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The Study Of Continuation And Transformation Methods For Aeromagnetic Gradients

Posted on:2010-10-25Degree:DoctorType:Dissertation
Country:ChinaCandidate:H X LiFull Text:PDF
GTID:1100360302979903Subject:Earth Exploration and Information Technology
Abstract/Summary:PDF Full Text Request
Aeromagnetic gradients survey has improved dramatically over the past three decades, in our country it will be developed as an important technique in the future, so it is necessary to study the methods for gradient data processing. In this paper, continuation and transformation methods for gradient data will be investigated.The most widely used method for level-to-level potential field continuation is the Fourier transform (FFT) technique, the upward continuation is very stable by FFT , however, the downward continuation is unstable, the distance of continuation is generally less than 3 times the grid interval. Most works of continuation between arbitrary surfaces are based on the equivalent source technique, generally, equivalent source techniques demand solving a large set of linear algebraic equations, so they are very time consuming. Besides, the downward continuation distance of these methods is limited. The measured gradients usually include more high frequency useful signal and noise than total field, therefore, the downward continuation of gradients need more stable compute technique. In this paper, iterative method and interpolation-iterative method will be introduced to the continuation of aeromagnetic gradients. Model test results demonstrate the effectiveness and stability of the methods, and the computation accuracy is good.Transformations between aeromagnetic total field and gradient components in frequency domain are investigated. Fast Fourier transform technique widely applied to calculating gradients from total magnetic anomaly, however, when applied to transforming between gradients or reconstructing total field from gradients, there are problems needs resolving. Because those expressions of transform operators are undefined when u or v equal to zero or u and v are simultaneously equal to zero (u is the frequency in x-direction and vis the frequency in y-direction), the operators function can not be sampled at these frequencies. Consequently, the transformation can not implement by fast Fourier transform technique directly. In this paper, shift sampling theory is employed for resolving this problem. Model test results show that the technique has good accuracy.The third part of this paper is about the transformations between total field and gradients on irregular surface. The major problem of existing techniques for potential field transformation on irregular surface is time-consuming, so it is difficult to apply these approaches to large data set. In this paper an efficient procedure is proposed for calculating gradients from total magnetic field anomaly and transformation between horizontal and vertical gradient on irregular surface. This method integrate interpolation-iteration method with frequency domain transformation method, it has high computation speed.The measured total field anomaly and gradients were used to test the validity of iterative method and interpolation-iterative method, the results demonstrate that the methods are stable and have good accuracy. Moreover, the computation speed is very fast, that ensure the practicability of methods. Transformations between measured total field anomaly and gradients were accomplished, the results indicate that anomaly shapes and amplitudes are very similar in the calculated gradients and measured gradients, but it is worth noting that the calculated results from transformation between gradients are better than those from total field. The total fields reconstructed from horizontal and vertical gradients loose some short-wavelength components compared with the measured total field, but the anomaly shapes agree in the general trends. The good consistency between transformation results and measured gradients and total field not only demonstrate the effectiveness of the method but the reliability of measured data. The real case test results give an evidence for the application of the methods.
Keywords/Search Tags:aeromagnetic total field anomaly, horizontal gradient, vertical gradient, continuation, frequency domain, transformation, irregular surface
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