Reflection seismic surveying at the volcanic geothermal system of Petra (Lesvos Island, Greece)

• Reflection seismic imaging defines the fault-bounded shape of the Petra basin. • Diffraction modelling helps identify faults and intrusions within volcanic rocks. • Traveltime tomography maps low-velocity zones, linked to fluid pathways. • Gravity data confirms low-density basin fill within the Petra depression. • Two major fault zones emerge as prime targets for geothermal drilling. The Petra morphological depression on Lesvos Island hosts one of Greece’s most promising low- to intermediate-enthalpy geothermal prospects, yet its internal structure and fluid pathways remain poorly constrained. To improve the characterization of the geothermal system, an integrated geophysical survey combining seismic, gravity, and magnetic methods was conducted in 2024 to delineate basin geometry and identify fault and fracture zones with enhanced permeability. Reflection seismic data were processed to image subsurface structure, while point-diffraction modelling was applied to constrain migration velocities and distinguish structural discontinuities from faults and small-scale intrusive bodies. First-break traveltime tomography provided complementary constraints on fault geometry and on the thickness of fresh volcanic units (Upper Lava), and residual gravity data were used to resolve lateral density variations. The integrated interpretation reveals a fault-bounded depression filled with low-density, low-velocity Upper Lava overlying higher-velocity, higher-density Lower Lava. Two major sets of steeply dipping faults define the Petra depression; minor and intersected faults are considered as conduits that enhance permeability and promote hydrothermal circulation. The Petra depression is interpreted as a small transtensional pull-apart basin that localizes geothermal fluid flow along its major fault zones. From a geothermal perspective, these results reduce structural uncertainty and highlight two domains along the northern and southern bounding faults as the most promising targets for future geothermal drilling.