Abstract: Cobalt-Rich Manganese Crusts (CRCs) is one of the important submarine mineral resources and has important application in the industry, so the exploitation of CRCs has potential mining value. The thickness of CRCs is an important index to evaluate the amount of resource in mining areas. In this paper, the prototype parametric acoustic probe is developed for in-situ measurement of CRCs thickness. The prototype transmits primary signals vertically to undersea sediment and receives the dual-channel echo signals of primary and difference frequencies. The envelopes of the primary signal and difference signal are extracted respectively to get the arrival time of the peak point of echo signal, and then the “time delay difference” method is used for the thickness measurement. The extraction of echo arrival time is the key factor of the measurement. Traditional methods of envelope extraction, such as Hilbert transform can precisely extract the envelope of the primary echo, but the extraction accuracy of the envelope of the difference frequency signal is poor. In this paper, the envelopes of the primary signal and difference signal are extracted by Hilbert transform and autocorrelation algorithm respectively, which improves the accuracy of thickness measurement. The pool test is conducted to analyze the performance of the algorithm. The sea trial result in a China voyage shows the effectiveness of the algorithm.