True Stratigraphic Thickness Calculator: TST From Measured Interval, Dip, and Borehole Geometry

Convert a measured drilling interval or outcrop traverse into true stratigraphic thickness using the Setchell equation, see how true thickness shrinks as dip increases for a fixed measured interval, or solve for the measured interval needed to capture a target true thickness. Everything updates live.

Why Measured Thickness Is Not True Thickness

A borehole rarely cuts a dipping bed at a right angle, and neither does a traverse across an outcrop. Whenever the line along which thickness is measured is not perpendicular to bedding, the measured interval is longer than the bed actually is. True stratigraphic thickness (TST) is the thickness measured perpendicular to bedding, the geologically meaningful quantity for correlating reservoir intervals, computing net pay, or comparing stratigraphic sections between wells.

This tool converts a measured thickness into TST for any combination of bed dip, dip azimuth, borehole inclination, and borehole azimuth, shows how sensitive true thickness is to dip in the common vertical-well case, and reverse solves for the measured interval a target true thickness requires.

The Setchell Equation

The standard formula for TST from a measured interval, following Tearpock and Bischke (1991), is TST = MT x [cos(dip) x cos(inclination) + sin(dip) x sin(inclination) x cos(borehole azimuth minus dip azimuth)], where MT is the measured thickness along the borehole or traverse, dip and dip azimuth describe the bed, and inclination and azimuth describe the borehole. This is commonly called the Setchell equation, and it is mathematically equivalent to taking the dot product between the borehole direction vector and the pole to bedding.

Two familiar special cases fall directly out of the general formula. For a vertical borehole, inclination is zero and the equation reduces to TST = MT x cos(dip), where MT is the true vertical thickness. For a horizontal traverse taken perpendicular to strike, inclination is 90 degrees and the borehole azimuth matches the dip azimuth, reducing the equation to TST = MT x sin(dip), where MT is the horizontal map width across the bed.

How Sensitive Is True Thickness to Dip

In a vertical well, the correction from measured to true thickness is small at low dip and grows quickly beyond about 30 to 40 degrees. A 100 ft measured interval only drops to about 87 ft of true thickness at 30 degrees of dip, but falls to 50 ft by 60 degrees and to under 20 ft by 80 degrees. This is why structural dip should always be checked before reporting bed thickness from a well that penetrates tilted strata, particularly in fold belts or near faults where dips can be steep.

Solving for the Required Measured Interval

Sometimes the question runs the other direction: given a target true thickness that a well needs to penetrate, how much measured interval must actually be logged. Rearranging the Setchell equation for MT gives a direct closed-form answer once the dip geometry and planned borehole trajectory are known.

Common Mistakes

Reporting measured depth interval as bed thickness without any dip correction, which overstates thickness in every well except a truly vertical one drilled through flat-lying beds.

Applying the simple cosine correction to a deviated well, when the full Setchell equation with borehole inclination and azimuth is needed once the hole departs meaningfully from vertical.

Mixing up dip azimuth and borehole azimuth, since the angle between them, not either azimuth alone, controls how much the two directions reinforce or cancel in the thickness calculation.

Ignoring changing dip across a thick interval, since the single-dip Setchell equation assumes constant dip and constant borehole trajectory across the measured interval, an assumption that weakens over long or structurally complex intervals.

Who Uses This

Wellsite and development geologists correcting measured pay intervals to true stratigraphic thickness for net pay and reserve calculations.

Structural geologists converting outcrop traverse widths into true bed thickness across dipping strata.

Directional drilling and geosteering engineers planning the measured interval needed to penetrate a target zone thickness.

Structural geology students and instructors working through true versus apparent thickness exercises.

Try It Above

Use the quick solve tab for a single TST and TVT calculation from any dip and borehole geometry, move to the thickness versus dip tab to see how sensitive true thickness is to dip in a vertical well or horizontal traverse, or use the solve tab when you need the measured interval a target true thickness requires.

Related Calculators and References

Continue building your workflow with the Elastic Moduli Interconversion Calculator. For background on the formulas used above, see Berg (2012), Calculating True Stratigraphic and True Vertical Thickness.