| http://www.w3.org/ns/prov#value | - rom a set that consist of positions less than ??/4 away from said first end and positions approximately n??/2 from said first end, where ?? is a wavelength associated with the carrier frequency and n is a positive integer; and an acoustic receiver mounted on said tubing string at a second selected position relative to said second end, wherein said second selected position is selected from a second
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