Integrated remote sensing and geophysical methods can provide detailed information about buried cultural heritage. We implemented an integrated survey protocol (IREGA, Integrated REmote-sensing and Geophysical prospecting for Archaeology) and tested the performance of the method in the area of the ancient Roman town of Aquileia, NE Italy, to define and characterize microareas of archaeological interest starting from macro-area observations. We used electromagnetic (GPR; ground-penetrating radar), magnetic and remote sensing (MIVIS; Multispectral Infrared and Visible Imaging Spectrometer) to image and characterize buried targets of potential archaeological interest in the depth range between 100 and 350 cm. We identified various geometrically coherent anomalies, possibly related to subsurface structures, through MIVIS data processing and found them in good agreement with the elements reported in the Aquileia archaeological map obtained from documentary evidence and excavations performed in the last century. Ultra High Resolution (UHR) MultiFold (MF) Ground-penetrating Radar (GPR) and magnetic surveys confirmed the MIVIS results and allowed imaging and mapping of buried structure related to different Roman remains (SE sector of the Circus, harbor and residential buildings foundations, roads).
The research of buried archaeological structures requires a detailed knowledge of the
shallow subsurface and a methodological approach designed to define areas of
maximum interest where surface data collection and successive excavation should be planned.
The definition of macro-areas can take advantage of remotely-sensed data or Digital
Elevation Model (DEM) characterized by metric spatial horizontal resolution and
millimetric spatial vertical resolution respectively
Aquileia is a Roman archeological site covered by alluvial sediments and
characterized by a shallow water table
During the last ten years, we have been collecting a large amount of UHR
geophysical datasets to identify archaeological structures buried in the area. Several studies
AQUILEIA
The proposed methodological integration (IREGA) for the non-invasive study of archaeological sites is based on the analysis of the following data: MIVIS, magnetic and GPR. At selected locations, Electrical Resistivity Tomography is also included, for an integrated analysis of the electric properties of the buried materials. The MIVIS system has 4 optical ports with 102 spectral bands - ranging from 0.431 to 12.7 m and a spatial resolution about 3.0 m/pixel with a fly height of 1500 meters above sea level.Its effectiveness in archaeological prospecting depends on the characteristics of both the landscape and the buried structures. In fact, archaeological buried objects can influence soil moisture and temperature, growth and health of the vegetation with anomalies marked out by different brightness (related to reflectance and temperature pixel value) and peculiar geometric shape. 50m
0m A
5.4m MAGNETIC Anomaly [nT]
10 B 0 -10 Fig. 2. Example of integrated geophysical surveys at site 6 (see Fig.1): A) GPR 200MHz stack section with magnetic data superimposed (in yellow); B) interpreted magnetic map. w1, w2 and w3: buried foundations remains; h1: contact between soil layers; dn : large dipolar magnetic anomalies due to shallow metallic materials; w4, w5: anomalies related to archaeological targets.
DISTANCE [m] 0.0 0.0m A
B
The magnetic method measures variations of the Earth’s magnetic field. It is an
effective technique in archaeological prospecting to detect variations of magnetic
susceptibility due to the presence of buried objects. Single-sensor and multi-sensor
gradiometer measurements can be performed to detect magnetic anomalies due to buried
archaeological remains
The term Ground-Penetrating Radar refers to a family of Ultra-Wide Band devices
that use electromagnetic (EM) waves in the frequency range between approximately 10
MHz and 6 GHz to image and characterize the subsurface
MIVIS digital images were provided by Regione Friuli Venezia Giulia. We show here the results obtained from the analysis of thermal Infrared bands (93 to 102), whose temperature pixel values in oC were calculated with linear interpolation between referenced values of two black bodies. Ground pixel size corresponds to 3.0 meters.MIVIS data processing encompassed radiometric correction by means of principal component (PC) method, bad bands removal and geometric correction. Magnetic surveys were performed with a cesium magnetic gradiometer (SMARTMAG model SM-4), with a sensitivity of 0.01 nT and an operating range from 15000 to 100000 nT. Measurements were performed with two sensor located at 30 and 130 cm above ground level on NS
Magnetic anomaly [nT]
100 m +
B oriented grids.. A low-pass filtering 3x3 matrix was applied to remove incoherent noise and enhance magnetic anomalies.
Magnetic and GPR data were both acquired on grids with 2 cm - 25 cm inline
crossline sampling interval. A Mala˚Geoscience GPR system equipped with 250 MHz
central-frequency antennas was used to acquire single- and multi-fold (average 1200%
fold) data. Minimum and maximum offset were set according to preliminary tests and
range between 40 and 240 cm. The basic GPR processing sequence included Wavelet
Transform based de-wow, background removal, amplitude analysis and corrections,
spectral analysis, time-varying band pass filter and predictive deconvolution with
operator length = 30 ns and prediction distance = 4 ns. The instantaneous attributes
(amplitude, phase and frequency) of the radar trace were calculated by Wavelet Transform
techniques
Examples of complementary (GPR and magnetic) and individual (GPR) results are given in Figs.2,3,7. Figs.4,5,6 show the outcome of the integration of different techniques: MIVIS, magnetic, GPR, ERT.
GPR and magnetic data frequently provide mutually consistent results, but in some cases they give complementary information. Fig.2 shows a comparison between GPR and magnetic data from site 6 (see Fig.1): the yellow line superimposed on GPR data in Fig.2A is the variation of the magnetic field measured along the GPR profile. W1,2,3 are the locations of buried wall/foundation remains that can be interpreted on the GPR data but do not show a clear signature on the magnetic record (W3). W4,5 in Fig.2B are small variations (in the range of 10nT from average magnetic field) that exhibit geometric coherence and are actually related to buried brickwork.
At the N limit of the cemetery of Aquileia, a large 3-D multifold GPR dataset provides a detailed subsurface reconstruction. Fig.3 shows an example of a highly elusive target, namely an unpaved road close to the walls of the imperial town, imaged by GPR. In this case, the contrast in petrophysical properties – porosity, fluid content – is given by the different compaction of sediments of the road compared to the surrounding materials.
An example of integration of remote sensing (MIVIS) and geophysical (magnetic) surveys is shown in Fig.4. Thermal infrared band 98 (Fig.4A: arrows and dotted line) shows a clear anomaly characterized by homogeneous temperature values lower than the surrounding ones. The anomaly shows a good correlation with the position of the roman circus tentatively reported on the Aquileia archaeological map (inset between Figs.4A and 4B). The filtered magnetic map (Fig.4B) shows an excellent correlation with the position of the roman circus as reported on the archaeological map, and with the thermal infrared data. Nonetheless, MIVIS and magnetic data do not convey information about the third dimension, i.e. the depth and vertical cross-sections of the anomalies are unknown. Such information is given by the GPR record in Figs.5A,C. Targets a, b, c in the B-scan (Fig.5A) can be interpreted as buried remains within, at the edge and outside the roman circus, respectively. The agreement with the magnetic data is apparent and further confirmed by the comparison between the GPR time-slice (Fig.5C) and the magnetic map (Fig.5B).
In front of the excavated sector of the Roman Forum, a large unexplored field has been used to test the integration of GPR and ERT (Fig.6). The GPR B-scan (Fig.6B) and the ERT section (Fig.6C) are in good agreement and give evidence of a bulky sector characterized by higher resistivity and low attenuation of the EM wave. Such characteristics are consistent with materials normally found in buried walls/foundations. A complete map of the surveyed area with the integrated interpretation of GPR and ERT data is shown in Fig.6C. The interpreted subsurface targets exhibit orientation coincident with the exposed remains excavated in the nearby Forum area. The highest resolution attainable by non-invasive methods is illustrated by the multi-fold 3-D GPR results shown in Fig.7. Slices of the processed data volume (Fig.7A) or of the attribute volumes that can be calculated from the processed data (e.g. Fig.7B) give clear evidence of horizontal and vertical distribution of the targets. The interpretation superimposed on the GPR data (Fig.7C) and the final results, combined with the archaeological map of the neighboring site (Fig.7D) illustrate the horizontal and vertical distribution of targets.
DISTANCE [m] 0 0.0 GPR Profile 4 20 []sn40 a T 60 W T 80 A 100
b
GPR Amplitude GPR Profile 4
25m GPR Profile 4 25m The tests performed in the Aquileia area indicate that the integration of different geophysical methods and the combination with remote sensing data analysis allow detailed and cost effective identification of targets of potential archaeological interest in large areas. MIVIS data allow the identification of areas of potential archaeological interest at macro-scale. Such information is obtained from the anomalies related to soil moisture and temperature. Ultra-High Resolution magnetic gradiometry and multi-fold GPR, with dense spatial sampling (i.e. average 5/25 cm in/cross-line respectively) are successively required to image and characterize targets before excavation. Areal geometry of the buried structures can be obtained by an integration of MIVIS, magnetic and GPR. Depth of the targets can only be obtained from GPR data, with accuracy depending on
SURVEY
AREA m 20
ELECTRICAL RESISTIVITY [Ohmxm] the depth of the target. In the present study, targets in the depth range between 80 and 250 cm were identified with absolute uncertainty 5 cm. Electrical resistivity tomography can further help in the characterization of physical properties of the materials, since higher resistivities are normally correlated with stone, brickwork and foundation remains.
In this work, we studied four sites of potential interest. The largest structures revealed by the geophysical survey are the buried remains of the Circus, which exhibit a clear signature in all of the datasets (MIVIS, Magnetic, GPR; Figs. 4 and 5)). The analysis of 3-D GPR datasets allows the identification of further targets, which are below the resolution threshold of remote sensing methods. This is the case of the buried foundations and remains of walls in the forum and harbor areas (Figs. 6 and 7). Buried remains characterized by low contrast in physical properties (see e.g. the compact soil corresponding to the road, Fig.3, are also revealed by 3-D multi-fold GPR due to the high S/N ratio of the final images and the high sensitivity of the method to minor variations in water content. In some cases (e.g., site 6, Fig.2) magnetic and GPR data can provide complementary information and help in the identification of highly elusive targets.
Future developments should benefit from the integration of the proposed methods with high resolution topographic surveys of the areas of interest by means of airborne laserscan.
The authors gratefully acknowledge the support of the BAAAS Superintendency of Friuli Venezia Giulia. They also thank Regione Friuli Venezia Giulia for permission of using MIVIS data of Aquileia and dr. L.Bertacchi for the permission of reproducing the archaeological map of Aquileia. The study was supported by the European contract MICCS and the Italian Foreign Office contract NICCOS.