I don't know much about the method you're using, so I cannot advise how to improve it. But it reminds me of seismic exploration by acoustic means.
Attach a sending transducer firmly. Send shock waves through the block, at various frequencies. Use one or more attached receiving transducers, to test for uniform echo response through the block.
A fracture within the block will result in non-uniform echo response.
Or, there will be two echo frequencies displayed.
Or, the pattern will differ in some way from that of a solid block.