Building A New Map of the Mahomet Aquifer

Our current understanding of the distribution and characteristics of the Mahomet aquifer is based largely on compilation and analysis of information collected from the driller’s descriptions of materials encountered during the drilling of thousands of private water wells, supplemented with small collections of research-quality data collected by geologists and geophysicists. This approach has been the state of the art in geologic mapping for many decades and the accuracy of these maps is limited by the high cost and very limited information provided by the research-quality data. In the past decade, significant advancements have been made in the collection of one type of geophysical data. This new data is much more accurate than previous versions and while still expensive, the amount and quality of information provided is incomparable. This new data is collected by helicopter and provides a type of aerial geophysical survey of the upper 600 feet of the subsurface. This new geophysical survey data, called HTEM for short (which stands for helicopter-based time-domain electromagnetics – a real mouthful!), is particularly beneficial for groundwater resource mapping studies. In the past several years HTEM surveys have been transforming our understanding of shallow aquifers around the globe!

For the past several years, the Illinois State Geological Survey (ISGS) has been working to build an HTEM mapping program. One of the largest ISGS priorities for this program is the HTEM-based mapping and characterization of the Mahomet aquifer. A new HTEM-based map will be much more accurate than the existing maps that were developed by traditional mapping methods. Because each survey is fairly expensive, the plan is to break the complete mapping effort into separate pieces, with each effort being separately funded and producing a new map describing the extent and characteristics of that portion of the Mahomet aquifer. Together, each project will incrementally produce the most comprehensive measurements and maps of the Mahomet aquifer to date. Data from each project will add to our understanding of the sediment distribution, allowing us to build detailed three-dimensional maps of the Mahomet aquifer system. The evolving three-dimensional map that comes from this program will be used to help answer important questions regarding sustainable water use, protection of water quality, estimation of groundwater recharge, and protection of resource security.

Success in Project Funding!

In 2022, the ISGS secured funding for the first HTEM survey of the geologic deposits for a portion of the Mahomet aquifer. This survey project targeted the deposits of Champaign County (Figure 1). The data collection was completed in January 2023, and the processing, analysis and interpretation of the data collected are scheduled to be completed in late 2024. More information will be available soon on the Champaign County HTEM Project page.

In 2023, the ISGS secured funding for a second HTEM survey project for Mahomet aquifer sediments across large portions of Iroquois, Ford, and Vermilion counties (Figure 1). This survey will be significantly larger, and data collection flights will be conducted in 2024 between late January and late February ­– weather depending. More information will be available soon on the Northern Mahomet Aquifer HTEM Project page.

Geophysical Mapping of the Mahomet Aquifer – Study Area

Flight Details

The survey will be conducted by a helicopter flying at an altitude of 100–250 feet (30–75 m) and a speed of 20 miles per hour (about 32 km/h). The helicopter will traverse across much of the multi-county study area, but it will not fly over buildings, developed areas, or highly populated areas. The geophysical equipment will be suspended under the helicopter at about 115 ft (35 m) above the ground. The noise on the ground from the helicopter flyover will be audible for about two to four minutes. The helicopter will fly back and forth across the study area in regular gridlines (flight lines) (Figure 1).

Safety

These geophysical surveys present no risks to humans, animals, or infrastructure. A very weak magnetic field is generated by the system and is measured by high-precision electronic equipment, all controlled by an onboard computer. The magnetic field generated during the measurements is weaker than what you would be exposed to if you were watching an LCD TV or blow-drying your hair, and it disappears within seconds!

Additional Resource

The Future of Science of the Mahomet Aquifer, 2018 [ISGS Circular 594]. Authors: Steven E. Brown, Jason F. Thomason, and Kisa E. Mwakanyamale. Download free: https://hdl.handle.net/2142/99079

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