The Magnetic Surveys

The Magnetic Surveys

There are three types of magnetic surveys: ground based, aero magnetic survey, and ship borne survey. When the target of interest requires closely spaced readings, the ground magnetic survey is preferred. The ground based surveys can be carried out on foot or on a vehicle.

The traverse direction and the station spacing should be chosen first. The traverse direction is across the strike direction of the target. The station spacing does depend upon dimension and depth of the target. Larger targets and larger depths need wider spacings. Similarly, the detailed surveys need narrow spacings.

All the metallic objects such as rings, watches, keys etc should be removed while doing the survey. Magnetic survey should not be carried out near power lines, railway lines, automobiles, etc. The location and time should be noted down by using a GPS.

In airborne magnetic surveying, the magnetometer should be mounted behind the aircraft by a cable of about 100 to 500 feet in an apparatus known as a bird. For mineral exploration, the flight altitude can be from 50 to 75 meters and for oil exploration it is maximum of 2 kilometers. The flight lines are closed contour loops. Their spacing depends upon several factors.

For ship borne surveys, the length of the cable should at least be twice the length of the survey ship. The magnetometer should be placed in an instrument known as a fish.

Proton precession, Fluxgate, and Optical pumping magnetometers are being widely used in magnetic surveys.

The proton precession magnetometer does measure the total magnetic field strength but not its direction. It shows a total magnetic field intensity anomaly. It is commonly used in the ground magnetic surveys. The sensor does contain a container and is surrounded by a coil. It should be filled with any liquid rich in hydrogen such as water or kerosene. The latter is preferred as it does not freeze in cold weather. The proton in the hydrogen does act as a tiny magnet and aligns with any magnetic field. The frequency of precession of the proton is proportional to the strength of the field. This instrument is being widely used for exploration purposes as the procedure is simple.

Corrections for variations in the earth’s field are known as the geomagnetic corrections.

The last step is the preparation of profile curves and anomaly contour maps which display the magnetic anomalies of the target or geologic interest.

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