Sonar Of Baleen Whales
To the best of my (D.M.) knowledge, no sonar has yet (in 2013) been discovered in the non-toothed, the "baleen," whales.
It is possible to build echoscopic sonar of a sort that would work for baleens. It seems likely that Nature has already done this. What remains to be found are sounds made by baleens that are sufficiently feature-constant. Probably the features are of much longer duration than the "fangs" of the duration 1/100,000 second, in the clicks of dolphins. Probably, in compatibility with the long click-features, the sensors are widely spaced (feet apart rather than fractions of an inch) on the head.