How a Magnetometer Works
All of our magnetometer surveys are performed with the Model GA-72Cd Magnetic Locator, which is manufactured by Schonstedt Instrument Company of Kearnysville, WV. The magnetometer detects the magnetic field of ferromagnetic objects by responding to the difference in the magnetic field between two sensors spaced apart about a distance of fourteen (14) inches. This instrument is unique in that it provides an audio signal and visual indications of both signal strength and polarity. The reason this is advantageous is that although most objects can be located using either one of these indications, simultaneous use of both types enables one to pinpoint a target, determine its orientation, and identify magnetically detectable, non-metallic duct and cable.
Sweeping for an Iron Marker
The figure below illustrates the use of the locator to detect an iron marker, the type which is commonly used for property line identification.
How a Signal Is Generated
As shown in the figure below, the magnetic field of the iron marker is stronger at Sensor A than it is at Sensor B. As a result, the frequency of the audio signal is higher than the idling frequency, forty (40) Hz, which exists when the field strength is the same at both sensors. This stronger signal also causes the digital indication to peak, in either the positive or negative direction, when the audio signal is at its highest frequency.
Performing a Sweep
To perform a sweep, the locator is swept from side to side. When the locator comes within range of an iron object, the audio signal will peak, the bar graph will expand positive or negative, and the digital readout will peak as shown in the photo below.
Advantages of Magnetometer Technology
- Quick and easy to use.
- Scans over uneven surfaces and heavily wooded areas that cannot be scanned with the GPR.
- Confirms whether or not objects positively identified with the GPR are metallic USTs or simply rocks or other non-ferrous anomalies.
Limitations of Magnetometer Technology
- Cannot find non-ferrous metals such as aluminum, brass, or copper.
- Signal interference is abundant at industrial sites, parking lots, and other areas housing a lot of metallic objects.
- Does not tell one the exact size of the object being located, like the GPR can.