The Research And Application Of Data Processing Technique For Potential Field Anomaly

As one of the basic means of geophysical exploration,potential field exploration(gravity and magnetic exploration)plays an important role in solving mineral resources exploration and geological problems.With the increasing demand for accurate data interpretation,it is imperative to develop and improve more stable and high-precision data processing methods for potential field anomalies.Aiming at improving the computational stability and accuracy of data processing,the difficult problems in the routine processing of real data,which contain data denoising,reduction to the pole at low latitudes,derivative conversion and edge detection,are studied and improved in this thesis.The validity of these methods and their advantages in improving the accuracy of results are proved by theoretical model tests.They have been applied to the data processing of aeromagnetic anomaly in Sulu Sea and gravity and magnetic anomalies in Changbai Depression of Yalu River Basin,and good results are obtained.The measured potential field data are composed of anomalies produced by geological bodies and noise.Most of the existing potential field data processing methods are sensitive to noise.The calculation results will be affected by noise and its accuracy will be reduced.Therefore,de-noising is an important means of data preprocessing.Nowadays,there are some problems in de-noising methods,such as incomplete de-noising or excessive de-noising.In order to solve this problem,A new de-noising method for potential field data,called VMD-SE method,is proposed,which combines the variational mode decomposition(VMD)and the sample entropy(SE).This method automatically determines the decomposition mode numbers of VMD method based on the sample entropy characteristics of anomaly and noise,and achieve the adaptive de-noising of potential field data.The model test shows that the VMD-SE method has higher calculation accuracy than other de-noising methods,and can retain the anomaly to the maximum extent and suppress the noise interference.It is a reliable method for de-noising the potential field data.Reduction to the pole(RTP)is a basic technique for magnetic anomaly data processing.It can simplify the anomaly form and facilitate interpretation.However,RTP is both an amplification operator and a phase shifting operator.When at low latitudes,the RTP processing becomes extremely unstable due to the amplification,resulting in poor result.Aiming at this,a Taylor series iterative(TSI)method for the RTP processing at low latitudes is proposed.The method constructs an approximate RTP operator of magnetic data at the geomagnetic equator based on the truncated Taylor series.By combining with the reduction to the equator(RTE),the RTP at low latitudes,especially at the magnetic equator,is achieved.By using the iterative method in wavenumber domain,the accuracy of RTP results is improved.By choosing the analytic signal amplitude(ASA)which is highly correlated with the theoretical RTP anomaly as the criterion,the number of iterations is determined by the correlation coefficient between the RTP results and the analytic signal amplitude of original anomaly.Theoretical model test proves that it is reasonable to use the correlation coefficient method to determine the number of iterations.The RTP result of Taylor series iteration method is closer to the theoretical value than other methods.In potential field data processing,derivative conversion has important physical significance.It is often used to separate superimposed anomalies and provide operational data for other potential field data processing methods(edge detection,Euler deconvolution,etc.).Due to the influence of measurement error and noise,the derivative conversion in wavenumber domain has the problems of unstable calculation process and poor accuracy.In order to solve this problem,a Chebyshev low-pass filter method for derivative conversion in wavenumber domain is proposed in this thesis.The method achieves an arbitrary-order derivative conversion by adding Chebyshev low-pass filter to routine derivative operator in wavenumber domain.Through analyzing the filtering properties of the filter and the characteristics of the radially averaged power spectrum of potential field anomalies,the parameters of the Chebyshev low-pass filter method are determined,which can reduce the interference of human choice.The noise model test shows that the root mean square error of the derivative calculated by this method and the theoretical value is the smallest compared with other methods.Edge detection is an important aspect in data processing and interpretation of potential field.Its results can provide the boundary location of geological body and help to solve related geological problems.Normalized standard deviation(NSTD),as a edge detection method belonging to mathematical statistics class,can recognize the boundary of geological bodies with different buried depths and has strong anti-noise ability,However it has the problems of inaccurate boundary location,low resolution and false boundary.For this,three improvements based on the routine NSTD method have been proposed: 1.reduce the occurrence of false boundary by adding constants to the normalization term;2.using first-order vertical derivative as input data to enhance the boundary resolution;3.improve the continuity of the edge by adding standard deviation of directional derivatives to the calculation formula.Through a series of theoretical model tests,it is proved that these three improvements improve the edge detection performance of the routine NSTD method in boundary resolution and accuracy.They have certain advantages over other edge detection methods and can be used as a reliable choice in real data processing.The Taylor series iteration method is applied to the RTP processing of aeromagnetic data.in Sulu Sea.Through comprehensive comparison,stable and reasonable RTP results are obtained,and the performance of the method in practical application is examined.Apply the other methods mentioned above to data processing of 1:50000 gravity and magnetic anomaly in Changbai Depression,Yalujiang Basin,and then the application effect of the new method is tested.Based on the processing results and combined with the geological,petrophysical and electrical data of the study area,the faults and tectonic units in this area are divided,which enhances the understanding of this area.