What is Geophysics?
Geophysics is a non-invasive survey method that allows the detection and mapping of archaeological, geological and modern man-made features beneath the ground surface. The techniques most commonly used by archaeologists include:
- Magnetic survey
- Earth resistance survey
- Electromagnetic survey
The information gathered can help archaeologists locate and map buried archaeological remains, without disturbing them through excavation. Geophysics can also help target excavations more strategically, delivering better value-for- money to clients.
- Magnetic surveys
- Earth resistance surveys
- Expert interpretation of survey results
- Production of high-quality plans and reports
We assess each site individually to determine the most appropriate and cost-effective survey strategy.
Ger Dowling of AMS has undertaken geophysical surveys at numerous sites, including:
- Drumanagh, Co. Dublin
- Lambay, Co. Dublin
- Dún Aillinne, Co. Kildare
- Lough Gur, Co. Limerick
- Brugh na Bóinne, Co. Meath
- Hill of Lloyd, Kells, Co. Meath
- Tara, Co. Meath
- Glendalough, Co. Wicklow
- Ferns, Co. Wexford
- Annegray, Haute-Saône, France
- N51 Dunmoe Phase 2 Scheme, Co. Meath
Geographic Information Systems (or GIS) are computer systems that capture, store, displace and generate geographical information as maps or data. This geographical information takes the form of vector data, raster data or geospatial databases.
GIS allows the archaeologist to digitise and create datasets, generating information that can be shared with other stakeholders as vector data, raster images and maps.
GIS can be used to access and query the large and freely- available heritage datasets from the Irish government, like the Sites and Monuments Record (SMR) and the National Inventory of Architectural Heritage (NIAH). GIS can also be employed with computer-aided design (CAD) technology to view and create vector data relevant to a project or development. It can also be utilised to analyse and model real-world environments for the purpose of identifying or predicting the presence of archaeology.
GIS allows the archaeologist to bring together and compare different geographical datasets, created for different purposes, easily and in a cost-effective way, with the intention of informing archaeological research questions and mitigation strategies.
Archaeologists use many different GIS software packages. Using QGIS (a powerful open-source GIS platform) and other software, AMS employs GIS extensively on projects to view CAD drawings of proposed developments; compare these with previously recorded archaeological sites and monuments records or historical maps; view the product of remote sensing surveys like geophysical surveys, LiDAR or photogrammetry; and to generate maps showing these relationships.
AMS uses maps extensively to share information with clients and statutory authorities, and to illustrate strategy documents, reports and publications.
Ground-penetrating Radar (GPR)
GPR can be used to map underlying foundations, structural remains and burials.
The technique involves transmitting a continuous electromagnetic pulse or wave of energy into the ground and recording reflections of that energy following its interaction with the objects and layers below the surface. The strength of reflections recorded is a function of 2-way travel time (the elapsed time for the energy wave to travel from the GPR to reflector and back to the surface) and is proportional to the conductive and dielectric properties of the layers and objects below the surface.
The system can be used manually in areas where access is limited, for example at the interior of existing buildings, and in combination with precision GPS or total station and quad bike for investigating larger urban/agricultural sites.
Examples of successful site usage include:
- Former cemeteries and burials
- Roman villa sites, castles and tower house remains
- Gallo-Roman temples
- Early church foundations and monasteries
What is LiDAR?
LiDAR (Light Detection and Ranging) is a remote sensing technique used to generate high-resolution 3D models of the ground surface which can help archaeologists identify and record various aspects of the historic environment.
The processed data can be manipulated by a range of techniques and software that can aid in the discovery of low-profile archaeological features, including those hidden beneath woodland canopy.
Some of the main visualisation techniques for LiDAR data include:
- Analytical hill-shading: artificial illumination is used to highlight surface changes such as upstanding features (mounds, walls etc.) and sunken features (ditches etc.). Greater illumination can be achieved by calculating analytical hill-shading in multiple directions equally distributed between 0° and 360°.
- Principal Components Analysis: a mathematical model that ‘summarises’ the information of hill-shade models from different directions.
- Sky-view factor: measures the portion of the sky visible above a certain point. In this way, flat terrain, ridges and earthworks which receive more illumination are highlighted and appear in light, while depressions appear dark because they receive less illumination.
- Slope gradient: represents the maximum rate of change between each data cell and its neighbours and can be calculated either as degree of slope or as percentage of slope. This technique works especially well in combination with hill-shading and also works well on most types of terrain.
- Openness: is a proxy for diffuse illumination. Positive Openness is similar to sky-view factor but with a more ‘flattened feel’, while Negative Openness provides additional information on convex features.
AMS uses the GIS program QGIS to store, manipulate and view LiDAR data. QGIS can be used to compare LiDAR data with other datasets, including CAD plans of developments, historical maps and the Sites and Monuments Record. AMS also employs the software Relief Visualisation Toolbox (RVT) to generate visualisations of LiDAR data, which can then be brought into QGIS for analysis.
Local History Research
What is Local History?
Local history sits between national history and family histories/genealogy, and generally deals with the origin and development of local communities. In the context of cultural heritage assessments, local history research aids in the identification and evaluation of heritage values, including archaeological and architectural assets.
Heritage researchers can use a whole range of historical sources,bothsecondaryandprimary,toidentifytheseassets and find out more about them. This information can then inform Environmental Impact Assessments, archaeological investigations, mitigation, and dissemination. Secondary sources include local history publications such as books and journal articles, which often deal with places of local interest that may not feature in regional or national histories.
Primary sources cover many different kinds of documents created at the time of an event, such as the letters written during the first Ordnance Survey of Ireland in the first half of the nineteenth century, which can contain information about antiquities that no longer exist or for which local knowledge may have been forgotten.