Print this pageModel 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.