Characterization and imaging of a hydrothermally active near-vertical fault zone in crystalline rocks based on hydrophone VSP data

Abstract

A shallow, hydrothermally active, fault zone embedded in fractured crystalline rocks of the Central Swiss Alps has been drilled and geophysically explored in view of its potential analogies to planned petrothermal reservoirs in the Alpine Foreland. Vertical seismic profiling (VSP) experiments have been performed using a 24-channel borehole hydrophone array with a sensor spacing of 1 m in order to image the fault core and to detect hydraulically open fractures. The dense sensor spacing allows for an accurate sampling of the wavefields. This, in turn, enables their separation through standard seismic processing methodologies, thus allowing for the generation of seismic body wave VSP profiles and a tube wave corridor stack, which maps brittle deformation in the surrounding rock mass.Multiple offset hydrophone VSP data were acquired with a crooked-line survey geometry and processed with 3D methods. Hereby, a laterally changing velocity cube, representative of the vertical structural variations associated with the fault zone, is generated and utilized for Kirchhoff pre-stack-depth-migration (PSDM) imaging. The resulting seismic image delineates various vertical structures, notably the target fault zone. The interpretation of the VSP data is complemented by and corroborated through a suite of geophysical borehole logs, which characterize the fine-scale petrophysical variations in response to brittle deformation in the fault core and the surrounding damage zones.This study exemplifies typical targets of hardrock seismic exploration, where the structures are frequently near-vertical, discontinuous and where acoustic impedance contrasts are commonly associated with deformation. As such, it illustrates the benefits of using borehole seismic methods in such environments and the use of borehole hydrophone arrays for targeting fractures and shear zones.