A test of basin-scale acoustic thermometry using a large-aperture verticalarray at 3250-km range in the eastern North Pacific Ocean

Broadband acoustic signals were transmitted during November 1994 from a 75- Hz source suspended near the depth of the sound-channel axis to a 700-m lon g vertical receiving array approximately 3250 km distant in the eastern Nor th Pacific Ocean. The early part of the arrival pattern consists of raylike wave fronts that are resolvable, identifiable, and stable. The later part of the arrival pattern does not contain identifiable raylike arrivals, due to scattering from internal-wave-induced sound-speed fluctuations. The obse rved ray travel times differ from ray predictions based on the sound-speed field constructed using nearly concurrent temperature and salinity measurem ents by more than a priori variability estimates, suggesting that the equat ion used to compute sound speed requires refinement. The range-averaged oce an sound speed can be determined with an uncertainty of about 0.05 m/s from the observed ray travel times together with the time at which the near-axi al acoustic reception ends, used as a surrogate for the group delay of adia batic mode 1. The change in temperature over six days can be estimated with an uncertainty of about 0.006 degrees C. The sensitivity of the travel tim es to ocean variability is concentrated near the ocean surface and at the c orresponding conjugate depths, because all of the resolved ray arrivals hav e upper turning depths within a few hundred meters of the surface. (C) 1999 Acoustical Society of America. [S0001-4966(99)04506-3].