The ground penetrating radar doesn’t work on electrically conductive host materials. Clay is a very common problem for the ground penetrating radar. If the radar doesn’t work well, you might have to rely on metal detectors or magnetometers. A magnetometer passively measures the earth magnetic field and could suffer greatly from noises in a city environment. Metal detectors are used more often.
A successful search for underground storage tanks involves a metal detector suitable for the site, an experienced operator, and luck. There are many metal detectors; they have different sensitivity, different investigation depth and different reactions to the noises. Very old underground storage tanks, such as gasoline and kerosene tanks, should be made of steel. Their capacity generally should not be less than 1000 gallon. The majority of them were buried less than 5 feet deep (from the top of the tank to the ground surface), although there are tanks buried much deeper. To avoid unnecessary noises, you should use a less sensitive metal detector; those used for searching coins are not good choices. If the metal detector is most sensitive in the depths where tanks could be buried (i.e., 2 to 5 feet), that would be ideal. The other important factor is the experience of the operator. Experienced operators sometimes could tell the responses of the metal detector are from large or small, deep or shallow, aboveground or underground metals. But still, sometimes there is just no way to tell whether the responses are from underground storage tanks or not.