What is Apparent Dip and how is it related to True Dip?

megalodon

Staff member
The dip measured in any vertical plane that is not perpendicular to the strike line is called Apparent dip. We can calculate true dip from apparent dip based on strike direction using Trigonometry. The Geological cross-sections can only contain true dip if they are drawn perpendicular to strike. The cross-sections in other directions contain apparent dip.

What is Apparent Dip?
Apparent dip is the inclination of the geologic beds recorded from any vertical cross-section that is not perpendicular to the strike of the beds. We can record the true dip in field when the vertical cross-section is perpendicular to the strike of the beds.

How to Calculate Trup Dip from Apparent Dip?
We can calculate true dip using Stereographic projection through a graphic method or using Orthographic projections through trigonometric functions. If you have limited data, then it's easy to calculate using stereographic projection. But if the data is huge or you want accurate results, then it's better to calculate using the formula. The beginners should practice both methods for a better understanding.

a = apparent dip
b = angle between and strike direction and apparent dip direction
c = true dip

True dip (c) = arctan(tan a / sin b)
Apparent dip (a) = arctan (sin b - tan c)

We can calculate true dip using apparent dip and vice versa using these formulae.

Why is an Apparent Dip always less than a True Dip?
True dip is the steepest angle of inclination relative to a horizontal plane recorded from a vertical cross-section perpendicular to the strike of the geologic beds. The angle of inclination calculated from any other cross-section is called apparent dip and it can never be equal to or higher than the true dip.

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