SEA BED REVEALED.

New Zealand Herald, Volume LXIX, Issue 21087, 22 January 1932, Page 9

SEA BED REVEALED.

UNIQUE METHOD USED. TESTS IN COOK STRAIT* STRANGE CONTOURS FOUND. THE MATAI'S NEW APPARATUS. Some surprising 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 testsi was made by the use of a British Admiralty echo sounder. In the pafct, when it has been necessary to take soundings along the route of a new cable across the strait, a rough and ready idea of tho rise and fall of the sea bed has been secured with the now obsolete lead and line. Tho Matai, equipped with the latest type of echo sounder, made a survey over the course of one of the telegraph cables across Cook Strait on Thursday of last week, 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 been possible to lay and maintain cables on such rugged formation. Captain H. R. 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 ho had taken soundings of the bed of the strait for many years in the course of his cable laying and repairing work, he stated yesterday that the records obtained were a complete surprise to him. A distance of almost 70 miles at a speed of about nine knots was covered by the Matai in these tests. Heaving the Lead Oukof-Date. When the Matai went into commission at) the end of 1930 she was fitted with a depth-sounding apparatus, but it could not be satisfactorily employed, mainly because the device was then in the transitional stage. Last year, however, Gaptain Hughes, while in England, made a study of the apparatus, and when" he returned last September lie brought with, him a set of the latest equipment That set is now fitted to the Matai. Tha tests carried out in pook Strait are considered to be entirely satisfactory. The new methods of depth finding all employ sound waves as a means of measuring depths, thereby saving considerable time and trouble,, and are steadily replacing the old method of heaving the lead. A sounding in 4000 fathoms can be made in about ten seconds with the new " echo " methods, whereas many hours of labour are involved to obtain a somewhat doubtful result bv the old lead system. Further, observations can be made when the ship is travelling at speeds up to 15 or 20 knots. r : The echo method involves the measurement of distance in terms of the time interval between the initiation of a sound impulse and the reception of an echo. Sound is reflected from the sea bed in the same way that it is reflected in air from buildings and cliffs, with the difference thai; in air sound takes about five seconds to travel a mile, whereas in water the timu is only about one second. Actually, in sea water, sound travels at tho rate of 4800 feet per second. Submarine Craters Detected. The apparatus is electrically operated so that at the moment the sound is transmitted the hydrophone is not listening,"but it is ready to receive the echo when it arrives. The transmitting apparatus emits three beats a second and takes 4J5 soundings eath 'minute. The soundings are electrically recorded on sensitised paper, producing an accurate chart of tho contour of 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 as 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 taken on the Matai are exceptionally interesting. All r sections of the paper chart show the marked irregularity of the sea bed, and 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 w hen the rising w r all- on the other side of tha giant depression is met. Th§ deepest (if the craters is just offshore to the south of Pencarrow at the entrance to Wellington Harbour. Each strip of chart has along its upper or water-level edge a series of sound markings quite distinct from the depth in cording?. Those are the deck sounds incidental to the working of the vessel, caught by the sensitive apparatus.