Wave-theoretic ocean acoustic propagation modeling is combined with the peak arrival approach for tomographic travel-time observables to derive the sensitivity kernel of travel times with respect to sound-speed variations. This is the Born–Fréchet kernel relating the three-dimensional spatial distribution of sound-speed variations with the induced travel-time variations. The derivation is based on the first Born approximation of the Green’s function. The application of the travel-time sensitivity kernel to an ocean acoustic waveguide gives a picture close to the ray-theoretic one in the case of high frequencies. However, in the low-frequency case, of interest in ocean acoustic tomography, for example, there are significant deviations. Low-frequency travel times are sensitive to sound-speed changes in Fresnel-zone-scale areas surrounding the eigenrays, but not on the eigenrays themselves, where the sensitivity is zero. Further, there are areas of positive sensitivity, where, e.g., a sound-speed increase results in an increase of arrival times, i.e., a further delay of arrivals, in contrast with the common expectation. These findings are confirmed by forward acoustic predictions from a coupled-mode code.

Passive and active acoustic measurements are effective and unique approaches to ocean observation. Both approaches have techniques that are well-established and suitable for immediate deployment as part of an operational observing system.Many of these techniques exploit the remarkable ability of lowfrequency sounds to traverse great distances in the ocean. Passive acoustic measurements of natural or man-made sounds are made for diverse purposes ranging from assessing the environmental impact of human activities, to geophysics, to monitoring changes in biodiversity in response to climate change. Active acoustic systems are employed for tracking instruments and acoustic tomography. Tomography has a role in the ocean observing system that is complementary to altimetry and profiling floats, particularly in the context of data assimilation and ocean modeling. Modest systems of acoustic receivers and sources deployed across ocean basins can make cost effective, sustained observations of a variety of ocean processes.

Individual ray arrivals were not observed. Rather, the arrival patterns consisted of a single, stable, broad arrival pulse of about 100 ms duration. Travel time variations of 60.15 s recorded the vigorous mesoscale environment of the region and the seasonal cycle. To estimate ocean temperature from the tomography data an inverse scheme employed a high-resolution ocean model for Fram Strait as the reference ocean. The information from the tomographic measurements is primarily average temperature. Estimated temperatures, averaged over 0-1000 m depth and over range, had a mean of 1.118C and variations of 60.338C; the uncertainty of the tomography estimates was about 60m8C. Agreement with an alternate inverse approach based on EOFs and a Markov Chain Monte Carlo inversion scheme relying on a matched-peak approach was excellent, indicating a robust estimate for ocean temperature. The inverse estimates for average temperature agreed with the equivalent estimates from hydrographic sections obtained along the acoustic path at the start and end of the program. Among other deficiencies, the ocean model greatly underestimated the intensity of the mesoscale fluctuations and exhibited a warm bias of about 0.388C in section-averaged temperature. Tomographic measurements in Fram Strait offer unique large-scale temperature constraints for ocean models through data assimilation. It is anticipated that these constraints will lead to more accurate estimates of the circulation and transports in Fram Strait.

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Recordings of pulsed sounds ͑clicks͒ from Cuvier's beaked whales are presented. Such recordings have not been reported in the literature before. Spectrogram analysis of data collected off SW Crete ͑Greece͒ from 1998 to 2000 revealed numerous sequences of clicks. Click pulses had durations of about 1 ms and their energy content in the audible spectrum presented a narrow peak between 13 and 17 kHz. Sequences of 35-105 clicks, with duration 15-44 s, were separated by short intersequence pauses of 3-10 s. Interclick intervals appeared fairly constant, primarily oscillating between 0.40 and 0.50 s. Characteristics of Cuvier's beaked whale clicks were consistent with echolocating cetaceans, suggesting that this species do echolocate.