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How can magnetometers detect buried objects during surveying?

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Surveying is an essential part of construction management, as it helps to determine the location, shape, and size of the land and the features on it. However, surveying can also encounter challenges when there are buried objects that may interfere with the measurements or pose a risk to the project. One way to detect these objects is by using magnetometers, which are devices that measure the magnetic field of the earth and any anomalies caused by ferrous materials. In this article, you will learn how magnetometers work, what types of objects they can find, and how to use them effectively in surveying.

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Allan Fairhurst

Regional Field Service Manager

1 What are magnetometers?

Magnetometers are instruments that measure the strength and direction of the magnetic field in a given area. They can be handheld, mounted on vehicles, or flown by drones or helicopters. Magnetometers can detect variations in the magnetic field caused by the presence of ferrous objects, such as pipes, cables, tanks, drums, or unexploded ordnance. These objects create a magnetic anomaly, which is a difference between the expected and the measured magnetic field. By measuring the magnitude and shape of the anomaly, magnetometers can estimate the location, depth, and size of the buried object.

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Allan Fairhurst

Regional Field Service Manager
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'Magnetometers' = Electro Magnetic Locators for Utility locating. The most effective way to locate is an 'Active' survey. Inducing a signal for example of 33khz / 131khz into a conductor such as a street light will then be picked up by the incoming/outgoing feeder cable. Using the receiver set to only pick up 33KHZ/131KHZ you can then trace the signal with distance and depth depending on the EM locator you are using. Limitations as much as capabilities are both equally important in understanding, multiple ferrous materials in close proximity can distort the signal, telling the difference requires skill/experience. Grounding (earthing) your induced frequency is often required.

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Akinsola Akinbode, PMP, PMI-SP, PMI-RMP

Certified Project Manager, breaking barriers, and delivering solutions.
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Magnetometers are valuable tools in various industries because of their ability to detect and locate ferrous objects. However, it is essential to be aware of their limitations and consider alternative methods when necessary. By understanding the strengths and weaknesses of magnetometers, users can make informed decisions about their applications in different contexts.

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2 How do magnetometers work?

Magnetometers work by using different types of sensors that respond to the magnetic field. The most common types of sensors are fluxgate, proton precession, and cesium vapor. Fluxgate sensors use coils of wire that generate an alternating current when exposed to a magnetic field. Proton precession sensors use a container of hydrogen atoms that align with the magnetic field and then precess at a frequency proportional to the field strength. Cesium vapor sensors use a glass cell filled with cesium gas that emits light when stimulated by a magnetic field. The intensity of the light is proportional to the field strength. The sensors are connected to a display unit that shows the magnetic field readings in units of nanotesla (nT) or gamma (γ).

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Akinsola Akinbode, PMP, PMI-SP, PMI-RMP

Certified Project Manager, breaking barriers, and delivering solutions.
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Each type of magnetometer sensor has unique advantages and disadvantages, making them suitable for different applications. Fluxgate sensors are reliable and affordable, making them ideal for many general applications. Proton precession sensors provide higher accuracy and sensitivity, making them suitable for geophysical surveys and high-precision measurements. Cesium vapor sensors offer the highest precision and sensitivity, but their complexity and cost make them more appropriate for specialized applications, such as NMR spectroscopy. Understanding the specific requirements of a given application will help select the most appropriate magnetometer sensor.

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3 What types of objects can magnetometers find?

Magnetometers can detect any object that contains ferrous metal, such as iron, steel, nickel, cobalt, and some alloys. These objects can be buried up to several meters deep, depending on their size, shape, and orientation. Examples of objects that magnetometers can find include pipes and cables that carry water, gas, electricity, or communication signals; tanks and drums that store liquids, chemicals, or waste; unexploded ordnance (UXO) that may pose a threat to safety or security; archaeological artifacts with historical or cultural value; and geological features that may indicate mineral deposits or faults.

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Akinsola Akinbode, PMP, PMI-SP, PMI-RMP

Certified Project Manager, breaking barriers, and delivering solutions.
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Magnetometers can be a valuable archaeological tool, providing non-invasive and efficient methods for surveying and detecting ferrous metal artifacts. However, it is essential to acknowledge their limitations and combine them with other archaeological techniques to ensure a comprehensive understanding of the site and its historical context. By doing so, archaeologists can better preserve and interpret the past, while also maximizing the efficiency and effectiveness of their research.

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4 How to use magnetometers effectively in surveying?

To use magnetometers effectively in surveying, you need to follow some best practices that will ensure accurate and reliable results. Before starting the measurements, plan your survey area and grid, using a map or GPS device to mark the boundaries and grid lines of your survey area. Additionally, calibrate your magnetometer before and during the survey, as well as avoid sources of magnetic interference in your survey area. Sources of magnetic interference can include power lines, vehicles, buildings, fences, or other metal objects which can distort the magnetic field and create false anomalies. When using a magnetometer, it is important to avoid wearing or carrying any metal items such as jewelry, watches, phones, or tools. Furthermore, record and interpret your data carefully; this includes recording your data in a systematic and consistent way using a data logger, notebook, or software program. Lastly, interpret your data using appropriate methods such as contour maps, profiles, or 3D models to identify and characterize the anomalies. Utilizing additional tools such as metal detectors, ground penetrating radar, or excavation can help you confirm and verify your findings. Magnetometers are valuable tools for detecting buried objects during surveying; understanding how they work and how to use them effectively can improve your surveying skills and outcomes.

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Akinsola Akinbode, PMP, PMI-SP, PMI-RMP

Certified Project Manager, breaking barriers, and delivering solutions.
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To use magnetometers effectively in surveying, it is crucial to follow best practices, such as planning the survey area and grid layout, calibrating the instrument before and during the survey, and avoiding sources of magnetic interference. By doing so, surveyors can ensure accurate and reliable results, leading to a better understanding and interpretation of archaeological features.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How can magnetometers detect buried objects during surveying?

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1 What are magnetometers?

2 How do magnetometers work?

3 What types of objects can magnetometers find?

4 How to use magnetometers effectively in surveying?

5 Here’s what else to consider

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