AN0016 Applications of The Grad601 Magnetic Gradiometer
AN0016 Applications of The Grad601 Magnetic Gradiometer
AN0016 Applications of The Grad601 Magnetic Gradiometer
Gradiometer
Principle of operation
All materials exhibit a property called magnetic
susceptibility, which is a measure of how magnetised
the material becomes under the influence of an external
magnetic field. If an object made from a material with
a high magnetic susceptibility is placed in a uniform
magnetic field then it will become magnetised and the
magnetic flux will be concentrated in the area around the
object. This will cause an increase in the strength of the
magnetic field close to the object and a corresponding
weakening of the field at some distance away from the
object.
The presence of materials in the soil with dissimilar
magnetic susceptibility will, under the influence of the
geomagnetic field, create local variations in the strength
of the magnetic field where this contrast in magnetic
susceptibility occurs.
A kiln or ferrous material may have an associated
remanent field that will also give rise to local distortions
of the magnetic field.
The problem of measuring local distortions with a
resolution of 0.1nT in the magnetic field of say, 50,000nT
over an area of interest is compounded by diurnal variations
of the background geomagnetic field. These are caused by
the solar wind giving rise to an increase in the geomagnetic
Applications
With a resolution of 0.1nT, excellent stability and a
non-volatile memory, the Grad601 geophysical survey
gradiometer is ideal for archaeological prospection and
the location of unexploded ordnance (UXO), waste drums,
pipelines and cables.
An example of a geophysical survey covering an area
of 90m x 120m for archaeological prospection is shown
below.
This area is used for arable crops and the survey was
carried out after cutting the crop. No visible remains are
to be seen at the surface but field walking after ploughing
had previously produced some pottery shards. The survey
reveals a buried Romano-British site with drainage
ditches, field boundaries and enclosures clearly seen.
Operation
The Grad601 is normally used in the survey mode where
the area of interest is divided into 10 x 10, 20 x 20 or 30
x 30m squares (termed grids). The gradiometer records
the magnetic gradient while being carried at normal
walking pace, covering the grid in a series of zigzag or
parallel traverses. Typically, data is recorded at intervals
of 0.25m along lines spaced 1m apart. This gives a good
resolution for anomalies with a dimension of greater than
1m and allows up to 36 grids to be stored in memory.
For some applications, e.g. the location of post holes in
archaeology, a higher resolution may be required and the
interval between lines can be reduced to 0.5 or 0.25m.
This will result in proportionally more data per grid and a
corresponding reduction in the number of grids which can
be recorded.
The unit may also be operated like a metal detector in a
scanning mode, where the amplitude of the gradient is
indicated by an audible tone and data is not saved to the
memory.
Setting up
In order to cancel the background field effectively,
regardless of the direction of the field relative to the
sensor, it is necessary to align the two sensor elements in
the gradiometer very precisely. The alignment is carried
out by rotating the sensor in a low-gradient area and
AN 0016 13/05