It has long been possible to determine He + /H + ratios from radio or infrared recombination lines, making it possible to measure ionic abundance ratios for highly obscured emission-line objects. It is not possible to determine the total helium abundance from such data alone, however, since the ionization correction factor (ICF), the correction for unobservable stages of ionization of He or H, must also be determined. Optical forbidden lines are usually used for this, limiting studies to relatively unobscured objects.This paper outlines a way to determine the ICF using only infrared data. We identify four line pairs, [NeIII] 36 µm/[NeII] 12.8 µm, [NeIII] 15.6µm /[NeII] 12.8 µm, [ArIII]9 µm/[ArII] 6.9 µm, and [ArIII] 21 µm/[ArII] 6.9 µm, that are sensitive to the He ICF. This happens because the ions cover a wide range of ionization, the line pairs are not sensitive to electron temperature, they have similar critical densities, and are formed within the He + /H + region of the nebula. We compute a very wide range of photoionization models appropriate for galactic HII regions. The models cover a wide range of densities, ionization parameters, stellar temperatures, and use continua from four very different stellar atmospheres.The results show that each line pair has a critical intensity ratio above which the He ICF is always small. Below these values the ICF depends very strongly on details of the models for three of the ratios, and so other information would be needed to determine the helium abundance. The [Ar III] 9 µm/[ArII] 6.9 µm ratio can indicate the ICF directly due to the near exact match in the critical densities of the two lines. Finally, continua predicted by the latest generation of stellar atmospheres are sufficiently hard that they routinely produce significantly negative ICFs.

The Charity Shoal structure (CSS) is a 1.2 km diameter, bedrock-rimmed circular depression in the lakebed of northeastern Lake Ontario (Fig. 1). The CSS has been interpreted as a possible Ordovician-age simple impact crater based on multi-beam imaging of the lakebed (Holcombe et al. 2013) but the subsurface structure was not resolved. Other possible origins include a karst sinkhole, glacially erosion (e.g. kettle hole), basement structural depression or Jurassic volcanic intrusive (Suttak, 2013). Detailed magnetic and high-resolution chirp and 1.5 kHz boomer seismic (>400-line km) surveys were conducted across a 9-km 2 area to investigate the subsurface structure. Total magnetic intensity (TMI) data (Fig. 2) reveal a large (>1400 nT) magnetic anomaly centered over the crater basin and a ring-like magnetic high (40-50 nT) corresponding with the raised bedrock rim (Fig. 3). Depth to basement below the structure was estimated at 600 m using extended Euler deconvolution (Fig. 3F).