Geophysical surveys and preventive archaeology

Ground-penetrating radar survey of pre-existing structures in the Bergoglio district (AL) under the Piazza D’Armi (Parade Ground)

The aim of the research is to identify buried remains of the Bergolio district under the present-day Cittadella of Alexandria. Given the context and the aim, ground-penetrating radar surveys were planned in the area of the parade ground, including the inner tree line. From November 2019 to January 2020, three surveys were completed on different dates, covering the eastern area of the parade ground. The data collected was processed and assembled in such a way that sections could be made at constant times, i.e. at constant depths, under the entire investigated area. The analysis of the results led to the following considerations: conducting the surveys on different dates, with different soil conditions highlighted a difficulty in making the data homogeneous; there are no evident traces under the investigated area of building plans with possible internal articulations (rooms and/or corridors); instead, scattered, quadrangular or aligned structures appear. The latter are oriented in a way that is not in line with the Cittadella’s current main axes. In order to achieve greater homogeneity of data below the parade ground, a further measurement survey was planned, using a different type of radar, which allowed the entire area to be scanned on two consecutive days. The results of the latter survey are being processed.



Scientific director


Chiara Colombero, Diego Franco


Research project

The aim of the research is to detect structures or indications of buried structures ascribable to buildings or artefacts belonging to the pre-existing Bergoglio district under the present-day Cittadella of Alessandria. In order to achieve this aim, a ground-penetrating radar survey was conducted, covering the area of the parade ground, including the surrounding inner tree line. According to a reconstruction proposed by E. Piccoli et al. (E. Piccoli, C. Tocci et al., Cittadella di Alessandria. Storia dell’architettura e della costruzione. Relazione intermedia, July 2018), beneath the scanned area there should be traces of external and internal walls of some of the Bergoglio buildings (Figure 1).


The extent of the areas to be investigated and the equipment chosen meant access to the site was required on different days. Changes in physical ground conditions due to weather events between one session and the next varied the ground-penetrating radar response. This condition, despite making the areas acquired in each survey readable, does not allow for a uniform representation and examination of all the data from all the surveys. For this reason, a final survey was conducted in October 2020 with special equipment that would allow data to be acquired over the entire area in a single day, limiting the variability of ground conditions as much as possible.

A total of seven measurement sessions were conducted over a period from May 2018 to October 2020. Apart from the first one, which involved segments of wall considered significant in some of the Cittadella’s buildings, the research focused on the parade ground (rectangle bordered in red) and an area adjacent to the playground (rectangle bordered in blue), as shown in Figure 2.


After a first partial measurement survey, conducted on 24 May 2018, activity resumed, planning the data acquisition for the entire eastern part of the central corridor of the parade ground. The acquisitions were conducted by sub-area, the vertices of which are shown in Figure 3.


After the surveys in the eastern area, conducted from November 2019 to February 2020, all radar profiles (468 profiles for about 17,000 m of radar tracks) were processed following the same procedure. The processing of all the profiles had an additional aim compared to the usual processing: to make the volume of the final data as uniform as possible, i.e. to eliminate as much as possible the differences, due to different soil conditions, between areas scanned at different times. The processed profiles were assembled in space, according to the acquisition coordinates, to obtain the data volumes below the eastern area. The volumes were sectioned at constant times to obtain ‘horizontal’ (timeslice) sections, in depth, of the reflection intensity.


The analysis of the timeslices allowed some reflective structures to be recognised, for example, in Figure 4, three well-defined although discontinuous reflective objects are highlighted. The equalisation of the radargrams obtained in the various surveys, however, is only partially successful. In fact, in order to minimise the effects due to acquisitions on different dates and to obtain more comparable information over the whole area, a further definitive survey was planned and conducted using a multi-channel radar over two days (21 and 22 October 2020), the data from which are still being processed.