Instrumenting wells with distributed acoustic sensors (DASs) and illuminating them with passive or active seismic sources allows precise tracking of temporal variations of direct-wave traveltimes and amplitudes, which can be used to monitor variations in formation stiffness and density. This approach has been tested by tracking direct-wave amplitudes and traveltimes as part of a carbon capture and storage project where a 15 kt supercritical CO2 injection has been monitored with continuous offset vertical seismic profiling using nine permanently mounted surface orbital vibrators acting as seismic sources and several wells instrumented with DAS cables cemented behind the casing. The results indicate a significant (from 15% to 30%) increase of strain amplitudes within the CO2 injection interval, and traveltime shifts of 0.3–0.4 ms below this interval, consistent with full-wave 1.5D numerical simulations and theoretical predictions. The results give independent estimates of the CO2 plume thickness and the associated P-wave velocity reduction.

Distributed temperature sensing (DTS) and distributed acoustic sensing (DAS) data recorded by a fiber-optic array installed during the decommissioning operations of the 1550 m Harvey 3 well in Western Australia reveal an abundance of valuable information about the course of the decommissioning process and the quality of the cement job. The DAS monitoring has detected vibrational disturbances during the cement's setting up, while DTS was used to assess setting up of the cement and curing times as well as uniformity of cementation from the distribution of temperature along the borehole. A weeklong trial acquisition of passive seismic data with the same array a year later shows an abundance of seismic events in a wide frequency range from below 1 mHz to above 200 Hz. The downhole DAS array provides traveltimes and amplitudes of these events, which include earthquakes, mine blasts, ocean microseisms, and local human activity. The amplitudes of waves from distant seismic events can be used to estimate and monitor physical properties of the media along the extent of the well. When used in combination with information from active vertical seismic profiling, these events can help obtain independent estimates of velocities and densities. Spectral analysis of low-frequency microseisms shows a strong correlation between passively recorded DAS and local weather observations. This shows that the ability to continuously record oceanic microseisms at low frequencies opens opportunities to employ such arrays for wave climate studies. In addition, the data contain peculiar in-hole reverberations likely caused by crossflow of groundwater behind the intermediate casing, which may indicate imperfections of the cement job. The results demonstrate that a downhole fiber-optic array installed in an abandoned well represents an opportunity to establish a permanent facility for continuous recording of passive and active geophysical data and for exploring various applications.