Area-Depth Strain Analysis is a valuable way to study structures that either
sole out into an underlying detachment which is parallel to regional stratigraphy (depth to detachment method),
Watch Niger Delta Area-Depth Example
form above a fault that cuts across the deep bedding (Fault Trajectory Method)
Mapping Fault Shapes with Area-Depth
Mapping Fault Shapes using the Fault Trajectory Method in StructureSolver
We demonstrate in this video how to use the Fault Trajectory method to determine not only the dip and location of the fault at depth, but also the the variation in the shape of the fault with depth. As discussed earlier, the Fault Trajectory method in StructureSolver incorporates the differences between a structure’s hangingwall and footwall elevations when estimating the depth and dip of the fault. Therefore, as we narrow the limits to include only the shallower part of the fault, we will get an estimate of the depth and dip of that part of the fault only. This allows us to map the variation in the shape of the fault with depth. To show how the Fault Trajectory Method is sensitive to changes in fault shape, we will walk through area-depth analyses of two fault-related folds with varying fault geometries.
The first example in this video uses the seismic section across a fault propagation fold from the Bermejo Basin shown in the previous video. As noted in that video, the pre-growth horizons in the distal hangingwall are elevated above the footwall but nearly flat. So the structural relief in this part of the structure is positive but essentially constant. This observation is consistent with a planar but dipping fault at depth.
As we move the hangingwall regional limit across this portion of the structure, the analysis immediately shows that the computed fault follows the deep seismic reflector to within 200 m.
This approach can be applied in situations where observed folding implies a variable fault geometry, but the fault itself is not directly imaged. In the second example, we use a seismic section across the Inner Moray Firth (Virtual Seismic Atlas1). We show how to apply the Fault Trajectory method, together with the StructureSolver restoration and kinematic modeling tools, to estimate with confidence the dip and location of the fault.
In this example, the main fault is not well-imaged at depth and interpretation of the major growth fault in the section is limited by the lack of data in the lower section. We start with the basic fault geometry and pre-growth horizon interpretation from the Atlas. We check the interpretation using the StructureSolver Restoration feature. Restoration of the upper pre-growth horizon reveals good seismic continuity in the restored section, which supports the interpreted correlations.
Visual analysis of the section shows gradual folding in the hangingwall that is maintained towards the right side of the image. In contrast to the distal hangingwall of the Argentina structure, the continual change in structural relief suggests a smoothly varying fault shape at depth. Structural relief between the hangingwall and footwall also decreases towards the right, indicating that the underlying fault may also flatten.
Area-depth analysis of the major growth fault estimates a shallower far-field dip than is indicated in the seismic. This is consistent with the hangingwall fold shape.
Next, we map the fault location for progressively smaller hangingwall regional limits, as shown and determine the envelope for the fault location. Using this envelope as a starting point, we create a kinematic forward model to test and refine the fault shape. Within a short time, we have developed a model that reproduces the pre-growth fold geometry and is consistent with the results from the Fault Trajectory analysis.
1 Virtual Seismic Atlas. www.seismicatlas.org. Survey: Fugro Inner Moray Firth IMF97 Profile #3
The technique works by analyzing the variation of excess structural area with depth. Under appropriate circumstances, this approach results in:
- A determination of the depth of the controlling detachment, or the location and dip of the controlling fault
- Estimates of the displacements along the detachment or controlling fault for both pre-growh and growth horizons
- Estimates of the layer-parallel strain for all horizons.
Area-depth strain analysis in StructureSolver is a valuable tool for interpretation and review of interpretations:
- area-depth strain analysis is not dependent on any assumptions such as constant bed length or bed thickness.
- It can be performed before the interpretation is completed.
- The area-depth strain analysis implementation in StructureSolver can be used easily and quickly for continuous quality control during the interpretation process to catch problems before they become costly mistakes.
Area-depth graphs can be created rapidly in StructureSolver for any interpretation satisfying these minimum conditions:
- At least two horizons have been interpreted across the structure(s) of interest.
- At least two of the horizons should be pre-growth horizons.
- In the case of the Depth-to-Detachment Method, the horizons should have returned to regional structural levels at both ends of the interpretation.
- In the case of the Fault Trajectory method, the horizons at both the footwall and and hangingwall ends of the structure should have returned to horizontal.
As in all StructureSolver features, results are dynamically updated if any changes are made to the interpretation.
Results from Area-depth graphs can be compared to results from structural modeling and restorations performed in StructureSolver. Since each method is independent, consistent results from all three indicate that the interpretations are robust.
What are the principles behind Area-Depth Strain Analysis?
Early structural geologists (Chamberlin, Bucher, Goguel, Laubscher, Dahlstrom) recognized that measurements of bed length and bed area could be used to provide estimates of detachment depth. However, Epard and Groshong (1993, Excess Area and Depth to Detachment, AAPG Bulletin, V. 77, No. 8.) made a significant advance when they recognized that plotting horizon excess area as a function of depth could be used to estimate both detachment depth and structural displacement parallel to the detachment. In StructureSolver, this is referred to as the Depth to Detachment Method of area-depth analysis.
Groshong and Epard (1994, The role of strain in area-constant detachment folding, Journal of Structural Geology, V. 16, No. 5) further recognized that a combination of displacements from area-depth graphs and from bed length measurements could be used to estimate how bed lengths may have changed during deformation due to layer-parallel strain within each formation. Groshong, Pashin, Chai and Schneeflock (2003, Predicting reservoir-scale faults with area balance: Application to growth stratigraphy: Journal of Structural Geology, v. 25, p1645-1658) demonstrated that displacements and strains could be also be estimated for growth strata.
The next advance in area-depth analysis was the recent work by Eichelberger, Nunns, Groshong and Hughes (2015, Predicting the dip and location of master faults beneath forced folds, Geological Society of America Annual Meeting) which extended area-depth analysis to include structures controlled by dipping faults. Referred to as the Fault Trajectory Method, this approach to area-depth analysis estimates fault dip in addition to fault depth, displacement, and layer parallel strain.
The publications quoted above show comparisons of the theoretical predictions with actual results for both numerical and synthetic structural models. In these examples, the area-depth-strain analysis reproduces the model fault locations and displacements. In those pubilcations, area-depth analyses of seismically imaged natural examples consistently give estimated fault depths and dips that match the imaged fault planes.
Area-depth analysis will yield:
- An estimate of the detachment depth.
- An estimate of the vector displacement along the detachment.
- Estimates of the vector displacements of each layer (growth as well as pre-growth).
- Estimates of the layer-parallel strain in each layer.
Area-Depth analysis can also be useful for identifying Interpretation errors:
- An area-depth point that does not fall on the linear trend shared by other horizons in the interpretation may indicate the the horizon has been mis-interpreted.
- Abnormally high or non-systematic layer-parallel strain values may also indicate line-length errors in the interpretation.
As a tool for structural balancing, area-depth analysis in StructureSolver is more flexible than many other techniques because it can be performed before the interpretation is complete. However, the Area-Depth Strain Analysis feature in StructureSolver does immediately provide full constant-area and constant line-length displacements without performing multiple measurements and calculations.
A strength of area-depth analysis is that the horizon displacements estimated from area-depth graphs only assume that area is maintained during deformation. This is in contrast to other common techniques for estimating displacement, such as balanced cross-sections, which assume constant bed length and thickness. Indeed, it appears from multiple studies that bed-length is rarely conserved in most common styles of deformation.
A fundamental assumption of the area-depth graph method to determine displacement and fault location is that area is conserved within the section plane during deformation. For example, if the cross-section being analyzed is not parallel to the principal displacement direction, then you might get inaccurate or unexpected results because this assumption may not be true.
Another common situation in which area is not conserved involves migration of shale or salt into the core of a detachment fold. In these cases, the flat detachment method may predict detachment depths that are too deep. However, if we know the approximate depth of the detachment, we can estimate the "area" of material that has migrated into the core of the fold. This can be done interactively in StructureSolver using the "Non-Zero Intercept" option.
The fault trajectory method for dipping faults is sensitive to regional dip so it is good practice to remove regional dip (for example, using the Restoration feature) to obtain reliable estimates of fault dip, depth, and displacement..
An additional assumption of the area-depth method is that each pre-growth horizon records constant displacement. If the structure has accommodated significant shear, this may not be the case. It is good practice to limit the regression fit to horizons outside of shear zones to reduce the influence of depth-dependent horizon displacements.
For fault-related folds, an area-depth graph is created by plotting of the structural area of deformed horizons (e.g. the area of a fold) against the "regional" horizon depth. In structural settings where area is conserved during deformation, the variation in structural area with depth is a linear relationship. This linear function can be used to determine structural displacement, horizon strains, as well as the location and dip of controlling fault at depth.
We recommend that the area-depth graph analysis is a good starting point to analyze and understand a folded or faulted section, when a pre-growth section is present and well-imaged. The analysis will immediately give you information about the structural styles and whether your section is balanced.
You can create an area-depth strain analysis of an (approximately) 2D folded structure in your area of interest in a matter of minutes:
- Generate an image of the seimic section, together with any horizon and fault interpretation.
- Create a new StructureSolver dolution with your saved image.
- Calibrate the image, and draw your interpretation
- Select the Create Area Depth graph menu item and add your horizons to the area-depth graph.
- The area- depth graph is instantly plotted on the section, together with the estimated fault position,displacement vectors and strain estimates.
Now you can start to explore the structural implications of your interpretation!
All components of the area-depth graph - the plot, the predicted fault location, the strain estimates and displacement vectors - update instantaneously as you modify your interpretation, or reconfigure the area-depth graph. The interactivity is great for exploring alternative interpretations, and identifying uncertainties and errors in the interpretation.
When you create an area depth graph and add horizons to it, the horizontal and depth limits of the area-depth analysis region are automatically set but can be interactively adjusted at any point in the workflow. StructureSolver uses regression analysis of the area-depth curve to identfy the pre-growth horizons that can be used to estimate fault location. Simple controls are provided so that users can interactively exclude horizons from the analysis and restrict the horizontal extent of the included horizons. Area-depth results dynamically update as soon as any of the controls of the area-depth graph (or the interpretation) are changed.
The minimum requirement for performing an area-depth graph analysis is that you have interpreted at least two horizons across one or more structures of interest. Furthermore, these horizons should be pre-growth horizons in order to estimate the fault location. The accuracy of the estimate will increase as more pre-growth horizons are included.
Area-Depth Strain Analysis works for complex, composite structures, provided that there is a common detachment or underlying fault. If there are multiple levels of detachment in your section, the technique may still work. However the analysis of the derived numerical data is more complex. Please refer to our private website material for further discussion.
If there is a mobile layer (e.g., salt or shale) immediately above a detachment, naive use of the area-depth graph technique will result in an incorrect estimate of detachment depth (usually too deep). We have extended the method to treat such cases.
You can use the Area Depth Strain analysis in StructureSolver in conjunction with any interpretation system. You will easily be able to:
- Rapidly assess balance of a structural interpretation
- Validate interpreted fault locations
- Identify high-strain horizons
- Guide fault interpretation in situations where the shallow structure is well imaged but deep fault location is unknown.
StructureSolver can provde you with detailed, rigorous analysis of 2D sections. In particular, StructureSolver can help you:
- Determine fault displacement history from growth strata
- Use layer parallel strain profiles to characterize kinematic style of deformation
- Map out variations in fault geometry such as ramp-flat-ramp transitions
- Quickly compare changes in displacement and strain for multiple sections within a 3-D volume.
You can use the 2D analyses to guide construction of full 3D structural models and save days or even weeks of time.
Our structural geologists have extended the area-depth graph method of Epard and Groshong to handle dipping faults. This method is implemented in StructureSolver only. Please contact us for scientific and technical details.
The area-depth analysis is more complex in the presence of multiple detachment levels, and requires careful interpretation. However, you can gain a lot of insight into the development of the structures by exploring the area-depth graphs for different sections.. Please contact us for examples and more information.
In StructureSolver, you can vary the Intercept area to account for mobile material. In many cases, this option will enable you to get limits on the amount of material that has flowed into the structure. Please contact us for examples.