Engineers continue to investigate 12-15 sinkholes that opened on April 25 in a gated community in Ocala, Fla. Residents have evacuated eight townhomes near a retention pond that was drained by one sinkhole, exposing several others in the pond. Sinkholes also opened on the slope above the pond and in a street on the other side of the townhomes, but no structures or buried infrastructure have been damaged or weakened. The displacements are temporary, says the president of Sentry Management Inc., which manages the development for the homeowners’ association.

Geo-Technologies Inc., Ocala, is investigating the sinkholes for the homeowners’ association and has issued a report on the holes near the building. The firm is preparing a second report on the larger holes near the retention pond. Geo-Tech declined to reveal the findings, which will go first to the association.

Sinkholes are common in Florida, where the bedrock is largely limestone, and this area has seen sinkhole activity before, including a 100-ft-dia hole in 2012, says Sean Lanier, Ocala city engineer. “These are also in a retention, where you’ve dug out the top sediment, so you’re right down on top of the lime rock. Those tend to have a propensity for having a sinkhole,” Lanier says.

Two techniques are common in sinkhole investigations: ground-penetrating radar (GPR) and electrical resistivity (ER), says Anthony Randazzo, University of Florida professor emeritus of geological sciences.

GPR can’t penetrate clay, so near-surface clay attenuates the radar signal, revealing little information about the subsurface. With ER, investigators pass an electric current underground and measure its resistance to flow, yielding a two-dimensional picture that can be analyzed for normal patterns or anomalies. These “reconnaissance techniques” are often used in tandem to target anomalies. Geo-Tech used only GPR because the limestone was covered by just 8-10 ft of sand, and clay layers were not prominent, says Jonathan Heath, Geo-Tech president.