Chapter 79 Structure Contours and Thickness Maps

Year 4 · Structural Methods · G445-L1

Construct simple structure contours, distinguish isopach and isochore maps and calculate dip from controlled observations.

Fold geometry and shortening in a schematic layered succession. Original Geology Hub teaching diagram.

This independent lesson follows the topic framework of the supplied University of Peshawar BS Geology curriculum, session 2004–05 onward. It is newly written study material, not an official university course or qualification.

Learning objectives

Distinguish elevation structure and thickness information.
Construct simple contours from control points.
Check units orientation and interpolation assumptions.

Different maps answer different geological questions

A geological map shows rock units and structures at a mapped surface, usually the land surface. A structure-contour map represents the elevation or depth of a particular geological surface. Every point on one structure contour has the same value relative to a specified datum. Before contouring, verify that observations refer to the same horizon. A correlation error can create a false fold or fault more dramatically than a small numerical measurement error.

An isopach map portrays true stratigraphic thickness, whereas an isochore portrays vertical thickness. Depth below ground, depth below a common datum and elevation are different quantities. Mixing them without conversion can distort subsurface geometry. An incomplete borehole penetration gives a minimum thickness rather than a full value. Keep such censored observations visible instead of treating them as complete measurements.

Interpolate with geology and inspect the result

Contours can be constructed by interpolating between control points under a stated assumption, such as approximately planar geometry between nearby measurements. Closely spaced contours imply a steeper surface when contour intervals and scales are consistent. Faults may offset the horizon and require separate domains rather than smooth interpolation across the break. Sparse data can fit more than one structure. Geological knowledge guides the model but should not conceal unsupported detail.

A planar surface has a strike parallel to its structure contours and dips in the direction of decreasing elevation. Use horizontal spacing perpendicular to contours to estimate dip through the tangent relation. Depth contours reverse the numerical sense, so label the convention clearly. Topographic and structure contours intersect where a horizon reaches the ground. This relationship helps check whether a proposed subsurface surface is compatible with its mapped outcrop trace.

Defining the surface and datum before contouring

Choose one geological horizon and one reference datum before combining measurements. A horizon at 200 metres below ground at a well whose ground elevation is 300 metres has elevation 100 metres relative to that datum. A second well with a different ground elevation needs the same conversion. Contouring raw depths below different ground surfaces can create a false structure. Preserve the original values as well as the converted elevations.

Check horizon identity using stratigraphy, logs or other evidence. Similar sandstone at different levels is not a valid common datum merely because its lithology matches. If a borehole ends before reaching the horizon, it provides a constraint rather than a measured horizon elevation. Show that distinction instead of assigning a convenient value to fill a gap.

Interpolating a planar example and recognizing faults

Between observations at elevations 100 and 160 metres separated by 300 horizontal metres, linear interpolation places the 120-metre contour one-third of the way from the lower point, or 100 metres away. This construction assumes a steady slope along the connecting line. Additional points can test that assumption. A curved or folded surface may require a different interpolation, while insufficient control should remain visible rather than being hidden by closely spaced contours.

A fault may separate the horizon into displaced parts. Smooth contours drawn across it can erase the displacement and invent a broad fold. Treat separate structural domains explicitly and use observed fault relationships to constrain the offset where possible. A fault map should distinguish a traced intersection from a projected continuation. Numerical interpolation cannot repair a wrong geological correlation.

Reading geometry and thickness separately

For a planar horizon, strike parallels its elevation structure contours, and dip is toward decreasing elevation. Use contour spacing measured perpendicular to strike to calculate tan dip = vertical interval / horizontal separation. Closely spaced contours imply a steeper surface only if scales and contour interval are comparable. On depth maps, increasing values indicate a deeper surface, so the numerical direction differs from an elevation map.

An isochore represents vertical thickness, whereas an isopach represents true stratigraphic thickness. For parallel beds dipping 30 degrees, a vertical interval of 20 metres corresponds to about 17.3 metres true thickness using 20 cos 30 degrees. This formula assumes the specified geometry and a vertical interval. Erosion, variable bed orientation and incomplete penetration require separate treatment.

Checking the map against independent relationships

Where structure contours intersect matching topographic contours, they can help locate the predicted outcrop of the horizon. Compare that trace with mapped contacts. A mismatch may indicate incorrect correlation, a fault, an unsuitable planar assumption or an input error. Investigate the cause rather than moving the contour until the map simply looks consistent.

Add control-point locations, datum, units, contour interval and uncertain boundaries to the final map. Test an alternative surface through sparse data and state which new observation would distinguish it. A useful map makes both the inferred geometry and its evidential limits inspectable.

Worked activity

Two structure contours on a fictional planar bed are at elevations 100 and 150 metres and are separated by 200 horizontal metres perpendicular to strike. The tangent of dip is 50/200 = 0.25, giving approximately 14 degrees. If the map scale is 1:10,000, 200 metres corresponds to 2 centimetres. Show both conversions. Then introduce a fault between the control points and explain why the original planar interpolation may no longer be valid. The arithmetic is only as reliable as the horizon correlation and structural assumptions.

Practice and interpretation

Build a small contour map from four invented elevations and state the interpolation method. Add the points used, datum, contour interval and any faults. Draw one alternative surface that also fits sparse control. Ask which extra observation would distinguish the alternatives. A useful contour map is not merely smooth: it is traceable to measurements, consistent with geological relationships and honest about the information missing between control points.

Self-check

What does a structure contour represent?
Can vertical thickness be substituted for true thickness in dipping beds?
Why check horizon identity before contouring?

Check your answers

Equal elevation or depth values on one defined geological surface.
Not generally; their geometry differs.
Mis-correlated points can produce a false subsurface structure.

Sources and further study

The explanations and teaching examples are original. Use these institutional resources to check terminology and explore the subject further.

Chapter 79 multiple choice questions

Choose one best answer for each question. Open each answer panel after making your choice.

Two structure contours differ by 50 metres in elevation and are 200 horizontal metres apart perpendicular to strike. What dip is approximately implied?
1. 14 degrees
2. 7 degrees
3. 45 degrees
4. 76 degrees
Answer and explanation
A. 14 degrees The tangent of dip is 50/200 = 0.25. The angle whose tangent is 0.25 is approximately 14 degrees.
At a map scale of 1:10,000, what map spacing represents 200 horizontal metres?
1. 0.2 centimetre
2. 2 centimetres
3. 20 centimetres
4. 200 centimetres
Answer and explanation
B. 2 centimetres Two hundred metres equals 20,000 centimetres. Dividing by 10,000 gives 2 centimetres on the map.
A borehole stops before reaching the base of a unit. How should the recorded penetrated thickness be treated?
1. As the full thickness because drilling has ended
2. As zero thickness because the base is unknown
3. As a minimum rather than a complete unit thickness
4. As true thickness independent of hole orientation
Answer and explanation
C. As a minimum rather than a complete unit thickness Incomplete penetration supplies a lower bound. Treating it as a complete thickness can distort the map.
For a planar bed mapped with elevation structure contours, which relationship is correct?
1. Strike is perpendicular to contours and dip is toward increasing elevation
2. Dip direction is independent of contour values
3. Every contour marks a fault trace
4. Strike is parallel to the contours and dip is toward decreasing elevation
Answer and explanation
D. Strike is parallel to the contours and dip is toward decreasing elevation A structure contour is a horizontal line on the geological surface. Its direction gives strike, and dip descends toward lower elevations.
Control points accidentally refer to two different horizons. Why is smooth contour interpolation unreliable?
1. Mis-correlation can create a false subsurface structure
2. Smoothness guarantees the horizon identities are correct
3. The error changes only the map colors
4. Different horizons necessarily have equal elevations
Answer and explanation
A. Mis-correlation can create a false subsurface structure Horizon identity must be checked before contouring. A smooth surface cannot repair incorrect geological correlation.

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