GPR prospection to locate a water well and a gallery in an urban historical site

BONOMO, NÉSTOR; MARTINELLI, PATRICIA; OSELLA, ANA; DE LA VEGA, MATÍAS

Geoacta

Association of Geophysicists and Geodesists

Año: 2016 vol. 41 p. 50 - 64

0326-7237

This paper describes the methods and results of a geophysical survey carried out with GPR at the historical site ?Manzana de las Luces?, located in the city of Buenos Aires, Argentina, whose main objective was to pinpoint a cistern and a gallery built during the 17th and 18th centuries. Another objective was to detect newer structures, such as parts of the floor and pipelines. The search was prompted by the need to have useful information for planning archaeological research and conservation actions. Besides, the results of the study will contribute to the preservation of the historic structures against possible architectural or civil engineering works that can be performed in the future. From the geophysical viewpoint, the site has a layer of soil with significant clay and moisture contents, which implied intermediate to high attenuation of the GPR waves and, consequently, limited penetration. Moreover, the site is in an urban area with nearby objects -located in the soil and air - and varied sources of radio frequency, which anticipated the possibility of obtaining complex data sections with numerous and close reflections, with different shapes and intensities. The expected characteristics for the reflections due to objects of interest and the proximity of events required a good resolution level in the data sections so that the main reflections could be distinguished and interpreted correctly. That made necessary to take some precautions, especially at the time of survey design and during the analysis and interpretation of the data. Tests performed before the definitive prospecting allowed defining a set of measurement parameters that made possible to detect intermediate to low- amplitude reflections with good definition. Furthermore, the simultaneous representation of the data in the forms of vertical sections and constant-time/horizontal slices favored detecting reflections that in principle were detectable in only one of these forms of representation. The reflections detected from these types of graphs were correlated between them, classified and finally interpreted in terms of their probable origins. This methodology allowed determining the locations of the cistern and the gallery with an accuracy of a few centimeters, as well as detecting floor discontinuities, different pipelines and the presence of a thin mesh in the floor structure. The types of investigated objects and environmental conditions in this study are common to different archaeological sites in Argentina and other countries. In this regard, the procedures and results shown in this paper serve as an example or reference case. A particular goal of this paper is to demonstrate the usefulness and limitations of the applied methodology, in relation to the conditions imposed by the type of application and environment. Other objectives are to illustrate the different types of reflections that can be obtained in these cases, to show a detailed analysis of these reflections and their interpretation in terms of probable origins. It is expected that these contents will be of interest to people working with GPR in this kind of sites, as well as other colleagues with a more general interest in these topics.