Did the Allies fail to scout the Dardanelles minefields?

Was the Dardanelles plan simply made without investigating the Ottoman mine defenses?

No; British destroyers repeatedly probed the straits, including on 19 February 1915, but reconnaissance did not solve the problem of sweeping mines under fire.

The straits were probed with destroyers on several occasions. Two destroyers entered on 19 February 1915 specifically to learn where the mines lay, and considerable effort went into reconnaissance. Knowing the minefield's general position did not make civilian-crewed trawlers willing or able to sweep it under concentrated fire and a strong current. The campaign's failure therefore cannot be reduced to planners never looking for the mines. The operational gap lay between information and the protected specialist force needed to act on it.