IN COOK STRAIT

Hokitika Guardian, 27 January 1932, Page 8

IN COOK STRAIT

■SEA BED REVEALED. matai makes tests. LATEST EQUIPMENT USED. .. WELLINGTON, January 25. Some suprising information concerning :the contour of the bed of the sea in Cook Strait was recently obtained by the Government lighthouse steamer Matai,' when a .series of depth tests was jmade by the use of a British Admiralty echo sounder. In the past, when it has been necessary to take soundings along the route of a. new cable across the Strait, a rough and ready idea of the rise and fall of the sea bed has been secured with the now obsolete lead and law. The .Matai, equipped with the latest type of echo sounder, recently made a ■urvey over the course of one of the telegraph cables across Cook Strait, and the recording chart showed such a mass of deep valleys and sharp peaks that officers of the Post and Telegraph Department who witnessed the test expressed surprise that it had oeen possible to lav and maintain cables on such rugged formation.. Captain H. 11. Hughes, who recently retired from the command of the cable steamer Iris, now known as the Recorder, was also on board the Matai, and, although he bad taken sounding l of the bed of the Strait for many years in the; course of his cable laying and repairing work, he stated' yesterday ‘hat the records obtained were a complete surprise to him. A distance of .almost 70 miles at a speed of.aboi.il nine knots wa-s covered by the..Matai in these tests.

HEAVING THE LEAD OUT-OF-DA'JT

When the Alatai went into commis sion at the end. of 1&30 she was fitted v\ ith a depth-sound :ng apparatus; but it could not be satisfactorily employed, mainly because the device was then in lie ti'ansicionaJ stage. Last year, however, Captain Hughes, while in England, made a study of the apparatus, and when he returned last Septe.mbei he brought with him a set of the latest equipment. That yet is now fitted to the -Matai. The tests carried out in Look Strait are considered to be entirely satisfactory. ihe new methods of depth finding all employ sound waves as a means oi measuring depths* thereby saving considerable time and trouble, and are steadily replacing the old method oi heaving the lead. A sounding in 4000 fathoms can be made in about ten seconds with the new "echo" methods, wherea s many hours of labour are. involved to obtain a somewhat dououu. result by the old lead system., Further, observations can bo made when the ship is travelling at speeds, up to 15 or 20 knots.

The echo method involves the measurement of distance in terms of the time interval between the initiation of. a sound impulse and the reception ot an echo. Sound is reflected from th sea bed in the same way that it is reflected in air from buildings and cliffs, with the difference that in air ■sound takes about five seconds to travel a mile, whereas in water the time is only about one second. Actually, in sea water, sound travels at the rate of '4BOO feet per second

SUBMARINE GRATERS DETECTED

The apparatus is electrically operates so that at the moment the sound is transmitted the hydrophone is not “listening," but it i s ready to receive the echo when it arrives. The transmitting apparatus emits three beats a second and takes 45 .soundings each minute. The soundings are electrics Jly recorded on sensitised paper, producing an accurate chart of the contour o. the bed of the sea.

The apparatus fitted to the Matai will record depths up to 133 fathoms, which is considered sufficient for ordinary navigation purposes a.s it enables vessels to pick up the 100-fathom line on approaching a coast. There is also an oceanic type recording up to about 4000 fathoms, which is sufficient to ascertain the depth of practically any part of the sea. Some of the records taken on the Matai are exceptionally interesting. All sections of the paper chart show tile marked irregularity of the sea bed, an I in places there is evidence of the existence of submarine craters. At such places the depth drops right off the chart, to be picked up again some distance, further on when the rising wall on the other side of the giant depression is met. Tire deepest of the craters i s just offshore to the south of l’ciicarrow .Head at the entrance to Wellington Harbour. Each strip of chart has along H s upper or water-level edge a series ol

sound markings quite distinct Irora the depth recordings. Those are the deck | sounds incidental to the working ol the vessel, caught by the sensitive apparatus.