Learning Module 3: Constructing a Geologic Cross Section
Geologic cross sections provided two-dimensional slice of Earth's subsurface and is used to help understand geologic conditions that occur in specific areas of the cross section. Creating and evaluating cross-section is a very important aspect of the geoscience profession. The following exercise presents an overview of how to create cross sections and use them as a scientific evaluation tool.
To construct a geologic cross section, you need a map showing the location of the boreholes from which the geologic data were obtained, and the borehole logs that contain the information concerning the underlying sediments and bedrock. The map will provide a horizontal scale, or distance between the boreholes and a projection of elevation. The space between the boreholes is interpreted from the map scale. If the cross section is to have the same horizontal scale as the map, the scales of the map and cross section are the same. However, if you want to enlarge the cross section order to see additional detail, you need to reduce the ratio of the scales. So, if the map scale is 1 inch equals 40 feet, making it larger would require changing the horizontal scale on the cross section to 1 inch equals 20 feet; in essence this doubles the size of the horizontal aspect of the cross section.
It is common to increase the vertical scale of a cross section in order to view the geologic details. Expanding the vertical scale is known as "vertical exaggeration". It is typical to increase the vertical scale ratio by an order of magnitude compared to that of the horizontal scale. So if the horizontal scale is 1 inch equals 100 feet, a typical vertical scale for the geologic cross section would be 1 inch equals 10 feet. An interesting note from the the Woburn trial relates to Judge Skinner's not being familiar with the standard professional use of vertical exaggeration when preparing geologic cross sections. He initially disallowed Dr. Pinder's court exhibits because they utilized vertical exaggeration to view the geologic data. The plaintiffs frantically re-created the cross sections at a one-to-one scale, only to have the cross sections cover nearly an entire wall of the courtroom.
Laying out the Data
An example of a geologic map and cross section is presented at the attached link to provide you with a typical layout for setting up the horizontal and vertical scales. This example is a copy of the Ohio bedrock geology map. You will note that symmetry of the geologic units on the western half of the state, with the older units being in the center of the symmetry. This symmetry and age relation indicates an structural anticline. The cross section shown at the bottom of the page, which reflects the subsurface structure between points A-A' also shows this up-arched sequence of rocks that form an anticline. Note the vertical exaggeration on the cross-section.
The following three files contain the data necessary to construct a geologic profile of the Aberjona River Valley. The first file is a map showing the location of the geologic cross section and location of the boreholes used for the lithologic logs. The second file presents the lithology for each of the boreholes, the borehole data is presented in depth from ground surface on the left (these elevations are relative to the ground surface, zero (0) value, increasing downward). The Land Surface Elevations and column labeled Top Elev., which are on the right side of the spreadsheet are referenced to average sea level (asl). An average sea level value of zero (0) represents the elevation at sea level, positive values extend above sea level. Negative values are below sea level. The third file is a template for constructing a cross section. The blank spaces below the cross section are intended to be color-coded to show the different colors used to identified each lithology presented on the geologic cross section.
- Base map (Excel 87kB Jun19 09)
- Lithologic logs (Excel 87kB Jun19 09)
- Cross-section template (Excel 1.4MB Jun19 09)
Follow these general rules for making a geologic cross section.
- Decide what the geologic cross section is going to be used for, and use this to guide you in selecting the appropriate scales.
- Choose appropriate vertical and horizontal scales.
- On the map, locate the well or borehole positions, land service elevations, depth of the well and the number of geologic units in each well bore.
- Transfer the geologic information from each will long to the cross-section. This information represents discrete points of knowledge about the subsurface geology. Part of the geologist skills is interpretation from these discrete points of knowledge to those areas that lie in between. Part of this exercise is determining what units can be lumped together versus those which should be split apart. Again, this is mostly related to the detail that is trying to be displayed.
- Correlate the geologic information between boreholes. Applying knowledge of the specific depositional features of the rock or sediments can be used to increase the accuracy of the model. Look for differences in lithology, texture, or sediment or rock properties as a guide to defining contacts between contiguous geologic units. Use solid lines to indicate reasonably certain relations between discrete data points. Dashed lines are used to indicate uncertainty or inferred data. Areas where does not exist are typically labeled with question marks.
- Incorporate a legend into the cross-section to explain the types of geologic materials present. The legend should be placed at the bottom of the profile.
- Use appropriate orientations and landmark information to help the viewer relate to the cross sections position in space relative to recognizable features (buildings, streets, streams, etc.)
- Include vertical and horizontal scales along with the statement of vertical exaggeration.
Some Things to Consider
After completing the cross-section referenced above, in reference to that cross-section answer the following questions.1. Which units in the geologic cross section appear to be aquifers and able to store and transmit significant volumes of water? Assume that sand and gravel units have the highest hydraulic conductivity and clay and bedrock have the lower hydraulic conductivity's.
2. Which units in the geologic cross section are confining layers that are not able to transmit and store significant volumes of water?
3. Assume that each of the wells shown in the cross section is open or screened to allow water to enter the wellbore only over the bottom 10 feet of the well. Use your geologic cross section to determine which wells appear to be hydraulically connected to the most permeable material such as sand or sand and gravel?