Report for Analytical Chemists
sûrement. Wind and wave conditions typically influence the temperature of this thin layer by as much as 0.5 ° C , even though the measuring system is inherently capable of a precision better by fivefold. Wide area temperature surveys are of considerable practical value. As a single example, .consider the herring industry, which is of much economic significance to Iceland. These fish feed in the zone of demarcation where the Atlantic and Polar Waters meet in the Norwegian Sea, and the exact location varies from year to year. In 1967, this region was so far from the mainland of Iceland that much of the harvest spoiled on the return trip. In 1968 an aerial system of remote temperature sensing was employed, by means of which it was possible to ascertain in twelve days the sea-surface temperatures around the entire island of Iceland out to a distance of 100 miles. The practical values are considerable, both in rapidly locating where the fish are and also in providing a basis for making appropriate arrangements for storage and transport back to the home base. Mapping of the ocean bottom has been accomplished for twenty-five years primarily by sonar methods, in spite of the fact that the velocity of sound is dependent somewhat upon the temperature of the water and by other conditions which may not be constant nor even known. An interesting alternative system, the "laser range gate," has been developed by Electro-Optical Systems. A pulse of laser light replaces the sonar emission. The approximate time for this pulse to return from the ocean floor is calculated in advance, and the receiver "gate" is opened only briefly at this time, thus minimizing the background "noise" from other random reflections and scatterings. Sonar systems to locate areas of high plankton density have been quite unsuccessful because of the fact that the sonar wavelengths so greatly exceed the typical sizes of plankton that the sonar scattering cross sections are virtually nil for most forms of plankton. Early attempts to use visible light, with its shorter wavelengths, have been
largely unsuccessful because of such factors as poor transmission through sea water, background light from other sources, bioluminescent organisms, and so forth. Accordingly, conventional methods of "analyzing" directly for plankton consist of sampling with nets and bottles, necessarily limiting the analyses to very small volumes or regions at a time. Once again, a new instrumental system employing laser light appears to be very promising. This device, called the "lidar," is essentially a device for radar-like plotting of three-dimensional maps of "clouds" of plankton in water at considerable depths. The lidar employs blue-green light, at 0.53 micron, which radiation is short enough for appreciable scatter from plankton and which penetrates water to a reasonable extent with minimal Rayleigh scatter from water molecules. This particular apparatus repetitively emits short pulses of high peak power at this sharply defined wavelength.' The photomultiplier detecting system is coordinated with the emitted pulses, so as to minimize "noise" from other sources of light. The fact that laser light is coherent is not significant in this application. The significance of this, and other newer methods of rapidly collecting masses of analytical data may be illustrated by the following statement which concludes an article on the lidar. "At a pulse repetition rate of 5,000 pulses per second and with one-meter resolution cells, it appears that in one minute of operation an airborne océanographie lidar will sample more volume elements of the ocean than have been sampled by all the net-and-bottle, scattering meter, and transmissometer measurements to date. A good many surprises may be in store" (10). A very versatile deep sea instrument capsule has been developed for the collecting of data on physical properties at or near the ocean floor (11). An unmanned, self-contained capsule is dropped to the ocean floor from a surface ship. For a predetermined period of time, ranging from days to months, data are automatically collected by appropriate sensors and recorded on
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VOL. 4 1 , NO. 7, JUNE 1969
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29 A
