Ultrasound doesn't have to be costly. There are depth gauges that don't do imaging, and should have caught this. It's a little box that looks much like a digital voltmeter, with a wand attachment and tells you depth to the next void. They seem to be a few hundred to a few thousand dollars... I found one listed for $55 but I am betting it's a typo.
This was a good catch and I'm glad you are not riding it.
__________________
Genesis 49:16-17
"Well, well!" said Holmes, impatiently. "A good cyclist does not need a high road. The moor is intersected with paths and the moon is at the full."