Patent ID: 8400159

Claim:
A method for making an electromagnetic induction survey of a formation surrounding a borehole having a casing comprising: deploying an electromagnetic transducer into a section of the borehole that is cased with a conductive non-magnetic casing having non-constant casing properties along the length of the casing; making impedance measurements relating to impedance of the electromagnetic transducer at a first frequency while deployed in the section of the borehole; making electromagnetic survey measurements of the formation; and compensating for varying attenuation in the electromagnetic survey measurements due to having non-constant casing properties of the conductive non-magnetic casing, the compensation being based at least in part on a relationship between a varying casing correction factor and a product of parameters relating to casing conductivity, casing thickness and electromagnetic frequency, the compensation being further based in part on a relationship between one or more impedance parameters and the product of parameters relating to casing conductivity, casing thickness and electromagnetic frequency, the product of parameters relating to casing conductivity and casing thickness being estimated based on the one or more impedance parameters, wherein either the parameter relating to casing conductivity or the parameter relating to casing thickness being independently measured and the other of the two parameters is determined based on the estimated product of the two parameters.