MODERN DISCOVERIES

Sun (Auckland), Volume III, Issue 685, 10 June 1929, Page 14

MODERN DISCOVERIES

SURVEYING BY SOUND ECHO FROM OCEAN BED Included in the multifarious ser- i vices of the Empire Marketing Board, upon which £1,000,000 a year is spent, | are many important researches into what lies underneath the surface of j the Empire as well as that which j happens on it. These deal with geo- j logical surveys on novel lines, and : surveying the ocean depths by sound. j One of the important new methods is that of seismological survey, which is in principle a straightforward matter, says a writer in the “Manchester Guardian.” It resembles a method of finding the depth of the sea by firing a blank cartridge just below the surface and measuring the time that elapses before the echo arrives from the bottom of the sea. There is an echo, because the sound waves, which travel much like the water waves we see, are turned back, or reflected, at the bottom of the sea. Sound waves are reflected at any surface where there is an abrupt change in the velocity of sound, just as light waves are reflected by glass because light travels faster in air than in glass. Suppose, then, that a charge is exploded in a cavity just below the ground; it will send out waves In all directions through the earth. These may be called sound waves, though actually two types of waves' can travel through a solid, and only one type corresponds to sound waves In air or water. Suppose further that, somewhere below the cavity, the surface rock gives place to another in which the velocity of sound is different; here the waves will be reflected and will return to the surface. The time the waves take in their double journey through the surface roclt can be measured; sue velocity of the waves Is known, and therefore the depth of the second rock can be found. Looking For Ore The geologist may be looking for ore imbedded in rock, or for oil imprisoned below limestone, but he is inevitably looking for something that differs entirely from Its surroundings, and consequently for something in which the velocity of sound Is different from what it is in the immediate neighbourhood. The method of listening for echoes is therefore applicable. The difficulties to be overcome are, however, considerable. We obtain a clear, undistorted image in a mirror because there is a perfectly definite surface separating the air and the glass—there is no doubt where the air ends and the glass begins—and because this surface is plane and not curved. This leads to regular reflection of the light waves. No man would elect to shave in a mirror with hills and valleys and cracks and so many air bubbles that it was really difficult to discriminate between the end of the glass and the beginning of the air. But whereas we can choose our shaving mirrors, the geologist has no choice in the nature of the surface that separates his objective from its surroundings. His. exploring sound waves will usually be reflected irregularly, and he can obtain information only by correlating the results of a number of artificial explosions. Sound Waves Seismological survey relies on the circumstance that ore differs in regard to the propagation of sound waves from the rock that surrounds it; hut the ore will differ in other respects, and upon any such differentiation a method of survey can be based. A metallic ore is, for example, an altogether better conductor of electricity than the ordinary rocks; if, therefore, an exploring current of electricity takes the place of the exploring’ sound waves, the proximity of ore will be revealed by irregularity in the flow of electricity.