Research ArticleENVIRONMETAL STUDIES

Radar interferometry offers new insights into threats to the Angkor site

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Science Advances  01 Mar 2017:
Vol. 3, no. 3, e1601284
DOI: 10.1126/sciadv.1601284
  • Fig. 1 The area covered by the TerraSAR/TanDEM-X SAR data is highlighted by the pink rectangle, and the area within the Angkor World Heritage site, where the detailed studies reported in this work were undertaken, is shown by the green rectangle (courtesy of the Shuttle Radar Topography Mission Digital Elevation Model data from the U.S. Geological Survey).

    The map was generated by ArcMap 10.0 (www.esri.com).

  • Fig. 2 Regional-scale Tomo-PSInSAR–derived annual deformation rates around the Angkor site (reference point is located at the terminal building of the Siem Reap International Airport, shown by the white star) for the 2011–2013 observation period (overlapped on the averaged amplitude of SAR imagery).

    Surface stability and/or mild subsidence was observed (less than −4 mm/year) surrounding the central archaeological zone, including the Preah Khan Temple, west of Angkor Wat, and other monuments around Srah Srang, which can be attributed to not only the restoration of the ancient hydraulic system but also maintaining or raising the water level of reservoirs. The locations of public groundwater pumping wells and observation boreholes are indicated by the black circles and pink squares (indicated by H1 to H15), respectively. Urbanization surrounding the densely populated areas was rapid, leading to a mild to moderate (−5 to −12 mm/year) surface subsidence in the Siem Reap City region. TerraSAR/TanDEM-X data were provided by Deutschen Zentrums für Luft- und Raumfahrt (DLR; http://sss.terrasar-x.dlr.de/) under the General AO project (CAL2073).

  • Fig. 3 Groundwater data on 15 boreholes observed in the vicinity of the central monument zone of the Angkor site (pink squares in Fig. 2), indicating a steady condition of the shallow aquifer system during 2009 to 2014.

    Seasonal variations of groundwater tables were significant and were modulated by the wet-drought monsoonal climate with values ranging from a depth of 4 to 0.5 m relative to the ground surface.

  • Fig. 4 Monument-scale Tomo-PSInSAR–derived results in Angkor Wat for the 2011–2013 observation period.

    (A) Annual deformation rates show the spatial motion heterogeneity (overlaid on a QuickBird imagery provided by DigitalGlobe, www.digitalglobe.com/). (B) Two examples of vulnerable monuments [marked by pink arrows in (A)] with cracks and where maintenance work was under way, as seen during field investigations in 2014. Photographs were provided by F.C. from the Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences.

  • Fig. 5 Geology map of the Angkor site.

    The map was generated by ArcMap 10.0 (www.esri.com).

  • Fig. 6 Observed triggering evidences of seasonal groundwater tables and the thermodynamics of stone materials.

    (A) Correlation between groundwater level and precipitation. An intense seasonal variation in groundwater level (−4.5 to −0.5 m), coupled with annually steady groundwater table, was seen detectable in the central archaeological zone of Angkor after the restoration of the Barays, which stabilized groundwater tables despite a decrease in annual precipitation from 2012 to 2013 (that is, 1183.8 mm in 2012 dropping to 1037.0 mm in 2013). (B) Annual deformation rates of the Angkor Wat Temple, indicating irregular fragmentary motions with values ranging from −3 to +3 mm/year; (C) thermal amplitudes in SAR line of sight direction, indicating spatial differences with values ranging from −0.25 to +0.25 mm/°C (overlaid on the averaged amplitude of SAR imagery); and (D) deformation time series of two representative PS points, PS1 with mild subsidence and PS2 with a steady trend, marked by pink stars on (B). A positive correlation between the seasonal variation of the groundwater table and the nonlinear motion of PSs was detectable. The co-occurrence of structural instabilities and thermal amplitude dispersions was also observed [highlighted by the pink arrows in (B) and (C)]. TerraSAR/TanDEM-X data were provided by DLR (http://sss.terrasar-x.dlr.de/) under the General AO project (CAL2073).

  • Fig. 7 Deterioration processes affecting ancient monuments because of the interaction of material decay, material thermodynamics, and seasonal variations of the groundwater table.

    The model shows that an entire rigid motion dominant in the early stages immediately after a temple had been constructed gradually gives way to irregular and fragmentary motions over the long term (for example, hundreds of years) due to variability in seasonal fluctuations of groundwater tables and thermal expansion of stones, which together trigger and/or aggravate structural instability and decay, increasing the risk of monument collapse.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/3/e1601284/DC1

    Supplementary Materials and Methods

    fig. S1. Collapsing monuments in Angkor site.

    fig. S2. Extension of urban land use estimated by the comparison of multitemporal Landsat images for the period 1985–2013.

    fig. S3. Field photos of wells for pumping groundwater.

    fig. S4. Monument-scale Tomo-PSInSAR–derived annual deformation rates in ancient temples (overlaid on the averaged amplitude of SAR imagery).

    fig. S5. Validation of Tomo-PSInSAR–derived motions using the PS heights confirmed by field investigations undertaken in 2014.

    table S1. Parameters of acquisition and interferogram formation of TerraSAR/TanDEM-X SAR images and the corresponding temperature data (10 a.m. local time) used in this study; the acquisition of 4 November 2012 (marked by the star) was selected as the reference image.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Materials and Methods
    • fig. S1. Collapsing monuments in Angkor site.
    • fig. S2. Extension of urban land use estimated by the comparison of multitemporal Landsat images for the period 1985–2013.
    • fig. S3. Field photos of wells for pumping groundwater.
    • fig. S4. Monument-scale Tomo-PSInSAR derived annual deformation rates in ancient temples (overlaid on the averaged amplitude of SAR imagery).
    • fig. S5. Validation of Tomo-PSInSAR–derived motions using the PS heights confirmed by field investigations undertaken in 2014.
    • table S1. Parameters of acquisition and interferogram formation of TerraSAR/TanDEM-X SAR images and the corresponding temperature data (10 a.m. local time) used in this study; the acquisition of 4 November 2012 (marked by the star) was selected as the reference image.

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