
(right brain) excels at holistic reasoning, pattern recognition, emotional and nonverbal expression, and
dominates imaginative and creative thinking. Overall, however, “the normal functioning of the mind involves cross-talk between the two sides of the brain”
(Siegel, 1999, p. 178). What is important here is that the
right brain plays the primary role in imaginative and
creative thinking, dominates penetrative visualization, and is therefore the primary generative source of
new interpretive concepts.
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Figure 1. Types of three-dimensional drawings.
(A) Isometric; high-angle and low-angle (“parallelogram
blocks”). (B) Perspective, with one, two, and three
vanishing points. (Modified from LeRoy and Crain, 1949).
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Using Freehand Three-dimensional Drawings 3
there is at least one theoretical vanishing point for
horizontal lines as shown in Figure 1B. Whether to
use isometric projection or one- or two-point perspective depends on the important elements of the
structure. But don’t let this decision be a dominant
left-brain one; let your right-brain creativity prevail,
at least at first. Then, after making your first sketch,
you can modify this sketch to maximize the 3-D effect
and consider whether or not perspective or isometric
rules might enhance the final drawing. Experimentation with shading, based on a selected directional
light source, is also worth some consideration. A little subtle shading can enhance the 3-D effect of your
drawing.
An example of a distant, moderately high angle,
single vanishing-point drawing is shown in Figure 2.
Because the freehand drawings displayed in this chapter were constructed to illustrate geologic structures of
interest to explorationists engaged in subsurface structural interpretation, they specifically use the elevation of
a key horizon as the surface of the drawing. This feature
of 3-D drawing is particularly relevant to petroleum
exploration, but differs from the block diagrams discussed in academic programs such as Reynolds et al.
(2006; see also Web site, http://reynolds.asu.edu/
blocks, for mini-movies of rotating block diagrams of
different structures). The figures in this chapter also
differ from the classic block diagrams of early geomorphologists (e.g., Johnson, 1931) whose beautiful
3-D drawings used topography as the surface of their
blocks.
THREE-DIMENSIONAL VISUALIZATION
Figure 2. A simple perspective three-dimensional (3-D)
drawing with one vanishing point, using two depthmigrated, east-west profiles from a 3-D seismic data
volume at Salt Creek oil field on the Casper Arch, Wyoming
(courtesy of Anadarko Petroleum). The drawing shows the
north half of the Salt Creek oil field, a basement-involved
thrust-generated fold, with oil-water contacts (OWC) of the
Second Wall Creek Sandstone and Tensleep reservoirs (dark
green lines). The hachured pattern marks the surface of the
Wall Creek Sandstone (no fracture system implied). Applying
the concept of self-same imaging, an animation of east-west
seismic profiles starting at the north tip of the structure and
moving south to the profile showing maximum slip and fold
wavelength at the center of the structure should replicate
the kinematic development of the Salt Creek fault-related
anticline that occurred in place at the center of the anticline.
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Many geologists have difficulty with 3-D perception, apparently more because of neglect during their
formal education and later inattentiveness than because of an absence of innate ability. Overemphasis
on today’s industry demands for computer skills (a
left-brain domain) and lack of encouragement from
mentors probably contribute to this neglect. However,
improvement in spatial perception can be developed
with dedication and practice (McKim, 1972; Kastens,
2006).
To verify the idea that many geologists may be spatially challenged, I conducted a simple (unscientific)
experiment, using the Kastens et al. (2009) block diagram shown as Figure 3A (while ­covering the other
parts of this figure) in a test of the 3-D visualization
skills of a dozen ­experienced petroleum geologists.
I asked each of them, “What is wrong with this 3-D
drawing?” ­Surprisingly, at least half of them did not
recognize the 3-D implications of the block diagram,
as they did not see how the dark outlines of Formation X on the front, side, and top faces of the block fit
together to outline a plunging anticlinal fold. Of those
who recognized the drawing as a 3-D block with the
dark Formation X penetrating its faces, none of them
noticed in about one minute’s viewing time that Formation X on the top face is drawn erroneously as
­thinner than in the vertical front face of the block. If
you think about the top face view as that seen on flat
­outcrop, it should be clear that along the axis of the
fold, Formation X should have been shown as at least
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4 Donald S. Stone
Figure 3. Views of an anticline plunging toward the observer. (A) Block diagram from Kastens et al., 2009, showing the
­ enetration of Formation X at the front, top, and side faces of the block. (B) Cross section along the axis of the fold in
p
(A) showing that if the fold plunges at 25°, the thickness of Formation A on the top face of the block is too thin. Formation
X should be shown as thicker than in the front face by a factor of 2.36, not thinner as indicated in (A). In (C), structural
contours are drawn on Formation X, based on the depth numbers in A with dashed contours extrapolated above the topface level. (D) Partial conversion of the data in (A) and (B) to a three-dimensional (3-D) freehand drawing by projecting the
plunging anticlinal fold above the level of the top face. (E) Full conversion of the plunging anticline to a 3-D freehand drawing, retaining only the front and right-side faces of the block diagram in (A) below Formation X.
twice as thick as in the front face (Figure 3B). (Compare the block diagrams in Reynolds et al., 2006).
Figure 3C is a structural contour map (Kastens et al.,
2009) on top of Formation X representing the plunging
anticlinal fold in A. In Figure 3D, the structure is converted to a 3-D sketch, in which the corrected, wider
Formation X top and bottom traces are indicated by the
dashed lines on the now hidden top face of the original
block diagram. In the final step, Figure 3E is a full conversion to a 3-D drawing of the plunging fold, retaining
only that part of the front and side faces of the block below the traces of Formation X. The 3-D geometry of the
plunging fold is clearly more easily visualized in drawing E of the structure than in A.
COMPUTER MANIPULATION VERSUS FREEHAND
DRAWING
As an aid to subsurface structural interpretation,
no one can deny the utility of modern computer
software programs that are capable of producing
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3-D images of routine subsurface databases such
as integrated well and seismic data, surface geology, digitized contour maps, and so forth. However, there are limitations to the visual transpicuity
of 3-D images created by these software programs.
Gibergues et al. (2009) assert that for the “structurally more complex formations”—“available 3-D exploration software—cannot accommodate a realistic
geometrical description of present-day geologic
structure and tectonic steps.” Many of these
computer-generated 3-D images contain too much
often questionable information, are too complicated, and are ultimately incomprehensible. Compare the 3-D images in Figure 4 with any of the
freehand drawings in this chapter that were purposely designed to highlight the most important
elements of specific complex structures mapped in
the subsurface and to present these elements in a
comprehensible way. A comparison of a freehand
3-D drawing with a computer-manipulated 3-D image of the structure in Figure 4A and B illustrates
the difference between these two 3-D modes.
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Using Freehand Three-dimensional Drawings 5
Figure 4. Comparison of freehand
and computer-generated threedimensional (3-D) images.
(A) 3-D freehand drawing of the
complex structure created by
the intersections of the Casper
Mountain, Casper Arch, and
subsidiary faulted structures in
central Wyoming (modified after
Stone, 2002). (B) 3-D image of
part of the same area generated
by computer manipulation using
Schlumberger’s Petrel software,
based on digitization of the
western half of the two structural
contour maps and several structural cross sections from Stone
(2002).
Figure 4 illustrates the two modes of 3-D imaging of the complex structure that surrounds the
Casper Mountain and Casper Arch thrusts along
the border of the Casper Arch and Wind River Basin, Wyoming (Stone, 2002). The colorful, digitally
manipulated, 3-D image in Figure 4B of most of
the same structural elements as in A was created in
Schlumberger ’s Petrel software. Digitization of the
west half of two structural contour maps on the top
of the Muddy and Madison horizons and several
structural cross sections from Stone (2002) provided
the input. This comparison, although perhaps not
quite equitable, nevertheless makes a point; the 3-D
aspects of Figure 4A can be easily visualized by most
geologists, whereas clear visualization of Figure 4B
requires a higher level of spatial skill, which according to Kastens et al. (2009) is “unevenly distributed,”
and as suggested by the test exercise discussed previously with Figure 3, is apparently uncommon.
However, computer software programs can rotate
3-D images, allowing views of the structure from
any selected angle (to what advantage is not always
clear), whereas a 3-D freehand drawing presents a
static view.
In Figure 5, the intricate details of the faulting
seem to overwhelm the visual senses to such an extent that the utility, practicability, and accuracy of
the images may be questioned. Manipulation of a
10711_ch01_ptg01_hr_001-014.indd 5
limited subsurface database entered into one of these
3-D software programs is dominantly a left-brain
function, whereas the construction of a freehand
3-D drawing highlighting the critical elements of a
structural interpretation is dominantly a right-brain
directed assignment and can inspire a more intuitive,
creative interpretation. A 3-D freehand drawing exercise is more likely to uncover new, obscure trap geometries not so easily recognized in labyrinthine, 3-D
digital images. A well-executed freehand drawing
can assist in unraveling the details of a comparable
computer-generated 3-D image of the same structure
because a successful penetrative visualization of the
subject structure implies that the interpreter has developed an understanding of the key elements. As
John Lorenz (2010) points out in his perspicacious
AAPG President’s Column on critical thinking, “computers can provide strong support for both learning
and scientific processes, but they can also be used to
replace them.” However, this examination of computer-generated 3-D images is not intended to minimize
the value of computer software manipulations but
rather to reinforce the value of right-brain directed
freehand drawings as a process of clarification and
verification of critical 3-D structural elements in local (prospect size) subsurface structures that may be
overlooked by interpreters using only computer software in their analysis.
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6 Donald S. Stone
Figure 5. Computer-generated 3-D
images of complex geologic
structures. (A) Image taken from
the cover of the AAPG Bulletin
(2001, v. 85, no. 5), generated
in 3DMODE (Midland Valley
Exploration Ltd.) and described
in a whimsical poem. (B) Image
from advertisement using
Schlumberger’s Petrel software.
CREATING 3-D DRAWINGS BASED ON INTERPRETATION
OF ONE- AND TWO-DIMENSIONAL SUBSURFACE
EVIDENCE
In the absence of 3-D seismic data volumes, 3-D interpretation and visualization must be based on one- and
two-dimensional information derived from well control, 2-D seismic data, and surface geologic mapping
(sometimes with secondary support from potential
fields surveys) combined in an integrated structural
analysis. The products required for structural interpretation generally consist of structural contour maps
on one or two horizons and selected structural cross
sections. In constructing these maps, it is critically important that the 2-D seismic data be migrated in depth,
the data moved to their correct position in 3-D space
at profile intersections, and tied to the well control.
Also useful in an integrated structural interpretation
are fault-plane contour maps (Figure 6) that can provide a check on interpreted fault intersections and
with key stratigraphic horizons and help to identify
fault branch lines and tip lines. After reviewing these
sources of subsurface information, a synthesizing
phase begins in the mind’s eye (the right brain) and
penetrative visualization of the structure is required
before there is mental grasp of how these data come
together in three dimensions.
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Figure 6. Fault surface contours showing fault intersection
lines. (A) Offset of Casper Mountain fault by Casper Arch
thrust with hanging wall and footwall cutoff lines (same
area as in Figure 4A). (B) Complex fault pattern enclosing
the Quealy Wrench Duplex (Stone, 1995), Laramie Basin,
Wyoming. Fault contours at +1000, Sea Level (S.L.), and
–1000 ft (–304.8 m) are highlighted to show direction of
fault dip.
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Using Freehand Three-dimensional Drawings 7
Figure 7. Two threedimensional (3-D) freehand
(isometric) drawings,
different views, of the
Quealy Wrench Duplex.
(A) Looking west (modified
from Stone, 1995).
(B) Looking north.
By scrutinizing and effectively synthesizing and
memorizing the contour maps, seismic profiles, borehole data, and surface geology, a mental 3-D image of
the critical geometric elements of the structure can be
developed. Then there is the selection of an angle of
view upon which a 3-D sketch of the critical area of
the structure can be attempted using a key horizon as
the surface of description. (Note that 3-D drawings
in which the surface of the diagram is identified as a
specific stratigraphic horizon seem to be scarce in the
geologic literature.) Multiple drawings sighted from
different angles of view can be beneficial (Figure 7). In
the process of constructing one or more completed analog drawings one might discover previously unrecognized structural relationships not easily identified
in an equivalent rendition produced by manipulation
with digitizing computer software.
OTHER ANALOG MODELING MATERIALS
Experimentation with other analog modeling materials can sometimes help in 3-D visualization of an interpreted subsurface structure. Using paper (dry or
wet), transparent plastic film, or water-based clay as
the medium, a simple 3-D model can be constructed
experimentally to aid in the 3-D visualization of the
interpreted structure.
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The paper model in Figure 8A illustrates a structural geometry that can be produced by a left-lateral
slip component on a finite wrench fault. This figure
was created by cutting a finite fault in a sheet of dry
paper, holding the lower side (south block) of the paper down on a table with the left hand, and pushing
the upper side (north block) to the left with the right
hand to create the drag folds along the fault. The variable fault separations produced by this fault movement
were then taped in place, the axes of the anticlinal and
synclinal folds marked with red and blue felt pens
(respectively), and the final fault geometry photographed. The purpose of this exercise is to show that
there must be vertical, as well as lateral, separation(s)
along a finite wrench fault.
Figure 8B is also a paper model, this one of the
Pierce-Black Hollow-New Windsor group of structurally controlled oil fields in the northern Denver ­Basin,
Colorado (Stone, 1985). With a light source on the
southeast, the shadows produced on the northwest
add to the 3-D effect of the folded-paper structures.
In Figure 8C, color-coded, key stratigraphic horizons from a number of variously oriented, true-scale
structural cross sections drawn across the Quealy
Wrench Duplex (Stone, 1995; see Figure 6 for locations of cross sections across the important, intersecting faults) were traced onto interlocking, transparent
plastic panels and taped down in their actual positions
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8 Donald S. Stone
Figure 8. Additional analog modeling materials. (A) Three-dimensional paper model of a finite left-wrench fault with drag
folds creating variable vertical separations and two null points (i.e., point of zero, and reversal of, vertical separation).
(B) Three-dimensional model of the Pierce-Black Hollow-New Windsor oil-field complex, looking east, with a light source
on the southeast producing shadows on the northwest sides of the folds. Direction of Laramide compression shown by
s1 arrows (modified from Stone, 1985). (C) Interlocking, transparent structural cross sections through the Quealy Wrench
duplex taped in their actual locations over the top-of-Muddy structural contour map (see Figure 6 for section locations
and fault-surface detail). (D) Water-based clay (= Precambrian basement) and salt-cornstarch-water (= sedimentary cover)
analog model of a basement-involved thrust-generated fold that has been shortened 18% in a homemade contractional
device (Stone, 1993). (Can you locate the vanishing point?)
across the underlying structural contour map drawn
on the Muddy Sandstone horizon. By looking through
and studying these interlocking sections close-up at
eye level, the 3-D relationships within the structure
were visualized and the exercise provided a check
on the construction of the freehand 3-D drawings
­(Figure 7) of the Quealy structural complex. It is recognized that the geometric elements of the structure in
the photograph (Figure 8C) are not discernible, but the
basic technique is nevertheless illustrated.
The clay model shown in Figure 8D has helped
visualize the 3-D kinematic development of basement-involved thrust-generated folds that provide
the trapping geometries for most of the oil fields with
Paleozoic reservoirs of the central Rocky Mountain
foreland province (e.g., the Salt Creek oil field structure in Figure 2). These analog clay model studies
have been discussed elsewhere (Stone, 1993, 2009;
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Russell and Stone, 1995, and numerous oral and poster
presentations).
Figure 9A explains the 3-D aspects of an animation
of the 3-D seismic data volume from the Red Wing
Creek impact structure oil field produced in Landmark’s computer software (Figure 9B). Animations
were created moving through the seismic block using
both vertical profiles and time slices. The multimedia mock-up of the impact structure in (A) includes
a transparent salad bowl representing the crater, an
overturned plastic cup representing the central peak,
and several vertical mat board seismic profiles.
The point to be made here is that these various
3-D modeling techniques can stimulate an initial 3-D
visualization of the final structural interpretation by
providing an intermediate step in the development
of freehand 3-D drawings. And since these are actual 3-D models, not drawings of the 3-D geometry
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Using Freehand Three-dimensional Drawings 9
Figure 9. Red Wing Creek impact structure in North Dakota. (A) Mixed-media analog model of the Red Wing Creek crater
(transparent salad bowl) with a central peak (overturned plastic cup) and spaced vertical seismic profiles (Mat board) starting
outside the crater on the east, and moving through the crater and central peak, to outside the crater on the west. This model
was used to explain the direction of the animation (movie) of vertical seismic profiles made from a three-dimensional (3-D)
seismic data volume collected by True Oil Company, operator of the Red Wing Creek oil field. (B) Three-dimensional cube created in Halliburton’s Landmark software showing a top-surface time slice near 1.00 sec two-way time and a vertical seismic
profile through the middle of the impact structure with the approximate outline of the crater and central peak indicated. Mmc
is Mission Canyon Limestone nearly flat-lying outside the crater area, and ~3000 ft (~914.4 m) structurally higher within the
chaos of the central peak. OW = Ordovician Winnipeg Formation.
executed on 2-D paper, the sequence of construction
can also be reversed; that is, the actual 3-D models
could be produced based upon freehand 3-D drawings and used as a check on the acceptability of the
drawings.
SOME SUCCESSFUL 3-D DRAWINGS
Figure 10 from Trevisan’s (1939) in-depth study of
the “Tectonics of Il Gruppo di Brenta (Trentino Occidental)” in northern Italy is an elegant freehand 3-D
drawing that clarifies the basic geometry of the illustrated fault-fold structure. Today this complex structure would be described by structural geologists as a
right stepover on a left-wrench fault system. Another
meaningful freehand 3-D drawing, certainly worth a
thousand words of description, is Quennell’s drawing
of the Dead Sea Rift pull-apart (1956; see also Girdler’s
1989 biographical article titled “A. M. Quennell, ­father
of transform faults and poles of rotation”). Quennell’s
3-D drawing is “an isometric block diagram of the
Arabian and Palestine blocks in their present relationships” (Figure 11). He also presents block-diagram
drawings illustrating the kinematic development of
the regional, fault-induced structure.
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Figure 10. A three-dimensional (isometric) freehand drawing described as a “representation of the surface tectonics
in the Tovel Lake belt,” Trentino Occidental of northern Italy.
Modified from Trevisan, 1939. Translations: campo (field);
costa (coast); lago (lake); malga (alp or big mountain);
tuenno (well); val (valley).
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10 Donald S. Stone
Figure 11. An isometric, freehand block
diagram of the Arabian and Palestinian
blocks in their present relationships
(modified from Quennell, 1956) showing the pull-apart graben produced by a
left-lateral slip component on the Dead
Sea rift.
Figure 12 presents the evidence in a 3-D sketch for
the identification of piercing points in the shallow,
gravity-induced thrust and tear-fault pop-up structures of the Bearpaw Mountains in northern Montana.
Figure 12A displays the line of intersection of the ramp
thrust plane and the Eagle Sandstone seismic horizon,
which produces a line that intersects the tear-fault
plane (i.e., the frontal face of the drawing) to produce
a piercing point. Note that this is a perspective drawing with a single vanishing point not very far above
(or to the north of) the drawing. Figures 12B and C
are maps of the fault traces (on the Eagle Sandstone
Figure 12. (A) Freehand threedimensional (3-D) drawing
(D. Stone) of the piercing point
produced by the line of intersection of the ramp-thrust surface
with the Eagle horizon, indicating right-lateral movement on
tear faults that offset detached,
gravity-induced pop-up structures
in northern Montana. (B) Map
showing traces of ramp thrusts
and tear faults (dark green) at
the level of the Eagle Sandstone
horizon, based on interpretation
of a 3-D seismic data volume tied
to surface geology and well control. (C) Palinspastic restoration of
thrust traces along two major tear
faults (maps B and C are courtesy
of Mark Caldwell, 2006).
10711_ch01_ptg01_hr_001-014.indd 10
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Using Freehand Three-dimensional Drawings 11
horizon) for the pop-up thrusts (trending east to west
and light colors) and tear faults (trending northwest to
southeast and dark green or black). The tear faults bottom on the basal detachment (flat) and carry important right-lateral slip components as indicated by the
offset piercing points in B. Figure 12C is a palinspastic reconstruction that illustrates how well these offset
thrust traces restore with one restorative left-lateral
shift along the two major tear faults.
The freehand drawing in Figure 13A illustrates a
common misinterpretation made by geologists in cases
where surface geologic mapping indicates that there
are important lateral separations on hanging-wall
faults that strike and dip at high angles to the strike of
the causative thrust and axis of the thrust-related anticline. This clear, freehand 3-D drawing (Paylor et al.,
1989) indicates that these high-angle faults are interpreted to cut the underlying thrust plane and extend
into the footwall. However, in this example there is no
evidence from the footwall block that might support
these footwall fault extensions. In Figure 13B, then, I
have modified the drawing to show a more likely interpretation, that is, that these high-angle faults are
true tear faults (according to Perry’s 1935 definition)
confined to the hanging wall of the master thrust.
Figure 13. Tear-fault definition as used here. (A) This
three-dimensional freehand (isometric) drawing shows the
commonly misinterpreted geometry of the term “tear fault”
(from Paylor et al., 1989). (B) Drawing in (A) modified to illustrate the true definition of tear fault as a high-angle fault
formed by differential movement within the hanging wall
of a thrust and striking in the general direction of tectonic
transport along the underlying thrust (Perry, 1935).
10711_ch01_ptg01_hr_001-014.indd 11
The classic interpretation made by geologists who
have mapped the Granite Ridge fault associated with
the Piney Creek thrust bordering the northeast flank of
the Bighorn Mountains (see Stone, 2003, for a reference
list) is that this east-striking, high-angle fault cuts and
offsets the regional-scale Piney Creek thrust and therefore extends into the footwall of the thrust. But at the
same time, this interpretation indicates that the Granite Ridge fault terminates on the east at the surface
thrust trace, forming a sharp right-angle corner (i.e.,
“Storey Corner”). Clearly, this interpretation is untenable because it requires abrupt termination of 30,000 ft
(9144 m) of dip-slip on the Piney Creek thrust and
10,000 ft (3048 m) of dip separation and ~3 mi (4.8 km)
Figure 14. Using freehand 3-D drawings to illustrate (A) the
two-step kinematic development of the Piney Creek thrust
and Granite Ridge tear fault (modified from Stone, 2003).
The green fault surface identifies the Piney Creek thrust
with 30,000 ft (9144 m) of dip slip, and the black vertical fault plane (L.S.) is the Granite Ridge tear fault with
3 mi (4.8 km) of left-lateral separation on the Madison
Limestone at the surface. The black vertical line through the
Piney Creek thrust is the Gulf Oil deep subthrust test well
that started in Precambrian granite in the hanging wall,
passed through the thrust, and bottomed in the Madison
Limestone in the footwall.
6/5/13 7:59 AM
12 Donald S. Stone
Figure 15. Freehand
sketches (a, b, c)
of the three-step
development of a
left-stepover on a
right-wrench complex in the Trentino
Occidental, northern
Italy. Modified from
Trevisan, 1939.
of left-lateral separation on the Granite Ridge fault
at its abrupt termination at the Storey Corner. If the
Granite Ridge fault were to cut though the Piney Creek
thrust with these large offsets, it would have to extend
eastward some double-digit miles into the (footwall)
Powder River Basin before dying out. But as in the
previous example, the footwall seismic and borehole
evidence does not support this extension. The solution
to this conundrum is simple: the Granite Ridge fault
is a true tear fault limited to the hanging wall of the
Piney Creek thrust as illustrated in the freehand 3-D
drawings of Figure 14A and B (Stone, 2003).
Figure 14A and B comprises a two-step kinematic
sequence in the development of the Granite Ridge tear
fault within the hanging wall of the Piney Creek thrust.
Another kinematic development sequence is shown in
Trevisan’s (1939) freehand drawings in ­Figure 15 A,
B, C. My purpose in including these ­illustrations is
to emphasize the tractability of freehand drawings in
kinematic restoration interpretations.
CONCLUSIONs
Three-dimensional freehand drawings based on 2-D
subsurface geologic information require penetrative
visualization of the structural geometry of the study
area interpreted from time-honored representational
displays. These displays consist primarily of interpretive structural contour maps on at least two different
stratigraphic horizons, and intersecting structural
cross sections oriented in several different directions
10711_ch01_ptg01_hr_001-014.indd 12
across the structure, constructed from borehole control and 2-D seismic data. Surface geology may also
come into play where primary stratigraphy crops out
with some topographic relief and reflects in some
meaningful way the underlying structural geometry.
Some benefits of creating freehand 3-D drawings as
a part of the final subsurface structural interpretation
include
• clarifying and verifying the 3-D implications of a
structural interpretation based on a 2-D database;
emphasizing the critical deformational geometries
of folding and faulting
• checking on the internal geometric consistency of
the structural interpretation
• examining fault relationships, fault separations,
fault cutoff and branch lines, and relative times of
movement
• revealing possible untested new trap possibilities
created by complex structural geometries not easily identified in the 2-D database
• facilitating the construction of a kinematic developmental sequence and illustrating geologic
concepts such as self-same imaging of fault-fold
structures, for example, Figure 2 (particularly
well-illustrated in animations of 3-D seismic data
volumes)
• suggesting possible across-fault migration paths
• furthering the deciphering and simplification of
bewildering 3-D images of the same structural
complex produced by computer manipulation using one of the current 3-D software programs.
6/5/13 7:59 AM
Using Freehand Three-dimensional Drawings 13
ACKNOWLEDGEMENTS
I would like to thank Connie Knight, Art Snoke, and
Jim Lowell for their positive comments and encouragement during the development of this manuscript.
I thank Leland Cress for his scrutinizing comments,
which were considered in revision of the manuscript.
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