def pr(vp,vs):
    """
    Computes the Poisson ratio

    Parameters
    ----------
    vp : array
        P-velocity.
    vs : array
        S-velocity.

    Returns
    -------
    pr : array
        Poisson ratio.
    """
    vpvs=vp/vs
    
    pr = 0.5*((vpvs**2-2)/(vpvs**2-1))
    
    return (pr)