The fundamental experimental requirement of ocean acoustic tomography is to make precise measurements of acoustic propagation in the ocean. Any of a variety of characteristics of acoustic propagation potentially can provide useful information about the ocean through which the sound has traveled, provided that the forward problem is thoroughly understood. The task of the measurement system is to provide estimates of whatever parameters are desired, with useful precision, together with estimates of the errors of the measurements.

In this chapter we focus on the use of broadband acoustic signals to measure the impulse response of the ocean with sufficient resolution (in time and/or vertical angle) to separate individual ray arrivals. The precision with which travel times and other parameters of the individual ray arrivals can be estimated is limited by the ambient acoustic noise in the ocean. In addition, and more important in most cases, small-scale ocean variability causes fluctuations in ray amplitudes, travel times, phases, and arrival angles. Travel time is the most robust observable and is the one we emphasize in this book. The inversion of travel-time data to obtain information on the ocean sound-speed and current fields was outlined in chapter 1. Because the expected magnitude of the travel-time perturbations is O(100 ms), travel times need to be measured with a precision of a few milliseconds, corresponding to a few parts per million over 1 Mm range.