A single slice tomography experiment using a long vertical array of receivers

Abstract

Broadband signals were transmitted from a near-axial source to a long vertical receiving array 1000 km distant in the North Pacific for 9 days in July 1989. The signals were received at 50 hydrophones spaced 60 m apart in the vertical, starting near the surface. Pulses were recorded at 10-min intervals for two periods, totaling 24 h, while hourly pulses were recorded for the duration, giving a total of about 320 pulses. Individual ray arrivals were resolved and their arrival times measured with a precision of a few milliseconds using phase-coded transmissions centered at 250 Hz with a bandwidth of 83 Hz (12-ms resolution). The pulse arrivals as a function of time and depth correspond well with predictions based on geometric (ray) and physical (WKBJ) optics. In each transmission, 600 to 900 ray arrivals can be identified with rays from numerical analysis, distributed along up to 24 distinct wave fronts. The ray arrivals evolve at the time scales of ocean processes. The longest period travel time changes can be inverted to infer changes in the mean sound-speed field between the source and receiver and in high-wave-number ocean features with wavelengths correspondng to ray loop lengths and their harmonics (70 km and shorter wavelengths). Simultaneous CTD and XBT surveys are available for comparison. Temporal and vertically lagged fluctuation statistics of the individual arrivals quantify internal wave and fine-structure effects. [Work supported by ONT.]