## 1 Introduction
In the mining sector, IIoT is used to improve worker safety [9; 10] and the production and exploitation processes [11]. Furthermore, [PERSON] et al. [12] shows the potential benefits of applying IIoT systems in underground coal mines to improve safety, productivity, and equipment automation. On the other hand, analyzing the information technology used in the mining sector and identifying the challenges regarding IIoT adoption, [13] proposes a unified architecture for the mining industry, considering IIoT frameworks and standards. We propose to design,develop, and implement an IIoT system that allows the monitoring of environmental variables in mining environments. The solution comprises static nodes in different mine locations and mobile nodes that the miners would carry. The nodes would communicate using a WSN and be sent to external monitoring centers over the Internet. The monitoring centers will be located in safety entities and with mine administrators. The deployment of sensor nodes in IIoT systems refers to the site of sensors' placement in the scenario. The node deployment methods fall into deterministic and random distribution [14]. In deterministic deployment, the locations are known according to some pattern or vector. In contrast, in random deployment, the sensors are scattered in the region of interest (RoI), following a randomized distribution [14]. Deterministic deployment schemes are ideal for network efficiency but impractical and impossible in large or inaccessible/harsh areas. For example, harsh environments like battlefields or disaster regions make deterministic deployment risky and infeasible. Random deployment becomes the only option, with sensors delivered from various sources such as planes, artillery shells, rockets, missiles, or ships [15]. The environment is the main criterion when defining which deployment method is used. However, it is also possible to consider the deployment objectives, such as coverage, connectivity, energy consumption, and network lifetime. Sensor deployment is a crucial design issue in IIoT based on WSNs and is considered critical because it determines the network's performance. The proper placement of sensors can optimize performance metrics such as coverage, network connectivity, network lifetime, energy efficiency, reliability, deployment cost, and fault tolerance. A good deployment improves performance in information gathering and communication. The application requirements and the deployment scenario mainly determine the method for node deployment. Monitoring underground coal mines' environment at work fronts and in transport tunnels is mandatory. Nodes must be deployed deterministically or randomly, but choosing only one deployment method causes performance issues such as coverage holes and weak network connectivity. It could also affect the deployment cost. Coverage indicates how well the sensors detect behaviors in the RoI [16] and is a function of the sensing range (r\({}_{\text{s}}\)). Connectivity refers to ensuring the node's communication with other nodes and the sink node and is a communication range (r\({}_{\text{c}}\)) function. To address these issues, a proper deployment methodology that combines deterministic and random deployment to support long-term planning is proposed in the present work. This paper proposes a methodology for deploying WSN-based IIoT systems for gas monitoring of explosive atmosphere for long-term underground coal mine planning. Using a deterministic and random deployment, the algorithm locates the sensor nodes from a long-term planning blueprint. The sensors are located in tunnel crossings and at work fronts in the deterministic deployment step. Then, using random distributions, it locates new sensors to cover the whole scenario in case it does not meet the coverage and connectivity requirements. The strategy considers the communication and coverage sensing ranges obtained from previous experimental studies.