Geological and buried manmade features or objects can cause local variations in the earth’s magnetic field, which can be measured with a magnetometer. Accordingly, a detailed magnetic survey can be used to effectively define areas of past human activity and it is a quick, cost effective technique. As such, it has become the industry standard for archaeological surveys.
However to obtain successful results from your archaeological (magnetometer) survey, several factors must first be taken into consideration. The most important is the nature of the ground conditions, especially when there are crops growing on the fields.
We have therefore set out below, a list of the perfect conditions for geophysics to help you get the most out of your archaeological survey.
In an ideal world, all survey sites would have the following ground conditions: -
1. Short pasture, or early season crops
2. ‘Stubble’ - short crops after the harvest
3. Lightly ploughed fields
Lightly ploughed fields which have been prepared for crops can be easily surveyed. But, if the plough is deep or the field contains clay soils, then it can become tricky or even impossible to survey.
So, what ground conditions are unsuitable for archaeological surveys?
Factors affecting the magnetic survey may include soil type, local geology, previous human activity, disturbance from modern services etc. The physical state of the ground itself can also significantly affect the result of the survey, especially rough or uneven surfaces. SUMO understands that the ‘perfect’ survey conditions will rarely be achievable due to the nature of individual sites, but we have listed examples of unsuitable ground conditions for archaeological surveys below:
1. Old potatoes fields/ Oilseed rape fields
2. Overgrown sites
3. Potato fields
4. Untidy, dumped material
Are there other factors which affect the carrying out of magnetometer surveys?
Livestock - for safety purposes, livestock should be removed from survey areas.
Agricultural work i.e. certain crop sprays can make fields unsafe for personnel to survey.
Above: Magnetic survey equipment which can either be towed by a quad bike (where conditions allow) enabling large amounts of data to be collected quickly, or man powered, which is somewhat slower! Smaller two sensor arrays which can be carried by an individual are also used in less open sites.
SUMO will always discuss the survey area with its clients to understand its current use and to tailor its approach wherever possible.
Most archaeological surveys are carried out using magnetometry but Ground Penetrating Radar (GPR) is being employed more often. While the same ground conditions apply for GPR as they do for magnetic survey, there are a couple of other issues which need to be considered.
Clay or clayey soils can severely restrict the use of GPR in that the energy waves cannot penetrate through dense soil structure.
A high water-table will also limit the depth of penetration.
Above: Ground Penetrating Radar (GPR) equipment.
Seasonal factors can also affect geophysics surveys both positively and negatively…
The spring months:
Early spring is an ideal time for a survey, when the weather is improving and the land is drier, but before the crops have grown significantly. New crops can be delicate and severely impacted by either cart systems or trampling under foot which can result in the landowner being unwilling to authorise a survey until the crop has matured or even been harvested.
The summer months:
Tall crops can interfere with magnetometer sensors. This can make it impossible for the operator to walk safely.
Maize fields are the worst! This is because despite the harvest, the remaining stalks can be extremely brittle. This causes problems for the surveyors walking the site.
It is impossible to collect underground GPR mapping data in potato fields. Other root crops are less of a concern unless the ground is heavily rutted.
The summer is also an ideal time for aerial surveys, when archaeological features can often be seen as a result of the differing moisture contents between archaeological remains and the surrounding land.
The autumn months:
Immediately after harvest is a perfect time for a survey, when the land is dry and the weather is good.
Why should you use SUMO?
With over 40 years’ experience at the forefront of geophysics, we pride our survey offering on the following:
We employ many of the industry’s leading survey professionals.
All our surveyors are multi-skilled to aid flexibility and tailoring of our services.
Our unique size and nationwide coverage allows us to tailor our service to a client’s individual needs and budget.