## 4 Conclusions and Discussion
Unless any of the \"noise\" signals are directly utilized as signals of opportunity, frequencies that are not heavily interrupted by noise are likely the best choice to maximize the SNR of scattered signals. A third important parameter is the type of sensor being utilized. The most common type of ELF/VLF sensors is loop antennas [24] that typically lose sensitivity as frequency is decreased. Thus, frequencies that are too low may not be easily detectable using standard sensors or may require either specialty or expensive equipment. In general, the frequency response of a hidden object and shield combination can be relatively complex. As such, the use of multiple frequencies is likely the most prudent methodology in practice. As a qualitative description, higher frequencies tend to achieve less penetration into a shielded container, but with the advantage of responding more to conductivity than permeability. On the other hand, the lower frequencies demonstrate a high degree of penetration and are considerably more sensitive to magnetic properties (permeability) relative to higher frequencies. For the purposes of this work, the 50 Hz frequency case was chosen based on a relatively good (empirically determined) trade-off between penetration and sensitivity to material properties and also due to the ubiquity of 50 Hz radiation from global power line networks. The time-domain signature of the magnetic field distortions plays an important role in imaging. However, the simulations in this case are run applying monochromatic excitation to ascertain the response to low frequencies in a more straightforward manner. Thus, any time-variation is encoded in the amplitude and phase profile of the measured signals. It is worth noting that this method can be extended to the time-domain in future work by running several frequencies and synthesizing the time-domain signal. The results of the work provide a basis for future inversion methodologies for imaging objects through shielded enclosures using ELF/VLF signals. Conceptualization, H.K. ; methodology, V.H. ; software, H.K. ; validation, V.H. ; formal analysis, H.K. ; investigation, H.K., V.H. ; writing--original draft preparation, H.K. ; writing--review and editing, V.H. ; project administration, V.H. ; funding acquisition, V.H. All authors have read and agreed to the published version of the manuscript. This research was funded by the Defense Advanced Research Projects Agency (DARPA), Imaging Through Almost Anything Anywhere (ITA3) grant number HR00111820016. Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. All data are available on Github ([[https://github.com/hoyoungrkiim/Numerical-modeling-of-nondestructive-testing-of-various-conductive-objects-inside-metal-enclosures](https://github.com/hoyoungrkiim/Numerical-modeling-of-nondestructive-testing-of-various-conductive-objects-inside-metal-enclosures)]([https://github.com/hoyoungrkiim/Numerical-modeling-of-nondestructive-testing-of-various-conductive-objects-inside-metal-enclosures](https://github.com/hoyoungrkiim/Numerical-modeling-of-nondestructive-testing-of-various-conductive-objects-inside-metal-enclosures)), accessed on 19 April 2021). The authors declare no conflict of interest.