Feasibility of <i>V</i> P, <i>V</i> S, density inversion by elastic FWI

Abstract We utilize a VP-VS-density parameterization in elastic full-waveform inversion (EFWI) to invert hydrophone seismic data for subsurface earth properties. This inversion is feasible due to the amplitude variation with angle (AVA) behavior of primary-to-primary reflection data, which directly relates to contrasts in elastic parameters across subsurface interfaces. EFWI provides a natural framework for this process. To achieve this, we employ an adjoint-state approach to compute objective function gradients, enabling joint simultaneous elastic inversion for VP, VS, and density without relying on well-log constraints. One of our primary goals is to eliminate the use of well-log constraints, offering a significant advantage over standard AVA analysis derived from migrated image gathers, which often depend on such constraints for accurate inversion. Our inverted acoustic impedance, density, and VP-VS ratios from EFWI show good agreement with well logs and standard AVA results from migrated gathers, indicating that EFWI is robust for determining elastic subsurface earth properties directly from hydrophone data.