In the measurement of soil resistivity

the earth surface potential（ESP）is an external embodiment of the internal structure of the soil.In order to explore the reflection of earth surface potential by every layer

the earth surface potential distribution of homogeneous soil and horizontally two-layer soil are studied in this paper.Their formulas of the earth surface potential are theoretically analyzed and deduced.It is found that the earth surface potential after treatment can eliminate the influences of the test current and the electrode spacing.This paper proposes a concept with the apparent resistivity distribution coefficient and the distribution coefficient curves of homogeneous soil and horizontally two-layer soil using the simulation data are portrayed.This paper finds that the distribution coefficient curve of the homogeneous soil is independent on the spacing between electrodes;the distribution coefficient curve of the horizontal two-layer soil is related to the reflection coefficient of soil interface

the thickness of upper layer and the electrode spacing.The distribution coefficient can be used to calculate the soil resistivity and thickness.The conclusion of this paper provides reference base for the measurement of grounding design.