Office of Levee Safety

Figure 2. EM geophysical unit on Gator UTV.

Electromagnetic (EM) Terrain Survey

EM surveys will be run at the top and base of levees (if conditions allow) identified for this effort. Due to the possibility of multiple passes (top and base), we anticipate up to 200 miles of linear EM data will be gathered along the up to 100 miles identified in the combined levee systems. The EM unit, mounted to a utility-task vehicle for efficient data collection (Figure 2), measures how the subsurface responds to electrical charge. This unit records measurements while in motion and can deliver thousands of data points in a short time. Results from the EM surveys are shown in easy-to-understand color images and are expected to highlight significant changes to levee composition and/or underlying geologic materials such as zones of sand and gravel, unusual groundwater flow paths, or voids that could suggest anomalies.

Figure 3. Generalized levee cross-section showing anticipated locations for geophysical data collection.

Electrical Resistivity (ER) Survey

The ER method involves physical connection with the ground (via metal stakes) and provides information about the levee and/or underlying materials. Figure 3 shows an example of where the ER will be collected in each of the areas. ER results are shown in cross-section view and provide information about the depth and extent of potential anomalous zones.

Geophysical Limitations

Electrical geophysical methods model how the subsurface responds to electrical charge. These methods provide the best results on undisturbed, natural ground. Data quality can become compromised when run near underground and overhead utilities, drainage structures, surface and subsurface metal, inhomogeneous fill materials, etc. We do expect these features to be present at the sites and will attempt to keep equipment away from these features whenever possible. Electrical geophysical methods are the most beneficial when there is significant electrical contrast between the subsurface target and its surroundings. Geophysical results represent an area generally beneath each transect and may not represent what lies in areas not covered by the geophysical survey.