Prospecting for archaeological sites is a central concern of archaeology, and the development of new techniques for detection, along with the continued application of well-established approaches, has put an understanding of landscape at the heart of archaeological research. Analysis of the results of the various detecting and imaging techniques described in this section have also enabled the visualisation of the past over a variety of scales – from recreating a carved stone slab, to digitally modelling Rosslyn chapel, to helping reconstruct entire landscapes of past human activity.
Detecting and imaging techniques can effectively complement other archaeological or environmental science approaches, including excavation, coring, field-walking and topographic survey, in order to help understand the formation of complex and multi-period landscapes. Recent developments have, for example, enabled the exploration of submerged landscapes, potentially revolutionising current understanding of prehistory. Developments in the speed and resolution of a variety of techniques have improved detection, providing a more reliable knowledge-base that can support detailed analysis and interpretation, and modelling and visualisation of the past. Advances in, for example, the application of laser technology has enabled rapid and accurate survey of artefacts, buildings and landscapes. These techniques are most effective if they (and the results of previous survey and analysis) are built into archaeological research from the beginning. The choice of suitable technique will depend on local conditions, the required resolution of data (dependant on the questions being asked), and cost. Several techniques can be used for both prospection and for interpretation, and the value of their data increases exponentially when part of an integrated archaeological or environmental strategy (including e.g. field-walking, excavation, coring and soil sampling).
Both relatively established techniques and emerging approaches are discussed below, beginning with those that operate at a landscape scale and produce extensive information cost effectively, and progressing to intensive and detailed techniques that necessarily operate at a site or artefact level. Underpinning this structure is the need to apply multi-scaled approaches that recognise cost/benefit and the applicability of techniques to context.
Aerial reconnaissance is the original form of extensive archaeological remote sensing. Beginning with O.G.S. Crawford in the 1920s, reconnaissance in light aircraft has revolutionised the understanding of archaeology, particularly in lowland arable areas where the majority of heritage assets may have been discovered from the air and recorded on obliquely taken photographs. Ongoing reconnaissance continues to record otherwise unknown sites, and there is a growing recognition of the importance of archival imagery collected for other purposes such as map making and infrastructure projects, in fortuitously recording archaeological sites and the broader landscape. The photographic record reveals a range of archaeological and landscape information, though the nature of that information varies according to scale and date of photography, terrain type, land-use, type of site, and climatic and seasonal conditions.
Geophysical survey has traditionally operated at a site level, though with the development of larger data-collecting arrays that can be towed by vehicles this technique is progressively achieving landscape-scale coverage. Geophysical survey encompasses a variety of techniques that can be used in combination through multi-sensor arrays. Gradiometry, measuring differences in the magnetic field (the magnetic gradient), is the most utilised technique for prospection, particularly for negative features (e.g. pits, middens, ditches). Magnetic susceptibility is used to measure the magnetic properties of soil – a proxy for human activity – and can therefore be used for locating sites and interpreting their use. Resistance survey is another prospection technique for features such as ditches and walls, measuring differences in the electrical resistivity of the soil (rather than magnetic properties). Ground Penetrating Radar (GPR) provides information on depth and stratigraphy – the data can then be used to enable 3D modelling of a buried site. Other techniques include Electromagnetic (EM) survey, detecting both resistivity and magnetic data and Electrical Resistance Tomography (ERT), providing similar information as GPR though working best in different conditions.
Underwater surface mapping – or bathymetry – can be carried out using a number of techniques, utilising either acoustic or magnetic signals and is particularly effective when combined with coring or grab sampling. Single Beam Echosounders measure acoustic pulses bounced off underwater surfaces to create a bathymetric map. Multibeam echosounders use the same principle though to a much higher resolution. Sidescan sonar enables the reconstruction of the seafloor as a whole by firing acoustic pulses from either side of the sonar. Magnetic Signature Analysis can be used to trace submerged metal, although it has had limited archaeological use as yet. Underwater sub-surface mapping can also be undertaken by technology that utilises acoustic pulses, with techniques for rendering 3D imagery of the results currently being developed. The interface between land and sea can also be explored through tidal geo-physics.
Laser scanning has the potential to revolutionise landscape scale archaeological survey, but in Scotland to date its availability remains limited and resolution is not always that required to realise its full potential. It works with a laser beam reflected off surfaces and measurement of the time taken for the return signal, supplemented by GPS positioning and inertial navigation in the case of airborne scanners. Highly accurate 3D records can be made of heritage assets, from artefacts through to landscapes. These records can be subsequently modelled and used for a variety of purposes, e.g. the replication of artefacts or virtual access to inaccessible locations. Airborne Laser Scanning (ALS) otherwise known as lidar (light detection and ranging is increasingly being used in archaeology to create highly accurate digital terrain models (DTMs) that can be manipulated to explore the topographical nuances of the land surface and reveal archaeology hidden to visual survey. The data it produces, which can be rendered to remove vegetation cover can help reveal archaeological information underneath.
Hyper- or Multi-Spectral remote sensing captures information from across the electromagnetic spectrum by recording variations in wavelengths of light invisible to the naked eye. This data can then be processed and analysed to detect and model archaeological information, through for example, identifying changes in crop growth that are difficult to detect visually. At present the sensors do not have adequate radiometric resolution to routinely pick up archaeological sites (i.e. they are too coarse grained) but improvements in sensor resolution has the potential to open up detection of heritage assets across a much broader range of the electro-magnetic spectrum than the narrow visual bandwidth.
Harnessing the opportunities afforded by these techniques requires suitable training and skills, together with an understanding of local and regional conditions. The data produced requires skilled training in order to convert it to archaeological purposes, and can be combined with other information to good effect, such as integrating the results of laser scanning with the photographic record. Powerful 3D models can be created to aid understanding of, for example, the architectural phases of a building, or to create a virtual research environment to bring together physically separate resources. These techniques can be used to visualise the currently invisible aspects of the past, reconstructing past environments through both invasive and non-invasive techniques.
The variety of techniques and approaches available for detecting and imaging heritage assets are considerable and powerful. Integration with other techniques, often working at different resolutions, and with other forms of archaeological analysis, provides a well-rounded picture of how people lived in the landscape. The archival resource of these techniques, often representing decades of work, represents a considerable research resource and data set. The results of such survey work build up a picture of landscape use and development, which can be developed into landscape characterisation. Survey also provides the parameters for future archaeological work and ground-observation in an iterative process. The results from certain techniques can be combined to provide a comprehensive understanding of the resource, while the results of, for example, laser scanning can provide detailed models of heritage assets that can be subsequently analysed and researched.
Collaboration is essential to ensure that the research resource is as effectively gathered, integrated, interrogated and disseminated as possible. Fruitful collaborations with the field of environmental science, or through processes of resource management, offer considerable potential for value added to research. Detecting and imaging techniques feed into the interpretation, presentation and visualisation of archaeology, and contribute considerably to research as well as management and public appreciation. The application of these techniques also provides a locus for interaction with other disciplines, such as architecture and engineering – the re-purposing of marine data from other industries for reconstructing past environments is a good example of cross-disciplinary and collaborative working. Exploring the developing role that the techniques outlined in this section, along with emerging technologies and approaches, will have in archaeology is important to ensure that these considerations are built in to the day-to-day considerations of archaeological research.