Method and apparatus for calibrating the output measurement of a logging tool as a function of earth formation parameters

#1. Field of the Invention

#This invention relates to an apparatus and method for calibrating an electromagnetic logging tool such as an induction logger, that is used to measure the conductivity of the surrounding formation while a borehole is being drilled therethrough. The apparatus is particularly useful as a secondary standard for field use in testing MWD loggers.

#2. Discussion of the Prior Art

#The novel features which are believed to be characteristic of the invention, both as to organization and methods of operation, together with the objects and advantages thereof, will be better understood from the following detailed description and the drawings wherein the invention is illustrated by way of example for the purpose of illustration and description only and are not intended as a definition of the limits of the invention:

#FIG. 1 shows the response curve for the measured signal phase angle difference between two receivers as a function of formation resistivity;

#FIG. 2 is the response curve that relates the measured signal amplitude ratio to formation resistivity;

#FIG. 3 is a schematic drawing of an induction logger assembly generally shown as 10. The assembly consists of a mandrel 12, which, in the case of an MWD tool is conductive but non-magnetic. Transmitter wire-coils T1 and T2 are wrapped coaxially on the mandrel in grooves (not shown) cut around the outer surface of the mandrel. The two coils may be about 1.7 meters (m) apart. Receiver wire-coils R1 and R2 are coaxially wrapped in grooves betwixt the transmitter coils as shown, separated by about 0.3 m. The respective transmitter and receiver coils are insulated from the conductive mandrel. Suitable electronics modules (not shown) are incorporated inside the logging tool.

#In operation, the transmitter wire-coils act as transmitting antennas for an electromagnetic field and the receiver coils serve as receiving antennas. For brevity, we shall refer to those items as transmitters or receivers respectively. For calibration purposes, a calibration fixture 14 is positioned around the mandrel, preferably between the receivers but not necessarily limited to that location. The calibration fixture consists of a plurality of closed conductive loops that are co-planar and concentrically mounted around the mandrel coaxially with the transmitters and receivers.

#Although the theory of operation of the induction tool is known, it will be briefly viewed here with respect to a single calibration loop. When a transmitter is excited in the absence of a calibration loop, a voltage V.sub.P is induced in the receivers. If we add a closed conductive loop, a voltage V.sub.L is induced in the loop which re-radiates a voltage contribution V.sub.S to the receivers. Then the total voltage, V.sub.t induced in the receivers with a calibration loop present, is