Geophysics—Magnetic Gradiometer

What’s great about Gradiometry? Remote Sensing technique Fast with high resolution Survey large areas to determine site boundaries and prime areas to investigate Great opportunity for you and cutting edge investigations of Iron Age hillforts in Galicia Goals—looking for structures, ramparts, pits, hearths

Magnetometry in Archaeology Based on the fact that the iron core of the earth produces a magnetic field The earth’s magnetic field changes over the long-term. It’s north-south dipole (like a bar magnet) changes alignment.

Magnetometry Magnetic features possess their own magnetic fields. Why? -Random alignments of iron oxides in materials -However, these are not very detectable by the gradiometer. BUT, iron oxide alignments can be changed and become very detectable. How?

Magnetometry Thermoremanent magnetism—firing such as hearths, clay, bricks Induced magnetism—caused by movement of magnetic soils and rocks or fired materials—earthworks, trenches, moats, house pits, midden fills, stone foundations

Magnetometry Results Hillfort in Britain

Burrough Hill Iron Age Hillfort (Britain)

Gradiometer Survey –Turkish city

Magnetic Gradiometer GeoScan FM-36 Fluxgate Gradiometer

Magnetic Gradiometer Measures variation in magnetic field in nanoteslas FM36 has fixed top and bottom sensors vertically separated by 0.5 m Measures difference in nanoteslas between two sensors Bottom sensor more sensitive to temporal, earth magnetic field variation while top sensor is more sensitive to spatial variation while moving

Magnetic Gradiometer Records local contrasts in the earth’s magnetic field near-surface Contrasts called “anomalies” (refer back to previous slide) Important—cannot wear metal including eye glasses, clothes with rivets and zippers and cannot use metal equipment; cannot have a credit card or keys in your pockets

Methodology Survey—walk 20m by 20m zig-zag grids with regular measurement intervals Processing—interpolation, smoothing and interpretation with software & topography