No abstract

Correlations of random seismic noise are now widely used to retrieve the Green's function between two points. Whereas this technique provides useful results in tomography and monitoring studies, it is mainly limited by an uneven distribution of noise sources. In that case, theoretical requirements are not completely fulfilled and we may wonder how reliable the signals are reconstructed, in particular for the purpose of estimating traveltime from correlations. We present in this study a way to quantify effects of a non-isotropic noise 1 field by estimating the arrival time error due to a particular non-isotropic distribution of recorded wave intensity. Our study is based on a theoretical prediction of this bias and we successfully test the theory by comparing the theoretical expectation to real measurements from seismic-prospecting data. In particular, we distinguish between the effects of source distribution and effects of medium heterogeneity between the sources and the region of receivers. We find relative errors in the order of a percent which may affect monitoring results especially where smaller relative velocity variations (smaller than 10 −3 for some applications) are investigated. Finally we see that correlation of coda waves helps mitigate the effects of a non-isotropic field, hence making the estimation of traveltime quite stable irrespective of the source distribution.2

International audienceRandom field cross-correlation is a new promising technique for seismic exploration, as it bypasses shortcomings of usual active methods. Seismic noise can be considered as a reproducible, stationary in time, natural source. In the present paper we show why and how cross-correlation of noise records can be used for geophysical imaging. We discuss the theoretical conditions required to observe the emergence of the Green's functions between two receivers from the cross-correlation of noise records. We present examples of seismic imaging using reconstructed surface waves from regional to local scales. We also show an application using body waves extracted from records of a small-scale network. We then introduce a new way to achieve surface wave seismic experiments using cross-correlation of unsynchronized sources. At a laboratory scale, we demonstrate that body wave extraction may also be used to image buried scatterers. These works show the feasibility of passive imaging from noise cross-correlation at different scales

A large part of global plate motion on mid-ocean ridge transform faults (RTFs) is not accommodated as major earthquakes. When large earthquakes do occur, they often repeat quasiperiodically. We focus here on the high slip rate (∼14 cm/yr) Gofar transform fault on the equatorial East Pacific Rise. This fault is subdivided into patches that slip during M w 5.5-6 earthquakes every 5 to 6 years. These patches are separated by rupture barriers that accommodate slip through swarms of smaller events and/or aseismic creep. We performed an imaging study to investigate which spatiotemporal variations of the fault zone properties control this segmentation in mechanical behavior and could explain the specific behavior of RTFs at the global scale. We adopt a double-difference approach in a joint inversion of active air gun shots and microseismicity recorded for 1 year. This data set includes the 2008 M w 6 Gofar earthquake. The along-strike P wave velocity structure reveals an abrupt transition between the barrier area, characterized by a damaged fault zone of 10-20% reduced V p and a nearly intact fault zone in the asperity area. The importance of the strength of the damage zone on the mechanical behavior is supported by the temporal S wave velocity changes which suggest increased damage within the barrier area, during the week preceding the M w 6 earthquake. Our results support the conclusion that extended highly damaged zones are the key factor in limiting the role of major earthquakes to accommodate plate motion along RTFs.

No abstract

In this paper, we present two experimental studies of mechanical wave propagation in a concrete building around 1 kHz. The first experiment is devoted to the observation of the coherent backscattering enhancement, which demonstrates the presence of multiple diffractions in the late part of the wave records. An application of multiple diffraction and reverberations is proposed in a second experiment. Thanks to their sensitivity to weak changes of the medium, the late records are used to monitor weak change in concrete wave velocity induced by thermal variations. The velocity change measurements have a precision of deltac/c=10(-4). Such a precision is difficult to obtain with direct waves. This experiment is the first step to other applications like stress, damage, aging, or crack monitoring in concrete structures.

When considering direct waves in the correlation process, the Green's function is reconstructed when using an even distribution of seismic sources or when the source distribution is restricted to the direction close to the alignment of the sensors. On the other hand, when considering records of coda waves, the convergence is achieved for any source distribution, as expected theoretically. We extract the expected amplitude decay along a seismic profile from the correlation functions when an even distribution of sources is considered or when the time window includes scattered waves.

International audienceA seismo-acoustic and self-potential survey has been performed in the hydrothermal area of the old Waimangu Geyser (New Zealand), which was violently erupting a century ago. Nowadays, no surface activity is visible there. We set-up an array of 16 geophones and recorded a high and steady acoustic ambient noise. We applied the matched field processing (MFP) approach to the acoustic data to locate the sources responsible for the ambient noise. The white noise constraint processor reveals the presence of a unique and well-focused acoustic source at a depth of 1.5 m below the seismic array. For this very shallow source, the application of MFP enabled the determination of both the source location and the dispersion curve of seismic velocity. The study was completed by self-potential (SP) measurements on several profiles around the acoustic noise source, which displayed a large positive anomaly above it. The results of the SP inversion gave an electric streaming current density source very close to the acoustic one. Both sources likely belong to a shallow hydrothermal structure interpreted as a small convective cell of boiling water beneath an impermeable layer. The joint application of these methods is a promising technique to recognize hydrothermal structures and to study their dynamics