A first arrival travel time inversion applied to the 1986 PASSCAL Ouachita experiment has been used to image upper crustal structure. Twenty-one shot points every 10 km were recorded along two 100-km-long segments with 400 seismographs on each segment. A splined velocity model parameterization allowed for the inversion of laterally inhomogeneous models but not velocity discontinuities. Over 800 travel times were used in the inversion with a variable data covariance weighting. The multiple source nature of the experiment design allowed the use of reciprocity relationships among the 21 shot points to enhance data correlation. A succession of models with velocities specified at 30, 90, 171, and 221 nodes were inverted for whose horizontal node spacing decreased from 100 to 12.5 km in the final 221 node model. Vertical node spacings of 3.0 and 1.5 km were used. The results of the inversions indicate changes in depth of isovelocity lines which are interpreted as Triassic fault structure based on nearby well data. The final RMS travel time residual is 0.045 s. Resolution using first arrivals deteriorates rapidly below 9 km depth. Three distinct velocity gradients can be correlated with geologic units. Surface exposed Cretaceous and Jurassic sediments with velocities from 2.0 to 3.5 km/s extend to 1.5 km depth. Triassic rift fill sediments, which do not outcrop, have inverted velocities ranging from 3.5 to 4.75 km/s between depths of 1.5--3.0 km. Paleozoic sediments are imaged by a decreased velocity gradient below a depth of 3.0 km. Lack of resolution at depths greater than 9 km necessitates detailed modeling of wide-angle reflected phases to further constrain deeper crustal structure. ¿ American Geophysical Union 1990