ALBERT

Description: Continuous monitoring of in situ reservoir responses to stress transients provides insights into the evolution of geothermal reservoirs. By exploiting the stress dependence of seismic velocity changes, we investigate the temporal evolution of the reservoir stress state of the Salton Sea geothermal field (SSGF), California. We find that the SSGF experienced a number of sudden velocity reductions (~0.035 to 0.25%) that are most likely caused by openings of fractures due to dynamic stress transients (as small as 0.08 MPa and up to 0.45 MPa) from local and regional earthquakes. Depths of velocity changes are estimated to be about 0.5 to 1.5 km, similar to the depths of the injection and production wells. We derive an empirical in situ stress sensitivity of seismic velocity changes by relating velocity changes to dynamic stresses. We also observe systematic velocity reductions (0.04 to 0.05%) during earthquake swarms in mid-November 2009 and late-December 2010. On the basis of volumetric static and dynamic stress changes, the expected velocity reductions from the largest earthquakes with magnitude ranging from 3 to 4 in these swarms are less than 0.02%, which suggests that these earthquakes are likely not responsible for the velocity changes observed during the swarms. Instead, we argue that velocity reductions may have been induced by poroelastic opening of fractures due to aseismic deformation. We also observe a long-term velocity increase (~0.04%/year) that is most likely due to poroelastic contraction caused by the geothermal production. Our observations demonstrate that seismic interferometry provides insights into in situ reservoir response to stress changes.

Description: Tailings dams are massive structures designed to contain the waste slurry remaining after processing ore at open pit and underground mines. These structures fail far more regularly than normal water-storage dams. In recent years, catastrophic tailings dam failures have occurred, causing significant damage to the environment and even loss of life. To mitigate these catastrophic events in the future, there is an urgent need to develop cost-effective methods to monitor the structural stability of these constructions over time. The lack of current cost-effective monitoring methods prompted us to investigate whether ambient seismic noise can be used to detect internal changes in a tailings dam wall during a period of heavy rainfall. We recorded three weeks of continuous seismic data with 10 short-period geophones at a tailings dam in Tasmania, Australia. Seismic interferometry was used on ambient noise to create virtual seismic sources. With these virtual source signals, small changes in seismic velocity were measured daily and compared to rainfall, seepage flow rates, and fluid pore pressure. The observed velocity changes were driven by fluid saturation, ground water level, increased loading from increased dam water level, and a sudden increase in fluid pore pressure in a section of the dam wall. The results suggest that this relatively inexpensive method can be used to monitor and locate small changes in the interior of the tailings dam wall, providing a valuable tool to remotely monitor the structural stability of tailings dam walls over time.