Natural hazard prediction and efficient crust exploration require dense seismic observations both in time and space. Seismological techniques provide ground-motion data, whose accuracy depends on sensor characteristics and spatial distribution. Here we demonstrate that dynamic strain determination is possible with conventional fibre-optic cables deployed for telecommunication. Extending recently distributed acoustic sensing (DAS) studies, we present high resolution spatially un-aliased broadband strain data. We recorded seismic signals from natural and man-made sources with 4-m spacing along a 15-km-long fibre-optic cable layout on Reykjanes Peninsula, SW-Iceland. We identify with unprecedented resolution structural features such as normal faults and volcanic dykes in the Reykjanes Oblique Rift, allowing us to infer new dynamic fault processes. Conventional seismometer recordings, acquired simultaneously, validate the spectral amplitude DAS response between 0.1 and 100 Hz bandwidth. We suggest that the networks of fibre-optic telecommunication lines worldwide could be used as seismometers opening a new window for Earth hazard assessment and exploration.

Distributed acoustic sensing (DAS) is a relatively recent development in the use of fiber-optic cable for measurement of ground motion. Discrete fiber-optic sensors, typically using a Bragg diffraction grating, have been in research and development and field testing for more than 15 years with geophysical applications at least 12 years old (Bostick, 2000, and summary in Keul et al., 2005). However, developments in recent years have sought to remove the need for point sensors by using the fiber cable itself as a sensor (Mestayer et al., 2011; Miller et al., 2012).

More than 50 000 tons of CO₂ have been injected at Ketzin into the Stuttgart Formation, a saline aquifer, at approximately 620 m depth, as of summer 2011. We present here results from the 1^st repeat 3D seismic survey that was performed at the site in autumn 2009, after about 22 000 tons of CO₂ had been injected. We show here that rather complex time‐lapse signatures of this CO₂ can be clearly observed within a radius of about 300 m from the injection well. The highly irregular amplitude response within this radius is attributed to the heterogeneity of the injection reservoir. Time delays to a reflection below the injection level are also observed. Petrophysical measurements on core samples and geophysical logging of CO₂ saturation levels allow an estimate of the total amount of CO₂ visible in the seismic data to be made. These estimates are somewhat lower than the actual amount of CO₂ injected at the time of the survey and they are dependent upon the choice of a number of parameters. In spite of some uncertainty, the close agreement between the amount injected and the amount observed is encouraging for quantitative monitoring of a CO₂ storage site using seismic methods.

At the CO2SINK site (Ketzin, near Berlin, Germany), the pilot study for onshore CO2 storage in saline aquifers includes monitoring of the storage reservoir and the structures above using physical, chemical, and microbial observations. Seismic and geoelectric measurements have delivered the structural framework and monitor CO2 propagation between two observation wells. Borehole temperature data serves to derive information about in-situ formation temperatures and to detect processes related to the injection and movement of CO2 in the subsurface. Pressure measurements aim at ensuring safe operations and characterization of the reservoir. For a complete characterization of the CO2 storage process, the physical observations have to be complemented by chemical and biological probing, as fluid/fluid and fluid/rock interactions and microbial processes play an important role possibly affecting the stability of the reservoir and caprock. A newly developed Gas Membrane Sensor detected the CO2 breakthrough on the first monitoring well. Microbial investigations contributed in optimizing the injection borehole after recognizing organisms reducing its injectivity

Surface-downhole electrical resistivity tomography (SD-ERT) surveys were repeatedly carried out to image CO2 injected at the pilot storage Ketzin, Germany. The experimental setup combines surface with downhole measurements by using a permanent electrode array that has been deployed in three wells. Two baseline experiments were performed during the site startup and three repeat experiments were performed during the first year of CO2 injection. By the time of the third repeat, approximately 13,500 tons of CO2 had been injected into the reservoir sandstones at about 650 m depth. Field data and inverted resistivity models showed a resistivity increase over time at the CO2 injector. The lateral extent of the related resistivity signature indicated a preferential CO2 migration toward the northwest. Using an experimental resistivity-saturation relationship, we mapped CO2 saturations by means of the resistivity index method. For the latest repeat, CO2 saturations show values of up to 70% near the injection well, which matches well with CO2 saturations determined from pulsed neutron-gamma logging. The presence of environmental noise, reservoir heterogeneities, and irregularities in the well completions are the main sources of uncertainty for the interpretations. The degradation of the permanently installed downhole components is monitored by means of frequently performed resistance checks. In consistency with the SD-ERT data, these resistance checks indicate a long-term resistivity increase near the CO2 injector. In conclusion, the investigations demonstrate the capability of surface-downhole electrical resistivity tomography to image geologically stored CO2 at the Ketzin site.