Lecture 4: Thickness and depth

Main Ideas

• True (stratigraphic) thickness, (T): The perpendicular distance between the two parallel planes that bound a tabular body, such as a sedimentary layer.
• Apparent thickness, (T’): The distance between the bounding planes measured in a direction other than perpendicular to them. It is always greater than or equal to the true thickness.
• Outcrop width, (W): The surface distance between the traces of the bounding planes, measured perpendicular to strike on horizontal or sloping ground.
• Depth, (D): The vertical distance from a reference level—normally the ground surface—to a point, line, or plane.

Ways to Determine Thickness

1. Direct Field Measurement

Stratigraphic thickness can be measured directly in the field using a Jacob staff, a stick of known length fitted with a clinometer.

To measure a sequence of beds:

1. Incline the staff so that the line of sight along its top is parallel to the layer’s dip direction.
2. Use the clinometer to match the dip.
3. The thickness between the top of the beds and the observer’s feet corresponds to the staff’s length.
4. Repeat as needed to cover a thicker sequence. The combined path gives the sequence thickness.

2. Indirect Calculation Using Trigonometry

For a traverse across a layer on horizontal ground:

• If the traverse is perpendicular to strike, calculate thickness from the traverse length and the layer’s dip.
• If the traverse is oblique to strike, also account for the angle between the traverse and strike.

For a traverse on sloping ground, account for both ground slope and layer dip:

• If the slope and dip point in opposite directions, use their sum.
• If they point in the same direction, use their difference.

In the most general case—an oblique traverse across sloping ground—thickness depends on the traverse length, ground slope, traverse direction relative to strike, and layer dip.

3. Converting Well Measurements to True Thickness

A well measures apparent thickness along its path. This can be converted to true thickness using the layer dip and the well’s drift from vertical. The appropriate calculation depends on whether the well is vertical, drifts down dip, or drifts up dip.

The lecture notes that these formulas can be difficult to remember and introduces linear algebra as a simpler approach in the next lecture.

Speakers and Sources

• Speaker: Unnamed lecturer or narrator.
• Source cited: “Reagan,” whose relevant sections the lecturer recommends reading.