Unveiling the Hidden Depths: Insights in Intermediate Depth Compaction from 50 years of Extensometer Data in the Netherlands

Subsidence research in the Netherlands primarily focusses on anthropogenic processes, specifically in the shallow and deep subsurface, resulting from land use, water management, and resource extraction. However, the 'intermediate' depth range, spanning hundreds of meters, has been relatively underexplored due to limited economic activities within this range. To accurately disentangle the overall human-induced subsidence, however, understanding the contribution from intermediate depth processes is essential.

We analyzed data from 20 extensometers monitoring subsurface movements between 1970 and 2021, at depth intervals ranging from ~10 to 400 meters. The extensometers were located in the hydrocarbon extraction areas operated by NAM (Nederlandse Aardolie Maatschappij) in Groningen, Friesland and Rotterdam.

Our findings highlight that secondary compaction, or creep, induced by overburden weight is the main driver of subsidence at intermediate depths. We found that subsidence rates of up to 0.6 mm/year in Groningen and 0.8 mm/year in Rotterdam account for over 10% of total measured subsidence in these regions. In Groningen, creep is attributed to Tertiary marine and Pleistocene eolian/fluvial deposits, whereas in Rotterdam, it is associated with Pleistocene shallow marine deposits. In all locations, the overburden weight of the Holocene deposits also affects the compaction at intermediate depth range.

Moreover, our analysis reveals cyclic responses in the extensometer data, including seasonal and tidal patterns. The strength of the seasonal signal corresponds to the inland salt-brackish groundwater boundary, while the tidal response is prominent near the coastline. Unexplained trend breaks, potentially linked to phreatic groundwater management, were observed in both regions.

Our analysis emphasizes the crucial role of intermediate depth contributions in obtaining a comprehensive understanding of the overall subsidence signal. Neglecting these contributions can lead to an incomplete interpretation subsidence, potentially overestimating the impact of mitigation measures. The intricate interplay of cyclic trends, salinity, water management measures, and secondary compaction emphasizes the necessity to expand monitoring efforts in the intermediate depth range.

 In response to the complexities identified in intermediate depth compaction analysis, we propose a monitoring setup with supplementary data sources, which maximizes the utility of the extensometer data. This setup is applicable to coastal plains and deltas worldwide, where a thorough understanding of subsurface processes is crucial for maintaining a sustainable living environment. Only with this comprehensive understanding can the total subsidence signal be accurately interpreted, ensuring that mitigation measures achieve their full effectiveness and avoid overestimation of other contributing sources.