Model of Near-Surface Velocity
, Southern San Joaquin Valley,
California
James G. Ferry
A model was developed that accounts for temporal and spatial variations in
the near-surface velocity
of the southern San Joaquin Valley. Significant
temporal variations in traveltime-vs.-depth measurements have been caused by
changing water-table levels over the last 50 years. Spatial
velocity
variations,
at any particular point in time, are due to spatial variations in geology and
water-table depth. A theoretical traveltime equation was fit to measurements of
traveltime-vs.-depth and water-table depth. This traveltime equation was derived
from the
velocity
model where z is the depth below the surface, Dw is the depth of the
water table, V is the
velocity
at one unit depth, Vw is the
velocity
below the water table, and B is the inverse-power coefficient. Fitting all of
the
data
from the area, an inverse model with parameters V = 268 m/sec, B =
2.60, and Vw = 2,170 m/sec was determined with an RMS traveltime
error of 17 m/sec. Inverse models were determined with RMS traveltime errors of
10 m/sec or less when the theoretical model was fit to subsets of
data
from
areas of similar geologic conditions. The sea level datum correctional
traveltimes calculated using the inverse models were found to correlate with
actual traveltime measurements much better than static corrections used in the
past. The inverse model acco nts for false time-structure as great as 180 m/sec
in seismic-reflection
data
that has been static-corrected using a sea level
datum correctional
velocity
of 1,830 m/sec.
AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.