Target of Investigation

Magnetometers 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 utilized these instruments because of their relative ease of use in locating subsurface features (e.g., traditional cable and pipe systems).⁽¹⁾ Magnetometers are also used in geological prospecting, underground detection, aerospace navigation, underwater navigation, land navigation, and submarine detection. Magnetometers 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. Response to the magnetometer varies by type of pipe; therefore, the pipe type should be considered in deployment of magnetometers. Table 1 presents magnetometer responses for different pipe types.

Table 1. Summary of magnetometer effectiveness for different materials.

Material	Detects	Response Strength
Metal/steel	Yes	Strong-medium
Plastic/polyvinyl chloride (PVC)	No1	None
Electrical line	Yes	Medium-low
Fiber Optic	No1	None
Concrete	No1	None

Description

Like other NDE methods, a magnetometer 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 handheld device with a set of internal magnetic coils that measure variations in the magnetic field’s strength and orientation. 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 indicating proximity to a buried utility.

Physical Principle

Magnetometers operate on the principle of detecting significant variations in the local magnetic field relative to the Earth’s magnetic field. Several types of magnetometers are in use, but the most common are the fluxgate magnetometers and the proton precession magnetometers.

Fluxgate Magnetometer

The fluxgate magnetometer is a device that measures the intensity and orientation of magnetic lines of flux in three orthogonal directions.⁽²⁾ Industries have used fluxgate magnetometers for many applications, such as geological prospecting, underground detection, aerospace navigation, underwater navigation, land navigation, and submarine detection. The fluxgate magnetometer has a ferromagnetic core surrounded by two wire coils in a configuration that resembles a transformer. As alternating current (AC) passes through one coil, called the primary, it produces an alternating magnetic field that induces AC in the other coil, called the secondary. When a change occurs in the external magnetic field, the output of the secondary coil changes. The extent and phase of this change can be analyzed to determine the intensity and orientation of the flux lines.⁽³⁾

Proton Precession Magnetometer

The proton precession magnetometer is the most commonly used instrument for measuring the Earth’s magnetic field.⁽⁴⁾ In its simplest form, the instrument consists of a bottle filled with water. A strong electric current is passed for a few seconds through a coil wound around the bottle. Water is a paramagnetic substance and can be thought of as containing an assemblage of tiny magnets that are executing random, thermally induced motions. When the current is turned off, the coil is used to record the current being produced by the tiny magnets as they process around the direction of the Earth’s ambient magnetic field. Digital counters in the device record the frequency of precession, which is proportional to the Earth’s magnetic field, thereby producing the value of the magnetic field.⁽²⁾

The proton precession magnetometer can detect only subsurface pipes with significant iron content, which means it cannot detect other metallic pipes or fiber optic, concrete, or PVC pipes on their own.

The feature indicating the presence of a buried utility is defined by the strength and orientation of the magnetic field measured by the receiver. The magnetic field’s strength and orientation 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 value constitutes a sufficiently strong signal against background noise for identifying a buried utility. Some modern magnetometers provide automated routines for threshold adjustment.

Magnetometers 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 to the utility decreases. The maximum tone occurs when the operator has 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 magnetometer should be carried out by an experienced and qualified operator. The lists in this section comprise the general steps required to achieve quality results during a utility investigation.

Fieldwork Preparation

These field preparation 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 location of buried utilities, operators should consider these steps as part of a systematic approach to locating subsurface utilities, adjusting for efficiency where possible.

1. Identify an area of interest for the investigation, using a coordinate system laid out with semipermanent marking paint to identify the area’s extents and major features. This coordinate system should have these characteristics:
   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-axes orientation for the coordinate system.
   3. Indications at regular intervals along each grid axis denoting the distance from the reference origin for use in identifying 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 the test area’s 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 field documentation of the coordinate system.
4. Complete the definition of the test area extents, even when using a magnetometer in combination with Global Positioning System (GPS)-enabled survey equipment, for quality control of the data positioning where 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 most magnetometers are polarized such that they can detect only pipes oriented perpendicular to the instrument itself. A typical spacing is 5 ft (1.5 m) in either direction, with a 2-ft (0.6 m) spacing used for high-resolution imaging.
6. Prepare marking tools for use in labeling locations of buried utilities detected by the magnetometer. The labels should remain visible long enough for detections to be documented and for use by the site owner.

System Operation

The following steps apply to using the passive location method:

1. Power on the magnetometer receiver per the manufacturer’s instructions and review all system components to ensure the device receives signals properly.
2. Adjust the locator’s 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, making sure 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 magnetometer receiver per the manufacturer’s instructions and review all system components to ensure the magnetometer is properly receiving a 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 magnetometer 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 magnetometer aligned on the appropriate start location on the coordinate system. Scan collection should be completed systematically along the test grid in both directions, with notes and documentation gathered where appropriate.
2. The magnetometer instrument should be swept side to side above the ground, with the operator 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 magnetometer before the location is marked, and then mark the point.
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 and mark an additional point, following the same process of maximizing the signal before marking.
5. Operators should trace the utility line in this way 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 locating, the operator returns to the original starting place along the survey grid and continues scanning, using these same field data collection process steps for other utilities encountered.

Data Processing

Magnetometer systems typically do not produce data files or results for processing at a later date. Also, the magnetometer system results could be considered subjective because the operator sets or adjusts signal thresholds used to identify pipe locations. Despite those factors, the generally accepted value of using magnetic locator systems lies in their immediate delivery of direct feedback onsite and ease of use. Magnetometer systems do not require operators to have advanced training and expertise to perform a successful utility location.

Data Interpretation

Magnetometer systems generally do not produce any data files or results for interpretation at a later date. Typical applications of magnetometers 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

• Advanced training for operation not required.
• Instant feedback, resulting in quick action.

Limitations

• Sensitive to a variety of local conditions, including the presence of buildings or other surface objects that may disturb the local magnetic field.
• Possible subjectivity in the process due to dependence on settings and thresholds selected by the operator.
• Can only detect pipes made with iron.

References

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. Talwani, M. and W. Kessinger. “Exploration Geophysics,” in Encyclopedia of Physical Science and Technology. 2003. San Diego: Academic Press.
3. Keesey, L. 2017. “NASA Technologist Develops Self-Calibrating, Hybrid Space Magnetometer” (web page). https://www.nasa.gov/feature/goddard/2017/nasa-technologist-develops-self-calibrating-hybrid-space-magnetometer, last accessed June 30, 2023.
4. Telford, W. M., L. P. Geldart. 1990. Applied Geophysics. 2nd ed. New York: Cambridge University Press.