Abstract: Due to its distinctive thermohaline structure, the Beaufort Sea creates a double duct acoustic environment, characterized by two typical sound propagation ducts in the surface layer of the ocean . This paper studies the propagation characteristics of modes in a double duct environment. It is found that the low-order modes in the sound field exhibit tunneling phenomena and significant spatial separation characteristics. Based on the characteristics that low-order modes preferentially fill a single duct, this paper constructs a method for depth discrimination of passive sound sources in a double-duct field environment. First, the modes captured by the two ducts are divided into different modal subspaces. Then, a subspace projection operator, based on eigenvalue decomposition modal filters, is introduced to project the received signal to the corresponding subspace. The duct where the target is located is determined by comparing the energy. Finally, the effectiveness of the proposed method is validated through Monte Carlo simulations and Arctic experimental data. The results demonstrate that the proposed method is robust for depth discrimination, even with limited array aperture when only amplitude information is available.