by the computer. A precision of 0.5% is 

 obtained. Calibration from leg to leg is well- 

 maintained. 



Radon — Excess radon gas is measured in 

 both surface and near-bottom profiles at each 

 station. It is extracted from samples by flush- 

 ing with helium and freezing at liquid nitrogen 

 temperatures. Four extraction systems are in 

 use at all times, and the extraction time is 

 approximately two hours. Following purifica- 

 tion to remove water and carbon dioxide, the 

 radon gas is transferred to a counting system 

 consisting of eight scintillation tubes. 



All shipboard automatic systems are de- 

 signed and programmed so that analyses may 

 also be done manually. 



In Situ Underwater Measurement 



In addition to measurements on board ship 

 from bottle samples, scientists use a newly 

 developed underwater sensor package (Fig- 

 ure 5] which includes: Conductivity-tempera- 

 ture-depth (CTD] sensor; bottom proximity 

 pinger, dissolved oxygen probe, nephelometer 

 and a rosette of 30-liter sampling bottles. 



BOTTOM PROXIMin 

 . PINGER" 



* fl on deep cast, 12 on other cast 



• Only on Oeep caal 



CTD SENSOR PBOItCIION CAGE 



Figure 5 GEOSECS underwater sensor package. 



Conductivify-Temperature-Depth. The CTD 

 sensor is an adaptation of one constructed at 

 the Woods Hole Oceanographic Institution. 



The instrument, originally intended for micro- 

 structure work, has a high rate of data accu- 

 mulation. The CTD profile is fed to the ship- 

 board computer. 



Bottom Proximity. Bottom proximity is de- 

 tected by a 12 KHz pinger mounted on the 

 rosette and monitored with a precision depth 

 recorder on board the ship. 



Dissolved Oxygen. An oxygen sensor that 

 works below 2,000 meters is not available 

 commercially. In an effort to overcome this 

 limitation, investigations were conducted on 

 the various sensors available. Results show 

 that the membrane-limited polarographic type 

 is the best available and will serve as the basis 

 for the GEOSECS probe. Preliminary investi- 

 gations indicate that the precision is better 

 than one percent, with a lower detection limit 

 of 0.05 ml/1 of oxygen. To obtain this accu- 

 racy, calibration facilities are maintained on 

 board ship. Figure 6 illustrates the results 

 taken with the O2 probe to a depth of 5,000 m 

 and compared with laboratory determinations 

 using the Carpenter-modified Winkler titrator. 



DISSOLVED OXYGEN PROFILE 

 GEOSeCS STATION El - SOUTH ATLANTIC 137 S 45°W| 



-JSOO 

 -4000 

 -«500 



-MOO 



BT DISCRETE 

 SUPLIIIC 



i 400 1 GOO 4 IX 



OXYGEN CONTENT {ml It 



Figure 6 



NepheJometry. To detect suspended par- 

 ticulate matter, a nephelometer is included in 

 the underwater sensor package. This instru- 

 ment uses a He-Ne gas laser beam light 

 source. A resistive photocell placed just out- 

 side the beam detects forward-scattered radia- 

 tion, 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. 



13 



