Geological formations forming a reservoir for the accumulation of hydrocarbons in the subsurface of the earth contain a network of interconnected paths in which fluids are disposed that may ingress or egress from the reservoir. To determine the behavior of the fluids in this network, knowledge of both the porosity and permeability of the geological formations is desired. From this information, efficient development and management of hydrocarbon reservoirs may be achieved. For example, the electrical resistivity of geological formations is a function of both porosity and permeability. Considering that hydrocarbons are electrically insulating and most water contains salts, which are highly conductive, resistivity measurements are a valuable tool in determining the presence of a hydrocarbon reservoir in the formation.
One technique to measure formation resistivity involves the use of electromagnetic induction using transmitters of low frequency magnetic fields which induce electrical currents in the formation. These currents in turn produce secondary magnetic fields which are measured in an adjacent wellbore (or at some distance away in the same wellbore) by a magnetic field receiver.
The performance of a magnetic field receiver positioned within a wellbore casing may be compromised by an electrically conductive casing's effect on the magnetic field to be measured. The measurable magnetic field may be highly attenuated due to presence of the electrically conductive casing, and the measurements made by the receiver may be influenced by variations in attenuation caused by variations in the conductive casing's properties. Often, a cased wellbore reduces the magnetic field signal to a level that is undetectable by standard receivers. Moreover, the variance in conductivity, permeability, and thickness along a length of the casing makes it difficult to determine an attenuation factor (which represents attenuation of the measurable magnetic field caused by the casing) at any selected point. The inability to determine an attenuation factor at a selected point along the casing may cause errors in field measurements that are not easily corrected.