geology · structural
Structure Gradient (Regional Dip Rate)
Gradient = |zB − zA| / D
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Inputs
ft
ft
ft
Description
Computes the structure gradient — the rate of change in depth of a structural marker between two picks, expressed as a regional dip rate (traditionally reported in feet per mile in older subsurface-mapping literature). This is the two-point counterpart to a full three-point or contour-based dip determination: a quick way to characterize regional dip trend between two wells or map locations without needing a third control point.
Variables
| Symbol | Unit | Description |
|---|---|---|
| G | ft/mi | Structure Gradient (Regional Dip Rate) |
| zA | ft | The depth of a structural pick (e.g. a formation top) at well or map location A, on a consistent datum (e.g. subsea true vertical depth). |
| zB | ft | The depth of the same structural pick at location B, on the same datum as location A. |
| D | ft | The straight-line horizontal (map-view) distance between locations A and B. |
Assumptions
- Both depth picks are the same correlative structural marker (the same formation top or contact), on the same depth datum.
- The horizontal distance is measured as a straight-line map-view distance between the two locations, not an along-hole or along-section distance.
- The two points lie on a reasonably planar or uniformly-dipping trend between them — this calculator reports the average rate over that span, not a local dip at either point.
Limitations
- Reports only the magnitude of dip rate along the line connecting the two points — unless that line happens to run in the true dip direction, this understates the true regional dip rate the same way an apparent dip understates true dip; conceptually this is the same true-dip-vs-apparent-dip relationship computed by apparent_dip_true_dip, just expressed as a rate over a real map distance between two wells instead of as an angle from a known strike. Use apparent_dip_true_dip when the true dip and strike are already known and an angle in a specific cross-section direction is wanted; use this calculator when only two structural picks and the distance between them are available and a regional rate (ft/mi or m/km) is the more useful output for structure-gradient mapping.
- Assumes uniform dip between the two points — any fold, fault, or dip reversal between locations A and B will not be detected and will bias the computed rate toward the two endpoints' net change only.
- Does not resolve a dip direction (azimuth) — only the magnitude of the rate along the specific line connecting the two picks.
Use Cases
- → Quick regional dip check between two wells: Characterize the average dip rate of a marker between two wells that penetrate it, without needing a third control point for a full three-point solution.
- → Structure-gradient mapping: Build up a regional structure-gradient trend (in ft/mi or m/km) across a field or basin from pairs of well picks, the traditional subsurface-mapping technique for characterizing regional dip away from local structural closures.
Related Calculations
Region Notes
Global
Typical gently-dipping regional trends in mature US onshore basins are commonly on the order of 10-40 ft/mile (e.g. published Kansas Geological Survey figures of roughly 12-20 ft/mile for near-surface Kansas strata) — a computed gradient far outside that order of magnitude for a similar tectonic setting is worth double-checking against the source picks before treating it as a genuine local structural feature rather than a data-entry error.
References
Primary source
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