Patent ID: 8009511

Claim:
A method of communicating through a fluid in a borehole between a first location and a second location, the method comprising: producing a first signal at the first location and a second signal at the second location, the first signal and the second signal traveling in opposite directions between a first position and a second position in the borehole; measuring a third signal at the first position indicative of the first and second signals; measuring a fourth signal at the second position indicative of the first and second signals; and using a result of filtering the third signal by a first filter with a result of filtering the fourth signal by a second filter and a third filter for estimating a value of the first signal at at least one frequency at a first output wherein the first filter, the second filter and the third filter are selected based upon a result at a second output of applying the first filter and the third filter to the third signal and applying the second filter to the fourth signal wherein the third filter has a phase shift substantially equal to π.