def get_average_dom_z_coords(geo):
    """Find average z coordinates for IceCube (non-DeepCore) and DeepCore
    "z-layers" of DOMs.

    A "z-layer" of DOMs is defined by all DOMs on all strings of a given string
    type with shared DOM (OM) indices.

    Parameters
    ----------
    geo : (n_strings, n_doms_per_string, 3) array
        (x, y, z) coordinate for string 1 (string index 0) DOM 1 (dom index 0) is found at geo[0, 0]

    Returns
    -------
    ic_avg_z : shape (n_doms_per_string) array
    dc_avg_z : shape (n_doms_per_string) array

    """
    ic_avg_z = geo[:78, :, 2].mean(axis=0)
    dc_avg_z = geo[78:, :, 2].mean(axis=0)
    return ic_avg_z, dc_avg_z