Method Study Of Accurately Detecting Close-Spacing Parallel Buried Pipelines Based On Magnetic Anomalies

Underground pipelines are the “lifeline” project for cities.Underground pipeline detections can provide dependable pipeline location information for urban underground excavation works to avoid accidents of underground pipeline being destroyed during construction.With the improvement of the sensitivity and precision of magnetometers,magnetic methods be used to detect sources with weak magnetism,such as underground pipelines.The rapid development of modern cities has resulted in less and less available underground space.In order to save underground space and facilitate management planning,most of the underground pipelines are laid in parallel at close spaced.In this paper,a comprehensive study on the methodology of magnetic methods in close-spacing parallel underground pipelines detections.The main works are as following:A buried pipeline is regarded as a line of mass to calculate its gravitational potential and its feasibility has be verified.Then its magnetic anomaly has be calculated by the Poisson formula between the gravitational and magnetic potentials.Compared with using the magnetic dipole reconstruction method to calculate the magnetic anomaly of the buried pipeline,this method has higher calculation efficiency and accuracy.When calculating the magnetic anomaly of ferromagnetic pipelines,the demagnetization cannot be ignored.The demagnetizing factor is a key parameter that affects demagnetization.The demagnetizing factor of magnetic bodies is calculated by the finite element method.The accuracy of the method is verified by comparing the numerical and analytical solutions of the demagnetizing factor of a spheroid.Studies show that the demagnetizing factor of a pipeline is proportional to its thickness-to-diameter ratio and is not affected by its outer-diameter and length.A forward model of gravity and magnetic anomaly measurements of parallel underground pipelines is established.Studies show that the curve shape of gravity and magnetic anomalies is only affected by the outer-diameter ratio,axis spacing,buried depth,and azimuth of parallel pipelines.The magnetic anomaly is more suitable for close-spacing parallel underground pipelines detection than the gravity anomaly because the detection accuracy and range of the magnetic anomaly are better than the gravity anomaly.The experiment of the magnetic anomaly measurement of two cast iron pipes also shows that it is feasible to use magnetic methods to detect parallel buried pipelines.The downward continuation methods in frequency domain,such as the Fourier transform,Tikhonov regularization and its iteration,integral iteration,are used to downward continue magnetic anomaly data to separate the superimposed anomalies generated by close-spacing parallel pipelines and enhance the horizontal resolution.The improved edge expansion method is used to expand the magnetic anomaly data to reduce the influence of the edge effect on the downward continued results.The improved Fourier transform method can prevent the downward continuation plane crossing the pipelines and reduce the influence of fast Fourier transform induced noise.The differential minimization of downward continued solutions as a stable functional of the Tikhonov regularization method and a criterion for regularization parameter choices.Theoretical analysis show that for noiseless magnetic anomaly data,the integral iteration has the best downward continuation performance.For noise-containing magnetic anomaly data,only the iterative Tikhonov regularization method can separate the pipeline superimposed magnetic anomalies and suppress the noise in the data.The closer continuation plane is to the pipeline,the more high-frequency components contained in the pipeline magnetic anomalies.However,these high-frequency components will be suppressed during the downward continuation process,which will cause the downward continued results to be distorted.Therefore,it is not that the closer the continuation plane is to the pipeline,the higher the horizontal resolution of the downward continued results.The spatial location of parallel underground pipelines is determined using the genetic algorithm and magnetic tilt angle method.The genetic algorithm uses total intensity magnetic anomalies as inversion data to estimate the buried pipeline depth;the tilt angle method uses magnetic anomaly components and is only valid to pole-reduced magnetic data.A tilt angle of 90° value marks the location of a buried pipeline while the depth is the distance between the location of the 90° and its adjacent 0° values.Compared with the traditional inversion algorithm,the magnetic tilt angle values are not affected by the amplitude of the magnetic anomalies,so the positioning accuracy of the magnetic tilt angle method is not affected by the demagnetization of the pipeline and the weakening of the magnetic anomaly amplitude during the downward continuation process.Finally,an accurate positioning method for close-spacing parallel underground pipelines from magnetic anomaly data is proposed.This method successfully locates pipelines on theoretical and experimental models.