The problem of electron density inversion of digital ionograms is reconsidered from the viewpoints of new possibilities and of modern requirements. The data processing system of an advanced ionosonde (the dynasonde) provides accurate measurements not only of echo group range but also of direction of arrival, among other physical parameters, thus yielding the three‐dimensional distribution of apparent echolocations in each ionogram recording. An iterative ray‐tracing approach is described here to recover the parameters of a quite sophisticated three‐dimensional (so‐called wedge‐stratified ionosphere) model of the local electron density distribution, characterizing its actual vertical N_e(h) profile together with horizontal gradients and general tilts. The power of a contemporary PC is sufficient to accomplish this analysis quickly. This approach is implemented in the algorithm introduced here, is named “NeXtYZ,” and is pronounced “next wise.”

We report on the results of an underwater acoustic field experiment conducted in December 2012 on the continental shelf off the Florida Keys in water of approximately 100 m depth. Ambient noise was recorded concurrently on three moored near‐bottom instruments with horizontal separations of approximately 5 km, 10 km, and 15 km. We focus in this letter on instrument pairs with 5 km and 10 km separations. Consistent with theoretical predictions, coherent sums of many realizations of cross correlations of ambient noise records at two measurement locations are shown to yield approximations to deterministic acoustic Green's functions that describe propagation between those two locations. The results presented demonstrate the feasibility of extracting information suitable for use in tomographic inversions from measurements of underwater acoustic ambient noise.

Group speed of sound in moving fluids depends on the propagation direction, which breaks acoustic reciprocity. Acoustic nonreciprocity provides a means to measure fluid motion. Using nonreciprocity, one can measure fluid velocities that may be small compared to uncertainties in sound speed. Interferometry of diffuse acoustic noise, with receivers replacing the transceivers employed in active techniques, offers a simple, low-cost means of measuring nonreciprocity. Here, the feasibility of using passive measurements of acoustic nonreciprocity to estimate current velocity in the ocean is experimentally demonstrated for the first time. Estimates of depth-averaged flow velocity are retrieved from cross-correlations of low-frequency noise recorded in the Straits of Florida by near-bottom hydrophones separated by 5 and 10 km.