We use cross-correlation of the ambient seismic field to estimate seasonal variations of seismic velocity in the Mississippi Embayment and to determine the underlying physical mechanisms. Our main observation is that the ∂t/t variations correlate primarily with the water table fluctuation, with the largest positive value from May to July and the largest negative value in September/October relative to the annual mean. The correlation coefficients between water table fluctuation and ∂t/t are independent of the interstation distance and frequency, but high coefficients are observed more often in the 0.3-1 Hz than 1-2 Hz because high-frequency coherent signals attenuate faster than low-frequency ones. The ∂t/t variations lag behind the water table fluctuation by about 20 days, which suggests the velocity changes can be attributed to the pore pressure diffusion effect. The maximum ∂t/t variations decrease with frequency from 0.03% at 0.3-1 Hz to 0.02% at 1-2 Hz, and the differences between them might be related to different local sources or incident angles. The seasonal variations of ∂t/t are azimuthally independent, and a large increase of noise amplitude only introduces a small increase to the ∂t/t variation. At close distances, the maximum ∂t/t holds a wide range of values, which is likely related to local structure. At larger distances, velocity variations sample a larger region so that it stabilizes to a more uniform value. We find that the observed changes in wave speed are in agreement with the prediction of a poroelastic model.