Evolution of fast bulk sound wave Fréchet derivative K_c, the shear wave derivative and the impedance derivative K′_ρ (top, middle and bottom rows, respectively) from the initial model m₀₀ (first column) through model m₀₄ (fifth column) for a poroelastic inversion using direct P-wave traveltime anomalies. Note how the amplitude of the gradient decreases with each subsequent iteration. A similar number of iterations is required for acoustic, elastic and Gassmann elastic inversions.