Geophysical surveys

types of geophysics and their uses

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What are geophysical surveys?

Techniques that detect features through their physical differences with the surrounding soil. 

Why are they popular? 

They are popular with many archaeologist because the preserve, and they save on the expense of excavation when very little may be found.

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Resistivity Survey

How does it work?   It passes an electrical current through the ground and compares the readings. It will then highlight anything that get in between the electrical current.

Why is soil moisture important?   It means that there is a higher conductivity of the soil. e.g. a ditch or grave will retain water better than a wall or road. 

What can be detected?   Ditches, graves, walls & roads

Which soils are best for resitivity surveys?  Dry soils

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Resistivity cont.

Which soils are difficult to read and why?   Waterlogged soils as there is little difference between that  and ditches and other pits in the ground. 

What other limitations are there for this technique?

1.The depth is can got to is limited

2.Readings are difficult to interpret

3.Sensitivity of meters below ground in poor

4.Need wider spaced probes than those available at the moment

5.Cost, it is a very expensive technique to use

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Magnetometer Survey

What is a magnetometer survey?  It picks up magnetic distortions in the ground caused by past human activity. The topsoil contains haematite (Fe2O3), an iron oxide.

How can past human activity distort the earth's magnetic field?  

1.A ditch will contain more haematite than the surround area as it will have been filled up with topsoil. 

2.Where the topsoil has been subject to considerable heat e.g. a fire or kiln.

What is the name of the modern and effective magnometer?  Fluxgate gradiometers

What is the depth of detection for these devices? 1 meter down

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Magnetometer Survey cont.

What must be measured first prior to any magnetometer survey? magnetic background of the soil 

What are the limitations of using magnetometer surveys?

1.Easily disturbed by iron

2.Background interference

3.Fluctuations of the earths magnetic field

4.Difficult to read and interpret findings

5.Slow process

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Caesium Vapour (CV) magnetometers

What are CV magnetometers?  They pump caesium vapour in to the ground and take measurements to find buried features by magnetite.

What are the benefits of CV magnetometers? 

1. Suffer less background noise

2. More sensitive than other magnetometers

What can be detected?  Wood & Stone

What is the main drawback of using them? Expensive

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Other methods to be used alongside the main method

What depth can metal detectors detect down to?   15 cm

What do archaeologist use them for?  Position of burial deposits

What is GPR?  Ground penetrating radar

How does it work?  Transmits pulses of energy in to the ground and records the time taken to return the signal.

What can be detected using GRP? landscapes and rivers which are buried.

What is sonar? It measures sound waves as they bounce back it reveals depth and form of sunken structures. 

What is bosing? It involves hitting the ground with a mallet and listening to the variations in the sound.

What is dowsing? It uses wooden rods to detect water and archaeology.

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