Phase velocities derived from ambient-noise cross-correlation are compared with phase velocities calculated from cross-correlations of waveform recordings of teleseismic earthquakes whose epicentres are approximately on the station–station great circle. The comparison is conducted both for Rayleigh and Love waves using over 1000 station pairs in central Europe. We describe in detail our signal-processing method which allows for automated processing of large amounts of data. Ambient-noise data are collected in the 5–80 s period range, whereas teleseismic data are available between about 8 and 250 s, resulting in a broad common period range between 8 and 80 s. At intermediate periods around 30 s and for shorter interstation distances, phase velocities measured from ambient noise are on average between 0.5 per cent and 1.5 per cent lower than those observed via the earthquake-based method. This discrepancy is small compared to typical phase-velocity heterogeneities (10 per cent peak-to-peak or more) observed in this period range.We nevertheless conduct a suite of synthetic tests to evaluate whether known biases in ambient-noise cross-correlation measurements could account for this discrepancy; we specifically evaluate the effects of heterogeneities in source distribution, of azimuthal anisotropy in surface-wave velocity and of the presence of near-field, rather than far-field only, sources of seismic noise. We find that these effects can be quite important comparing individual station pairs. The systematic discrepancy is presumably due to a combination of factors, related to differences in sensitivity of earthquake versus noise data to lateral heterogeneity. The data sets from both methods are used to create some preliminary tomographic maps that are characterized by velocity heterogeneities of similar amplitude and pattern, confirming the overall agreement between the two measurement methods. @en

Phase velocities derived from ambient-noise (AN) cross-correlation are compared with phase velocities calculated from earthquake data via the two-station method (EQ). The EQ approach involves crosscorrelating recordings of teleseismic earthquakes whose epicenters are approximately on the station-station great circle. The AN and EQ methods can be combined effectively to cover a broad period range from about 4 to 250 s. The comparison is conducted for both Rayleigh and Love waves using over 1000 station pairs in central Europe. The analysis shows that at periods between 20 and 40s, AN-based phase velocities are systematically slower by about 1.5% compared to the EQ measurements. We revise the theory, explaining possible biases from the stationary-phase approximation and from using horizontal components of the noise wavefield. A series of numerical tests of the AN method is conducted, which allow us to evaluate the effects of the variable distance between station array and noise sources, of the illumination pattern, and of horizontal anisotropy and attenuation. We finally determine phase-velocity maps based on AN vs. EQ data; we find that discrepancies between such maps are far weaker than the mapped anomalies, and too small to result in significantly different geological interpretation of the mapped structure.@en

Continental rupture processes under mantle plume influence are still poorly known although extensively studied. The Gulf of Aden presents volcanic margins to the west, where they are influenced by the Afar hotspot, and non volcanic margins east of longitude 46° E. We imaged the crustal structure of the Gulf of Aden continental margins from Afar to Oman to evaluate the role of the Afar plume on the evolution of the passive margin and its extent towards the East. We use Ambient Noise Seismic Tomography to better understand the architecture and processes along the Gulf of Aden. This recent method, developed in the last decade, allows us to study the seismic signal propagating between two seismic stations. Ambient Noise Seismic Tomography is thus free from artifacts related to the distribution of earthquakes. We collected continuous records from about 200 permanent or temporary stations since 1999 to compute Rayleigh phase velocity maps over the Gulf of Aden.@en

We apply two different algorithms to measure surface wave phase velocity, as a function of frequency, from seismic ambient noise recorded at pairs of stations from a large European network. The two methods are based on consistent theoretical formulations, but differ in the implementation: one method involves the time-domain cross-correlation of signal recorded at different stations; the other is based on frequency-domain cross-correlation, and requires finding the zero-crossings of the real part of the cross-correlation spectrum. Furthermore, the time-domain method, as implemented here and in the literature, practically involves the important approximation that interstation distance be large compared to seismic wavelength. In both cases, cross-correlations are ensemble-averaged over a relatively long period of time (1 yr). We verify that the two algorithms give consistent results, and infer that phase velocity can be successfully measured through ensemble-averaging of seismic ambient noise, further validating earlier studies that had followed either approach. The description of our experiment and its results is accompanied by a detailed though simplifed derivation of ambient-noise theory, writing out explicitly the relationships between the surface wave Green's function, ambient-noise cross-correlation and phase and group velocities

@en

We analyze ambient seismic and acoustic noise from a broadband passive OBS survey acquired over an oil reservoir in the norwegian North Sea. We analyze the data with respect to azimuthal variation of the incident wavefield in order to find directions of possible dominant sources to evaluate the validity of the assumption of a diffuse wavefield. Whitening of the spectra yields the most clear Green's functions for higher frequencies more important, gives us the complex coherency. Following the approach of Prieto et al. 2009 we are able to determine the phase velocities by fitting Bessel functions to the real part of the complex coherencies. Fitting the distance-dependent coherency to a Bessel function, we are able to estimate the surface-wave slowness of the area; an important strength of this method is that it will ultimately allow us to evaluate the quality factor Q as well.@en