CAA Conference

Recent study under the Portus Project (www.portusproject.org; www.heritageportal.eu) has provided the opportunity to look at the pattern and extent of natural and man-made features across the Tiber delta as a whole. The area represents one of the most prominent coastal zones on the west coast of Italy, and forms an important area of past and present human activity from the early Neolithic onwards. The aims of the present research are to understand the effect of human influence on the natural environment, and assess the establishing of patterns of settlement in relation to the changing geomorphology of the delta. An integrated strategy of analysis and fieldwork has been conducted, with assessment of air photographs held by the ICCD, and access to remotely sensed satellite data to map the archaeology and geomorphology of the area. Extensive fieldwork has also been conducted between 2008 and 2011 across the Isola Sacra in the central area of the delta, principally using magnetometry to map at high resolution all archaeological features over an area of 150 hectares. This is in addition to previous fieldwork between 1998 and 2006, where over 220 hectares of the Roman port and its immediate surroundings were mapped. Results of this integrated strategy have revealed a number of extensive features related to the natural development of the delta, and to past human interaction with the landscape. They include a map of the prograding deposits across the central portion of the delta, and patterns of deposit associated with the meandering course of the Tiber, together with man-made channels and field divisions, and a later complex of canals associated with connecting the Roman port at Portus with the river Tiber and the sea. Recent discoveries also indicate possible connections between the port and the town of Ostia at the mouth of the Tiber.

The application of geophysical methods to investigate the near-surface soil layers containing anthropic manufactures has been recognized as an important element of archaeological research by the international community. Geophysics can be used to rapidly delineate the presence of archaeological buried structures without invasive and expensive stratigraphic excavation. In particular, resistivity surveying can be used to understand the geometry and the depth of the anthropic element buried in the subsoil, due to the different resistivity properties between the potential archaeological targets and the surrounding environment; by GPR methods it is possible to easily produce high definition maps of buried remains, thanks to the transmission of high frequency radar pulses from a surface antenna into the ground. Geoelectrical data are traditionally acquired with a galvanically-coupled resistivity system. The most practical difficulty is to emplace electrodes in the soil; this operation is time consuming and prevents the fast realization of the investigation, especially in case of a three-dimensional survey. This problem can be avoided using the new OhmMapper (Geometrics Inc.) capacitively-coupled resistivity system designed to be pulled along the ground as a streamer that realizes an almost continuous profile. The main problem of the OhmMapper resistivity-meter is the length of the instrument and the consequent necessity of a lot of space to use it. To avoid this problem in small spaces, we use the multi-frequency GPR RIS MF HiMod (IDS S.p.A.), top level product dedicated to underground mappings: the 200 and 600 MHz dual frequency antenna is the ideal solution to quickly realize an accurate mapping of the subsoil in archaeological context. In order to verify the presence of buried structures and bedrock in the subsoil of the ancient town of Lixus (Larache - Morocco), we carried out some geophysical surveys in different areas using GPR and OhmMapper resistivity-meter. The site of Lixus is located on the northern Atlantic coast of Morocco, on the bank of the Loukkos River, approximately 80 kilometres south of Tangier and adjacent to the city of Larache. Thanks to the archaeological excavation carried out on the hill of Lixus it appears that the archaeological site had different building stages from the Phoenicians (8th century BC) until the Marinid dynasty (14th century). We decided to use GPR in restricted areas to detect buried structures under the foundation of the visible archaeological structures; we set a grid characterized by parallel and perpendicular lines close to each other to intercept the walls: we created several resistivity maps at different depths from a few centimetres from the ground surface to two meter deph. Instead we used the OhmMapper resistivity-meters along the dirt roads of the hill in order to locate the buried section of the city walls and the depth of the sandstone bedrock, which is untouched by anthropic activities. The resistivity and GPR data were also compared with magnetics data acquired in the ancient town of Lixus. This multi-method approach permits us to check the data from several independent measurements and increase the geometrical and physical information useful for interpretation.

Abstract Electrical resistance geophysical surveys are known to produce variable results at different times of the year. This is a problem which can often lead to a misinterpretation of an archaeological feature under investigation. The dynamic relationship between a natural soil matrix and an archaeological feature is a complex one which greatly affects the success of its detection through electrical resistance. For electrical surveys, the change in contrast is mainly a function of moisture and ion concentration within the soil, the geology and the environment in which the archaeological deposits are situated. This paper will present the initial stages of a study monitoring the gradual variation of moisture by electrical methods over a selection of test areas. Monthly earth resistance surveys have been conducted over the last six months over known ditch features on four sites.Introduction The success of an electrical resistance (ER) survey is dependent on many pedological, ecological, geological and archaeological variables which exist within the natural landscape. The prospection method of resistance survey is greatly affected by soil moisture content, which changes, both diurnally and seasonally throughout the year. Past studies have not looked at archaeological detection in traditional 'problem' areas such as those on clay geologies or areas of low contrast. Past research has thus concentrated on large ditch features in areas of high contrast and not fully investigated why the detection problems exist.Benefitting from a multi-disciplinary framework, as part of The DART Project, the research includes monthly geophysical data collection from four sites, and will incorporate data from parallel investigations into archaeological detection using hyper-spectral and spectro-radiometry survey, soil analysis, and time domain reflectometry (TDR) and weather measurements. This will allow a full environmental and geophysical case history for each site to be built, and enable the targeting of ER surveys at the 'best detectable' times, based on telemetry from the TDR and weather data.At the four test areas, a Twin-Probe multiplexed earth resistance survey has been conducted every month since June 2011 and will continue until October 2012. By using a multiplexer to increase the separation of the mobile probes at each data station, deeper volumes of earth can be measured. Each resistance survey yields a dataset from 4 sequential depth investigations.Preliminary resultsThe key of detection is 'contrast' between the feature under investigation and the surrounding soils so initial data analysis from the surveys is looking at the data collected over the ditch features and comparing these readings to a selected 'background' response as well as the complete dataset. By calculating the percentage difference between these populations, we are able to assess how the contrast between the ditch and background is changing and becoming more (or less) detectable.The results will be compared to the local weather in the area to assess its impact on the changing geophysical contrast.

Aerial and geophysical methods are widely used and valuable techniques for the detection, mapping and curation of the fragile archaeological resource. However, the changing geophysical properties of the soil, which vary spatially, seasonally and throughout the day, and their effect on sensor responses are poorly understood. A long term monitoring strategy to record the changing conductivity, permittivity and temperature, and the differences in these factors between an archaeological ditch feature and the surrounding soil matrix at different depths was thought to be of some benefit for informing future use of these technologies. The use of geotechnical properties, such as moisture content, density and particle size distribution, along with local weather data collected on site can be linked and used as a proxy for the geophysical response and feature contrast, through the use of dielectric mixing models. This paper aims to present a novel method for examining these contrast factors using Time Domain Reflectometry (TDR), a long standing electromagnetic technique used to monitor soil moisture in environmental, engineering and soil science research. The design and development of a monitoring station, suitable for a long term data collection strategy, and the methodology for field installation will be discussed. Particular attention will be given to the challenges faced, and the methods used to solve them. The large raw waveform datasets collected by the stations also present a challenge for interpretation, requiring processing to produce geophysical properties. Attention will also be given to the mathematical methods for interpreting these raw waveforms and the development of an automated script in MATLAB, capable of converting the waveforms to permittivity and conductivity values. Finally, some preliminary data from the monitoring stations will be presented to highlight the type of information the project is producing, and its significance is briefly discussed. This research forms part of the DART project (Detection of Archaeological residues using Remote sensing Techniques), a three year, multi-university and multi-disciplinary project. It aims to enhance knowledge of the science behind the detection of archaeological features using aerial and geophysical methods, particularly on soils which are traditionally unresponsive, such as those with high clay content.

A new multi-channel digitizer for fluxgate gradiometer arrays is presented. It is characterised by a very high measuring resolution, broadband ADCs of 24 Bit bandwith, sampling rates up to 500 Hz and flexible GPS interfaces. Extensive field tests with several sensor types have been realized since 2010. The ruggedized and waterproof digitizer was succesfully applied in several large-scale archaeological prospection projects in Germany, Spain, Portugal, Italy, Jordan and Turkey. The advantages of the new geomagnetic system are shown by large-scale prospection examples of archaeological structures of very low magnetisation. Especially sites situated in sandy environments are often characterised by unfavorable conservation conditions for organic remains due to an increased acidity of the soil. But compared to other fluxgate arrays even very small magnetic anomalies in the range of ±1 nT can be detected in these soil types. Case studies from several sites in Germany and Turkey are presented. The economic advantages of fluxgate magnetometers especially in large-scale prospections of archaeological sites and landscapes provided the starting point of the efforts to improve multi-channel fluxgate systems. In contrast to alternative magnetometer types applied in archaeological research like caesium (Cs) or SQUID probes, fluxgate magnetometers can be assembled to large arrays (6 to 16 probes) with comparatively low costs. Only such arrays allow the fast and efficient prospection of large areas. The most important precondition for the successful application of fluxgate arrays in archaeological research is a high-quality data logging exploiting the dynamic range and the maximal resolution of the probes to a maximum extend. Using a high-resolution broadband data logger with high sampling rates (up to 1000 Hz) the lower measuring accuracy of fluxgate sensors compared to Cs or SQUID sensors can be fully compensated. Fluxgate sensors can provide equivalent results to Cs sensors in archaeological prospection. The approx. twentyfold better accuracy of Cs sensors (ca. 0.01 nT) compared to fluxgate sensors (ca. 0.1 to 0.2 nT) take only in few cases effect in archaeological applications. The reason for this is, that the crucial factor for the successful detection of archaeological structures and objects is the the ratio of their proper and the ambient magnetization. The ambient magnetization (caused by geological and anthropogenic structures) superimposed by the magnetic anomalies of the archaeological targets determines the practical range of the required measuring resolution. Due to this limitations the dynamic range of archaeological magnetograms rarely drops under ±1nT. Below this range of ±1nT the magnetograms are usually oversaturated i. e. geological and anthropogenic influences dominate the results. From 2009 to the beginning of 2011 the new multi-channel digitiser LEA D2 for geophysical measuring systems was developed. The project was funded by the ZIM program of the German Federal Ministry of Economy. By the end of 2011 the digitiser was prepared for a broader commercial exploitation.