Structural measurement: the definition and measurement of true dip, apparent dip and strike; the recording of orientation data; the calculation of the true (vertical and stratigraphic) thickness of a bed from its outcrop width, dip and the slope of the ground; the use of trigonometry in structural calculations.

What this dot point is asking

OCR wants you to define and measure true dip, apparent dip and strike, to record orientation data, and to calculate the true thickness of a bed from its outcrop width and dip using trigonometry.

The answer

True dip, apparent dip and strike

These three terms describe the orientation of a tilted bed:

The true dip is the steepest line down the plane; the strike is the horizontal line across it (perpendicular to the dip). An apparent dip, measured in any oblique direction, is always less than the true dip, because no direction climbs more steeply than the true dip, and along the strike the dip is zero.

Orientation is recorded as the dip amount and direction (for example "$30^{\circ}$ to $120^{\circ}$"), measured with a compass-clinometer.

Calculating true thickness

The true thickness of a bed is measured perpendicular to the bedding, not across the ground, so a dipping bed's true thickness is less than its horizontal outcrop width. For a bed measured horizontally across its outcrop (at right angles to the strike) on flat ground:

where $t$ is the true (perpendicular) thickness, $w$ is the horizontal outcrop width and $\theta$ is the dip. This comes from the right-angled triangle in which the true thickness is the side opposite the dip angle.

If the bed is measured along a borehole or the ground slopes, the geometry adds or subtracts the slope, but the core relationship is the sine of the dip times the measured width.

Why this matters

True thickness is the real stratigraphic thickness of the unit, needed to compare sequences, estimate volumes (for example of an ore or reservoir) and correlate beds. Confusing outcrop width with true thickness over-estimates the bed.

Examples in context

Example 1. Estimating a reservoir's thickness. Calculating the true thickness of a dipping reservoir sandstone from its outcrop width and dip gives the real stratigraphic thickness needed to estimate the volume of rock (and so the hydrocarbon or water it could hold).

Example 2. Apparent dip in a road cutting. A bed seen in a road cutting that runs obliquely to the strike appears to dip more gently than its true dip; recognising this prevents under-estimating the true dip from an oblique exposure.

Try this

Q1. Define the strike of a bed. [1 mark]

• Cue. The compass direction of a horizontal line on the bedding plane, at right angles to the true dip.

Q2. A bed dips at $40^{\circ}$ and its horizontal outcrop width (perpendicular to strike, flat ground) is $30\ \mathrm{m}$. Calculate the true thickness. [2 marks]

• Cue. $t = w\sin\theta = 30 \times \sin 40^{\circ} \approx 30 \times 0.643 \approx 19.3\ \mathrm{m}$.

Q3. Explain why an apparent dip is always less than the true dip. [2 marks]

• Cue. The true dip is the maximum slope of the plane (perpendicular to strike); any other direction across the plane climbs less steeply, so the apparent dip is shallower (zero along the strike).