Robust observables for mode tomography

Deep water tomographic inversions should include the low modes to improve estimates of the axial sound speed. Unfortunately, the low-mode signals are strongly affected by internal wave fluctuations, making it very challenging to estimate their arrival times. This talk describes the use of matched subspace detectors (MSDs) to measure mode travel times for ranges up to 400 km. The MSD approach was first proposed by Scharf and Friedlander [IEEE Trans. Signal Process. 42, 2146–2157 (1994)] as a way to detect signals that lie within a particular subspace. In this work the subspace for each mode is defined by analyzing simulations of mode propagation through independent realizations of the internal wave field. Simulations show that the MSD-based travel time estimation approach yields higher accuracy and less variance than other methods such as peak-picking. The end result is improved inversions for sound speed near the sound channel axis. In addition to the simulation study, mode signals from the 2004 Long Range Ocean Acoustic Propagation EXperiment (LOAPEX) are used to invert for the sound speed profile (SSP) across the LOAPEX path. The inverted SSP is compared with environmental measurements made during LOAPEX. [Work sponsored by ONR.]