Echograms are produced by aligning the envelopes of short sections of audio. For odontocetes, these sections of audio are selected based on the detected clicks from a tagged whale or one recorded by a towed hydrophone. Echoes from targets form sequences of arrivals which appear in the echogram (Johnson et al., 2004; Johnson et al., 2006; Johnson et al, 2009; Madsen et al, 2013). For data collected by an animal-borne tag, echograms can be used to analyze how the animal was using echolocation to navigate and search for or capture prey.
In the example you provided, my understanding is that the energy along the x axis is that from the outgoing clicks produced by the tagged animal.
The energy and interval between clicks can vary- which may change what is seen in the echogram. My interpretation is that in Figure A the faint white line is the prey, getting closer to the porpoise before the buzz, and then attempting to escape just after the buzz. In both, the stacked wavy lines are the energy of the subsequent clicks.
I cannot explain the variations in resolution, but below I've provided some examples and references which I used to try to understand echograms.
Johnson, M., Madsen, P. T., Zimmer, W. M. X., Aguilar, A. and Tyack,
P. (2004). Beaked whales echolocate for prey. Proc. Roy. Soc. Lond. B 271,
Johnson, M., et al. Foraging Blainville's beaked whales (Mesoplodon densirostris) produce distinct click types matched to difference phases of echolocation. The Journal of Experimental Biology 209: 5038-5050.
Johnson, M., Aguilar de Soto, N., and Tyack, P. 2009. Studying the behaviour and sensory ecology of marine mammals using acoustic recording tags: a review. Marine Ecology Progress series 395: 55 – 73.
Madsen, P.T., Aguilar de Soto, N., Arranz, P., and Johnson, M. 2013. Echolocation in Blainville’s beaked whales (Mesoplodon densirostris). Journal of Comparative Physiology A 199: 451-469.
Figure 3: A) The envelope of the FM and buzz clicks during the approach and capture phase as a function of time from the end of the buzz. Note the dramatic change in acoustic gaze via a large reduction in amplitude (two orders of magnitude) and increase in sampling rate (two orders of magnitude) to reduce the auditory scene to a single target in the buzz. B) Echogram of the approach to and capture of a prey. All clicks are time aligned at range 0, and energy is then color coded as a function of the two way travel time (TWTT) and hence range from each click. Note how several targets can be seen in the approach phase with slow but power full FM clicks, and how the transition to a buzz leaves only a single target left for fine scale tracking. The spurious v-shaped structures at range 3m and 5.5 m in the buzz are generated by the next clicks. C) Inter click intervals (ICIs) of FM and buzz clicks showing the lack of TWTT adjustment in the approach phase, but close tracking in the buzz phase. The blue line is the TWTT derived from the echo delays in (B).
I have used echograms to study sperm whale dive behaviour, and specifically look for echoes off of the sea surface and seafloor. Below are some examples from my work.
Clicks produced by a tagged sperm whale descending from the surface were used to generate the echogram below. Outgoing clicks are visible along the x axis, and subsequent clicks produced by the tagged whale are visible. Echoes from the sea surface are visible. As the whale descends, the sea surface echo is recorded after longer and longer intervals.
Here is an echogram produced from clicks detected by a towed hydrophone array. Each vertical bin includes one of the detected clicks. By including 20 ms pre-click and 80 ms post click in each vertical bin, the plot reveals the multiple pulses of the sperm whale click and reflections from the sea surface.