8125 20260611182940.0 CTALK-2026-0119 Van Noten, Koen Estimation of Carboniferous Bedrock Depth Using Borehole-Calibrated H/V Spectral Ratios for Geothermal Assessment in Lille Metropolitan Area, France 2026 2026-06-10 The geothermal potential of the Lille Metropolitan Area (France), specifically for open-loop applications, strongly depends on the geometry and depth of the Carboniferous limestone basement, which forms the main regional target reservoir in northern France. Understanding the structural continuity, the presence of basement faults, and the thickness of the overlying sediments is essential for designing geothermal systems, preparing drilling strategies, and assessing risks linked to deep karstic or tectonic structures. The region exhibits a pronounced geological heterogeneity, with Meso-Cenozoic sediment thickness varying laterally from a few to 200 meters over short distances due to the influence of the Variscan basement structural network, the Meso-Cenozoic transgression and depocenter location, and later the Cenozoic tectonic reactivation phase. To address the scarcity of borehole information and to provide a systematic, spatially continuous estimation of the depth to the Carboniferous bedrock, a large-scale passive seismic survey was conducted across the Mélantois and Pévèle areas in Northern France. The applied method is the Horizontal-to-Vertical Spectral Ratio (HVSR) analysis of recorded ambient seismic noise. HVSR exploits the contrast in seismic impedance between the sedimentary cover and the underlying bedrock to extract the fundamental resonance frequency which is empirically linked to sediment thickness. Combining HVSR analysis with bedrock depth information from boreholes hence allows developing resonance-to-depth relations. These relations are subsequently used to construct “virtual boreholes” from ambient noise measurements, enabling bedrock depth estimations at locations where no direct geological information exists. This extended abstract presents the results of the depth conversion of 459 HVSR measurements, calibrated on 63 boreholes reaching the Carboniferous, providing a high-resolution view of the bedrock paleorelief and its implications for geothermal development in the Lille Metropolitan Area. NO H/V Spectral Ratio Method Ambient Seismic Noise Geothermal Exploration Boreholes Graveleau, Fabien Chanier, Frank Bouka Biona, Tchianik Bossennec, Claire Watremez, Louise Lecocq, Thomas World Geothermal Congress, Calgary, Canada koen.vannoten@ksb-orb.be 5534111 http://publi2-as.oma.be/record/8125/files/Cross-section VDA-CAP.png 3182 http://publi2-as.oma.be/record/8125/files/Cross-section VDA-CAP.gif?subformat=icon icon 6132 http://publi2-as.oma.be/record/8125/files/Cross-section VDA-CAP.jpg?subformat=icon-180 icon-180 Contributed CTALKCONT