OR/19/043 Discussion and conclusions

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Novellino, A, Terrington, R, Christodoulou, V, Smith, H and Bateson, L. 2019. Ground Motion and Stratum Thickness Comparison in Tower Hamlets, London. British Geological Survey Internal Report, OR/19/043.

With the recent surge in the availability of free SAR data, millions of measurement points are routinely being generated with short revisiting time. There is therefore a need to develop methodologies to understand the complexity of the components within the time series of the data. The clustering analysis used within this study is one such methodology upon which new case studies can be developed and new insights can be generated.

In particular, by exploiting the high temporal density of InSAR displacement data, this study illustrates how average displacement rates and cluster analysis can be used to investigate the characteristics of the human-induced deformation in the Tower Hamlets area and the relationships with the thickness map of the local geological units.

According to our results, ALV is the unit that is mostly affected by the deformation as proven by the relatively large number of MPs classified in to group E but, overall, the cluster groups we identified do not have correlation with the thickness of the units. In particular, no correlations between the AMG thickness and the MPs deformation has been found, probably because the foundation depth of the buildings in this area lie well below its lowest occurrence so these buildings tend to respond to strains affecting a deeper source rather than the top layer.

We can then conclude that the thickness of the lithological units directly above the Chalk Group does not have overall influence in the ground deformation behaviour.

Our first results for the Tower Hamlets area show also that the analysis of the average motion alone is not a good indicator for highlighting any influence between the unit thickness and ground motion because of:

  • the short time interval considered, which does not allow the recognition of large temporal trends such as compaction of unconsolidated deposits.
  • The narrow range of deformation and the small area of study, the latter limited by the availability of the new 3D geological model, that does not allow to encompass the whole area of uplift which extends eastwards (see Figure 6) and to extend our conclusion to the whole Greater London area.

Therefore, further work may be considered in order to spatially and temporally extend our analysis (i), to account for the actual saturated thickness of the layers above the Chalk Group (e.g., LC and LMBE; ii) and to consider the impact of urban features (e.g., building type, building age; iii) that show local patterns of subsidence (Old Spitalfields Market and Tobacco Dock) almost impossible to be detected without the clustering analysis.

The results presented here, although not conclusive, do show the huge potential of clustering analysis in analysing large matrix generated by InSAR data for displacement pattern recognition and in highlighting the strong influence of dewatering activities on ground deformation, thus providing more easy-to-interpret data to stakeholders (e.g., engineering consultants) or public organisations that are in charge of coordinate urban policies and hazard mitigation strategies.