1. A method comprising: providing at least one acoustic sensor located in an ocean layer below an ocean thermocline, the at least one acoustic sensor operating at a first frequency regime and at a second frequency regime, the first frequency regime being above a cut-off frequency of the ocean layer, the second frequency regime being below the cut-off frequency of the ocean layer; recording acoustic data from a moving broadband noise source at the at least one acoustic sensor; generating a plot of acoustic intensity as a function of frequency versus time from the acoustic data; generating a first waveguide invariant density distribution for the first frequency regime and a second waveguide-invariant density distribution for the second frequency regime from the plot using curve detection; and classifying whether the noise source is located below the thermocline based on the first waveguide-invariant density distribution and the second waveguide-invariant density distribution.