Passive ranging of sound sources in deep water has attracted considerable attention. This paper presents a passive ranging method using vertical double hydrophones, which estimates the range of near-surface sources based on the time delay between direct and bottom-reflection paths. Traditional methods commonly assume a stable seafloor reflection structure and use the visible seafloor depth directly. However, low-frequency sound penetration into the seabed and sub-bottom sound speed variations lead to an uncertain effective reflection interface and degraded ranging accuracy. To overcome this problem, we introduce and estimate an equivalent reflection depth to replace the actual seafloor depth in the ranging model. The proposed method is verified by numerical simulations and a sea experiment in the South China Sea, and parametric analyses are also conducted. Experimental results show that the estimated source ranges agree well with the ground truth, demonstrating the effectiveness of the method.
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