and the radon counters can be operated in the manual mode. Backup 

 nutrient analysis facilities will be provided by manual spectrophotometry. 



In-Situ Underwater Measurement 



In addition to measurements on board ship from bottle samples, an 

 underwater sensor package has been developed which includes : a) Con- 

 ductivity, temperature, and depth sensor; b) Bottom proximity detector; 

 c) dissolved oxygen; and d) nephelometer. The package consists of eleven 

 30-liter sampling bottles in a rosette. The bottles are tripped individually 

 by an electronic signal from the surface. Confirmation of each trip is 

 provided. 



a) Conductivity-Temperature-Depth 



The CTD sensor is an adaptation of one constructed at the Woods 

 Hole Oceanographic Institution. The instrument was originally intended 

 for microstructure work and has a high rate of data accumulation. The 

 CTD profile is fed to the shipboard computer. 



b) Bottom Proximity 



The bottom proximity detector is a special acoustic device used to 

 measure vertical distance between the sensor package and the sea floor 

 in the bottom 500 meters of the water column. It has a resolution of 

 0.5 meters. This instrument is required because of noise pollution gen- 

 erated by RV's Melville and Knorr. The unit transmits and receives the 

 ping, converts the delay into a digital signal, and transmits this informa- 

 tion up the wire along with the CTD and other data. 



20 c) Dissolved Oxygen 



An oxygen sensor that works below 2,000 meters is not available 

 commercially. Investigations have been conducted of the various sensors 

 available. Results show that the membrane-limited polarographic type is 

 the best available, barring a completely new development program, and 

 will serve as the basis for the GEOSECS probe. Preliminary investiga- 

 tions indicate that the precision is better than one percent, with a lower 

 detection limit of 0.05 ml/1 of oxygen. To obtain this accuracy, calibra- 

 tion facilities will be maintained on board ship. 



d) Nephelometry 



To detect suspended particulate matter a nephelometer is included in 

 the underwater sensor package. The nephelometer uses a ruby laser beam 

 light source. A resistive photocell placed just outside the beam detects 

 forward-scattered radiation, and the response is a measure of the par- 

 ticulate matter in the water column. A similar photocell placed in the 

 back-radiating beam of the laser monitors the emission. 



Data are transmitted from the underwater sensor package up the 

 coaxial lowering cable by frequency shift keying. On ship, a preprocessing 

 computer converts the data to appropriate engineering units, screens the 

 raw data for bad points, and averages over depth intervals of one meter. 

 These data are then transferred to the shipboard computer for storage 

 and display. Various displays are required. Temperature, salinity, 

 oxygen, and light scattering are displayed as a function of depth; tem- 

 perature is displayed as a function of salinity; and density is displayed 

 as a function of depth. A diagram of the underwater sensor package is 

 shown in Figure 9. 



