If it is difficult to measure the depth of the ocean (see enigma 2), how can we get an idea of the structure of the seafloor before coring ?
We succeed to penetrate the acoustic signal in the rock layers of the ocean floor. The signal entering  in the deeper layers will be reflected at each discontinuity in the structure. For this, we made use of explosives … the signal can be recorded directly on the bottom of the ocean by seismometers or on the boat by drift sensors.

Thus, we can hope to ‘look’; the physical structure of the seafloor. The seismic reflection process reveals layers. Of course the technique is successful when there is discontinuity in the nature of the basement rocks. This difference is manifested by a change in the properties of wave propagation in these rocks. We observe a succession of ‘reflectors’; corresponding to different geological layers superimposed. Something like X-ray image of the seafloor.

It remains to identify geological structures highlighted. One approach would be to know the speed of compression waves generated by explosive when they are running through different rocks.

In sea water (see enigma 2), the waves velocity is 1500 m / s. What happens through the rocks of the ocean floor?.

See below a new enigma !