System for measuring earth formation resistivity through an electrically conductive well bore casing

The invention relates to a system to measuring resistivity of earth rock stratification stratum through conductive drilling well bore hole casing pipe. It includes plural cases that passing through the bore hole. At least one electrode is distributed on case. The electrode electrically contacts to the inner section of pipe. The system includes current source, digital voltage measuring circuit and switcher. The switcher is located between one electrode and current returning. The electrode is selected to take corresponding voltage measuring the selected axial direction distance and selected traversal depth.