DISCUSSION 



Several conclusions can be drawn from fig- 

 ures 7 through 10. First, the presence of a void 

 surrounding an access tube is most easily de- 

 tected when the porous medium is saturated; 

 this condition gives an error of the largest mag- 

 nitude as shown in figures 7A, 8A, 9A, lOA, 

 and table 2. The magnitude of the error at any 

 level, and the magnitude of the total error, de- 

 crease as the water level falls with respect to the 

 void. If the void is empty and the porous 

 medium drained, the magnitude of the negative 

 error is significantly less than the magnitude of 

 the positive error. 



The effect of the two densities of porous 



media is shown in figures 7A, 8A, 9A, lOA, and 

 table 2. The material that has the highest bulk 

 density gives the smallest value of positive 

 measurement error and the lowest value of 

 negative error. Figures 7A and 8A show that 

 the total pore space of the coarse sand is 45 

 percent and figures 9 A and lOA show that the 

 total pore space of the fine sand is 40 percent. 

 The increased amount of water in the coarse 

 material caused a higher positive error. The 

 smaller values of negative errors associated with 

 the fine material were probably caused by re- 

 duced pore space. 



14 



