Most geological data is generated by complex physical and signal processing models and simulations. "Ground truth" means they physically retrieved samples and measurements instead of inferring the data, in this case probably core samples from the drilling that allows them to measure the dimensions of the matter deposited by the megaslide on the sea floor.

Ground truth data is generally something used as an offset for some other data. e.g., if I told you that you moved 1 meter to the right and your old position, you'd be able to tell your new position from the ground truth.