Structure Contours and Isopachs

In addition to traditional geologic maps, which represent the surface of the earth, there are maps that can be used to describe underground rock layers. These maps typically take advantage of the concept of contours. Just as contours can be used to show changes in elevation, they can be used to show any value that changes depending on where you are.

For example, a weather forecaster might contour temperatures or forecast precipitation:


Geologic Contour Maps

Geologists typically contour things having to do with rock layers. One type of contour map used by geologists is called an isopach map.

Isopach Maps

Isopach maps are used to show variations in the thickness of a stratigraphic unit.

For example, a sandstone layer containing oil might “pinch out” against an overriding layer. In this case, an isopach map might be useful because it shows how thick the layer is at any given point. Notice in the following illustration how the black I-beam-style lines measure the bed thickness perpendicular to the angle of the bed.

Isochore Maps

In contrast to an isopach map, which shows the stratigraphic thickness of a layer, an isochore map shows a layer’s vertical extent, measured straight down through the bed no matter at what angle the bed is situated. For steeply dipping beds, this vertical thickness can be much greater than the bed’s stratigraphic thickness.

1-5-isochore-example

Understanding a unit’s thickness underground is useful when trying to decide where to drill. However, sometimes geologists are most interested in learning about the depth to a given layer (to determine how deep to drill), or the shape of a layer (to determine the most effective place to drill into an oil trap). That’s where a structure contour map is useful.

A structure contour map represents the surface of a specific structure. Imagine a salt dome, an underground structure that has steeply sloping sides and a domed top. It’s covered by many layers of rock.

Imagine stripping away all these layers until only the dome remains. A topographic map can be constructed that represents the shape of the dome. A structure contour map is just like a topographic map, except that it traces an underground structure rather than the earth’s surface.

1-5-cross-section-map-section

Images: “Folded chert layers, Oregon coast” by CarbonBrain via iStock; “Real-time Mesoscale Analysis” by NOAA National Weather Service; “Illustrations” by Top Energy Training