Efficient and High-Resolution Detection for Urban Underground Space Using a Grounded Wire Source Frequency-Domain Electromagnetic Gradient Method

In the development of urban underground spaces (UUSs), comprehensive exploration of geological conditions is crucial for assessing the structural stability. While geophysical electromagnetic (EM) prospecting plays a pivotal role in this regard, urban settings pose unique challenges due to heightened requirements for measurement efficiency and interpretation precision, compounded by constraints on instrument deployment. To address this issue, this article proposes an innovative approach termed the urban-suitable frequency-domain EM gradient (UFEMG) measurement and quick imaging method. The method involves the measurement of array-type magnetic fields, computation of spatial and frequency gradients, and derivation of magnetic field gradient-based apparent resistivity (MFGBAR). The effectiveness of the method was tested, and the key influencing factors of its application effect were analyzed using a 3-D model. To further validate its practical utility, a UFEMG instrumentation system was designed and implemented. The system integrates multifrequency high-efficiency transmission (MHT) technology to enhance efficiency and vertical resolution during transmission and utilizes a movable and deformable towage receiver array (MDTRA) configuration to ensure efficient magnetic field measurement and precise gradient acquisition, thereby improving lateral resolution. Field surveys conducted at Cultural Square Light Rail Station and Yuhua Park in Changchun, China, demonstrated the effectiveness of the method in detecting subterranean features such as subterranean cavities, artificial ponds, and reservoirs. Comparative analysis with traditional EM prospecting and apparent resistivity imaging results reveals the superior anomaly recognition of the UFEMG method, and the better consistency with known geological information that further affirms its advantages of high efficiency, resolution, and precision. This research demonstrates the significance of the UFEMG method for urban subsurface exploration and provides technical support for enhancing the planning and development of UUS.