Maybe you can use water which is coloured by a tracer and cut the the rock after a time period. Then you will see the depth of penetration.

Dear is it a really research topic or just  testing the colleagues. If research what will be utility of finding??

Ultrasonic devices are used to detect objects and measure distances.

I would like to add something to my answer. When the penetration depth of the liquid into a solid is examined with a tracer, the color line usually smears when cutting the solid with a saw afterwards. Therefore, it is better to break the solid with a hammer. Then the boundaries of the coloration are clearly recognizable.