1. A method for determining a suitable differential transfer function (DTF) between first and second hydrophones, comprising: (a) determining a plurality of possible DTFs based on corresponding trace pairs from the first and second hydrophones; (b) generating a product by applying one of the plurality of possible DTFs to one trace of a portion of the trace pairs, the one trace being from the second hydrophone; (c) separating the product into an up-going component and a down-going component; (d) calculating a cross-correlation between absolute values of the up-going and down-going components; (e) repeating steps (b) through (d) for each trace pair in the portion of the trace pairs; (f) summing the cross-correlations for all the trace pairs in the portion of the trace pairs; (g) repeating steps (b) through (f) for each possible DTF in the plurality of possible DTFs; and (h) selecting the suitable DTF from the plurality of possible DTFs based on the summed cross-correlations.