Stability of the penetrometer was obtained by having the center of 

 gravity 8. 5% of the body length below the center of buoyancy and by 

 having fins 8 feet up from the nose. The fins aid stability in two 

 ways: first, by providing a righting moment when the penetrometer 

 deviates from the vertical and, second, by helping to break up vortices 

 that may tend to form in the wake of the penetrometer. 



A hemisphere was chosen for the nose shape because it offered 

 minimum drag (Hoerner, 1965) and a relatively easy-to-manufacture 

 shape. 



Instrumentation Development 



Initial work on instrument concepts centered on the use of accelerom- 

 eters to measure the dynamics of the penetrometer. Two methods of 

 transmitting the accelerometer data to the water surface were investigated. 

 One method involved using a wire- link (similar to expendable bathyther- 

 mograph wire-links); the other, acoustic transmission. 



In the use of wire- links two problems were encountered: first, the 

 electrical performance of . the cable was found to be undesirable for the 

 purpose and, second, the design payout rate of the wire was only one- 

 fifth the payout rate required. Considerable engineering development 

 would have been required to overcome these problems. Therefore, the use 

 of expendable wire- links was not considered further. 



Acoustic transmission where the frequency of an acoustic projector 

 varied according to deceleration of the penetrometer was determined to 

 be feasible. The cost of such a system, however, appeared to be high 

 because of the number of components required: an accelerometer, a signal 

 conditioner, a power supply, amplifiers, and an acoustic projector. 

 Another problem with this system was that the required range of operational 

 frequency of the acoustic projector was very wide to achieve a high 

 signal-to-noise ratio. Considerable noise came from Doppler frequency 

 shifts caused by the change in the velocity of the penetrometer relative 

 to the listening device as the penetrometer entered the seafloor. 



An alternative to both of these potential instrumentation systems 

 was to utilize the Doppler shift of a constant frequency sound source on 

 the penetrometer to determine the velocity of the penetrometer as it 

 penetrated into the seafloor. The advantages of an instrumentation 

 system utilizing the Doppler principle were that no trailing wire was 

 required and that cost would be significantly less than the other 

 concepts studied. 



A prototype sound source was designed and fabricated. It consisted 

 of an acoustic projector, a power supply, and electronic circuitry to 

 control the frequency of the projector to an accuracy of plus or minus 

 one part in 100,000. The housing was an aluminum cylinder. A vacuum 

 test point for checking the 0-ring seals was placed in the base of the 

 housing. The instrument was designed to turn on when it entered the 

 seawater by using the seawater to complete a starting circuit. 



10 



