Abstract: The Jintang subsea tunnel of the Ningbo-Zhoushan expressway second line is notable for its large size, exceptional length, and significant depth. The project site presents complex terrain, varying ocean currents, and diverse geological conditions, compounded by fluctuating weather and heavy maritime traffic, which make exploration challenging while demanding high precision. The authors outline a comprehensive exploration methodology that incorporates extensive data collection, thorough analysis of the site environment, and field tests in representative sections. The methodology primarily utilizes single-channel seismic methods, with multi-channel seismic methods serving as support, alongside the integration of marine transient electromagnetic techniques and depth-measuring methods at multiple speeds. During implementation, a substantial volume of high-quality raw data is collected, followed by advanced processing techniques to enhance data accuracy and reliability. Scientific analytical methods, including the integrated interpretation of single- and multi-channel seismic data and cross-validation of various techniques, are employed to systematically analyze results and derive reliable geological insights. Regarding engineering applications, geophysical findings are first used to evaluate the overall geological structure, which is then verified through drilling to refine stratigraphic distribution characteristics. This approach facilitates the dynamic optimization of tunnel alignment design by enabling accurate subsea stratigraphic classification, precise identification of bedrock undulations, scientific zoning of geological environments, and the development of a high-resolution three-dimensional engineering geological model. These results enhance tunnel design, lower construction costs, and shorten survey cycles, thereby offering strong technical support for subsea tunnel projects.

Key words: complex marine environments, ultra-large diameter shield tunnel, offshore seismic prospecting, refined stratigraphy division, three-dimensional geological modeling, integrated survey method