What is the maximum possible distance between the access tubes required for cross-hole sonic logging?
PDI's 45 kHz nominal frequency probes have been tested in tubes up to 3.0 m (10 ft) apart. Larger distances between tubes may be possible, but so far have not been tested nor reported to us. It should be kept in mind, however, that defects could be missed with large distances between the probes, since reflection and refraction in the aggregate may cause waves to bend around the defect. The larger the spacing the larger the defect that could be missed.