In order to perform tomography analysis on cross-hole sonic logging data, is it necessary to do additional scans with the emitter and receiver somewhat offset in the vertical plane?
Whenever defects are seen on parallel (not offset) scans, it is generally good to also obtain additional data from offset scans. If the shaft has 6 tubes or more, offset scans of diagonal profiles are not necessary even if defects are found. The TomoSonic software can give results using only the parallel scans, but if offset scans are available Tomosonic will use them to improve results; this is helpful around the perimeter in particular. An offset distance of about 300 mm (1 ft) to at most 500 mm (1.6 ft) is generally sufficient. Note that while it is possible to obtain the additional offset scan data at a later time, this procedure requires obtaining data using exactly the same Pile Name. It is therefore recommended, if a defect is suspected, to immediately collect the extra offset data while the probes are still in the tubes.