How would you accomplish this?

To add to @didiermazier ’s recommendation: 

If more randomness in the displacement is needed one can use a random Gradient Layer and the Filter Mosaic to create a Displacement Map. 

I believe two images are utilized for the final effect. You can apply the above suggestion and add another image on top, which will be masked to reveal only certain sections of the underlying photo. That's one method to achieve the desired result.

You cluld :

1- make a reference image. B&W 8 bits

2 -Apply a displacement filter based on this image