In a large pot melt butter over medium heat. Add onion, celery, fennel and garlic. Sauté until starting to soften but not brown, about 10 minutes. While vegetables are cooking prepare corn by peeling off corn husks and removing silk. Stand corn on end and remove kernels by running a large knife along the body, cutting at the base of the kernels. Trim ends off bare cobs and cut cob in half. Add kernels and cobs to pot along with vegetable broth, water, thyme sprigs and 1/2 tsp salt.