Fracture System Characterization Using Wave-mode Conversions and Tunnel-surface Seismics

A tunnel-to-surface experiment was conducted to test the recording capabilities of a newly developed 3C-MEMS (micro electro mechanical system) seismic landstreamer in noisy environments and to characterize the rocks in between. The tunnel portion of the seismic line was covered using a combination of 333-planted 10 Hz geophones and the 3C landstreamer. The landstreamer was located at a depth of -160 to -200 m below sea level, targeting known fracture systems intersecting the tunnel. On the surface, 75 wireless recorders connected to 10 Hz geophones and MEMS sensors were used. This setup enabled simultaneous recording of the seismic wavefield inside the tunnel and on the surface and tomographic imaging of the rockmass between the two. Geophone data recorded inside the tunnel show severe electric noise pickup, while the landstreamer data appear unaffected. First-break traveltime tomography results show decreased velocity associated with known fracture systems, indicating that high-resolution imaging of the rocks between the surface and the tunnel using this source-receiver setup is possible. Strong mode-converted energy (P-S and S-P direct- and reflected-waves) was observed from the fracture systems, suggesting significant elastic property contrasts compared to the host rock. 3D raytracing modeling supports that mode conversions originate from the fracture systems.