def porosity_sim(vol_pore=0, vol_bulk=1.0, vol_matrix=0.7):
    """Returns the simple porosity of volume given the bulk and matrix volume,
     or the user can give the pore volume and bulk volume."""
    if vol_pore > 0:
        return float(vol_pore/vol_bulk)
    else:
        return float((vol_bulk-vol_matrix)/vol_bulk)