Detection of buried manhole covers/ valve covers, etc. can be a time-consuming process. Many standard metal detectors don’t have the detection depth range required and may struggle to distinguish between buried aluminum drink cans and the ferrous materials which make up most manhole covers. Use of a detector based on a magneto-meter design is one way to overcome this problem. These devices are created to detect changes in the earth’s magnetic field caused by the magnetic effect of the buried metal. The improved sensitivity of these devices enables man-holes to be detected at depths of more than two meters (yards) while still managing to reject aluminum and other non-ferrous materials.
A manhole, for instance, will develop a north and south magnetic pole when left in the ground. Its effect on the earth’s magnetic field is maximized at the edges. This means the outline of the manhole can be determined with a null in the center.
There is also a polarity (north and south pole) to the peak response. Since the manhole will develop a magnetized characteristic when left stationary, we can detect which is north or south. This effect is particularly useful in determining if the buried object is a single lump of metal or two smaller objects, as smaller objects will tend to have one peak and the same polarity. For setting land perimeters easily, ferrous bars or even permanent bar magnets may be used. These accentuate the effect on the Earth’s magnetic field. It is best if these are always buried with the same polarity, (in the same direction) to cause less confusion.
One frequently asked question is, “Can a magnetometer device locate cast iron pipe?” The answer is probably “yes.” People ask this question because it can be difficult to locate pipes that have insulated mechanical joints using standard electromagnetic pipe locators. Also, cast iron pipes are often un-coated and therefore not insulated from the ground. This results in difficulty getting a detection signal to travel along the pipe. The signal will often “leak off” into the ground over just a few meters.
The magnetometer does not succumb to these situations and, in fact, has the added benefit that no tracing signal need be applied to the pipe. The mag-netometer detects the position of the pipe and will often give a larger response at each end of a pipe section. A segmented pipe generally has sections that are the same length, so if the peaks are regular and consistent during a locate, it helps to confirm the correct pipe is probably being detected.
Steve Benzie is Technical Director for Vivax-Metrotech Corp. He trained as an engineer in the Ministry of Defense and achieved higher national certification in electrical and electronic engineering. He has more than 30 years’ experience in the locating buried utilities industry.