# Analysis of Small-Loop Electromagnetic Signals to Detect Subsurface Anomaly Zones
## 6 Conclusions  
In this study, cavities, areas of ground softening, and underground facilities were detected using the small-loop EM exploration method. The anomaly zones were qualitatively detected by visual observation using colored contour maps. In addition, a statistical method for detecting anomaly zones quantitatively was employed. The main conclusions of this study are as follows:  
A method for taking survey measurements at several heights and superimposing the results was utilized to increase the number of measured depths to compensate for the capacity weakness of the EM exploration equipment. Thus, survey results with high resolution can be obtained for multiple depths using only one piece of equipment.  
Figure 17: Verification of anomaly depth by the RSD method using a box plot. (**a**) Absence of anomaly zone (Section No. 1). (**b**) Existence of one small anomaly zone (Section No. 3). (**c**) Existence of more than one or a large anomaly zone (Section No. 2).  
The ER gradually decreased as it penetrated farther down into the natural ground. This was attributed to the fact that the EM wave decreased as a result of changes in the water content with the depth.  
The top of the cavities, which were simulated using polystyrene hemispheres, exhibited a much higher ER than that of the surrounding ground. However, the lower part of the polystyrene hemisphere showed a similar ER to that of the surrounding ground. It was estimated that the higher ER that appeared only at the top of the polystyrene hemisphere was caused by the current channeled at the location. Interestingly, the softened ground, which was simulated using a mix of sand and polystyrene particles, exhibited a higher ER than that of the surrounding ground over its entire area. It was estimated that the higher ER that appeared at the entire area of softened ground was also caused by the current channeled at the top of each sand and polystyrene particle.  
The nonconductive facilities installed in the ground showed a significantly higher ER than the surrounding ground, whereas the conductive facilities exhibited a lower ER. The steel plate and reinforced concrete drainage pipe showed a significantly higher ER than the surrounding ground over a much larger area than their actual size. Hence, the locations of these facilities cannot be accurately detected. Their existence, however, could be easily verified.  
A statistical method for detecting cavities, areas of ground softening, and underground facilities using a box plot and the RSD method was employed. The positions of the anomaly zones were identified by plotting the coefficients of each term of the cubic functions of the ER according to depth on a box plot and identifying the outliers. In addition, the depths of the anomaly zones were detected by plotting the RSDs of the ER according to the position calculated for each depth on a box plot and identifying the outliers.  
It was determined that the portable EM exploration equipment used in this study can be applied to small-scale roads with narrow widths, park roads, sidewalks, etc., which are different from the applications of large-scale exploration equipment. In addition, it was determined that cavities, areas of ground softening, and underground facilities can be detected quantitatively using the statistical method, which was an improvement on the previous method that detected anomaly zones qualitatively by visual observation of the 2D ER contour map.  
Supervision, J.-H.J.; writing--original draft preparation, D.-H.K.; writing--review and editing, K.-H.C.; data curation, investigation, C.-H.L.; All authors have read and agreed to the published version of the manuscript.  
This research was sponsored by the Korea Research Foundation (Task No. 2016R1D1A1B03934177) and was sponsored by the INHA UNIVERSITY research grant.  
The authors declare no conflict of interest.