What is the maximum possible distance between the access tubes required for cross-hole sonic logging?

Engineers have tested PDI’s 45 kHz nominal frequency probes in tubes up to 3.0 m (10 ft) apart. It may be possible to test with larger distances between tubes. Nevertheless, PDI has not tested or heard reports of its success. Defects can be missed with large distances between probes. This is because 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.