物理超聲波問題!!

Ultrasonic waves are used due to their short wavelength. Since the frequency of ultrasound is very high, the ultrasound waves can be sent out in the form of a beam. It is thus possible to radiate out a large amount of energy to a distance point at the ocean bottom.
If ordinary sound waves are used, the sound waves would spread out. Because of the absorption of wave energy by the sea water, the waves weaken rapidly. The reflected waves from the bed of the ocean would become too weak to detect.

主要原因:
水分子能吸收任何一重電磁波的能量(橫波)
但不會吸收超聲波的能量(縱波)

所以不會利用電磁波來測量海洋

次要原因:
縱波的速度在水行得比空氣中快