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4. Processing and visualisation of data

GPR Coherence Imaging Examples: The Good, the Bad, and the Ugly

Alois Hinterleitner, Immo Trinks, Erich Nau, Lars Gustavsen, Mario Wallner et Klaus Löcker
p. 251-254

Résumé

– Large-scale high-resolution multi-trace GPR coherency imaging enables novel data visualisations to enhance the contrast of certain archaeological structures that otherwise are difficult to see.

– We will illustrate why data coherence visualisations can give good results at some sites and bad, if not ugly at others.

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Abstract

1By using efficient motorised multichannel ground-penetrating radar (GPR) array systems with cross-line spacings of 10 cm and less, it has become possible to generate highly detailed three-dimensional datasets of the subsurface covering entire archaeological sites (Trinks et al., 2018). Amplitude visualisations of the reflected energy in form of GPR depth-slices of only a few centimetres thickness have revealed fascinating traces of buried archaeological remains not only in case of urban Roman structures (Neubauer et al., 2018), but also in case of entire Viking Age settlements in Scandinavia (Trinks et al., 2013 ; Gustavsen et al., 2020).

2Nevertheless, the visualisation of the amplitudes of the reflected energy alone cannot depict all traces of archaeological structures contained in the data: in particular backfilled pits in sandy soils, with little geophysical contrast between the archaeological structure and the surrounding soil, are often difficult to detect because hardly any GPR pulse reflections occur at the interfaces of such structures.

3The computation of GPR multi-trace coherence of adjacent traces and visualisation in form of corresponding coherence depth-slices can image specific archaeological structures, such as backfilled pits in different geological soils, clearly visible (Trinks & Hinterleitner, 2020). This is because the GPR pulse propagation velocity and frequency content of the radar pulse are affected by these little reflective structures. Thus, the visualisation of GPR coherence in depth-slices can provide very good images for certain sites. For other sites, however, coherence mapping resulted in very noisy data images, from which hardly any valuable information could be retrieved.

4The many different sites prospected at high resolution over the past 11 years by LBI ArchPro in collaboration with the Norwegian Institute for Cultural Heritage Research (NIKU), the Vestfold and Telemark County Municipality, Norway, and the Central Institute for Meteorology and Geodynamics (ZAMG), Austria, provide extensive georadar data sets that have been visualized in both ways and compared according to their archaeological significance.

Coherence computation

5When computing the coherence of GPR data we use two different ways. In “inline coherence”-mode we calculate the coherence for each trace relative to the preceding and following adjacent GPR traces. In “in- and crossline coherence”-mode we also include the coherence relative to the two adjacent data channels to the left and right of each GPR trace acquired by the efficient 16-channel 400 MHz Guideline Geo MALÅ Imaging Radar Array (MIRA) systems used for extensive high-resolution surveys. The coherence (C) is calculated as the difference of the correlation of the wavelets within a certain time range (COR) of 1.0: C = 1.0 – COR. The chosen time range corresponds typically to one to two wavelengths of the nominal frequency of the radar antennas. In general, the “in- and crossline coherence” computation gives better results for data collected with the motorised MIRA systems. The “inline coherence” only is used, when there are strong influences of antenna coupling due to uneven ground, or when a GPR system with a cross-line spacing larger than 20 cm was employed.

6As an example, GPR prospection data from a Viking Age site in Norway, Gjellestad, is compared with data from a Roman site in Austria, Carnuntum. Both were surveyed in highest possible imaging resolution with a 16-channel 400 MHz MIRA system.

The Viking Age site of Gjellestad

7The GPR survey at Gjellestad in Norway, carried out by NIKU (Gustavsen et al., 2020), is an example of archaeological structures that became more clearly recognisable or only visible through coherence imaging. The ground at Gjellestad consists of marine sediments that formed cambisols, clay soils and brown earths, today used agriculturally. The absolute GPR amplitude depth-slice generated from data acquired at Gjellestad (Fig. 1A) shows only few archaeological structures aside of the modern drainage pipes/trenches and geological background variations. In contrast, the visualisation of the GPR multi-trace coherence using the same data set and depth-range (Fig. 1B) clearly shows several, regularly palced, large pits that belong to the buried remains of two Viking Age hall buildings, H2: 28 m long, 6.5 m width, H3: 38 m long, 7 m width (Gustavsen et al., 2020), as it has been confirmed by subsequent excavation.

Figure 1. 50 m × 50 m section of GPR data from Gjellestad, depth-slice from 60-80 cm depth. A: Visualisation of the absolute reflection amplitude (strong reflectivity is shown in dark). B: Visualisation of multi-trace GPR coherence (incoherence is shown in dark).

Figure 1. 50 m × 50 m section of GPR data from Gjellestad, depth-slice from 60-80 cm depth. A: Visualisation of the absolute reflection amplitude (strong reflectivity is shown in dark). B: Visualisation of multi-trace GPR coherence (incoherence is shown in dark).

Roman Carnuntum

8The extensive high-resolution GPR surveys conducted at the site of the Roman city of Carnuntum in Austria, carried out by LBI ArchPro and ZAMG (Wallner et al., 2021), provide the opposite example for coherence imaging. The local soils consists of calcareous sandy tip soils on top of wall rubble of Roman ruins. The visualisation of the absolute GPR amplitudes (Fig. 2A) shows very clearly the remains of buried wall structures and floors that easily can be interpreted as houses, roads, open spaces etc. The visualisation of the GPR coherence on the other hand (Fig. 2B) represents a bad or ugly imaging of the numerous walls, if at all, and the entire image appears very noisy compared to the visualization of the absolute amplitudes.

Figure 2. 80 m × 80 m section of GPR prospection data from Carnuntum, depth-slice from 60-180 cm depth. A: Visualisation of the absolute reflection amplitude (strong reflectivity is shown in dark). B: Visualisation of the multi-trace GPR coherence (incoherence is shown in dark).

Figure 2. 80 m × 80 m section of GPR prospection data from Carnuntum, depth-slice from 60-180 cm depth. A: Visualisation of the absolute reflection amplitude (strong reflectivity is shown in dark). B: Visualisation of the multi-trace GPR coherence (incoherence is shown in dark).

Summary

9The analysis of this and other examples shows that it is not only important that the archaeological structures do cause a change in velocity or frequency of the GPR pulse, but that the character and homogeneity of the surrounding soil is equally important, as are differences in ground coupling, reflections from within the arable topsoil layer, or strong reflections from structures in a highly inhomogeneous subsurface. In summary, from the many measurements that we have compared we can conclude so far that coherence data visualisations show very good results when

  • the high-resolution GPR measurements have been performed carefully and data of great quality has been collected,

  • the spatial sampling at the surface has been highly regular,

  • the arable top-soil layer is geophysically homogeneous,

  • the archaeological structures are relatively homogeneous, cause only little reflections but changes in the frequency spectrum and phase shifts.

10Coherence visualizations often appear very noisy when

  • the subsurface is stony or geophysically inhomogeneous,

  • the antenna connection varies strongly because of a rough ground surface, e.g. caused by plough furrows,

  • the archaeological structures are geophysically inhomogeneous,

  • the archaeological structures cause strong reflections.

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Bibliographie

Gustavsen, L., Gjesvold, P.E., Mannsåker Gundersen, S., Hinterleitner, A., Nau, E., Paasche, K., 2020. Gjellestad: a newly discovered ‘central place’ in south-east Norway. Antiquity, 94(378), 1520-1537.

Neubauer, W., Löcker, K., Hinterleitner, A., Wallner, M., Gugl, C., Trausmuth, T., Vonkilch, A., Jansa, V., Kröhl, M., Doneus, N., Tencer, T., Lugmayr, A., Aldrian, L., Verhoeven, G., Kucera, M., Trinks, I., Seren, S., Humer, F., Doneus, M., Bornik, A., 2018. Die nicht-invasive Erkundung der archäologischen Landschaft Carnuntum. In J. Drauschke, E. Kislinger, K. Kühtreiber, T. Kühtreiber, G. Scharrer-Liška, T. Vida (eds.), Lebenswelten zwischen Archäologie und Geschichte – Festschrift für Falko Daim zu seinem 65. Geburtstag, Monographien des Römisch-Germanischen Zentralmuseums, Schnell & Steiner, 969-984.

Trinks, I., Hinterleitner, A., Neubauer, W., Nau, E., Löcker, K., Wallner, M., Gabler, M. Filzwieser, R., Wilding, J., Schiel, H., Jansa, V., Schneidhofer, P., Trausmuth, T. Sandici, V., Ruß, D., Flöry, S., Kainz, J., Kucera, M., Vonkilch, A., Tencer, T., Gustavsen, L., Kristiansen, M., Bye-Johansen, L.-M., Tonning, C., Zitz, T., Paasche, K., Gansum, T., Seren, S., 2018. Large-area high-resolution ground- penetrating radar measurements for archaeological prospection. Archaeological Prospection, 25(3): 171–195.

Trinks, I., Hinterleitner, A., 2020. Beyond Amplitudes: Multi-Trace Coherence Analysis for Ground-Penetrating Radar Data Imaging. Remote Sensing, 12(10): 1583.

Trinks, I., Neubauer, W., Nau, E., Gabler, M., Wallner, M., Hinterleitner, A., Biwall, A., Doneus, M., Pregesbauer, M., 2013. Archaeological prospection of the UNESCO World Cultural Heritage Site Birka-Hovgården. In W. Neubauer, I. Trinks, R. B. Salisbury and C. Einwögerer (eds.), Archaeological Prospection. Proceedings of the 10th International Conference on Archaeological Prospection. Wien, Austria, 29.05.-02.06.2013. Wien: Verl. der Österr. Akad. d. Wiss., 39-40.

Wallner, M., Löcker, K., Gugl, C., Trausmuth, T., Vonkilch, A., Einwögerer, C., Jansa, V., Wilding, J., Pollhammer, E., Neubauer, W., 2021. The ‘Archpro Carnuntum’Project – Integrated Archaeological Interpretation of Combined Prospection Data, Carnuntum (Austria). Építés- Építészettudomány 49, Vol. 1-2, Akadémiai Kiadó, Budapest, 77-95.

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Table des illustrations

Titre Figure 1. 50 m × 50 m section of GPR data from Gjellestad, depth-slice from 60-80 cm depth. A: Visualisation of the absolute reflection amplitude (strong reflectivity is shown in dark). B: Visualisation of multi-trace GPR coherence (incoherence is shown in dark).
URL http://journals.openedition.org/archeosciences/docannexe/image/9914/img-1.jpg
Fichier image/jpeg, 1,8M
Titre Figure 2. 80 m × 80 m section of GPR prospection data from Carnuntum, depth-slice from 60-180 cm depth. A: Visualisation of the absolute reflection amplitude (strong reflectivity is shown in dark). B: Visualisation of the multi-trace GPR coherence (incoherence is shown in dark).
URL http://journals.openedition.org/archeosciences/docannexe/image/9914/img-2.jpg
Fichier image/jpeg, 2,5M
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Référence papier

Alois Hinterleitner, Immo Trinks, Erich Nau, Lars Gustavsen, Mario Wallner et Klaus Löcker, « GPR Coherence Imaging Examples: The Good, the Bad, and the Ugly »ArcheoSciences, 45-1 | 2021, 251-254.

Référence électronique

Alois Hinterleitner, Immo Trinks, Erich Nau, Lars Gustavsen, Mario Wallner et Klaus Löcker, « GPR Coherence Imaging Examples: The Good, the Bad, and the Ugly »ArcheoSciences [En ligne], 45-1 | 2021, mis en ligne le 16 août 2021, consulté le 15 novembre 2025. URL : http://journals.openedition.org/archeosciences/9914 ; DOI : https://doi.org/10.4000/archeosciences.9914

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Auteurs

Alois Hinterleitner

Corresponding author, Central Institute for Meteorology and Geodynamics (ZAMG), Hohe Warte 38, 1190 Vienna, Austria; Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology (LBI ArchPro), Hohe Warte 38, 1190 Vienna, Austria

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Immo Trinks

Vienna Institute for Archaeological Science, University of Vienna, Franz-Klein-Gasse 1, 1190 Vienna, Austria; Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology (LBI ArchPro), Hohe Warte 38, 1190 Vienna, Austria

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Erich Nau

Norsk institutt for kulturminneforskning (NIKU), Storgata 2, 0155 Oslo, Norway

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Lars Gustavsen

Norsk institutt for kulturminneforskning (NIKU), Storgata 2, 0155 Oslo, Norway

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Mario Wallner

Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology (LBI ArchPro), Hohe Warte 38, 1190 Vienna, Austria

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Klaus Löcker

Central Institute for Meteorology and Geodynamics (ZAMG), Hohe Warte 38, 1190 Vienna, Austria; Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology (LBI ArchPro), Hohe Warte 38, 1190 Vienna, Austria

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