The weight of the sample now will be less, since the sample will be buoyed up by the amount of water it is displacing. If you want to know more about the scientist who determined that this procedure would work, go to Archimedes . This is Archimedes' Principle. These are obtained with a probe that forces a cylinder into the soil. The soil can be removed from this cylinder and cut into sections and placed in another cylinder for transportation back to the laboratory.
Care needs to be taken to avoid compacting the soil during and after obtaining the core.