Taking the 2D FFT of axial displacement data

Hi,

I am aiming to recreate a protocol found in a manuscript to look at the k-space map of axial displacement data in MATLAB. The snippet of the paper that I am following is the following:

I have generated the axial displacement data (the map is below and the raw data is here), similar to that described by the paper I am using as reference:

I am struggling with two key things: 1) the 2D fft that they describe is not matching mine (the sampling of the 2D fft map is far too coarse and I am not sure why), 2) I am not able to then convert my plots in phase velocity as they do. They do state that they are only taking the 2D fft of half of the displacement map (that is why I do it below).

My code to determine the 2D kspace map is:

axial_displacement_FT = fftshift(fft2(axial_displacement_map(:, 2048:end), 4096, 4096));

The resultant image is:

In their manuscript, their workflow is similar to mine except for the 2D fft image and their conversion to phase velocity:

Thus, could someone help me with my code for the 2D fft map and provide any insights into how to convert this to the phase velocity? I think the issue is fundamental but I am missing something.

Thanks!