| http://www.w3.org/ns/prov#value | - A tracer solution comprising:a) a gadolinium salt of a carboxylic acid of general formula ##STR4## where R is an alkyl of not less than four carbon atoms, and R1 and R2 are selected from the group consisting of hydrogen and an alkyl group; b) a free branched-chain carboxylic acid; c) a non-polar solvent and d) a viscosity stabilizer selected from the group consisting of tributyl citrate, oleyl alc
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