Applications of first break turning‐ray tomography to shallow seismic reflection data processing and interpretation

Shallow seismic reflection surveys commonly suffer from poor data quality in the upper 100 to 150 ms of the stacked seismic record due to shot-associated noise, surface waves, and direct arrivals which obscure the reflected energy. Nevertheless, insight into lateral changes in shallow structure and stratigraphy can still be obtained from these data by using first arrivals picks in a refraction analysis to derive a near-surface velocity model. We have used turning-ray tomography to model near-surface velocities from a high-resolution seismic profile recorded in the Trans-Pecos region of west Texas, near Fort Hancock. The result of this analysis is an interval velocity model for the upper 200 ms of the seismic profile that locates several geologic features which were not interpretable on the original stacked record. This model helps locate an unconformity near 70 ms two way travel time and strong lateral velocity variations above water table that may be due to a perched water table or soil horizon development. In addition, the interval velocity model leads to improved time-to-depth conversion, and when changed to stacking velocities may aid in producing a better stacked record at early travel times.