Estimating seismic velocity variations in the Mississippi embayment from
analysis of the ambient seismic field
Abstract
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.