def basic_src(model, u, **kwargs):
    """
    Basic source for linearized modeling

    Parameters
    ----------
    u: TimeFunction or Tuple
        Forward wavefield (tuple of fields for TTI or dft)
    model: Model
        Model containing the perturbation dm
    """
    w = -model.dm * model.irho
    if model.is_tti:
        return (w * u[0].dt2, w * u[1].dt2)
    return w * u[0].dt2