u 



UDT . Underwater Demolition Team (USN) . 



UHF . Ultra High Frequencies (300 - 3000 megacycles). 



ULTRA-GRAVITY WAVES . Those waves having periods 

 ranging from 0.1 second to 1.0 second. 



ULTRA LOW FREQUENCY OCEAN WAVE RECORDER . The in- 

 strument consists of a five-inch stand-pipe mounted 

 vertically along a pier pile with its open end 

 fifteen feet beneath mean sea level. Most of the 

 pipe is filled with a highly refined petroleum oil. 

 The movement of the oil column is impeded by a 

 capillary. An air chamber above the air column is 

 connected to an air reservoir, the volume of which 

 can be readily adjusted. A fine capillary permits 

 leakage of air between the reservoir and the at- 

 mosphere. The remaining part of the instrument 

 consists of glass cylinders mounted above the deck 

 level of the pier. The differential between the 

 air pressure in the last glass cylinder and the 

 atmospheric pressure actuates a strain gauge pres- 

 sure transmitter, the output of which is recorded 

 on shore. The capillary in the stand-pipe tends to 

 suppress any short period vertical oscillation of 

 the oil column due to wind waves or swell. The 

 oil column does respond to the rise and fall of 

 the tides, but due to the leakage of air through 

 the air capillary in the reservoir, this very slow 

 movement does not lead to appreciable pressure 

 changes in the air pocket above the oil column. 

 The pressure changes in the air pocket result most- 

 ly from periods intermediate between those of the 

 swell and tides. The oil capillary connecting the 

 first two glass cylinders suppresses the transmis- 

 sion of short period waves to the second air reser- 

 voir, whereas, the air capillary in the third cylin- 

 der reduces pressure changes of long periods. Each 

 additional "stage" of the two cylinders thus has 

 the characteristics of a band pass filter. (30) 



ULTRAPLANKTON . See MARINE LIFE. 



ULTRASONIC FREQUENCY . An ultrasonic frequency is a 

 frequency lying above the audio frequency range. 

 Thus it extends from roughly 20,000 cycles per 

 second upward. (9) 



ULTRASONICS . Ultrasonics is the technology of 

 sound at frequencies above the audio range; i.e., 

 above 20,000 cycles per second. (9) 



ULTRASONIC SHIPBOARD WAVE HEIGHT SENSOR . A self- 

 contained electronic system to measure and record 

 the relative displacement of sea surface from mean 

 sea level. 



UN. 



United Nations . 



UNCC . United Nations Cartographic Conference. 



UNCONFORMITY ICEBERG . An ICEBERG consisting of 

 more than one kind of ice, such as blue water- 

 formed ice and NfVE. Such an iceberg often con- 

 tains many crevasses and silt bands. (17) 



UNCONSOLIDATED SEDIMENTS . Deposits consisting of 

 uncemented clastic or organic material. (27) 



UNDERTOW . A CURRENT, below water surface, flowing 

 seaward; also the receding water below the surface 



from waves breaking on a shelving beach. Actually 

 "undertow" is largely mythical. As the backwash of 

 each wave flows down the beach, a current is formed 

 which flows seaward, however it is a periodic phe- 

 nomenon. The most common phenomena expressed as 

 "undertow" are actually the rip currents in the 

 surf. Often uniform return flows seaward or lake- 

 ward are termed "undertow" though these flows will 

 not be as strong as rip currents. (11) 



UNDERWATER CAMERAS . Underwater cameras may be clas- 

 sified under two general categories; those operated 

 by divers and swimmers, and those that are lowered 

 with a winch and remotely operated by bottom con- 

 tact, messenger, or mechanical means. The first 

 category includes both still- and motion-picture 

 cameras using either color or black and white film. 

 The second category usually includes only still 

 cameras. One type is the single shot, taking only 

 one picture on each lowering. Another type is the 

 multiple shot, taking a series of pictures during 

 the lowering operation. 



The single-shot, bottom-contact camera is de- 

 signed to obtain photographs of the ocean bottom 

 at great depths. It will take one picture on each 

 lowering, and the camera shutter is tripped by 

 contact with the bottom. The tripping mechanism 

 obtains a small PHLEGER-TYPE core at the time of 

 tripping. The camera is contained in a heavy pres- 

 sure case capable of withstanding water pressures 

 up to 12,000 pounds per square inch. (35) 



'JNDERWATER SOUND PROJECTOR . An Underwater Sound 

 Projector is a transducer used to produce sound in 

 water. The conversion from electrical energy to 

 sound is achieved through the use of either the 

 piezoelectric or magnetostrictive effect. When a 

 piezoelectric material is used to generate under- 

 water sound, the electric potential across the 

 sample material is varied periodically at the fre- 

 quency of the desired sound signal. Under this 

 condition the piezoelectric material vibrates 

 mechanically at this frequency; this vibration when 

 properly coupled to the water, produces the sound 

 of the desired frequency in the water. 



If a rod of ferromagnetic material is brought 

 into a magnetic field parallel to its ion axis the 

 length is changed. This effect is called magneto- 

 striction. Magnetostriction is applied to the 

 production of sound waves as follows: a nickel rod 

 is subjected to an alternating magnetic field by 

 winding a coil of wire around it and sending an 

 a.c. current through the coil. The rod is shorten- 

 ed periodically in response to the changing field. 

 The natural frequency N of a rod of length L is 

 1 /El 1/2 E - Modulus of elasticity 



N - 22 y?'' 



(30) 



density of material 



UNDERWAY BOTTOM SAMPLER. See SCOOPFISH. 



UNDISTURBED WATER LEVEL . See STILL WATER LEVEL. 



UNESCO . United Nations Educational, Scientific and 

 Cultural Organization. 



UNIDIRECTIONAL HYDROPHONE . A unidirectional hydro- 

 phone is a hydrophone that is responsive predomi- 

 nantly to sound incident from a single solid angle 

 of one hemisphere or less. (1) 



UNIDIRECTIONAL TRANSDUCER . In many practical trans- 

 ducer designs the active surface is mounted in a 

 structure in such fashion that radiation or recep- 

 tion takes place on one side only. In such cases 

 the unit will show maximum response along a single 

 .bearing only and is said to be unidirectional. (4) 



UNILATERAL TRANSDUCER. A unilateral transducer is 



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