Computation of refraction static corrections using first‐break traveltime differences

Abstract

Differences in first‐arrival traveltimes between adjacent records in multifold reflection surveys can be used to compute the depth and velocity structure of near‐surface layers. The procedure uses the redundancy of first‐break data in multifold surveys to enable a statistically reliable refraction analysis to be undertaken for either end‐on or split‐spread recording geometries. The traveltime differences as a function of source‐receiver offset provide a direct indication of the number of refractors present, with each refractor being defined by an offset range with a constant time difference. For each refractor, the time‐difference value at a common receiver from two shotpoints is used to partition the intercept time into the delay time at each shotpoint. This procedure is repeated until the delay times at all shotpoints and for all refractors have been computed. Refractor depths and velocities are evaluated from this suite of delay times. A surface‐consistent static correction to a selected datum level is then calculated at each surface station, using a replacement velocity equal to that of the deepest refractor. In a case history from the Canadian Rocky Mountain foothills, short‐ and intermediate‐wavelength weathering static anomalies were resolved successfully. Elevation and weathering static corrections of up to 40 ms were computed, with an estimated error of less than ±3 ms.