Delineating fracture zones using surface-tunnel-surface seismic data, P-S, and S-P mode conversions

A surface-tunnel-surface seismic experiment was conducted at the Aspo Hard Rock Laboratory to study the seismic response of major fracture systems intersecting the tunnel. A newly developed three-component microelectromechanical sensor-based seismic landstreamer was deployed inside the noisy tunnel along with conventional seismic receivers. In addition to these, wireless recorders were placed on the surface. This combination enabled simultaneous recording of the seismic wavefield both inside the tunnel and on the surface. The landstreamer was positioned between two geophone-based line segments, along the interval where known fracture systems intersect the tunnel. First arrival tomography produced a velocity model of the rock mass between the tunnel and the surface with anomalous low-velocity zones correlating well with locations of known fracture systems. Prominent wave mode converted direct and reflected signals, P-S and S-P waves, were observed in numerous source gathers recorded inside the tunnel. Forward travel time and 2-D finite difference elastic modeling, based on the known geometry of the fracture systems, show that the converted waves are generated at these systems. Additionally, the landstreamer data were used to estimate V-p/V-s, Poisson's ratio, and seismic attenuation factors (Q(p) and Q(s)) over fracture sets that have different hydraulic conductivities. The low-conductivity fracture sets have greater reductions in P wave velocities and Poisson's ratio and are more attenuating than the highly hydraulically conductive fracture set. Our investigations contribute to fracture zone characterization on a scale corresponding to seismic exploration wavelengths.