## 1 Introduction  
In underground mines, the ground may become unstable owing to vibrations from blasting and mining operations. Furthermore, harmful gases or dust may be generated depending on the mined material; these factors are hazardous to mine workers, essentially resulting in the loss of life and money. In Korea, the number of mine accidents has decreased since the 1980s; however, accidents continue to occur. In Korea, the number of accident victims in the past 10 years was approximately 60 [1]. In the United States, which has more mining workers compared with Korea, 43 occupational fatalities were reported in 2014 and 26 in 2015 [2]. As various factors, such as machinery, haulage, ground subsidence, and sliding rocks, can cause accidents, the environment must be monitored. Essentially, the environmental factors must be estimated, and appropriate countermeasures must be adopted to prevent potential accidents in mines. In particular, the current state of a mine must be understood by monitoring the environment and worker conditions. For example, by monitoring vibrations, collapse can be prevented by reinforcing structurally unstable areas [3; 4; 5]. By monitoring the amount of gas or dust, an appropriate ventilation system can be established by installing a ventilator in an area where a large amount of harmful gas is generated [6; 7; 8]. If the location and heart rate of a worker are monitored, an optimal response can be achieved at the earliest in case an accident occurs [9; 10; 11]. However, in underground mines, a GPS connection is not possible, leading to difficulty in tracking a worker's location. In addition, a connection with an external network is impossible unless an additional network is configured in the mine. Therefore, several attempts have been made to combine various sensors and Internet of things (IoT) to build networks in underground mines [12; 13; 14]. To build a smooth wireless sensor network (WSN) in an underground mine, the issues regarding power supply and communication stability between devices must be addressed. First, sufficient and continuous power must be supplied to an IoT device to produce an appropriate output. In the long term, installing wires is the most useful strategy; however, applying wires in small mines is challenging because of high construction costs. An alternative is to supply power through batteries to minimize the power consumption of IoT devices. To improve the communication stability of WSNs, dense networks must be developed by increasing the number of IoT devices. When the communication between devices is smooth, a better communication network can be built using relatively few devices. Therefore, to obtain an optimal effect within a limited budget, the obstacles between devices should be reduced to the maximum possible extent. The absence of obstacles between devices implies that the line of sight (LOS) is connected between the devices. The LOS is an imaginary line between an observer and target. In communication, it represents a direct path from the transmitter to the receiver. Several studies in various fields have analyzed the visible area from an observer or analyzed the LOS for smooth communication in consideration of the topography [15; 16; 17; 18; 19]. Similarly, before installing communication devices at an underground mine site, the LOS must be analyzed in advance to determine the areas with good communication. Thus, the arrangement of communication devices can be designed efficiently, which can be useful when installing additional devices or performing maintenance tasks in the future. Several studies [20; 21; 22; 23] have been conducted to effectively install communication devices of various frequencies in underground mines. These studies actually installed communication devices in the field and analyzed communication strength considering LOS. However, most of these studies have focused on detailed strength analysis in a small area, and there have been few studies that have considered the 3D space for a large area.