Publications

Assessment of Saturation Changes Around Cavities and the Impact on Seismic Waves

Bonal, Nedra B.; Desilets, Sharon L.

According to theory, seismic methods should be able to identify cavities. The high impedance contrast between the rock (fast velocity, high density) and the air (slow velocity, low density) should produce strong reflections and refractions off cavity walls. In practice, however, cavities are not always easily detected. Changes in pore saturation, fracturing, and stress near the wall of the cavity may decrease the impedance contrast and diffuse the rock/air interface. To determine the impact of these effects on seismic waves, we first aim to understand the hydrological processes in the medium surrounding the cavity, a tunnel in this case. We simulate flow through the vadose zone to provide insight into how a cavity alters saturation. Next we examine how this saturation affects seismic velocities. Finally, we use a wave propagation code to simulate seismic waves through the saturation models. Results are presented from hydrology models for three cases: the tunnel far above the water table, the tunnel just above the water table and within the capillary fringe, and the tunnel below the water table with the tunnel being drained. We also present the results of seismic wave propagation through these models to determine the impact these effects have on seismic waves.