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The LSDARC Method of Seismic Refractor Analysis, and the Resolution of Lateral Variations in Refraction Velocity
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 6th SAGA Biennial Conference and Exhibition, Sep 1999, cp-221-00011
Abstract
The LSDARC method of seismic refraction analysis uses both the reciprocal and a least-squares<br>difference method to optimize estimates of time-depth terms. The difference method estimates<br>differences between time-depth terms at adjacent receivers in which errors in source timing are<br>eliminated, thus permitting the effects of such timing errors to be minimized. Time-depth terms<br>need only be defined at some receiver locations, and the gaps are filed in by the difference<br>method, thereby allowing flexibility in survey design. Refractor velocities are estimated by<br>subtracting time-depth values for each source and receiver from the first-break times. This<br>procedure projects each source and receiver on to the refractor surface. A damped least-squares<br>inversion procedure is then used to calculate and apply corrections to the times for each shot<br>gather to make all times appear to be produced by a single shot placed on the refractor surface<br>at one end of the profile. Refractor velocities can be estimated from the corrected times using<br>distance windows that are independent of the length of the recording spread, and which can be<br>adjusted to get a preferred trade-off between variance in velocity and window length. An<br>alternative method of velocity determination involves preparing slowness profiles in opposite<br>directions, and using a model for refraction across a dipping interface to compute true refractor<br>velocities. The advantage of the alternative approach is that refractor velocities can be reliably<br>determined without knowledge of the time-depth terms. The main features of the method are<br>illustrated using a hammer seismic survey undertaken near Ladismith in the Western Cape.