Measuring Sand Volumes of Chicago Beaches Using Ground Penetrating Radar

Ground penetrating radar (GPR) was deployed across Chicago beaches in the summer of 2022 as part of an Illinois State Geological Survey regional sand-assessment project coupling onshore and offshore geophysical data for sand volumetrics. Here, we focus on the onshore portion of the project. The primary goal was to identify and characterize the base of the sand unit for regional mapping purposes.

A 200 MHz unit (by GSSI) was used, with data collected from over a dozen beaches in time mode, with an internal GPS unit tracking location. Data were processed using the Radan 7 software program. Processing steps included (1) noise band removal, (2) an IIR vertical filter, and (3) application of exponential range gain. The dielectric constant was set to 25 (standard for wet sands) for the depth conversion, appropriate for our position at or below lake level.

Preliminary insights into the subsurface data show that GPR facies and sand thicknesses vary from beach to beach and, in some instances, across individual beaches (e.g., in the alongshore direction towards downdrift groins). Sandy deposits are identified as packages of continuous, subparallel to gently inclined radar reflections. These distinct packages are not recognized or resolved in underlying layers, where the return signals are incoherent or reflection-free. Thin, parallel layers within the upper 1.5 meters of subsurface appear as low-amplitude reflections across 63rd St. on Chicago's South Side, for example, where packages of sand between 1.5 and 4 meters in depth are higher amplitude, longer in wavelength, and contain lakeward-dipping radar reflections. A disruption in signal beneath 4 meters is attributed to the underlying clay till or limestone bedrock layers. This abrupt transition is mapped as the base of the sand.

Sand thickness is greatest at Montrose Beach, an example location from Chicago's North Side. The contact between sand and underlying unit here is mapped between 6 and 8 meters in depth, where a lack of signal is interspersed with strong parallel reflections that suggest the base of sand may even extend to greater depths (locally). Further processing is necessary to better capture the unique identities of and map GPR facies across Chicago's beaches before sand volumetrics can be undertaken to help guide future sand management.