Fifty Years of Intermediate-Depth Compaction in the Coastal Plain of the Netherlands: Detailed Analysis of Twenty Extensometers

In this study, we analyzed data from 20 extensometers located in the coastal plain of the Netherlands, monitoring subsurface movement between 1970 and 2021 at depths ranging from ∼10 to 400 m in regions associated with the Groningen and Rotterdam gas and oil fields, operated by NAM (Nederlandse Aardolie Maatschappij). Our findings indicate that compaction induced by overburden weight is the main driver of subsidence at these intermediate-depths. Subsidence rates of up to 0.6 mm/year in Groningen and 0.8 mm/year in Rotterdam account for approximately 10% of the total measured and modeled subsidence from previous studies in the respective areas over similar time periods. In Groningen, the compaction rates correlate with the thickness variance of tertiary marine (Oosterhout Formation), Pleistocene eolian/fluvial deposits (Boxtel Formation) and soft soil top layers (Holocene). In Rotterdam, the compaction rates correlate to the thickness variation in Pleistocene shallow marine deposits (Maassluis Formation) and soft soil top layers (Holocene). Additionally, the data show cyclic responses—seasonal and tidal, with a spatial variation in strength of the trends. The cyclic patterns and their spatial variations are important factors to consider when analyzing groundwater and subsidence data sets in the studied areas. The data analysis of this study illustrates the significance of considering intermediate-depth contributions when looking into surface subsidence. Neglecting these contributions leads to an incomplete understanding of the total subsidence signal.