On the limitations of interstation distances in ambient noise tomography

Ambient noise tomography (ANT) has recently become a popular tomography method to study crustal structures thanks to its unique capability to extract short-period surface waves. Empirically, in order to reliably measure surface wave dispersion curves from time-domain cross-correlations, interstation distances between a pair of stations have to be longer than two/three wavelengths. This requirement imposed a strong constraint on the use of ANT at the long-period end at local-and regional-scale tomography studies. In this study, we use ambient noise data from USArray/Transportable Array recorded during 2007-2012 to investigate whether dispersion measurements from cross-correlations of ambient noise at short interstation distances are consistent with those at long distances and whether the short-path dispersion measurements can be used in tomography, especially in local-and regional-scale tomography. Our results show that: (1) surface wave phase velocity dispersion curves measured by a frequency-time analysis technique (FTAN) from time-domain cross-correlations are consistent with those measured by a spectral method tracing the zero crossings of the real part of cross-spectrum functions in frequency domain; (2) dispersion measurements from time-domain cross-correlations with short interstation distances, up to only one wavelength, are consistent with and also reliable as those with interstation distances longer than three wavelengths and (3) these short-path measurements can be included in ANT to improve path coverage and resolution.