Detecting Buried Ferrous Utilities

Finding buried manhole covers, valve box covers, cleanouts, or tanks can be a time-consuming process. Many standard metal detectors don’t have the detection depth range required and struggle to easily distinguish between buried aluminum cans and ferrous material that make up most manhole covers, etc.

Overcoming these issues is easily solved by using a magnetometer-designed locator such as the VM-880 or VM-585. The VM-880 being a metal detector, while the VM-585 is a metal detector and pipe and cable locator. The improved sensitivity in these units enables manholes to be detected at depths of more than six feet (1.8m) while still rejecting aluminum and other non-ferrous materials.

The VM-585 combination locator and ferrous metal detector

Magnetometer devices detect changes in the Earth’s magnetic field caused by the magnetic effect of the buried metal. A manhole, for instance, will develop north and south magnetic poles when left stationary. The impact 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 middle. The response over a manhole will be similar to the below illustration.

Notice also there is a polarity to the peak response. That is to say, there is a north and south pole. Remember that the manhole will develop a magnetized characteristic when left stationary, so we can detect which is north or south.

A typical display is showing the field’s polarity. In this case, +ve (i.e., North pole) and a bar graph the indicating signal strength.

This effect is beneficial 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. A typical response from a small valve cover is shown in the below illustration.

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 (i.e., the same way up) to cause less confusion.

Detecting cast iron pipes

A frequently asked question is, “Can a magnetometer device locate cast iron pipe?”. The answer is probably “yes.” People ask this question because using standard electromagnetic pipe locators can be difficult with pipes with insulated mechanical joints. Also, cast iron pipes are often uncoated 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 has the added benefit that no tracing signal needs to be applied to the pipe. The magnetometer will detect the position of the pipe and will often give a larger response at each end of a pipe section. This is very useful as it helps identify that a pipe is being detected as the sections will generally be the same length, so if the peaks are regular and consistent, it confirms that the pipe is probably being detected.

For information on Ferrous Metal Detectors, please visit the web pages of the VM-880 and VM-585.

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