Apparatus and method for deep transient resistivity measurement

An apparatus for processing electromagnetic signal data, the apparatus comprising:

• a downhole tool configured to be disposed in a borehole in an earth formation, the downhole tool including a conductive carrier, a transmitter, a first receiver disposed at a first axial distance L1 from the transmitter, and a second receiver disposed at a second axial distance L2 from the transmitter that is less than the first axial distance;
• and
• a processor configured to receive transient electromagnetic (TEM) signal data representing a first EM signal S1(t) detected by the first receiver and a second EM signal S2(t) detected by the second receiver in response to inducing a pulsed current in the earth formation, the processor configured to perform:
• estimating an initial bucking coefficient based on at least the first axial distance L1 and the second axial distance L2;
• combining the first EM signal S1(t) and the second EM signal S2(t) using the initial bucking coefficient to estimate an initial formation signal;
• selecting a plurality of bucking coefficient values from a range of bucking coefficient values, the range of bucking coefficient values including the initial bucking coefficient and estimating a plurality of formation signals, each formation signal corresponding to one of the plurality of bucking coefficients;
• and
• selecting an optimal bucking coefficient from one of the initial bucking coefficient and the plurality of bucking coefficients based on the plurality of formation signals, the optimal bucking coefficient providing suppression of parasitic signals due to the at least one conductive component, the optimal bucking coefficient selected by identifying a formation signal from one of the initial formation signal and the plurality of formation signals that shows a smallest effect due to the at least one conductive component, and selecting a bucking coefficient corresponding to the identified formation signal as the optimal bucking coefficient.

A non-transitory computer readable medium comprising computer-executable instructions for processing electromagnetic signal data by implementing a method comprising:

• receiving transient electromagnetic (TEM) signal data representing electromagnetic (EM) signals detected by at least two receivers in a downhole tool disposed in an earth formation in response to inducing a pulsed current in the earth formation, the downhole tool including at least one conductive component;
• estimating an initial bucking coefficient based on relative positions of the at least two receivers;
• combining the TEM data using the initial bucking coefficient to estimate an initial formation signal;
• selecting a plurality of bucking coefficient values from a range of bucking coefficient values, the range of bucking coefficient values including the initial bucking coefficient and estimating a plurality of formation signals, each formation signal corresponding to one of the plurality of bucking coefficients;
• and
• selecting an optimal bucking coefficient from one of the initial bucking coefficient and the plurality of bucking coefficients based on the plurality of formation signals, the optimal bucking coefficient providing suppression of parasitic signals due to the at least one conductive component, the optimal bucking coefficient selected by identifying a formation signal from one of the initial formation signal and the plurality of formation signals that shows a smallest effect due to the at least one conductive component, and selecting a bucking coefficient corresponding to the identified formation signal as the optimal bucking coefficient.