A monitoring device is attached to one end of a long steel rod that is partially submerged in sea water. When a hammer strikes the other end of the rod, sound travels simultaneously through the steel and through the surrounding water toward the device.
(i) Derive, in symbols only, an expression for the time difference Δt between the arrivals of the two sound pulses at a distance d from the point of impact, given that the speed of sound in steel is vsteel and in sea water is vwater.
(ii) Using vsteel=6000m/s and vwater=1500m/s, calculate the minimum distance dmin for which the sound through water will arrive at least 2s after the sound through steel.
(iii) Explain two practical advantages that this time separation provides to engineers who need to distinguish between vibrations travelling through solid structures and those travelling through the surrounding water.