You can assume there are many other photographs of this area taken at this time, other than this one. The investigators may have to subpoena them, however.
If a large grout pumping operation took place and created this large block of concrete, there is some record somewhere of the occurrence. And someone who was on the job who can testify under oath as to what happened.
That is exactly what is happening, but not by subpoena. People who worked on the repairs (witnesses), stating depths of 6 foot voids in the main spillway, and 4 foot voids in the Upper spillway, are coming forward to others to get the information out there. These workers are afraid of retribution as they have asked for their names not to be revealed. But they are well known by the person who they revealed this information to. Scott Cahill has revealed this publicly, but is keeping the individual's names protected (i.e. people coming to him in confidence on the repairs & the details).
This also raises the question of the Independent Forensic Review Team. What will be in their reports? If there is supporting evidence that this "void filling" occurred, then they will need to address this evidence in their reports. OR will there be great pressure from DWR? OR will there be information that somehow is not "volunteered" to the Forensic Team?
Keep in mind the physical evidence has been pulverized in the recent blasting.