[JockoManning]

Those frequency graphs don't look all that single frequency like, to this ignorant layman.

What do the geologists say?

Seems to me if the cause was natural like a volcano--the frequency graphs would not be ONE frequency. But what do I know.

[Cletus.D.Yokel]

Well, actually on my first glance, the “period” (peak to peak) DOES look consistent. Compare that to the surrounding “peaks that appear very close together.

While the peak ^heights^ are inconsistent. This indicates the true attenuation of the water, density layers and the doppler effect.

[AZLiberty]

Those frequency graphs don't look all that single frequency like, to this ignorant layman.

The graphs show not frequency but amplitude (the size of the jiggle). I presume the black peaks before 9:36 and after 9:46 are background noise. The busy stuff in the middle is presumably the signal of interest. If I measure between the peaks of that signal, I find that they are roughly 17 seconds apart (+-2 seconds given the resolution), consistent with what others are saying.

[Not a geologist, but the university I graduated from calls me an electrical engineer.]

[amorphous]

Those frequency graphs don't look all that single frequency...

They are unusually regular. Look at the spaces between pulses (Frequency/Wave period).

Agree they vary quite a bit in peak amplitude( distance up & down from the horizontal center in layman's terms).

Mo info:

1. Peak amplitude
2. Peak-to-peak amplitude
3. Root mean square amplitude
4. Wave period (not an amplitude)