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Noninvasive geophysical techniques based on the dispersion of surface waves in layered media are commonly used approaches for measuring shear-wave velocity profiles of the subsoil. Acquiring surface waves is a simple task, but the interpretation of their dispersion curves poses a number of challenges. In an increasing number of cases, shear-wave velocity profiles are derived from the inversion of dispersion curves of surface waves and single-station passive horizontal-to-vertical (H/V) spectral ratios, mostly using a blind joint fit of the two sets of curves. Here we emphasize the benefits of carrying out H/V surveys prior to any array acquisition. We propose to start by collecting at least two H/V recordings at a site to verify the 1D plane-parallel soil condition, as this is essential in dispersion curve inversion/modeling. Then, we look for the diagnostic features of velocity inversions in the H/V curves: when they occur, the interpretation of dispersion curves is made difficult by mode splitting/superposition and Love wave arrays will not be effective. Then we inspect the shape of the H/V curves: flat curves acquired on rock usually imply poor dispersion curves. Large receiver spacings are recommended in the arrays and Love wave arrays will not be efficient. Flat curves on soft material sites represent gently increasing [Formula: see text] gradients and Rayleigh wave arrays should be preferred. H/V curves with high frequency peaks indicate shallow impedance contrasts: this makes Love wave arrays efficient for the soft layer characterization, but provide little information at depth. H/V curves with low frequency peaks indicate deep bedrock and their inversion can provide approximate [Formula: see text] profiles down to greater depths than from an array. Equipped with the information coming from accurate H/V observations, practitioners could make better-informed decisions about array acquisition geometries, source/surface wave types, and inversion strategies.

The basic property of passive imaging is that, given any two points, one of them can be taken as the source of the waves and the other as the recording station. This property can be derived from the statistical self-alignment of the rays along the vector joining the two points, and applies also to nondiffuse wavefields like seismic tremor. It provides a statistical basis for the use of the stationary phase integral, allowing passive interferometry under the mild constraint of mechanical homogeneity at a local scale. Combined with the tremor’s large spectral bandwidth, it allows one to recover the local Green’s function from spatial correlation. Furthermore, combining this property also with the azimuthal isotropy of either the wavefield or the array, and using the statistical mode as the estimator, provides a new technique to measure the local velocity dispersion in the subsoil. This technique exploits the potential of spatial autocorrelation (SPAC) and refraction microtremor (ReMi), allowing one (1) to use sparse small-aperture arrays with simple geometry, (2) to dispense with the fitting of Bessel functions, and (3) to measure, in a few minutes, the local (phase and group) wave velocity as a function of frequency of potentially all the wave-propagation modes — body and surface — and not just of the one prevailing at each frequency.