Borehole Magnetic Gradient Method Based on Detection of Deep Underground Pipeline

At present, borehole magnetic gradient is one of the commonly used methods for the detection of deeply buried metal pipelines, which is controversial due to its unsystematic theoretic basis and absence of quantitative analysis. In view of these challenges, this paper aims to present our study involving the following three aspects. (1) The relationships of the vertical component, horizontal component between the magnetic fields of surface and boreholes were derived, and the borehole magnetic gradient formula of deep underground pipeline was deduced, starting from the strength of the magnetic field of infinite horizontal cylinder and the transformation of coordinate system. (2) The relationship of the magnetic gradient changes with the inclination of effective magnetization and the position of hole was simulated according to the formula of magnetic gradient. (3) Three kinds of relationship were determined based on the statistical analysis of the forward modeling data, namely, (a) the relationship between maximum ratio of magnetic gradient and the inclination of magnetic gradient, (b) the relationship of the horizontal position of deep underground pipelines, the extreme distance of magnetic gradient and the inclination of magnetic gradient, (c) the relationship of the ratio of distance between the depth of deep underground pipelines, the depth corresponding to the maximum absolute value of magnetic gradient, the extreme distance of magnetic gradient with the inclination of magnetic gradient. These relationships constitute the estimation formula to estimate the depth and horizontal position of deep underground pipelines. Finally, an example of estimating the depth and horizontal position of deep underground pipeline is given. The feasibility of the method is shown by the results of forward modeling based on the estimation results.