Patent ID: 7030617

Claim:
A method of conducting an electromagnetic induction survey of a geological formation penetrated by a borehole lined with a conductive casing, said method comprising the steps of: positioning a transmitter in the borehole, whereby the transmitter generates a transmitter magnetic moment; positioning a distant receiver external of the borehole to detect a magnetic field induced by the transmitter, whereby the distant receiver is disposed across part of the formation from the borehole; positioning an auxiliary receiver in the borehole proximate the transmitter to detect a magnetic field induced by the transmitter and attenuated by the conductive casing; determining a first casing attenuation factor applicable to the magnetic field measured by the auxiliary receiver from a ratio of the measured magnetic field at the auxiliary receiver and the transmitter magnetic moment; determining a second casing attenuation factor applicable to the measurement of the magnetic field at the distant receiver from a non-linear relationship between the first casing attenuation factor and the second attenuation factor, wherein the second attenuation factor is less than the first attenuation factor; determining a formation attenuation factor applicable to the measured magnetic field at the distant receiver from a relationship between the magnetic moment of the transmitter, the second casing attenuation factor, and the measured magnetic field at the distant receiver; and correlating the determined value of the formation attenuation factor to a resistivity characteristic of the formation between the distant receiver and the transmitter.