1. Geophysics—Magnetic Gradiometer

2. 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

3. 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.

4. 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?

5. 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

6. Magnetometry Results Hillfort in Britain

7. Burrough Hill Iron Age Hillfort (Britain)

8. Gradiometer Survey –Turkish city

9. Magnetic Gradiometer
GeoScan
FM-36
Fluxgate
Gradiometer

10. 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

11. 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

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