Bayesian Inversion of H/V Spectral Ratios for Constraining Shallow Subsurface Structure in Geothermal Exploration

Accurate subsurface characterization is fundamental to the successful development of geothermal systems. Such comprehensive knowledge allows determining geological structures that govern local fluid circulation and heat transport. As drilling represents one of the largest cost factors in geothermal development, ensuring that wells target zones of high hydraulic conductivity and permeability can substantially reduce exploration risk and overall project costs. Passive seismic techniques, being both inexpensive and non-invasive, have proven to be effective tools for both geothermal exploration and monitoring. Among them, Horizontal-to-Vertical spectral ratios (H/V) are often used to map subsurface topography. Their interpretation and inversion, however, often rely on prior knowledge of local shear-wave velocity or subsurface layering.

In this case study we employ a trans-dimensional Bayesian framework to invert H/V curves from more than 70 survey points across a prospective aquifer thermal energy storage system in rural Belgium, which will supply about 160 housing units. Our approach enables the generation of pseudo-2D shear-wave velocity profiles across the site without requiring additional information to constrain the inversion. Low velocity zones are identified which can be related to karstification and geological layering suggested by geological maps. The results are further validated with direct field measurements. Borehole logs from exploration wells drilled on the basis of our results indicate high hydraulic conductivity and are supported by water table measurement from cone penetration testing. The derived profiles offer valuable information to guide well placement and optimize drilling decisions by reducing uncertainty in subsurface conditions. Our findings demonstrate that passive seismological techniques, combined with probabilistic inversion approaches can serve as a cost-effective tool in support of the energy transition.