Abstract: To address the challenges of distinguishing underwater/surface targets in shallow water environments where the scattering characteristics of target sources are unknown, this paper proposes a method that leverages the differences in the acoustic fields generated by modes excited at varying depths due to the presence of a thermocline. Based on a generalized field matching approach, this method differentiates between underwater and surface targets by matching the phase characteristics of the complex sound pressure ratios between any two sensors in a horizontal array, effectively mitigating the influence of target source scattering characteristics. Theoretical and numerical simulation results indicate that the method is suitable for shallow water environments with a negative sound speed gradient in the thermocline. As the sound speed gradient increases, the depth resolution capability improves, and the sensitivity to environmental parameter mismatches decreases. When the difference in sound speed across the thermocline exceeds 20 m/s and the signal-to-noise ratio is greater than 4 dB, the accuracy of depth resolution for underwater targets using a dual sensor configuration can reach 95%. Analysis of sea trial data demonstrates that the proposed method can accurately distinguish underwater targets, validating its effectiveness in practical applications.