Stress-induced temporal variations in seismic anisotropy observed in microseismic data

Abstract

S U M M A R YIn situ stress monitoring of crustal rocks is desirable as it yields insights into earthquake mechanisms, volcanic eruptions and changes in hydrocarbon reservoirs. Shear wave splitting, induced by stress-controlled cracks in the shallow crust, provides a way to infer this stress. Temporal variations in these observations can be difficult to quantify due to scatter in the data and discontinuous observations. Here we present evidence of temporal variations in shear wave splitting from a continuous time-series with a high occurrence of microseismic events recorded in a borehole. We interpret these observations in terms of stress-controlled cracks and are able to infer changes in stress and, via modelling, suggest the cause of the anisotropy. Possible origins of the temporal variation in per cent anisotropy are tidal or oil-field production processes. Our results suggest that shear wave splitting is a viable probe for inferring changes in crustal stress in cracked rock.Observations of crack-induced seismic anisotropy provide insights into crustal stress, and its temporal variation yields insight into changes in that stress (Gupta 1973). However, convincing measurements of temporal variations are difficult to obtain because of scatter in the data and non-continuous recording intervals. Here we present observations of anisotropy inferred from microseismic events at the Valhall oil reservoir (North Sea). The data set provides an ideal opportunity for studying stress changes as we have a continuous 56-day time-series with an average of 15 events per day, recorded on an string of six three-component geophones. The area is prone to large stresses-the seafloor has subsided by more than 4 m since production started and borehole breakout is common in this area. Large variations in shear wave splitting, which is arguably the best indicator of anisotropy, are observed and interpreted in terms of stress modulation of at least one crack set.Shear wave splitting occurs when a shear wave enters an anisotropic medium. The shear wave is split into approximately perpendicular fast and slow components and is akin to optical birefringence. The polarization of the fast direction (φ) is related to the orientation of the anisotropic symmetry axes whereas the lag time * Current address

The Erua earthquake cluster and seismic anisotropy in the Ruapehu region, New Zealand

“…An automated program developed by Savage et al [2010b], and based on the algorithms of Silver and Chan [1991] and Teanby et al [2004], was used to perform all SWS measurements in this study. The program grades each measurement and marks any null measurements in which no splitting result is obtained.…”

Anisotropy in the Shallow Crust Observed around the San Andreas Fault Before and After the 2004 M 6.0 Parkfield Earthquake

“…Several shear-wave splitting studies near recently ruptured fault zones have claimed to see temporal evolution of the crack field following a large magnitude earthquake or prior to earthquake swarms (e.g., Gao et al, 1998;Tadokoro et al, 1999;Teanby et al, 2004). However, a variety surveys have seen no postseismic change in anisotropic parameters in the months following large magnitude earthquakes (e.g., Cochran et al, 2003;Peng and Ben-Zion, 2004).…”

Linking microseismic event observations with geomechanical models to minimise the risks of storing CO2 in geological formations

“…It is well known that seismic velocities and anisotropy are modulated by 61 non-hydrostatic stress changes (e.g., Nur and Simmons, 1969;Zatsepin and 62 Crampin, 1997; Teanby et al, 2004b;Verdon et al, 2008a), so observations 63 of shear wave splitting -a key indicator of anisotropy -made on waves 64 generated by microseismic events can also be used to inform geomechanical 65 models. (White, 2009).…”