Frequency-domain free-field sound-speed estimates from (a) receiver column 2, and (b) receiver column 3. The primary (left) y-axis shows microphone height. Sound speed is encoded by the colour map; the colour-bar ticks are identical to those of the secondary (right) y-axis. For each grazing angle, dashed lines denote the mean sound speed and error bars indicate the range of estimates, both referenced to the secondary y-axis.