Unusual passive electromagnetic data results while performing a ground-penetrating radar void detection survey on the Lake o' the Pines, Texas emergency spillway

Abstract

The Lake o' the Pines emergency spillway in Texas was demonstrating signs of under-seepage, leading to a request to perform a ground penetrating radar (GPR) survey to detect voids under the spillway. A GPR survey was performed with a Geophysical Survey Systems, Inc. (GSSI) SIR-4000 controller attached to a digital 350 HS antenna. As we also had an GSSI LineTrac sensor that was attached to the 350 HS antenna to assist with detection of active power lines when mapping out utilities, we left the LineTrac sensor attached to the antenna but with no expectation that it would provide any useful data. We performed an initial GPR survey on a small 10 foot by 10 foot grid to determine the rebar spacing and assist with the line locations to maximize depth of penetration by surveying between the rebar. While performing the survey of the entire spillway we noticed an unexpected response in the LineTrac data as we passed over the seams between slabs where water could be heard flowing under the slab. We surveyed the entire spillway at 1 foot line spacing and the LineTrac data was consistent with the audible seepage. We also performed a photogrammetry survey of the spillway with a small unmanned aerial system (UAS) to improve the interpretation. The integrated interpretation of the GPR data, the LineTrac data, and the photogrammetry will be discussed along with ideas for the origin of the unusual passive electromagnetic data.