A 500 m deep multipurpose research borehole and monitoring station in Delft, the Netherlands

The 500 m deep DAPGEO-02 multifunctional research borehole and monitoring station was drilled in Delft in spring 2022. Dinoflagellate biostratigraphy reveals that the borehole goes through the Neogene Upper North Sea Group, including the Waalre, Maassluis, Oosterhout, Diessen, and Groot Heide formations. Below, a stratigraphic gap exists between the late Miocene Diessen and the middle Miocene Groote Heide formations, followed by a much larger gap between the Groote Heide and the lower Eocene Dongen formations. The Dongen is preceded by the Landen Formation, and the borehole ends in marl of the Danian to possibly Upper Cretaceous Ommelanden Formation. The drilling had an extensive coring programme using the Smet coring system sampling in PVC core liners and Shelby tubes with a total of around 75 m of recovered core. Core shoe samples were taken where possible, and cuttings were taken every metre. The downhole logging programme included short and normal resistivities, spontaneous potential, induced polarisation, single-point resistance, electromagnetic induction, deviation, total and spectral gamma ray, full waveform sonic, P- and S-wave suspension, acoustic borehole imaging, and nuclear magnetic resonance. The borehole makes up the central location of a seismic-monitoring station with the lower 70 m uncased and equipped with geophones. These three-component geophones at 440 and 490 m depth are accompanied by an immediately adjacent 200 m deep borehole with geophones every 50 m depth and two orthogonally oriented 100 m laterally offset boreholes, each with a single geophone at 50 m depth. Optical fibres laterally connect the boreholes and are also placed in the 50 and 200 m boreholes. DAPGEO-02 has several scientific aims, including the detection and characterisation of any seismicity from nearby geothermal projects, as well as an investigation of the subsurface properties and the changes therein, providing material to allow an investigation of the impact of heterogeneity on high-temperature storage and an investigation into the hydro-chemo-mechanical properties of the Neogene and Paleogene clays. The data from this borehole are publicly available at the 4TU data repository, and physical sample requests can be made.