cable. This position is estimated by extrapolating backward along the 

 path of the vessel. Assuming a position for the detector vehicle at 

 the beginning of the survey, the position of the detector at each fix is 

 computed by linear interpolation from the present position of the vessel 

 to the last computed position of the detector. The distance the detector 

 lies astern of the survey vessel depends on the distance from radar mast 

 to the stern post, where the cable is attached, and on the horizontal 

 component of the cable which is a function of water depth. Because 

 present field operations are conducted in very shallow water, or at 

 nearly the same water depth, a constant depth value is used. 



Before processing through RAPLOT, each set of data is visually 

 scanned and an estimated background count is determined and entered on 

 the data control card. The count is used to set an upper and lower bound 

 on the range of background radiation. All counts between these bounds 

 are averaged to obtain the mean background count. Background is sub- 

 tracted from the radiation value of each fix, and the remainder (if 

 greater than zero) is corrected for decay since the time of injection. 



CC = NC (e Xt ) 



where A 



natural log of 2 (0.693 ' 



Tk 



CC is the corrected count, NC is the net count (observed radiation value 

 less background), t is the elapsed time from the injection to the count, 

 and Tjg is the isotope halflife in hours. 



The output from RAPLOT is in two forms . The first is a printed out- 

 put which gives all of the values of the control parameters used to con- 

 trol the computation of the data and the flow of the program. Also 

 included are some summary statistics on the radiation values and the 

 position of the center of the radioactive material (analogous to the 

 center of gravity of a mass ) . The final part of the printout is a list 

 of the fixes, which gives the fix number, the coordinates of the fix in 

 the California Lambert Coordinate Grid and the corrected radiation value. 



The second type of output consists of maps drawn off-line on a 

 plotter driven by a plot tape generated by RAPLOT. There are three 

 types of maps. The first is a plot of the trackline followed by the 

 survey vessel. The second is a plot of uncorrected radiation values. 

 The third shows the radiation values corrected for background count and 

 decay since injection. The two radiation maps plot the fixes after they 

 have been corrected for the distance that the detector is towed astern 

 of the survey vessel. The choice of which maps to produce, as well as 

 scale and format, is controlled by values fed in on the plot control 

 card. A Fortran V source language listing of the RAPLOT program is 

 found in Appendix D. 



Position of the detector is computed at each fix time by linear 

 interpolation from the present position of the survey vessel to the 



24 



