Geophysical survey (x-ray)
There are many different types of geophysical survey using a range of equipment, and each is suited to different aims, objectives and conditions – all focused on providing a glimpse below the ground without disturbing the surface. The most commonly used in commercial archaeology is magnetometry, largely because it is the most reliable in the consistency of results, it is fast, and can be done comparatively cheaply. It can be hand-held (which is good in tricky conditions) or pulled behind a quadbike on a cart loaded with sensors.
A magnetometry survey produces a black and white image on which the positive or negative spikes in the data show where archaeological features may be present. Features such as filled-in ditches and the buried remains of walls are often the strongest results, but other features can include roundhouses, enclosures and pits. Graves often do not show on this type of geophysical survey because they are backfilled so quickly there is almost no discernible difference between the fill and the surrounding soil.
If the survey identifies the presence of archaeological remains, then it is often necessary to progress to the next stage of investigation: trial trenching.
Trial trenching (keyhole surgery)
Trial trenching investigations consist of a series of long rectangular holes – often 30m or 50m long by 2m wide – excavated across a site in a strategic pattern and covering a given percentage of the square meterage of the area.
The purpose of trial trenching is to test the geophysical survey results: are the features definitely archaeological or actually geology? How well are any archaeological remains preserved; is there any indication of the date of the archaeological remains in the form of artefacts (e.g. pottery)? Trial trenching helps to establish the form and function of any archaeological remains present: is it a settlement, iron working site, military site, etc? It also helps to establish the extent of the remains across the site: do they cover the whole area? Do they extend beyond it or are they confined to a small part of the site?
It may be determined in coordination with the county archaeologist that the record produced from the trial trenching is sufficient and proportionate enough to the significance and importance of the archaeological remains that no further work is necessary. If it is determined that further investigations and recordings are necessary, then works progress to the final level of archaeological investigation: mitigation.
Archaeological mitigation (open surgery)
During archaeological mitigation, the modern overburden of an area – defined in coordination with the county archaeologist – is stripped down to either the natural ground level or the first archaeological horizon, whichever is encountered first. The term ‘natural’ refers to the undisturbed ground formed naturally over millions of years through geological processes.
In most cases, surviving archaeology is visible as features cut into this natural layer, appearing as darker deposits on the surface. However, archaeological remains may also be found in layers above the natural ground. These layers are known as archaeological horizons, and they are the level of the ground where archaeology begins to be visible.
Once the site has been stripped, any archaeological features will be visible on the exposed surface. The next step is to investigate and record these remains. This is typically done by hand-digging slots in the features to determine their date, function and significance within the wider archaeological context of the area.
The methodology of this process is outlined in the Written Scheme of Investigation. Typically, a 20% sample of linear features is investigated, 50% of discrete features and 100% of burials, structural remains or any features of particular importance (although this may vary depending on the specifics of the site and the agreed methodology). All archaeological remains must be recorded through photographs, drawings, surveying and detailed notes, creating a database of the uncovered archaeological features and the recovered artefacts.
Once all necessary fieldwork is complete, a final report is produced. This report presents the results of the excavation and interprets the function and significance of the site within the wider archaeological setting.
.jpg)
.jpg)
.jpg)
.jpg)