The earths magnetic field is genrally uniform in any one place. however local magnetic distortions can be caused by past human activity. All topsoil contains haematite, iron oxide.
e.g. A ditch that has been filled up with topsoil will contain more haematite than the surrounding area.
Asses magnetic properties of features and objects beneath the grounds surface. picks up walls and ditches.
all data tell in increase/decrease in numbers that are converted by a PC.
Archaeologists have to interpret the data
- small and large pits
- stone walls
- building platforms
- Not limited by the weather
- Detect greater range of features than Resistiviter
- magnetometers are used for detailed investigation
- rapidly scan large areas to highlight anonmalies
- detect small features up to 1m down
- Only the person that carries out the survey can set up the machine
- metal and electricity can interfere with the equipment giving a faulse reading
- Expensive £40,000 per machine
Magnetic Suseptibility (mag sus)
A more accurate metal detector that looks at magnetic properties beneath grounds surface. all data is sent to the PC.
- Anyone can use them (unlike magnetometer)
- no training needed
- scanning is simple
- Shallow depth
- setting up takes a long time
- time consuming
There are some differences in the amount of electricity that a soil can conduct. we can detect this by passing an electrical current through the ground and comparing the readings. Electricity is conducted through the soil by mineral salts contained in the water. the more moisture the better the conductivity.
a ditch/grave will usually retain water better than the surrounding soil. a burried wall or road will conduct poorly and resist the current more than the surrounding soil. The electrical current flows close to the surface so it can be measured using shallow probes.
the method works better with some soils better than others e.g.clay retains moisture well
plants,rocks and variations in the depths of the soil can create misleading readings
- anyone can use it
- anyone can set the machine up
- good at detecting walls and ditches
- readings can be effected by poor contacts
- ground that stoney may not help the contact
- false readings is upper soil level is saturated
- time consuming
- won't recognise evidence of burning e.g. harths or small features such as post holes