
\section{Supplementary material}

\begin{figure*}[h!]
\includegraphics[trim=0cm 0cm 0cm 0cm,clip, width=1.0\linewidth]{Figure_Supp_1.pdf}
\caption{\label{fig:Snapshots} Experimental wave-fields for the two setups in Fig. 1. a) Snapshots at three time-steps of a propagating S-wave pulse (top row) and PII-wave step (middle and bottom row). The top and middle row show the particle velocity and the bottom row shows the z-gradient of the particle velocity. The gradient cancels the PI-wave and its reflection because their displacements are retrieved as nearly constant (PI-wave travels at the same speed as the tracking ultrasound) b) Corresponding time-space representation by summation orthogonal to x (top) and z (middle and bottom). In the top row, the S-wave (S) and in the middle and bottom row, the first (PI) and secondary compression wave (PII) can be identified. $u_z$ - Direction of particle motion. $k_{x/z}$ - Direction of wave propagation.}
\end{figure*}

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