Rock mass deformability is evaluated by Young's modulus (E), which provides the bases of stability assessment for designing and developing large engineering structures. Modulus of elasticity (E) is the most commonly used input parameter for rock mass characteristics and classification systems, stability analysis of surface/underground engineering structures, and rock failure criteria. However, E is conventionally obtained from the borehole tests, which have significant limitations and do not provide a thorough evaluation of rock mass deformability for lateral and vertical coverage of large areas. Conventional determination of in-situ geomechanical parameters is a complex system problem, which has always been a challenge under uncertainty and data lack. Alternatively, throughout the last decades, several attempts were made to assess the subsurface geology via geophysical-based approaches. Geophysical approaches are more cost-effective, quicker, and user-friendly and offer volumetric data of the subsurface. In this contribution, for the first time, we advance a non-invasive geophysical approach of controlled-source audio-frequency magnetotellurics (CSAMT) for quick estimation of 2D/3D E to evaluate the complex geological subsurface over one km depth. These results are important to better understand the complex engineering geological conditions, to assess the failure probability in the early stage, and to provide safety, stability, and cost evaluation support for successful development of the deep underground engineering infrastructures. Our approach fills the gap between accurate geotechnical models and insufficient geological information, gives more objective indices, and provides a reference for more accurate design of engineering structures in areas lacking sufficient mechanical drilling data.