Spectrograms of a single sperm whale click as recorded from a vertical line array of 4 hydrophones.

Why does it look like two clicks on the deeper hydrophones? Actually you are seeing one click as it arrives from its direct propagation path, and from its reflection off the ocean surface. The time between these two arrivals gets longer the deeper your hydrophone.

Also notice that the click is first received by the deepest hydrophone. Why? Where must the whale be? Somewhere deeper than the deepest hydrophone, right?