Ambient noise interferometry is a branch of seismology which extracts useful information from the ambient seismic field that always exists on Earth, and which was traditionally regarded as noise. Full-wave or generalized ambient noise interferometry, is a rigorous modelling-based approach, which aims to overcome the pitfalls (under realistic distributions of noise sources) associated with the widely used assumption of Green's function retrieval from interstation cross-correlations. Instead, it seeks to invert for noise sources as well as Earth structure, by explicitly modelling the cross-correlations themselves. Waveform attributes are related to source and structure parameters, through finite frequency sensitivity kernels.
Datta et al. (2019, 2023) have provided methods to recover ambient noise source distributions by waveform inversion.
Amplitudes of seismic waves recorded at any location depend not only on the source that generated them (e.g. an earthquake), but also on the types of geological structure the waves pass through, as they travel from the source to recording location. These so-called path effects can be significant for surface waves, which propagate parallel to the Earth's surface. However, they are typically ignored in standard estimates of seismic hazard, which have traditionally focussed on body waves and local site effects. Modelling of path effects requires modelling lateral variations in Earth structure.
Datta (2018) provided algorithms for semi-analytical solutions of surface wave propagation in simplified 2-D Earth models. These algorithms account for mode conversion at sharp lateral discontinuities and can be used to study, for example, basin edge effects in the context of surface wave amplification by sedimentary basins.
The horizontal-to-vertical (H/V) amplitude ratio or ellipticity of Rayleigh waves, is a surface wave observable that complements the widely used observables of phase and group velocity dispersion, due to its shallower sensitivity to Earth structure. We are interested in incorporating Rayleigh wave ellipticity in surface wave data analysis, to improve the vertical resolution of exisiting techniques for crustal imaging.
Surface Wave Tomography (SWT) is one of the primary tools for studying the shear-velocity (Vs) structure of the Earth’s upper mantle, which in turn is critical for addressing such fundamental Earth Science problems as mantle convection, evolution and deformation of the Earth’s surface, and natural hazard assessment. However traditional SWT methods, built on the approximations of ray theory, typically discard a significant fraction of available data — high frequencies and higher surface wave modes or overtones — which are prone to complex wave propagation phenomena, beyond the realm of ray theory. This places theoretical limits on the resolution of Earth models produced by such techniques. One such theoretical challenge comes from the scattering of surface waves by strong lateral variations inside the Earth, leading to conversion or coupling between different modes. Datta et al. (2017) analysed this mode coupling issue and found that the approximations of ray theory break down at high frequencies (period 30 s or less), thereby corrupting waveform inversions that include fitting of surface wave overtones.
Datta (2019) presented array-based methods to extract multi-mode surface wave dispersion. Higher mode surface waves are difficult to measure on observed data because they are not well separated in time and frequency.
Bharath Shekar, IIT Bombay
Sharavan Hanasoge, TIFR Mumbai
Supriyo Mitra, IISER Kolkata
Department of Science and Technology, Govt. of India
Science and Engineering Research Board (SERB), Govt. of India
Swiss National Science Foundation