Generally induction logs measure formation conductivity rather than resistivity and sands or sandstones forms one of the reservoirs rock types. Therefore the question of which between clay and sand will have the higher induced potential will surely depend on the content of the reservoir (in this case sand) pore fluid. If a sand or sandstone is saturated with water, then the IP will be higher, however, if it has being saturated with nonconducting fluids such as air and oil then the IP will be lower because you are now measuring conductivity potential induced by your induction log. On the other hand, shales and or clays pores in most conventional sense are filled in with water, therefore if that is the case and if the water content is not fresh water (meaning the water is salty), then the IP will be high.

Generally, induction logs work well in wells containing nonconducting fluids in the borehole.

Under normal conditions, Clay has a higher IP than sand because clay is more polarizable. When current is introduced into a clay formation and the curent is then switched off, the voltage does not immediately drop to zero but decay in a manner similar to how a capacitor will behave. This is what we mean by clay is more polarizable and has a higher IP than clean sand under normal condition.

In a very normal situation clay is more polarizable and more capacitive than sand. Therefore, it has higher IP than sand.