def acoustic_reflectivity(vp, rho):
    """
    The acoustic reflectivity, given Vp and RHOB logs.
    Args:
        vp (ndarray): The P-wave velocity.
        rho (ndarray): The bulk density.
    Returns:
        ndarray: The reflectivity coefficient series.
    """
    upper = vp[:-1] * rho[:-1]
    lower = vp[1:] * rho[1:]
    return (lower - upper) / (lower + upper)