# Analysis of Small-Loop Electromagnetic Signals to Detect Subsurface Anomaly Zones
## 1 Introduction  
Recently, sinkholes have significantly increased in urban areas as a result of the development of cavities and ground softening in the soil under road pavements. It has been reported that the majority of sinkholes are not triggered by natural causes [1]. Soil erosion which may be caused by damage to water and sewage pipes, and changes in underground water levels as a result of construction work have been identified as major causes of sinkholes [2]. Cavities can be formed in the ground by the ingress of soil into pipes, and areas of ground softening can be formed by water leakage from damaged or deteriorated underground water and sewage pipes. In addition, the excavation work that occurs prior to the commencement of construction activities in urban areas could lower the groundwater level and consequently cause the development of subsurface cavities, and sinkholes could develop over wide areas [1].  
Sinkholes pose a threat to life and property. In addition, the national and/or regional economy may be impacted by the cost of the repair work to fix these issues. Therefore, water and sewage pipe conditions, underground water levels, underground facilities, subsurface cavities, ground softening investigations, etc. should be managed systematically [3]. Physical exploration techniques have frequently been used to detect subsurface cavities, areas of ground softening, and underground facilities as it is difficult to detect these features using only visual observation. In addition, these types of techniques have been used to detect the characteristics of rocks, sedimentary layers, etc. that existin the ground. Recently, physical exploration techniques have been used to detect the locations of underground water, minerals, cables, etc. [4]. All physical exploration techniques can detect the location, size, and properties of the target objects. However, the appropriate technique for the target object should be selected among the various types of physical exploration as there may be significant differences in the physical properties between the target object and surrounding ground.  
The physical exploration techniques that have been used to detect cavities, areas of ground softening, and underground facilities include ground penetrating radar (GPR), electrical resistivity (ER), and electromagnetic (EM) explorations [5; 6]. GPR exploration has the advantage of a fast survey speed because it uses a non-grounding transmitting and receiving system. However, the surveyed depth is relatively shallow, specialized skill is required to accurately analyze the measured data, and the equipment is expensive [7]. Even novice users can detect anomaly zones in the ground using the ER exploration technique because the measurement results are expressed as a colored contour map. The technique also has the advantage of a relatively deep survey depth. However, the survey speed is slow as it uses penetration or a grounding transmitting and receiving system. The EM exploration technique was selected for this study because it has a relatively deep survey depth, subsurface anomaly zones can be easily detected as the measurement results are expressed as a colored contour map, exploration of small areas is possible, the equipment is inexpensive, and the survey speed is high because it employs a non-grounding transmitting and receiving system [8; 9; 10].  
The EM survey for this study was performed in a testbed in which cavities, areas of ground softening, and underground facilities were installed in the ground under road pavement. The anomaly zones were detected by visual observation for the colored 2D ER contour maps and comparted with the locations and sizes of the objects embedded under the pavement. A statistical method quantitatively detecting the anomaly zones was also employed to improve the method qualitatively detecting the anomaly zones by visual observation.