In a large pot, heat 2 tablespoons of the oil. Add the shrimp shells and cook over moderate heat until they turn pink, 3 minutes. Add the onion, fennel, celery, smashed garlic and saffron and cook until softened, 5 minutes. Add the Pernod and brandy and cook until evaporated. Stir in the tomato paste, add 6 cups of water and bring to a boil. Simmer until the broth is reduced to 4 cups, 45 minutes. Add the tarragon and parsley sprigs and let stand off the heat for 15 minutes.