Changes

:<math>\mbox{TST}  = (\mbox{TVD}_{b} - \mbox{TVD}_{t}) \times (\cos \delta') - [(\mbox{NSD}_{b} - \mbox{NSD}_{t})^{2} + \, (\mbox{EWD}_{b} - \mbox{EWD}_{t})^{2}]^{1/2} \times \sin \delta')</math>

:<math>\mbox{TST}  = (\mbox{TVD}_{b} - \mbox{TVD}_{t}) \times (\cos \delta') - [(\mbox{NSD}_{b} - \mbox{NSD}_{t})^{2} + \, (\mbox{EWD}_{b} - \mbox{EWD}_{t})^{2}]^{1/2} \times \sin \delta')</math>

In this equation, δ′ indicates the apparent dip of the bed in the direction of the horizontal displacement ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig6.png|Figure 6]]), which is written as

In this equation, δ′ indicates the apparent [[dip]] of the bed in the direction of the horizontal displacement ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig6.png|Figure 6]]), which is written as

:<math>\delta' = \tan^{-1} [\tan \delta \cos (\beta - \varepsilon)]</math>

:<math>\delta' = \tan^{-1} [\tan \delta \cos (\beta - \varepsilon)]</math>

==True vertical thickness==

==True vertical thickness==

''True vertical thickness'' (TVT) is the thickness of a geological unit in a well measured in the vertical direction ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig2.png|Figure 2]]). It is a valuable measure for volumetric calculations because it is unaffected by variations in the dip of the unit and can be derived by subtracting computer-gridded structural horizons. In a deviated well with a nonhorizontal unit, the TVT is difficult to calculate because, as the well steps out horizontally, it no longer cuts the bottom of the unit vertically below the point where it penetrated the top of the unit ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig5.png|Figure 5]]). If the dip is in the same direction as the deviation, the unit will appear thicker than it actually is, whereas if the dip is in the opposite direction, the unit will be shortened. The TVT is calculated according to the following formula:

''True vertical thickness'' (TVT) is the thickness of a geological unit in a well measured in the vertical direction ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig2.png|Figure 2]]). It is a valuable measure for volumetric calculations because it is unaffected by variations in the [[dip]] of the unit and can be derived by subtracting computer-gridded structural horizons. In a deviated well with a nonhorizontal unit, the TVT is difficult to calculate because, as the well steps out horizontally, it no longer cuts the bottom of the unit vertically below the point where it penetrated the top of the unit ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig5.png|Figure 5]]). If the dip is in the same direction as the deviation, the unit will appear thicker than it actually is, whereas if the dip is in the opposite direction, the unit will be shortened. The TVT is calculated according to the following formula:

:<math>\mbox{TVT}  = (\mbox{TVD}_{b} - \mbox{TVD}_{t}) - [(\mbox{NSD}_{b} - \mbox{NSD}_{t})^{2} + \, (\mbox{EWD}_{b} - \mbox{EWD}_{t})^{2}]^{1/2} \times \tan \delta'</math>

:<math>\mbox{TVT}  = (\mbox{TVD}_{b} - \mbox{TVD}_{t}) - [(\mbox{NSD}_{b} - \mbox{NSD}_{t})^{2} + \, (\mbox{EWD}_{b} - \mbox{EWD}_{t})^{2}]^{1/2} \times \tan \delta'</math>