ESI uses Ground Penetrating radar that can Identify Metallic & Non Metallic objects to a depth of 18 feet. A metal detector is not Ground Penetrating radar. ESI performs tank sweeps around the who property not just the perimeter of the structure. Oil and gas tanks can be anywhere.
Metal Pipes discovered during a GPR Investigation in Bay Head NJ required further excavation and soil sampling. No tanks were discovered but the property required extensive remediation due to gasoline & lead contamination in the soil and groundwater. The first contractor missed these pipes with his metal detector as they were buried below 3 feet.
Recently ESI during a Tank Sweep anomalie excavation identified a buried freezer from the 1950's. The freezer was buried approximately 20 feet from the house and was missed by another companies tank sweep using a metal detector. The Asbestos insulation was also buried in the area that the refrigerator was buried & needed Asbestos remediation to protect contact from asbestos fibers in the soil.