## 1 Introduction  
Kinmen is a small island. During the 43 years of military control, the Kinmen government constructed various defence works and military camps, as well as many spiritual landmarks. Military installations, bunkers, and tunnels can be seen across the island. During the military administration, there were at least 1000 berracks, 22 large-scale monuments, 28 memorial pavilions, and 10 large underground Halls in the Kinmen area [1]. As it was in a state of combat readiness for an extended period, 120,000 soldiers were stationed in Kinmen. After the 1990s, the government regained local autonomy and democratic governance, and the original historical sites became the resources of local tourism development. In recent years, with the reduction of military garrison and opening of military spaces, the pace of development has accelerated. It is estimated that over 1000 vacant barracks will be released over the coming years. At least 300 barracks have been or are in the process of being released, and about 50 to 80 barracks are planned for annual release. Although the vacant barracks are being released, the numerous underground bunkers are still closed or covered with weeds. In recent years, there have been frequent occurrences of collapsing underground bunkers that endanger private property [2]. Public works were frequently halted due to the discovery of underground bunkers during excavation [3] and ammunition depots excavated during public works [4]. The original data on these underground bunkers, including their planimetric positions, massing sizes, and underground depths, are difficult to obtain or simply incorrect. These difficulties and inconsistencies result from frequent changes of documents keepers and mistakes in handover during army withdrawal. Therefore, the Kinmen County Government faces a challenge in providing a detailed report on underground bunker investigation necessary when planning public works, thus delaying the planning and design schedule. If an underground bunker is discovered during the construction, the works must be halted. Without such reports, the building schedule cannot resume, which significantly delays the construction progress and affects the efficiency of government administration. In addition, the army faces similar problems of unknown or unclear locations of underground bunkers when returning the land to private owners, thus affecting the military handover schedule. Civilians also worry about whether there are any underground bunkers nearby, endangering their homes and safety. Over the past decades, despite societal progress, urban disasters occurred frequently. The collapse of underground bunkers across Kinmen is an example of such disasters. Detailed and correct geological data of the site can enhance the effectiveness of urban disaster prevention and reduce the potential damage these disasters may cause. The geotechnical engineering investigation targets the geological conditions of the site. The conventional geotechnical investigation uses mainly the geological drilling method, but the drilling cost is high and provides only a single point of information. Sometimes it needs to be combined with the geophysical exploration method to gain the complete geological picture. In recent years, nondestructive geophysical technology has been gradually introduced in different underground environmental surveys. In combination with document information, the information of underground \"surface and space\" can be obtained [5]. In the past, geophysical exploration was used in the geotechnical investigation primarily for seismic detection (refraction, reflection, and surface wave techniques). The elastic wave velocity of strata is closely bound with the material engineering characteristics [6; 7; 8], but the seismic detection is likely to be influenced by ambient vibration noise. Moreover, seismic refraction detection cannot positively detect the low-velocity layer under the high-velocity one, and the shallow layer (less than 50 m) reflection seismic detection is challenging to implement.