How then does the geophysieist play his part? Fundamentally, he has got to rely on the thing which he seeks being in some way different from the rock beds which cover it. Suppose that the search is for oil deep down. In this case

SOUNDING 'niE EARTH'S eR.UST 51 it proves to be impracticable to make use of the special physical properties ofthe oil itself; hut geologica1 experience indicates that certain rock beds, such as limestone, are like1y to be impregnated with oil if they are humped upwards, so as to form, as it were, an underground hill. The problem thus becomes one of locating the summit of this buried limestone hump. There are two physieal methods of doing this by observation carried ·out on the earth's surfaee.