| http://www.w3.org/ns/prov#value | - A tracer solution suitable for use in measuring flow velocities in a borehole, includes: a) a gadolinium salt of a carboxylic acid of general formula ##STR1## where R is an alkyl of not less than four carbon atoms, typically C4 or C5 alkyl, and R1 and R2 are hydrogen or an alkyl group, typically hydrogen, methyl or ethyl; b) a free branched-chain carboxylic acid such as an excess of the acid used
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