What Does A Geophysical Survey Do
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- 20-02-2023
What is a Geophysical Survey?
Geophysical surveys are integral to determining the extent of new archaeological sites and help us to understand what lies hidden beneath the soil on a physical level. A geophysical survey gives us a window underground, allowing us to assess the scale of an archaeological site in a non-intrusive way. We can conduct them quickly without disturbing the ground, making them ideal for large areas where it would be too time-consuming and expensive to excavate.
Archaeological geophysics
The term "geophysical survey" is an umbrella term for a range of detection methods. Whichever method you use, the results are mostly the same.
These surveys essentially chart the differences and contrast between archaeological remains underground with the surrounding soil. Depending on the instruments you use, there are several ways to measure this contrast between solid archaeological materials and soil.
Some of the most common measuring methods include charting the contrasts in electromagnetic, magnetic and moisture content of subsurface objects and archaeological features. This equipment can intuitively recognise cultural and natural patterns.
The instruments used to measure these include magnetometers, Ground Penetrating Rader (GPR) and earth resistance meters. These measuring systems can be used relatively quickly and cover large areas, such as entire Roman forts or towns.
With a detailed understanding of the archaeological features beneath your ground, you can implement appropriate land management or excavation methods to work around them and protect them. Any detection method based on geophysical examination and taking readings of physical differences between existing geology and buried features can reveal things such as:
Barrows
Stone Circles
Hillforts
Roman Camps
Roman Forts
Roman Villas
Ancient Industrial Sites
Villages
Historic Gardens
Types of geophysical survey
Again, there are several types of geophysical surveys you can choose from, each with its own application and dedicated set of equipment. You can conduct these surveys remotely from the air, physically on the ground and even by drilling holes into the soil you want to survey. If you opt for the aerial approach, your geophysical survey will likely involve magnetic, gravity, radiometric or electromagnetic equipment or optical remote sensing technology.
These airborne surveys are usually conducted using helicopters or smaller aircraft that can hover above the area you want to survey in a grid pattern. Ground-based surveys, however, come in many forms, including:
Seismic surveys are non-intrusive procedures and involve using geophones, which are essentially sensors connected with wires, in strategic patterns across the ground. These geophones will then gather raw numeric data about the physical properties of rock formations many miles below the surface, which you can turn into interpretable maps.
To generate this information, your survey team will either use a truck mounted with vibrating weights on a vertical component to send shockwaves down to the rock formations or small explosions. When these vibrations hit the submerged rock, the geophones will pick up on them as they pass through the different layers and return the information obtained to the survey team for analysis and interpretation.
Ground-Penetrating Radar (GPR) works in much the same way as seismic surveys but is less disruptive than using vibrating weights or explosions. Essentially, the light radar signal in question is really an electromagnetic pulse that is fired into the ground on your survey site. If there are any archaeological objects in that area, they will reflect the electromagnetic pulse back to sensors above the ground.
This will tell you where the objects are and how deep they lie in the soil. GPR surveys are best for soils with a lot of sand content, as clay, silt and other heterogeneous sediments can interfere with the signals and skew your results through metallic interference.
Magnetic surveys, as the name suggests, are best used to detect metallic objects submerged beneath the ground. This geophysical method detects variations in the Earth's magnetic field caused by the presence of magnetic materials. Underground conductivity metres respond strongly to very subtle anomalies caused by the different magnetic content of submerged archaeological finds and metals. These help your technicians and surveyors determine what types of rock they are dealing with and what resources are contained within them.
Given their range, magnetometers used for magnetic surveys are best used on small areas, searching for features that are near the soil's surface. They can accurately pick up materials such as steel, iron, brick, burned soil and other types of rock common in archaeological finds. They are also excellent at finding subtle changes in material caused by disturbed soils, decaying organic materials, historic fire pits, or campfires.
As radiogenic materials decay deep within the Earth, they constantly emit gamma rays up to the surface of the soil. Radiometric surveys use this constant stream of gamma surface waves to detect anomalies and materials below the ground. Radiometric surveys are usually limited to the top 30 centimetres of the soil.
They can be performed from the air or a ground-based survey to find the metallic or industrial minerals beneath the ground.
Gravimeters measure the Earth's gravitaty field in a given area to accurately determine the extent of rock densities beneath the soil. Your technicians will take gravity measurements across a pattern of regular intervals over a distance and measure the differences in height across the reference points.
In most instances, gravity surveys are conducted to determine areas that are rich in minerals or energy resource deposits, like oil and natural gas or for mineral exploration.
Benefits of geophysical surveys
Geophysical surveys offer several benefits and can be used for a wide range of purposes. They are the ideal solution when you need to identify features in the subsurface of the soil accurately. They are also much easier to perform than more intrusive excavation methods, such as digging or drilling, which is a bonus for areas where such methods would be too destructive.
For example, in areas where the soil is delicate, with fragile materials hidden below or areas where there may be unexploded ordnance, geophysical surveys provide a safe and non-intrusive way to assess underground conditions and features.
Speed & Efficiency
Geophysical surveys are usually quick and easy to perform. Professional geophysical surveyors can obtain more geophysical data about a given area in less time than other, more intrusive methods might be able to achieve.
Site Management
Given that geophysical surveys can reveal micro topography of what lies beneath your site, this allows you to make better site management plans and mapping and mitigate against any potential issues for field campaigns.
Cost-Effective
The cost of a geophysical survey is much more affordable than digging and drilling. The ability to quickly and thoroughly scan coverage on an area of interest means you can take a more precise approach to your survey. Your next steps will therefore be more informed, and you will know exactly where you should and shouldn't dig, further reducing the overall costs of your project.
Non-Intrusive
Using geophysical survey methods means you do not have to disturb the ground on the site you want to assess. There will, therefore, be no impact on the survey area.
You can detect buried features and analyse them through data processing without damaging the landscape or the features themselves, which is a vital role if those features are of great value, such as those of archaeological record.
This is the main reason why geophysical surveys are generally the first stage in any archaeological digs, heritage assessments or site developments.
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