Research-2014

Geophysics Techniques

Archaeological survey is a key component of archaeological research, and is increasingly used as both a tool for large scale exploration of landscapes and urban settlements, and as a precursor to excavation.

The service provided by the British School at Rome and APSS allows for the integration of different non-destructive techniques, comprising topographic survey, and the principal methods of geophysical prospection: magnetometry, resistivity, tomography and ground penerating radar. This work is backed up by a data geo-referencing and mapping service, which includes analysis and interpretation of results, and the rapid production of high quality, well-researched survey reports.

Specialist methods offered include:

  • Resistivity survey
  • Resistance tomography and multiplexed resistivity
  • Magnetometer survey
  • Magnetic susceptibility
  • Ground Penetrating Radar (GPR)
  • Topographical survey
  • Production of images and interpretation in a variety of formats
  • Fully researched interim and final reports, produced with full academic backup

Resistance and Tomography

Electrical Resistance and Resistivity Tomography surveys are based on the ability of sub-surface materials to conduct an electrical current passed through them. All materials will allow the passing of an electrical current through them to a greater or lesser extent. There are extreme cases of conductive and non-conductive material, but differences in the structural and chemical make-up of soils mean that there are varying degrees of resistance to an electrical current. This technique is particularly suitable for locating buried structures, such as walls and floor surfaces, and changes in the water content of the near surface which often indicate differential fills.

The two electrical techniques utilized by the BSR complement each other and the other survey methods offered. Electrical resistance surveys collect a series of data points in a high resolution grid pattern across the surface of a site or landscape, offering near surface data in two dimensions from a plan view. Electrical resistivity tomography (ERT) involves collecting a vertical section of data to explore deeper into the subsurface along linear profiles that can be hundreds of meters in length. While both methods can be used to generate two dimensional plans of archaeological features ERT is able to offer depth data that allows for interpolation to three dimensions, often reaching greater than five meters below the surface.

The BSR utilises a Geoscan Research RM15 Resistance Meter for resistance surveys, often in a twin electrode probe formation, however surveys can also be undertaken using other arrays, such as Wenner and dipole-dipole. For ERT surveys the BSR uses an Allied Associates Tigre 64-probe system which allows for long profiles reaching deep into the ground.

Magnetometry

Magnetometry survey detects changes in the magnetic field across the surface of a site or landscape. The sensitivity of this technique is able to detect buried features up to three meters below the surface depending on the strength of the signal and the surface conditions. This method is particularly efficient at individuating burnt features (e.g. kilns, hearths), and negative anomalies such as ditches and pits. The traces of buildings can be detected when they are constructed of fired brick, or if they have been buried in more soil with a distinctly different magnetic signal. High concentrations of ferrous material on or near the surface and modern structures such as fences can limit the effectiveness of this technique by obscuring the archaeological signals.

This is the most commonly used geophysical survey technique by the BSR. Although area coverage depends upon local terrain and survey conditions it is able to collect data over a larger area than the other techniques employed given the same time constraints and can often complement the other techniques employed. The BSR uses the dual probe Bartington Grad 601-2 which can collect high resolution grids of data across over a hectare per day.

Ground Penetrating Radar

Ground Penetrating Radar (GPR) is based upon the propagation of an electromagnetic radar wave through the soil to search for changes in soil composition and the presence of structures, measuring the time in nanoseconds (ns) taken for the radar wave to be sent and the reflected wave to return. This technique has been applied successfully on a range of archaeological sites, in particular over substantial urban archaeological remains.

While the use of GPR is more time consuming than the application of magnetometry or electrical resistance it provides extremely high resolution data including depth values that allow for a detailed modelling of the subsurface. It is recommended that this technique is used to target particular areas of interest at an archaeological site where archaeological deposits can be anticipated at a deeper level, or where it can complement or replace the other more coarse survey techniques.

The BSR operates a GSSI radar system with a SIR 3000 with 400MHz antenna, which allows propagation of radar waves down to a depth of approximately 5-6m depending on the nature of the sub-surface materials. Other antenna frequencies can be applied depending on the specific conditions and research goals for each project, either reaching deeper or surveying at a higher resolution.

Topographical Survey

Topography is the study of the shape of the Earth’s surface. The modern ground surface often contains important information on the conditions and nature of an archaeological site, and the potential existence of structures buried beneath the soil. Changes in the shape of the surface can have a great influence on determining the nature of features in a geophysical survey. It is vital to conduct a detailed and complete topographic survey as a key component of the integrative field work that the BSR conducts for every field project.

Obtaining a detailed model of the desired surface with the goal of emphasizing archaeological features generally entails the recording of elevations at regular intervals at a high resolution, but also the recording of points on known breaks of slope. Topographical survey is usually undertaken by the BSR using a Total Station or Global Positioning System (GPS) to record survey points. Computer software is then used to produce topographic plans and Digital Elevation Models (DEMs) of the results. The resolution is determined on a site-by-site basis and depending on this evaluation, the technique and site conditions many hectares a day can be covered.