An 800-pound dart-shaped instrument is the latest tool to map beneath the surface of levees in a partnership between the U.S. Army Corps of Engineers (USACE) Vicksburg District and the U.S. Geological Survey (USGS).
For the first time, a regional geophysical survey will provide a full overview of 800 miles of the Mississippi River mainline levees. Sometimes called a bird, the frequency domain system suspended from a helicopter will use frequency technology similar to LiDAR to differentiate between various soil layers and penetrate about 150 feet under the surface.
Data will be collected in three passes: along the riverside by the levee toe (the area where the base meets the ground), along the landside by the levee toe, and offset from the landside toe of the levee.
District project manager Kent Parrish explained that the survey data will provide a valuable resource that helps determine how geotechnical crews can prioritize their efforts, as well as help anticipate seepage around the levees.
“Borings show us the general lay of the land under the surface, but using this technology is like getting an MRI on your brain,” Parrish said. “It can tell me whether it senses silt, sand, clay, or gravel and show me data between the boring areas, like where the clay blanket might be too thin.”
The clay blanket beneath the land side of a levee is crucial to preventing seepage. As river levels rise near a levee, the increased pressure pushes water down through coarse-grained soils and up against the clay blanket. If the clay is too thin, it can rupture and allow water to escape from the protection of the levee. This increased flow can also undermine the levee’s foundation, resulting in a breach or collapse that causes levee failure.
In the past, the district mitigated seepage by building smaller structures called berms alongside the affected levees. Data from geotechnical borings, or large-scale drilling designed to reveal soil levels without disturbance, are another major tool. Conducted approximately every 500 feet along the levees, borings provide a small snapshot of the subsurface in a given area.
The survey is in part a result of the Mississippi Alluvial Plain (MAP) Water Availability Study through the U.S. Geological Survey. Appropriated by Congress in 2017, the project seeks to better understand how groundwater resources in the 29,000 square mile area are impacted by large amounts of agricultural activity. USGS conducted a trial survey near Shellmound, Mississippi, earlier this year to map out the U.S. Department of Agriculture-Agricultural Research Services’ Groundwater Transfer and Injection Project (GTIP). The successful trial paved the way for the regional survey, which began in Sikeston, Missouri, and will collect data as far south as Baton Rouge, Louisiana.
The system itself was contracted by the U.S. Geological Survey (USGS) through Canadian-based geoscience technology company CGG, which was recently acquired by Xcalibur Multiphysics Group. Powered by an external battery, four main parts work in tandem to extract and record a wealth of data. The transmitting section inside the rounded nose tracks six different frequencies from the subsurface ranging from 400 hertz to 140,000 kilohertz. The lower the frequency, the deeper the penetration into the earth. The bucking section cancels any external interference the system may pick up while in flight, including noise from the helicopter and various defects on the ground. These parameters are recalibrated before each flight to ensure accuracy. Next, a magnetometer records the changes in magnetic field throughout the surveyed area. Finally, the rear receiver section records the signals from the ground as a result of the original transmission.
The survey design and the system itself are thirty-five to forty years old, but the inner technology continues to be updated.
Will McRae, a civil engineer with the district’s Levee Safety Center, described the survey as a way to connect the dots between current data.
“For the most part, soil conditions underground may not change much, but you never truly know what’s between different borings. The idea behind the survey is to get a continuous look at the subsurface,” McRae said.
Support equipment for the survey include a Eurocopter AS350-B3, the single-engine light utility helicopter used for transport. On the cabin floorboard is the spectrometer pack, a dense, rectangular instrument the size of a rollaboard suitcase that will measure potassium, thorium, and cesium in the soil throughout the flight. Above it is the data acquisition rack, a cart of consoles reminiscent of an old school home theater system. The system’s findings are recorded there, and a hulking motherboard allows a pilot or passenger to modify settings as needed. Highly specialized equipment aside, the pilot’s navigation over the levees will be directed by something more familiar to the average person: an Android smartphone.
Following the survey, USGS will process the trove of raw data before passing it along to geotechnical professionals at the USACE Engineer Research and Development Center (ERDC), which will identify anomalies like areas with thin clay layers through careful analysis. The results of the analysis can then be used by multiple districts to determine if further action is required.
“The different layers from the data are a visual representation of how the alluvial valley was laid down over centuries,” Parrish said.