Rapid inversion of waveguide environmental parameters using an analytic adjoint method

A method is presented for inverting sound‐speed perturbations in a stratified waveguide environment over several kilometers range. An adjoint Green’s function formalism is used to backpropagate the difference between a measured and modeled received field through the modeled waveguide environment, in a manner reminiscent of acoustic time‐reversal methods. Using a set of linearized equations similar in form to the Born approximation, the backpropagated error field is converted into an estimate of an error surface gradient, or Frechet derivative [S. J. Norton, J. Acoust. Soc. Am. 106, 2653–2660]. The gradient can then be used to locate the error surface minimum via a series of iterations. The use of a normal‐mode waveguide Green’s function enables a considerable analytic simplification of the equations. The similarities and differences between the adjoint approach, matched‐field processing, and diffraction tomography are discussed. [Work sponsored by ONR.]