NONLINEAR SYSTEMATIC ERRORS IN BOREHOLE INCLINOMETER MEASUREMENTS

Abstract

Research Object and Relevance. Inclinometer measurements occupy an important place in geotechnical monitoring of hydraulic structures. Borehole inclinometers are used on tailings dams - they allow you to determine horizontal displacements, shifts, curvature of the casing pipe throughout its entire depth. An important part of the processing of initial data is the identification of various systematic errors. The authors of the article faced the problem of interpreting the displacements at the tailings dam of The Erdenet Mining Corporation - after the standard linear correction, some boreholes had arcuate displacements inexplicable from the point of view of geomechanics throughout their depth. Our first assumption was that this was caused by unaccounted for measurement errors. In March 2022, four series of inclinometer measurements were carried out within one day at the deepest borehole of the observation station in the main direction. Based on the results of the obtained data, non-linear systematic errors were identified. Research Methods. The article shows the functional dependencies of the main sources of systematic errors in inclinometric measurements: sensitivity drift, bias shift, rotation error, depth positioning error. We used a parametric adjustment with the inclusion of a systematic error model. This approach allows not only to determine the quantitative values of systematic errors, but also to evaluate their quality and check for statistical significance using a t-test. Results. As a result of the study, statistically significant systematic non-linear errors of inclinometer measurements were revealed. Statistically, this error is well described by dependence on the square of the interval number. They can also be partly attributed to the rotation error and the depth error (it may be better to define these errors as rotation sensitivity and sensitivity to the installation of the pulley on the casing head). Also in the article, we show graphs of residual errors and Q-Q plots for borehole position error without correction and with the corrections discussed in the article. The linear component of the systematic error was up to 0.065 mm per measurement interval. The non-linear systematic error was up to 0.018 sq. mm. Conclusion. As shown in this article, finding and correcting only a linear error does not allow one to be satisfied with the result (clearly seen on the Q-Q plots). It should be noted that if linear systematic errors affect the determination of displacements and shifts, then non-linear errors also distort information about changes in the curvature of the borehole. At the same time, in order to apply the method proposed by the authors for identifying nonlinear systematic errors between series of observations, strict consistency with the geomechanical model of potential deformation of the monitored object is required.