Seismic interferometry applied to ambient-noise measurements allows the retrieval of the seismic response between pairs of receivers. We studied ambient-noise seismic interferometry (ANSI) to retrieve time-lapse reflection responses from a reservoir during CO 2 geologic sequestration, using the case of the experimental site of Ketzin, Germany. We applied ANSI to numerically modeled data to retrieve base and repeat reflection responses characterizing the impedances occurring at the reservoir both with and without the injection of CO 2 . The modeled data represented global transmission responses from band-limited noise sources randomly triggered in space and time. We found that strong constraints on the spatial distribution of the passive sources were not required to retrieve the time-lapse signal as long as sufficient source-location repeatability was observed between the base and the repeat passive survey. To illustrate the potential of the technique, ANSI was applied to three days of passive field data recorded in 2012 at Ketzin. Comparison with the modeled results illustrated the potential to retrieve key reflection events using ANSI on field data from Ketzin. This study supports the idea that the geologic setting and characteristics of ambient noise at Ketzin may be opportune to monitor CO 2 sequestration.

We show application of passive seismic interferometry (SI) using P-wave coda of local earthquakes for the purpose of crustal-scale reflection imaging. We process the reflection gathers retrieved from SI following a standard seismic processing in exploration seismology. We apply SI to the P-wave coda using crosscorrelation, crosscoherence, and multidimensional deconvolution approaches for data recorded in the Malargüe region, Argentina. Comparing the results from the three approaches, we find that multidimensional deconvolution based on the truncated singular-value decomposition scheme gives us a substantially better structural imaging. Although our results provide higher resolution images of the subsurface, it shows less clear images for the Moho in comparison with previous seismic images in the region obtained by receiver function and globalphase seismic interferometry. Above the Moho, though, , we interpret a deep thrust fault and the possible melting zones which are previously indicated by active-seismic and magnetotelluric methods in this region, respectively. The method we propose could be an alternative option not only for crustal-scale imaging, e.g., in enhanced geothermal systems, but also for the lithospheric-scale as well as basin-scale imaging, depending on the availability of local earthquakes and the frequency bandwidth of their P-wave coda.

The theory of seismic interferometry predicts that crosscorrelations of recorded seismic responses at two receivers yield an estimate of the interreceiver seismic response. The interferometric process applied to surface-reflection data involves the summation, over sources, of crosscorrelated traces, and it allows retrieval of an estimate of the interreceiver reflection response. In particular, the crosscorrelations of the data with surfacerelated multiples in the data produce the retrieval of pseudophysical reflections (virtual events with the same kinematics as physical reflections in the original data). Thus, retrieved pseudophysical reflections can provide feedback information about the surface multiples. From this perspective, we have developed a data-driven interferometric method to detect and predict the arrival times of surface-related multiples in recorded reflection data using the retrieval of virtual data as diagnosis. The identification of the surface multiples is based on the estimation of source positions in the stationary-phase regions of the retrieved pseudophysical reflections, thus not necessarily requiring sources and receivers on the same grid. We have evaluated the method of interferometric identification with a two-layer acoustic example and tested it on a more complex synthetic data set. The results determined that we are able to identify the prominent surface multiples in a large range of the reflection data. Although missing near offsets proved to cause major problems in multiple-prediction schemes based on convolutions and inversions, missing near offsets does not impede our method from identifying surface multiples. Such interferometric diagnosis could be used to control the effectiveness of conventional multiple-removal schemes, such as adaptive subtraction of multiples predicted by convolution of the data.

SummarySeismic interferometry applied to surface reflection data (with source and receivers at the surface) allows to retrieve virtual-source gathers at the position of receivers, where no source was shot. As a result of the crosscorrelation of all primary and multiple reflections, the virtual-source gathers contain retrieved physical reflections as well as nonphysical (ghost) reflections also called spurious multiples. We show that a significant part of the ghost reflections can be suppressed by using surface-related multiple elimination on the active data advantageously. The method that we propose consists in retrieving the strong ghost reflections mainly from the crosscorrelation of primaries only and in subtracting this result from the virtual-source gather retrieved from all the data. The resulting new virtual-source gathers provide a better estimate of the reflection response since it is now less polluted by undesired non-physical events that may bring ambiguity in the interpretation. This is better to make a more effective use of the virtual-source gathers, for example for imaging.

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