This profile spans 36 meters in an undeveloped area of the University of South Florida campus in Tampa, Florida, called the USF Geopark. The geology of west-central Florida is described as covered karst: limestone bedrock is overlain by clays that are in turn overlain by sands. Sinkholes form where the limestone is dissolving away due to mildly acidic groundwater and the sands flow down into the voids in the limestone. (In the photo a piece of the deep limestone has been artificially placed on the surface, as an exhibit.)

Photo from http://hennarot.forest.usf.edu/main/depts/geosci/facilities/

Photo from http://hennarot.forest.usf.edu/main/depts/geosci/facilities/
Figure modified from Kruse et al., Geophysical Research Letters, 2006.

The GPR profile shown below was run near the right end of the diagram above. The strong irregular reflector at ~20-30 ns travel time corresponds to the top of the silty clayey sand. That layer shows a lot of diffraction hyperbola tails because the layer itself is very rough, and any corner or jaggedness will cause a diffraction.

The shallower diffractions at < 10 ns are caused by roots from the live oak trees, as can be seen in the photo.