Abstract: An apparatus and method for determining the range and depth to an acoustic ource from a sampling site in a medium. A linear array of pressure transducers monitors the acoustic field produced by the acoustic source. A measurement processor utilizes the measured acoustic field to produce an initial field condition for each member of the array. An environmental model includes environmental data in the form of acoustically pertinent variables that define acoustic energy transfer characteristics through the medium at different ranges and depths. A backpropagation processor utilizes the initial field condition and the acoustically pertinent variables to produce, for each incremental range and depth, an amplitude function. An index processor normalizes the amplitude functions for each incremental range and depth, and an index extreme value processor selects the amplitude function with the maximum amplitude.