Patent ID: 8456166

Claim:
A downhole tool system for making electromagnetic measurements in a borehole comprising: a first downhole transceiver adapted to be deployed in a borehole having a conductive casing comprising at least a uniform section and non-uniform section and induce a magnetic field through the conductive casing and into a surrounding subterranean rock formation; a second downhole transceiver adapted to be deployed in the borehole at the same time as the first downhole transceiver, and to inductively detect the magnetic field in the rock formation through the conductive casing; means for operating the first downhole transceiver at a plurality of frequencies; means for detecting response of the second transceiver at each of the plurality of frequencies; data processing means in signal communication with the means for detecting and adapted to identify the uniform and non-uniform sections of the conductive casing based on the amplitude of the magnetic field detected by the second downhole transceiver, and estimate the formation resistivity based on the magnetic field in the rock formation through the uniform sections of the conductive casing; wherein each of the non-uniform sections of conductive casing comprise at least one centralizer and at least one casing collar, the data processing means for identifying the at least one casing collar and the at least one centralizer based on a frequency response of the magnetic field in the non-uniform sections.