Innovation in Magnetic Measuring Instruments

Applications of the Grad601 Magnetic 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

field during the day and a corresponding decrease during the night. The changes are unpredictable and may be of the order of 100nT in an area with a geomagnetic field of 50,000nT. In order to record changes in the local field to the required resolution it is therefore necessary to subtract the background field from the measurements using a distant reference magnetometer, or by measuring only the vertical component of the magnetic field gradient.

The Grad601 magnetic gradiometer sensors use two carefully aligned single-axis fluxgate magnetometers with one sensor positioned 1m above the other. The difference in the output of the two sensors represents the magnetic gradient; variations in the background field, being common to both sensors, are removed by subtraction.

The instrument is suitable for the measurement of anomalies to a depth of up to 3m within which archaeological features and other buried objects are typically located. The instrument is available with one gradiometer sensor or with two gradiometer sensors spaced 1m apart. The dual version allows surveys to be conducted in half the time and with half the walking compared to the single gradiometer version.

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.

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Innovation in Magnetic Measuring Instruments

AN 0016 13/05

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

setting the adjustment for a minimum change in output. Any remaining offset error is also removed.

Before setting up the instrument it should be switched on and left running for about 20 minutes to allow the temperature of the sensors to reach equilibrium with the external environment. The operator will normally be setting out the grids during this period.

The Grad601 uses an automatic electronic adjustment, prompting the operator to align the sensor in several directions in sequence and indicate when ready by pressing a button. A dual gradiometer takes less than 5 minutes to complete the set-up procedure. The procedure should be repeated if a sensor height is changed relative to the beam or if the offset increases significantly due to large changes in temperature. Typically it is only necessary to repeat the set-up procedure every four hours or so, depending on the environment.

Downloading and visualising the data

The Grad601 data logger has an RS232 serial interface for downloading the data to a PC running the Grad601 download program supplied. This program allows the data to be saved to files in one of three formats compatible with specialist data processing and visualisation software such as TerraSurveyor. The TerraSurveyor software enables data to be downloaded directly without importing files created by the Grad601 software.

The sensitivity and stability of the Grad601 ensures that the acquired data requires a minimum of processing. Typically the data is clipped to remove any spurious high values associated with ground clutter, such as nails or horseshoes, and a zero-mean-traverse function is applied to remove any ‘stripes’ in the data between alternate traverses. These stripes are the result of sensor offset and alignment errors giving alternate offset values during zigzag traverses. Further processing may be applied, including a low-pass filter to remove unwanted noise.

The data processing software should include the ability to assemble grids into a composite picture, display the results in several modes, and include visual effects to enhance the appearance of the anomalies of interest.

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