Applications of algebraic reconstruction techniques to crosshole seismic data

Abstract

Tomographic imaging techniques were applied to two crosshole data sets to determine the velocity structures and the reliability and resolution of the algorithms on real data. The experiments were carried out at the Retsoff salt mine in New York and at the underground radioactive waste study site at the Stripa mine facility in Sweden. The traveltimes at Restoff were high quality and were obtained over raypaths of up to 500 m in length. The structure was quite complicated with velocity contrasts up to 50 percent. The Stripa site was in granitic rock with velocity contrasts of only a few percent. The dimensions of the experiment were small with maximum ray lengths of just over 10 m. The data at this site were collected with very high accuracy, source and receiver locations were measured to better than 1.0 mm, and traveltimes were read to 0.001 ms. A number of algorithms similar to the algebraic reconstruction techniques (ART) used in medical imaging have been applied to the data. Some modifications of the algorithms, such as the application of weighting schemes, damping parameters, and curved raypaths, were performed. The resulting velocity fields were compared to the known fields and with each other to determine an optimal method. The algorithms were found to be a rapid, reliable means of reconstructing the slowness field of real data. Low‐velocity zones were recovered with accuracy in location and value. It was also found that great care was necessary in application of the techniques to ensure that proper damping parameters are used and the proper number of iterations taken; otherwise poor reconstructions will result.