## 3 Evaluation Indicators
### Underground Structures  
Underground structures are primarily categorized into modern and ancient structures (Figure 2). Various protection requirements may restrict the development of UUS or complicate their implementation [82]. Three-dimensional information of modern structures must be acquired by collecting municipal data and integrating it with geophysical detection efforts, whereas the buried information of ancient structures is obtained solely through geophysical detection. (1) The modern structures  
Modern structures can be classified into several categories based on their functional uses: underground transportation facilities, municipal facilities, commercial and public spaces, and protective projects [19]. For instance, when classified by burial depth [98], structures can be divided into shallow underground structures (burial depth \(<\) 15 m), which primarily include underground parking lots, comprehensive pipeline corridors, and commercial streets; middle underground structures (burial depth 15-50 m), which mainly encompass subways, tunnels, and underground water storage reservoirs; and deep structures (burial depth \(>\) 50 m), which include deep buried laboratories and high-level nuclear waste disposal facilities. Generally, the deeper the underground structure, the greater the requirements for the development and utilization of UUS, which in turn influences the intended purpose of that space. For shallow underground structures, classification by burial depth typically involves methods, such as open excavation [99], cover excavation [100], and shield construction. When evaluating the geological suitability of large regions, it is essential to assess the feasibility of underground open space development and reuse in built-up areas, taking into account the degree and scale of development alongside planning requirements [25,101]. Additionally, existing engineering and environmental geological issues must be considered, along with potential secondary hazards, such as uneven formations, backfill stability, local ground settlement, and impacts on underground pipelines and other structures [102]. Medium-level underground structures, such as subway tunnels constructed using the shield method and underground reservoirs, are often viewed as limiting factors for development [103]. Furthermore, local considerations may include the avoidance of geothermal wells, pump stations, and other vertical shaft projects. In the long term, deep underground structures should be regarded as absolute restrictions, given the unique characteristics of their functions and the potential for development and utilization of UUS to cause irreversible changes in regional geological conditions, such as soft soil consolidation and rock breakage. The diversity of underground infrastructure imposes differentiated demands on geological conditions. For instance, metro tunnels must avoid water-rich sand layers to prevent water inrush during construction, while underground oil storage facilities depend on low-permeability shale layers to effectively seal oil and gas [31]. In soft soil regions, such as Shanghai, metro construction requires the use of the freezing method to reinforce the ground; conversely, in bedrock regions, like Hong Kong, the wear rate of tunnel-boring machine (TBM) cutters is directly related to rock hardness. Therefore, geological suitability assessments must customize indicator weights based on facility types--for example, assigning a higher weight to the 'rock mass integrity coefficient' for energy facilities, while placing greater emphasis on the'soil corrosion grade' for municipal facilities. (2) The ancient structures  
Ancient structures can be repurposed for cultural displays, tourism, and other functions, thereby enhancing the utility of UUS. Conversely, these structures complicate the development and utilization of underground areas due to challenges, such as foundation treatment, increased costs, and restricted development potential.