This chart presents the electrical signal from the underwater SOSUS array.
It is listening to ship traffic (and presumably with more interest in submarines). The scale across the bottom reveals a span of 8 or minutes of recording in hardcopy, colored format (in the old days, it was all black and white). Above the minute marks the scale at the left reveals the frequency of detection, within the field of color, the brighter the color, the louder the source.
As such, the span from very low frequencies up to 50 Hz (a power line kind of hum) has a bright patch across all time with very white (thus very loud) source noise from propellers that scar the field with horizontal lines showing at 30 Hz and 40 Hz (and other frequencies).
The picture below is more representational of the LOFARgrams observed during the 50s, 60s, 70s …. The aspect is as viewed on the Vernier (chart recorder) with the paper being advanced up as new data was burnt (no ink, the paper is thermally sensitive) into a line at the bottom. In this case, louder is equal to darker (more burning). With many Verniers running plots along bearing angles, there was a lot of burning going on, continuously.
Not labeled are the propeller shaft noise profiles. The shaft’s fundamental frequency is at the far left, about an eighth of an inch from the edge. Its first harmonic is an eighth of an inch to its right (and many more barely visible harmonics across the page).
As of this time, the picture is incorrectly labeling the propeller blade fundamental as an harmonic. Given its harmonics fall within groups of four shaft harmonics, it would seem to me that there were four blades on the shaft.