Guerre di meraviglie

BERTHED: at her home port, Woods Hals, Masa, the research ship Atlantis was taking on cargo for another trip to sea. It was a most peculiar cargo. Heavy aluminum-alloy cylinders about three feet long looked warlike, but turned out to be harmless. Inside were complicated seismograph instruments, tightly water-sealed, later to be lowered three miles down to ocean bottom. Nearby on deck was another pile of cylinders, also harmless. But each had a running mate which if handled with undue carelessness might mean sudden death. They housed batteries and time clocks. And to each before lowering will be attached a deadly bomb. When the Atlantis reaches suitable experimental waters out at sea she will heave-to. At stated intervals apart, the heavy and expensive equipment is lowered piece by piece to bottom. Presently, Dr. Maurice Ewing, pioneer in development of the new seismic method, demonstrates that the arts of seismography and bombing can apply themselves to ends less destructive than earthquakes and war. Explosions three miles down can’t do any harm. The whole point of this unusual proceeding is to record the sound vibrations sent out by each of several submerged explosions. Called seismic waves, they are the same sort of vibrations which on a larger scale are so widely distributed by an earthquake. Dr. Ewing wishes to learn in each case the direction in which the seismic waves travel, where they go and more particularly, how long it takes them to get there. He may discover in a certain instance that after having traveled at a velocity of about 20,000 feet per second they have suddenly slowed down. Then, after having penetrated the earth’s submerged crust for some distance at this lower velocity they have quickly picked up speed again and proceeded at the original rate, Nor does he have to wait long to read an illuminating travel-log of these speedy seismic waves. All the time a ticker tape in each deeply submerged seismograph has been steadily taking it down. Dr. Ewing reels off a few yards of seismogram and plots a graph. He then knows where the seismic waves have gone and the varying velocities of speed at which they traveled. “There are two distinct methods of recording the waves,” he explains. “One is the seismic reflection method, sometimes termed reflection shooting. These are the vertical waves. Following an explosion on sea bottom they take a course directly downward. Then, upon encountering changes in rock strata, they are reflected, bounced upward. Aboard ship we get an echo. It is the water wave meeting surface of water. In fact there is a series of echoes. This means the sound from the explosion is being bounced back and forth between water’s surface and the bottom. “The other method of recording waves is known as the refraction method. The waves in this case are essentially horizontal in character. That is to say, they penetrate downward for a certain distance, but before coming to the water’s surface they travel horizontally for quite a considerable distance. More important results have come from these refraction measurements than from reflection shooting. However, both methods have proved satisfactory.” The heavy and expensive apparatus must be retrieved from three miles down on ocean bottom before the story told by the waves can be learned. “Until recently we used a cable from ship for lowering and hoisting,” says Dr. Ewing. “It proved a constant source of trouble. Roll of the ship was the chief difficulty. This was transmitted down the cable to seismographs resting on bottom. We would pay out excess cable to better matters but a slack cable takes kinks when hoisting begins and is quickly damaged. The only satisfactory solution, we realized, was to do away with wires and cables.” After much experimentation, the eventual solution was found in the invention of an unusual type of float. These consist of black rubber casings seven feet long, each filled with about 25 gallons of gasoline to make for buoyancy. To the float is attached sufficient ballast to overcome buoyancy and to sink the float together with the attached apparatus to bottom. After explosions are over a release drops the ballast, and the float brings them up again to water’s surface, where they are retrieved. Early experiments conducted by Dr. Ewing in an old limestone quarry served as beginnings for important results to follow. He discovered that seismic waves would travel through limestone with a velocity of 19,600 feet per second until encountering a zone of irregularity where they would slow down. The slower zone meant softer material. The harder the material, the greater their velocity. These experiments have provided geologists with much valuable data. In the economic field a new science known as “seismic prospecting” has taken a definite and firm hold. If you plan to drill for oil or even dig a deep ditch it may prove an asset to know the relative hardness or softness of the material you will get into. As for the high seas, it is only five years since the first experiments aboard the Atlantis were launched. Yet, geologic findings have already been noteworthy. The seismic method has scored as a proved working tool in a field where proved working tools are few. Considerable attention has been devoted to instruments for undersea exploration. An outstanding achievement of the past was developed because of the U-boat menace during the last war - the sonic system of sounding for ocean depths, now of inestimable value to all navigators.