UNDERGROUND STORAGE TANK SURVEY

Recently a large environmental consulting company asked us to conduct geophysical evaluations at multiple gas station sites. The purpose of the survey was to define the surface expression of buried underground storage tanks (USTs) for the purpose of locating environmental sampling locations/borings. The approximate locations of the USTs were known, and the client needed to know how close to drill without hitting them or encountering pea-gravel. Encountering pea-gravel with a hollow stem auger slows drilling progress and may cause a void to form beneath the asphalt.

As you can see from the GPR records, the fiber glass USTs were readily detectible even though they were located under a heavily reinforced concrete slab. The reinforcing causes much of the radar signal to be reflected immediately back to the antenna. Since the radar signal passes easiest through loose sands and fine gravels, and poorly through clays and silts, the detection of USTs and product lines through reinforced concrete is simpler at newer service stations. This is because the USTs and product lines are buried in excavations backfilled with pea-gravel. Older gas stations, on the other hand, are less likely to have reinforced concrete and more likely to have steel tanks. Therefore, the electromagnetic and magnetics instruments are highly effective at older stations.