1. A method for characterizing a dissolved gas-liquid ratio of a fluid downhole using a correlation equation expressing the dissolved gas-liquid ratio in terms of at least sound speed, wherein the correlation equation was derived chemometrically from a training data set prepared by generating a plurality of output values from a plurality of input values to a set of multiple simultaneous existing correlation equations, the method comprising: receiving at least one input signal representing sound speed of a fluid downhole; processing the input signal with a computer using the correlation equation expressing the dissolved gas-liquid ratio in terms of at least sound speed wherein an output signal representing the dissolved gas-liquid ratio is produced; and outputting the output signal.