Early Techniques and Problems

Most of the developments in the techniques of exploring the sea bed have been witnessed over the last 150 years. However, it is known that the Vikings – such as Eric the Red, used ‘sounding weights’. These were lead weights attached to a rope – or line, which were lowered to the bottom from the side of the boat. Often, the weight had a hollow bottom so a sample of the sediment could be collected. It was a straightforward matter to measure the length of line lowered in order to estimate the depth. The line length was divided up into sections which were equal to a man’s arm span from fingertip to fingertip, a length of 1.83m [ 6 feet], this measurement was called a fathom, a term which is still used today.

This method, of taking ‘soundings’, was used until the mid 1800s. During the mid 19th century governments and scientists launched systematic surveys of the world’s oceans. You can read all about the Challenger Expedition here.

Consider this...

	Imagine yourself to be a scientist on the Challenger - you are in a small boat, possibly in rough water with currents flowing and the wind blowing.. What would be your concerns about the accuracy of your depth measurements?
	How accurately do you know your position? Limitations of navigation techniques and the effects of currents and wind would cause some uncertainty about your position.
	How accurately would you know the depth? The effects of the waves and tide would create a range of depth measurements even in the exact same spot. Added to this is the possibility of the ship drifting off position and making the line go down at an angle. Also, as the lines were so long, some stretching may occur.

Developments over the last 150 years

Scientists on board the Challenger Expedition (1872-1876) took a huge amount of measurements and made a phenomenal amount of discoveries. They took many samples of sea life, discovering lots of new species. They also took many measurements of the sea’s temperature, salinity and depth.

The Challenger was the first ship to be recorded as using wire released from a special machine, for taking soundings. This was a step forward as, up until then, the ropes were made of hemp. On a separate expedition the hemp rope hold became flooded and the rope rotted. Many of the soundings and dredges taken with this rotten rope were lost as the rope broke!

Shortly after this time improved wire sounding machines were designed, notably one by Sigsbee, which was used to produce a bathymetric map of the Gulf of Mexico.

During this period, another technique to find out about the seabed was to use a dredge. This is a large scoop, usually with a metal leading edge, attached to a line. It is simply pulled along the bottom of the sea by the ship and the dredge bucket is then winched back to the surface and the catch explored. Early lines were made of rope but these were replaced by wire during this period. The method has its limitations but can be useful in sampling the sediment or surface rock and slow moving animals. In 1866, the deepest recorded dredge was to 300 fathoms (550m), by 1869 this had been extended to 2400fathoms. The huge surprise was that living specimens were found even at this depth. Dredges are still used on research vessels today to take samples of the seafloor.

Sigsbee sounding machine in operation (1880)	Dredging for seafloor rocks on a modern research vessel	Dredging brings up rocks and benthic creatures

Consider this...

	Can you think of three advantages of using wire instead of hemp for the dredging line?
	Can you think of three disadvantages of using wire?
	Estimate how much wire, in metres, you would need to take a dredge at a depth of 2400 fathoms. [ 1 fathom = 1.83m, assume the angle of drag to be 45 degrees.]

Echo sounding

The first tests of echo sounding took place in 1919 by the French. It was only three years later in 1922 that echo sounding was being used on ships to survey the sea bed for cable laying. Echo sounders can also be called fathometers.

Echo sounders work using the relationship: Distance = speed x time

As the ship travels on the surface of the water, a transducer on the hull of the ship emits a pulse of sound, a ‘ping’. As you know, sound reflects off hard surfaces – we call this an echo. The ping is reflected back from the seabed, to a receiver on the ship’s hull and the time taken for this to happen is recorded by the device. It is then able to automatically calculate the depth by using the equation above and this is converted into a picture on a display screen or into a pen trace onto a roll of paper. In this way the ship can plot the depth of the sea along a particular line. A simple echo sounder uses a cone of sound which spreads out at about 5 degrees as it moves from the ship.

It is important to know the speed of sound in sea water. As the water particles are closer together than gas particles in air, sound travels faster in water than in air – at about 1500m/s. It is possible to use different frequencies of sound for different purposes. For measuring depth the frequency of 12kHz is used. Scientists discovered that low frequency sounds have a greater penetration of the sea bed, a frequency of 3.5khz can give information about the material up to 50m below the surface of the sea bed.

Consider this...

	Will sound travel at the same speed in tropical waters compared to the cold polar waters?
	In using echoes to measure distance, you must halve the time taken for the original sound to be detected as an echo. Why is this?
	Calculate the depth if the time between the signal being generated and the echo being detected is 0.5s and the speed of sound in water is 1500m/s. Will sound travel faster or slower through rock than through water?