  We consider the inverse cracks scattering problems from the far field
patterns with a fixed incident direction. We firstly show that the sound-soft
cracks can be uniquely determined by the multi-frequency far field patterns
with a fixed incident direction. The proof is based on a low frequency
asymptotic analysis of the scattered field. One important feature of the
uniqueness result is that the background can even be an unknown inhomogeneous
medium. A modified Newton method is then proposed for the numerical
reconstruction of the shapes and locations of the cracks. Compared to the
classical Newton method, the modified Newton method relaxes the dependence of a
good initial guess and can be applied for multiple cracks. Numerical examples
in two dimensions are presented to demonstrate the feasibility and
effectiveness of the modified Newton method. In particular, the quality of the
reconstructions can be greatly improved if we use the measurements properly
with two frequencies or two incident directions.
