Using a magnetic gradiometer in a borehole inclinometer to compensate for external magnetic interference

Gasanov O.V., Sitnikov V.N., Khamatdinov V.R., Grinyov I.V., Korolyov A.B. Using a magnetic gradiometer in a borehole inclinometer to compensate for external magnetic interference. Scientific and Technical Journal of Information Technologies, Mechanics and Optics, 2025, vol. 25, no. 6, pp. 1177–1184 (in Russian). doi: 10.17586/2226-1494-2025-25-6-1177-1184

In measurements of the magnetic azimuth of the borehole axis, calculations are based on the superposition of the Earth’s magnetic field and parasitic fields from the remanent magnetization of the geophysical tool assembly and the drill string. At high latitudes the horizontal component of the geomagnetic field is very small. As a result, even weak parasitic fields — on the order of 1 % of the geomagnetic field — can cause azimuth errors of 4° or more. Many methods to mitigate this effect have been reported in the literature. However, almost all of them require either additional equipment and preliminary measurements, or knowledge of the exact values of the magnitude and inclination of the geomagnetic field at the survey location. In connection with all of the above, there is a problem of creating a compensation method that would not require preliminary measurements of the parameters of the parasitic field or the modulus and inclination of the geomagnetic field. This paper proposes using an additional magnetometer in the inclinometer to measure the gradient of the superpositional magnetic field. From simulation, and using the measured gradient, an equivalent magnetic source in the form of a circular current loop is determined. The calculated field of this loop is then subtracted from the reference magnetometer readings. In the laboratory experiments, ring neodymium magnets (three variants with different magnetic flux densities) placed on the inclinometer axis were used as parasitic-field sources. A magnetic gradiometer was formed by two magnetometer sensors spaced 0.307 m apart. In experiments, the developed algorithm identified parameters of current loops equivalent to the sources in terms of magnetic effect. This enabled compensation of the reference magnetometer readings and improved azimuth accuracy from −1°15′36″ (source 1), −3°9′36″ (source 2) and +12°30′36″ (source 3) to ±0°39′ for all sources. In the experiment the field magnitudes at the reference magnetometer location were 0.42 %, 1.59 % and 5.60 % of the geomagnetic field, respectively. The proposed method increases azimuth measurement accuracy without requiring measurements of parasitic or geomagnetic field parameters. In addition, the use of the method allows reducing the length of nonmagnetic collars on both sides of the inclinometer during drilling. Thus, the method can be implemented in a sensor that computes and compensates for parasitic fields in real time during logging or drilling.

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