PROBLEM OF THE GREAT AUSTRALIAN ARTESIAN BASIN. 189 



There have been instances again in which new bores have 

 caused either a diminution or cessation in yield in one or 

 more of the adjoining wells, as about Longreach (II, §§ 

 2135-2147) and Saxby Downs (II, §2807). 



An approximate solution of the interesting and practical 

 problem of the limiting radius of interference of a group 

 of bores can be obtained in the following way. 



Firstly, assume that the wells are arranged at equal 

 distances R apart, in a chain parallel to the isopotentials, 

 and that the effective velocity of the underground flow is v, 

 and the thickness of the water-bearing beds t. Slichter 1 

 taking the case of a stratum in which there is a regular 

 flow in one direction, has worked out the plan of the lines 

 of stream-flow when water is being removed by a single 

 well, and it will be observed on examining his graphic pres- 

 entation, that all the water is being drawn from between 

 two limiting lines of flow which are at first parallel, and 

 then converge, and close upon and around the bore; the 

 water further to right and left escapes. The captured 

 portion may be considered as entering with the normal 

 velocity v upon an arc of radius r and centre a little beyond 

 the bore, within which its speed increases rapidly, and it 

 will be found that this sector subtends an angle of about 

 120°, and that r will about equal R/2. 



The water withdrawn by the bore crosses a sector of a 



2 7? 



cylinder -n x -— along the length of arc and t in height; 

 o A 



let this equal Q. 



Then Q =^rr Rtv. 

 o 



If Q = 1,000,000 gallons per day, t = 300 feet, and v = 

 150 feet per annum or about 0'4 feet per day, then ft = 

 1,273 feet or almost one-third of a mile. 



1 C. S. Slichter, U.S.A. Geo!. Sutv. 19th Ann. Rept., pt. ii, pi. 17, 1899. 



