How are geologic maps made?

Types of data used to make a geologic map

Well construction reports (WCRs)

Well construction reports (WCRs) are completed by drillers for wells drilled in Wisconsin. WCRs contain information about the location of the well, the geology encountered, and water-table elevation. The sheer number of WCRs makes them a useful tool for interpreting Wisconsin’s bedrock geology when it is buried under glacial sediments.

Ground-penetrating radar (GPR)

Ground-penetrating radar uses the reflection of high-frequency radio waves (radar) waves off of boundaries between different types of rock or sediment buried under the earth’s surface. Geologists use GPR to interpret the geometry and distribution of these rock or sediment boundaries.

Aeromagnetic and gravity surveys

Aeromagnetic and gravity surveys measure physical properties of bedrock. Aeromagnetic surveys (left) show the distribution of magnetic minerals in the rocks while gravity surveys indicate variations in rock density over an area. Geologists use knowledge of the distribution of minerals and rock density to interpret what rock types are present buried under the earth’s surface.

Well logs

The graph at left shows an example of a geophysical log. Geophysical logs are created by lowering a tool into a well hole to measure certain rock properties. The rock properties are displayed as a graph, with the depth of the well hole on the y-axis, and the rock property on the x-axis. This example shows a gamma ray log, which measures gamma radiation produced by certain minerals contained in the rock column.

Rock cores

The cylindrical rock core in this box came from a hole drilled into the earth. In Wisconsin, much of the bedrock is hidden by a thick pile of sediments that were deposited during the last ice age. Cores allow geologists to study a continuous column of this buried rock.

Other sources of data

Geologists also look at the rocks exposed in road cuts, outcrops, quarries, and river cuts when mapping an area.

Putting the information together

Geologic information is drawn on top of a base map. The base map orients the user in space by showing the location of rivers, lakes, roads, hills, and valleys.

The following images show a base map (aerial photo), a geologic map, and the same map combined with a satellite image to highlight the topography.