Westray 2001: The Geophysical Survey

Geophysics on the Links of Noltland

with Marcus Smith

The geophysical investigation on Westray this year takes the form of a magnetometry survey, concentrating (to begin with, at least) on the area of the Links of Noltland, inland and to the south of the beach at Grobust. It is conducted using a Fluxgate Gradiometer, a sophisticated and sensitive (read: temperamental) passive sensor used for ground-scanning. This formidable piece of equipment measures, quantifies and records local variations in Magnetic Flux Density, measured in Teslas (rather than Magnetic Field Strength, measured in Webers, which would make it a mere metal detector).

The archaeology on the Links is sufficiently subtle that for the most part it has been necessary to set the machine to a fine resolution of 0.1 nanoTeslas (nT). The survey is conducted over a series of 20×20m grid squares, in a series of 20 traverses, beginning at the (site) north-west corner and working to towards the south-east corner, walking east. The Gradiometer has a timed trigger, set to take readings every half-metre, and timed to stop at the end of each traverse. It is therefore necessary for the operator to walk the traverses at a very steady pace.

At the end of the day, the results are downloaded to the computer, and plotted in three dimensions, the third dimension given as an absolute value in nT (adjusted to compensate for any bias between grid squares) and displayed as either a flat greyscale plot as shown here, or an isometric line graph.

Some features are clearly visible on the plot (such as the possible Neolithic house towards the east of the grid) and others are less clear. Strong metal "spikes" can clearly be seen as localised extreme high or low readings. Areas of burning can be similarly identified by their dipolar nature, loosely aligned with true north. This shows up as two immediately adjacent points of extreme high and low readings. The effect occurs because the ground beneath a burn site is "reset" to zero following the burning; it has no magnetic properties. Gradually, over time, the null area begins to re-magnetise under the influence of the earth's magnetic field, and like a magnetic piece of iron, it develops a north and a south pole.

Point the mouse at 'view' to see the feature highlighted.

• Basic grid without highlighted features View
• The possible Neolithic house View
• Another possible Neolithic house, although less distinct View
• A rectangular worked stone slab, too thick to be a floor level, possibly an ancient feature, or possibly modern View
• Many (but by no means all) of the other smaller distinct features are stone kelp-burning pits, of more modern origin.

Point the mouse at 'view' to see the feature highlighted

• Basic grid without highlighted features View
• The "rectangular cairn" View
• Other interesting features, as yet undefined View
• The electric fence View
• Many (but by no means all) of the other smaller distinct features are stone kelp-burning pits, of more modern origin.

Point the mouse at 'view' to see the feature highlighted

• Basic grid without highlighted features View
• The "rabbit proof" metal fence View
• Metallic interference View
• The three main identifiable features View
• Possible evidence of field boundary lines View