Before the early part of the 20th century, the only known method of determining the depth of the sea at any point was by means of a cable. The ship stopped and a hemp line or wire with a weight at the end was lowered to the bottom. This was a slow, laborious task and not very accurate.
Today man has developed innumerable instruments that help him to go deep into the seas and acquire more and more knowledge about their bottoms. Do you know how the depth of the sea is measured with the help of modern instruments?
The equipment which is used to measure the depth of the sea is called a ‘fathometer’. It is installed on a ship. It produces sound waves having frequencies of more than 20,000 hertz. These are called ultrasonic waves. They cannot be heard by human ears. These waves are transmitted inside the sea. They are reflected back by the bottom of the sea. The reflected waves are received by a receiver and the total time taken by them to travel from the sea surface to the bottom and back to the surface is measured. Half of this time multiplied by the velocity of sound in sea water gives the depth of the sea at the point of the experiment. Thus the depth of the sea can be measured at any point. This technique is known as echo sounding or echo ranging.
This technique has been used to measure the depths of various seas and the deepest spot in each of the seas has been located using this technique. The Pacific Ocean is the deepest ocean. The average depth of this ocean is 4,282 metres (14,048 feet). The deepest spot in this ocean is located near Guam and has a depth of 10,668 metres (35,000 feet). Guam is an island in the Pacific Ocean. It is about 5,000 kms (300 miles) to the west of the Hawaii islands. A more recent survey located a spot with a depth of 10,924 metres (35,839 ft.) — as per a data obtained by Japan Maritime Safety Agency, using a narrow multi-beam echo sounder. Using depth as a criterion, the Indian Ocean occupies the second place. Its average depth is 3963 metres (13,002 feet). The Atlantic Ocean, coming third, has an average depth of 3926 metres (12,880 feet). Amongst the shallow seas, the Baltic Sea occupies the first place. Its average depth is 55 metres (1 80 feet).
Studies of the oceanic crust have revealed that they are also as uneven as the surface of the earth. They are full of the skeletons of the aquatic animals as well as big and small sea plants. Volcanic ash is also found there.
The most modern method of studying the oceanic crust is to use the laser beams. The laser beams are transmitted into the water of the ocean and the time taken by them from the surface to the bottom and back is measured. On multiplying the velocity of speed of light with this time, one gets twice the distance of the bottom of the sea. This method is very accurate.