Magnetic Method
Magnetic Surveys are commonly used for the following:
- Locate and delineate buried metal objects including USTs, storage drums, and utilities
- Locate and map buried metal debris
- Locate abandoned or lost metal well casings
- Identify archaeological ruins
- Landfill investigations
- Detect buried ordinance
Magnetic surveys consist of measuring and mapping changes in the earth’s natural magnetic field throughout an area. Buried ferrous metal objects distort this field in their vicinity, allowing for detection. Many magnetometers operate by measuring the total magnetic field intensity. In the gradient mode, two sensors are used, one mounted above the other, to suppress unwanted cultural noise from above ground objects such as fences and parked cars, enhancing the response from shallowly buried objects. Surveys are typically conducted with either proton or cesium vapor magnetometers.
Advantages
Gradient measurements are very sensitive to small objects. Magnetic surveys are set up and conducted quite easily. Large sites can be investigated quickly. Hand held metal detectors can be used to follow up the magnetic results to provide further information about detected objects.
Limitations
Exploration depth is limited to approximately 15 feet below ground surface. Detection ability depends upon magnetic variations above and beyond those caused by above ground features. In congested, urban areas parked cars, buildings, fences and utilities contribute interfering magnetic signals that can mask detection of buried metal objects. This limitation can be overcome by also using GPR.