Target of Investigation

Magnetic locators are nondestructive evaluation (NDE) technologies that detect and identify buried cables and pipes based on the measurement of the magnetic field surrounding them. The construction and utility service industries have used these instruments for applications similar to traditional cable and pipe locator systems due to their relative ease of use in locating subsurface features.⁽¹⁾ Magnetic locators are generally portable and cost-effective systems for providing an operator with clear, instantaneous feedback for marking and mapping utility lines ahead of construction or maintenance operations.

Description

Like other NDE methods, a magnetic locator makes use of a transmission source, a receiver instrument, and a data acquisition system to organize, present, and even interpret incoming signals. For these locators, the transmission source is a combination of the Earth’s magnetic field and the local magnetic field of the subsurface area under investigation. The receiver instrument tends to be a hand-held device with two sensors at a fixed distance apart that measure the magnetic field strength at their respective points within the device. The data acquisition system, which is typically integrated into the receiver device, provides a visual display of the automatically processed signals for the operator as well as an audible tone that indicates proximity to a buried utility.⁽²⁾

Physical Principle

Magnetic locators operate on the principle of detecting the difference in the magnetic field present at two sensors spaced a fixed distance apart. In the absence of a buried object, the field at both sensors is the same. The presence of an object made of iron or steel results in a difference between the magnetic field at the top and the bottom sensors. The difference detected is called a gradient.⁽³⁾ Magnetic locators indicate the detection of a gradient by changing the pitch of an audio tone and, depending on the model, by using a bar graph on a visual display. The size of the buried object impacts the size of the magnetic field difference, or gradient. Likewise, the orientation of the buried object also affects the size of the gradient detected. The same object oriented vertically generally produces a larger difference than one oriented horizontally. Some locators also indicate polarity, which can be useful in determining whether the target is oriented vertically or horizontally. To make this determination, the operator observes the polarity change at each sensor. The fixed spacing between the sensors determines the overall length of a locator. That spacing impacts the field difference detected and the sensitivity of the locator. The farther apart the sensors, the more sensitive the locator becomes.⁽⁴⁾

While magnetic locators can detect metallic pipes, they cannot detect fiber optic, concrete, and polyvinyl chloride (PVC) pipes on their own. Some utility providers overcome this limitation by attaching a metallic tracer cable alongside the nonmetallic utility line to provide an accessible method for magnetic locators to detect them.⁽²⁾

The strength of the signals returned to the receiver indicates the presence of buried utility lines. Signal strength depends on the depth, size, and material type of the pipe as well as the presence of other confounding electromagnetic sources in the area. The operator may manually adjust the threshold for what constitutes a sufficiently strong signal against background noise for identifying a buried utility, though automated routines for adjusting this threshold are available on some modern magnetic locators. Magnetic locators typically relate the location of a buried utility by way of a visual cue on the instrument combined with an audible tone that grows in intensity as the distance from the utility decreases and maximizes when the operator holds the instrument directly over the utility. The operator marks this location onsite with marking paint, a flag, or some other marking tool at regular intervals to map the lateral position of the buried utility.

Data Acquisition

Data collection with a magnetic locator should be carried out by an experienced and qualified operator. The following list provides the general steps needed to achieve quality results during a utility investigation.

Fieldwork Preparation

The following steps assume that no prior or reliable information on the utility locations is available. While historical documents and onsite landmarks are useful in making informed decisions about the approximate locations of buried utilities, these steps should be considered in taking a systematic approach to locating utilities, adjusting for efficiency where possible.

1. Identify an area of interest for the investigation with a coordinate system laid out using semipermanent marking paint to identify its extents and major features. This coordinate system should have the following features:
   1. A well-defined, easily identifiable, and reasonably permanent origin from which all lateral measurements across the test area can be referenced.
   2. An intuitive selection of the x- and y-axis orientation for the coordinate system.
   3. Indications at regular intervals along each grid axis to denote the distance from the reference origin and to identify the start location and orientation of a given scan.
2. Document the coordinate system with a sketch from the field showing the appropriate field measurements of its extents, reference origin, x- and y-axis orientation, and a north arrow. In addition, document the weather conditions, observed soil conditions, and other relevant details for context when analyzing the gathered data.
3. Develop a reference table for recording measurements, saved data file names, and other context data during the data collection process. Generally, a base template for this table worksheet is created ahead of mobilizing to the jobsite with the template being adapted in the field to complement the coordinate system.
4. Complete the definition of the test area extents, even when using a magnetic locator in combination with Global Positioning System (GPS)-enabled survey equipment, for quality control of the data positioning. In this case, the extents are locked in using appropriate GPS equipment and relevant procedures for coordinate lock-in operations.
5. Determine and document the intervals along the test area at which scans will be gathered, forming what is referred to as a test grid. Scans should be taken in both the x-direction and y-direction along the grid because magnetic locators are generally polarized such that they can only detect pipes oriented perpendicular to the instrument. A typical spacing is 5 ft (1.5 m) in either direction, with 2-ft (0.6 m) spacing used for high-resolution imaging.
6. Prepare appropriate marking tools to label locations of buried utilities detected by the magnetic locator. The marks should remain visible long enough for documentation and use by the site owner.

System Operation

The following steps apply to using the passive location method:

1. Power on the magnetic locator receiver per the manufacturer’s instructions and review all system components to ensure the locator is properly receiving signal.
2. Adjust the settings to ensure indicators, such as the visual and audible cues for a detected utility, are sufficient for observation while performing the inspection.
3. Tune any threshold settings for when the instrument should provide an alert, ensuring the instrument is over an area where a utility line is not suspected to be present, to appropriately set the threshold above the local noise floor. The noise floor is the point at which features of interest in a signal become indistinguishable from irrelevant background responses and cannot be detected.

The following steps apply to using the active location method:

1. Follow the manufacturer’s advised methodology for setting up the transmission system for inducing current in a given buried utility.
2. Power on the magnetic locator receiver per the manufacturer’s instructions and review all system components to ensure the locator is properly receiving signal.
3. Adjust the settings to ensure indicators such as the visual and audible cues for a detected utility are sufficient for observation while performing the inspection.
4. Tune any threshold settings for when the instrument should provide an alert, making sure the instrument is over an area where a utility line is not suspected to be present to appropriately set the threshold to be above the local noise floor.
5. Set the magnetic locator to the appropriate frequency range for detecting the transmitted signal in a buried utility line on an as-needed basis.

Field Data Collection

1. Following the manufacturer’s instructions on system operation, begin with the magnetic locator aligned to the appropriate start location on the coordinate system. The scan collection should be completed systematically along the test grid in both directions, with notes and documentation gathered where appropriate.
2. The operator should sweep the magnetic locator instrument side to side above the ground and be alert for any indication of a possible buried utility.
3. When the instrument detects a possible buried utility, the operator should sweep the instrument around the general area to maximize the indication signal strength on the magnetic locator before the location is marked. Mark the point of maximum indication.
4. After the operator locates the utility, the operator should continue to sweep the instrument around the marked location to identify the orientation of the pipe with an additional point, following the same process of maximizing the signal before marking.
5. Operators should trace the utility line using the previous steps for the full length within the area of interest, marking at regular intervals with lines and arrows that indicate orientation and are visible from a distance.
6. Once a line is sufficiently marked and mapped, its position should be recorded in the documentation developed during the coordinate system layout and sketching.
7. If other utility lines require location, the operator will then return to the original starting place along the survey grid and continue scanning, using the same process as described above when other possible utilities are encountered.

Data Processing

Magnetic locator systems typically do not produce data files or results for later processing. While this lack of raw data can mean a lack of repeatability as well as reliance on the subjective choices of the operator, the general value of using magnetic locators lies in their immediate delivery of direct feedback onsite and ease of use. Magnetic locators do not require operators to have advanced training and expertise to perform a successful utility location.

Data Interpretation

Magnetic locator systems generally do not produce any data files or results for later interpretation. Typical applications of these instruments focus on the marking and mapping of utilities onsite for immediate use by site owners and operators, though drawings of the mappings may be generated from notes and sketches gathered in the field.

Advantages

• Well-established field data collection process.
• Advanced training not required for operation.
• Direct feedback onsite and results for taking action.
• A reliable and repeatable detection method, especially for utilities made of metal.

Limitations

• The process can be subjective depending on the settings and thresholds selected by the operator.
• A magnetic locator cannot detect PVC, fiber optic, or concrete utilities that lack metallic tracer cables.

Reference

1. Reiter, D., V. Napoli, J. Cohen, S. Boone, P. Moseley, A. Alhasan, and J. Salerno. 2023. Availability, Feasibility, and Reliability of Available Nondestructive Evaluation (NDE) Technologies for Detecting and Locating Buried Utilities. Washington DC: Federal Highway Administration.
2. National Academies of Sciences, Engineering, and Medicine. 2009. Encouraging Innovation in Locating and Characterizing Underground Utilities. Washington, DC: The National Academies Press. https://nap.nationalacademies.org/catalog/22994/encouraging-innovation-in-locating-and-characterizing-underground-utilities, last accessed August 16, 2023.
3. Telford, W. M., L. P. Geldart, and R. E. Sheriff. 2010. Applied Geophysics. Cambridge, UK: Cambridge University Press.
4. Milsom, J., and A. Eriksen. 2011. Field Geophysics. Hoboken, NJ: Wiley Publishing Company.