A Civil Action - The Woburn Toxic Trial > Learning Modules > Geologic Setting > Creating a Geologic Cross-Section

# Learning Module: Geologic Setting

## Student Assignment

Geologic cross sections provided two-dimensional slice of the 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.

### Getting Started

To construct a geologic cross-section, you need a map showing the location of the boreholes from which the geologic data was obtained, and the borehole logs which contain the subsurface information related to soils 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 scale. So if the map scale is 1 in. equals 40 ft., making it larger would require changing the horizontal scale on the cross-section to 1 in. equals 20 ft.; 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 in. equals 100 ft., a typical vertical scale for geologic cross-section would be 1 in. equals 10 ft. An interesting note from the the Woburn trial was related to Judge Skinner's not being familiar with using vertical exaggeration when preparing geologic cross-sections. He initially disallowed Dr. Pinders court exhibits because they utilized vertical exaggeration to view the gelogic 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 geology 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. Note the vertical exaggeration on the example. Go to Woburn geologic log data to obtain the borehole logs for this exercise. At that same location there is a scaled map of the Woburn area that shows the location of the wells, this map can be used to layout the horizontal profile of the cross-section.

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.
- transferred 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.
- 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.

- What sedimentary unit appears to be the most significant in this geologic cross-section?
- What geologic unit underlies the W. R. Grace location? How would this unit affect migration of fluid to the sedimentary unit that underlies it?
- What type of geologic material underlies the Aberjona River? How will this material effect discharge through the river bottom to the subsurface?

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