Magnetometry

Magnetometer surveys are typically performed to map buried metal objects and to provide geological information. Typically magnetic dykes are mapped as well as changes in the depth of magnetic bedrock due to faults and displacements. Magnetic data is normally collected on a grid, with the station spacing determined by the depth and width of the target. Stations are laid on either on a grid or, in mapping linear anomalies, on profiles perpendicular to the magnetic strike at an interval of several times the station spacing.

A magnetometer survey for hydro geologic and engineering applications is conducted on foot, by one operator. Spacing of traverses and readings depends on the width of the expected anomaly. For instance, buried drum searches may be conducted at a 1 meter spacing while geologic mapping may be conducted at a 10 meter spacing

Applications

• Mapping steeply dipping geological contacts
• Mapping regions of potential stress amplifications (eg. faults)
• Mapping landfills
• Mapping of magnetic dykes
• Mapping archeological sites (buried metal and steel objects)
• Mapping buried drums, steel pipes and other ferromagnetic objects
• Locating sand and gravel deposits that contain heavy minerals

Limitations

• Although the magnetometer is a powerful tool, its most significant limitation is interference caused by high magnetic field gradients such as those resulting from steel-reinforced concrete buildings.
• One also has that power lines interfere with measurements. In areas with extensive metallic debris scattered over the surface no distinction can be made between surface debris and buried debris
• It is not recommended to conduct a magnetic survey during a magnetic storm due to large and unpredictable interference.
• Magnetic anomalies are often presented as dipole anomalies which can be difficult to interpret if one compares it with EM anomalies which show as monopole anomalies.

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