The Analysis Methods Of Potential Field Spectra And It's Application Based On Discrete Cosine Transform

Potential field is the main research content of physical geography field, which reflects the changes of the earth's crust structure and component from especial point of view and different sides. Potential field data processing is a important component part for gravity and magnetics prospecting, which is developing together with the ceaseless enhancement of computer science, computer mathematics, gravitology and magnetics and their measure methods. With the eternal enhancement of the geologic object and the precision of settling geologic question,the new methods and techniques that how to enhance data processing precision and resolution has been a important topic for persons who engage in geophysical prospecting.Spectrum analysis and filtering are important methods among geophysical prospecting data processing methods that have a great relation to the development of computer. With the development and perfect of computer technique and signal analysis theory, the methods of potential field data processing are not merely used Fourier spectrum analysis theory. At recent years, data processing technique has infiltrated through signal analysis theory, for instance, wavelet analysis, Hilbert transform, Hartley transform, which have improved the precision of data processing.Discrete cosine transform (DCT) excel non-sine transform in theoretical and applied value, and it has excellent capability, so it hold leading status in orthogonal transformation. DCT can avoid plural operation to real continuous function, and it has similar capability to Karhunen-Loève transform that can remove relativity and reserve superior energy of original signal. At potential field data processing, except author and collaborator have used DCT to gravity-magnetic anomaly derivate conversion, forward and inversion of density interface gravity anomaly, DCT spectrum frequency division of gravity-magnetic and DCT spectrum inversion of regular body gravity anomaly and the other relative components, at present, we have not found any correlative literature that used DCT to analyze spectrum of potential filed and to the process the data of gravity and magnetic anomaly.This paper take data processing method and technique that can determine geologic structure as central aim, take potential field spectrum analysis based on DCT as foundation and take improving data processing precision and resolution as cardinal line, we study and develop potential field data processing method and technique based on DCT.1. setting up potential field DCT spectrum analysis theoryPotential field spectrum analysis and conversion are important components of gravity-magnetic anomaly data processing, which include two sides, one is using the anomalies of observation surface to calculate it's n degree derivatives, the other is using the anomalies of observation surface to calculate it's above potential field anomalies and it's n degree derivatives.A method of magnetic potential spectrum analysis based on the cosine transform is proposed in order to improve the calculating accuracy of magnetic anomalies diversion. We derived the relations of cosine transform spectrum among gravity potential, magnetic potential and field constituent and deduced the cosine transform spectrum formula of analytic continuation and potential n degree derivatives using the cosine transform, so set up potential field DCT spectrum analysis and conversion theory.The horizontal and vertical first derivatives of magnetic anomalies of an infinite cylinder are calculated by the cosine transform method at model experiment, in which the maximum errors of them are -0.28 nT/m and 0.47 nT/m respectively, and the percent errors are generally within -3.57%~3.27% and -1.94%~1.88% respectively except several data of boundary and part are bigger because of remains of Gibbus effect. The calculating curve and theoretical curve are approximately coincident, and there is no influence by effective magnetic dip angle in computing. But the errors with the Fourier transform method are -10.62 nT/m and -14.42nT/m, there is large departure between the calculating curve and theoretical curve and evident influence by effective magnetic dip angle in computing. It indicates that the calculating accuracy of magnetic anomalies derivatives calculated by cosine transform is higher than Fourier transform, and the computing stability is excellent. By the same token, magnetic anomaly upward continuation calculated by cosine transform has also higher accuracy.2. using DCT to calculate normalized full gradient of gravity anomalies and phaseNormalized full gradient of gravity anomalies and phase can be used to find the location of source and inspect hydrocarbon structure because of its characteristic of"one less value between two high value". Traditionally,В.М.Березкинmethod and Fourier transform were used primarily, but using DCT to calculate normalized full gradient of gravity anomalies and phase is a new method.This paper present gravity anomalies conversion formula based on DCT, in succession study GH field characteristics of these models that include infinite horizontal cylinder, symmetrical-density sphere-coronal and asymmetric-density oil storage sphere-coronal. These experiments have proved that the more is the depth, the smaller is the harmonic wave member, at the same time singular point Maximum move downwards and has linear attenuation. GH field contour line of asymmetric-density oil storage sphere-coronal has presented typical characteristic of"one less value between two high value". GH field calculated by DCT has excellent stability and can enhance the depth of downward continuation. This paper present use DCT to calculate normalized full gradient of gravity anomalies and phase that are researched fracture in conversion of potential field. We research GH field and phase characteristics of typical infinite vertical step, finite vertical step and normal fault. These model experiments have proved that DCT is viable and valid, at the same time we gain these rules that estimate special location and horizontal distance of fracture. GH field has max. entrap and phase mutation in fracture location, and if structure have several fractures, then phase curves have mutations, and distance of two phase mutation can probably identify horizontal distance of fracture, and if we connect phase of different continuation depth that can reflect fracture trend. This method was used abnormal data processing of Xingkai-Hulin, and delineated 13 fracture according to it's GH field characteristics.3. the frequency division method of gravity-magnetic anomaly cosine transform spectrumThe paper put forward a frequency division method of matched filter based on discrete cosine transform in order to improve the accuracy and reliability for separating off gravity or magnetic anomalies. It is feasible to divide frequency of gravity-magnetic anomaly cosine transform spectrum using the matched filter method by analyzing the relation of amplitude spectrum between Fourier and cosine transform. The calculating formula of frequency division is obtained on the ground of characteristic analysis of anomaly cosine transform of vertical rectangular cylinder. In the experiment of double body (two rectangular cylinder), the biggest error of regional anomaly separated by Fourier transform is 12.71×10-6 ms-2, the mean square error is 5.63×10-6 ms-2 and the maximum percent error is 10.9%, and the errors of calculating points are all bigger than 9.3% besides 20km. But the anomalies divided by cosine transform are excellent to fit with academic anomalies of model and the relative errors are 1.65×10-6 ms-2, 0.067×10-6 ms-2 and 6.12% respectively, the bigger percent errors are only several data at the bounder of profile line and the calculating errors of the other points are all less than 3.0%. It is shown that the matched filter based on the cosine transform has higher precision.4. a new technique for elaborate explanation of faulted structures: three-direction small subdomain filteringHow to use gravity-magnetic field data to settle elaborate compartmentalization and explanation of geologic structures, the paper propose a new method of tree-direction small subdomain filtering. We analyze the offset technique, in which the phase of gravity anomalies of vertical dike displaceπ/2, and deduce theoretical formulas, in which planar gravity anomalies for characteristics of vertical dike are extruded and using the DCT method make their phases transformπ/2. It is shown in model experiments that the planar position of the most gradient for the displacement result where coordinates are transformed is the same as the projective position of fractured surface of vertical step. Using the conventional method of small subdomain filtering to check gravity gradient zones in offset processing results of each direction and to stand out boundary characteristic of three directional anomalies, so the numerical calculation of three-direction small subdomain filtering can be achieved, this method propose more abundant information for the identify of faulted structures horizontal position.Heilongjiang Hulin basin is one of the Daqing peripheral basins, it's gravity field characteristics are important researching content.Author analyzed structural setting of Hulin basin areal geology, basic characteristics of Bouguer anomaly and compartmentalized region for gravity field, the prospect area were delineated four anomaly regions and ten anomaly zones. In order to study the distributing characteristics of faulted structures and deposit characteristics of this basin, author disposed Bouguer anomaly data of Heilongjiang Hulin basin using proposed method at this paper. We used potential anomaly upward continuation and potential spectrum frequency division to separate and analyze anomaly, studied regional and residual field characteristics of basin, validated method's viable and veracious, at the same time obtained base depth of basin by inversion using obtained residual anomaly. In the application, we used the conventional small subdomain filtering and three–direction small subdonmain filtering to identify the fractures, the conventional small subdomain filtering can only check 11 faults, but the three-direction small subdonmain filtering can check 33 faults in Heilongjiang Hulin basin. It is shown that the technique of three-direction small subdonmain filtering can identify more information for planar location of fault structures. Simultaneity, we synthetically analyzed the characteristics of Bouguer anomaly sectorization, regional anomaly and residual anomaly, configuration of basal, configuration and distributing characteristics of faulted structure, finally, basin was delineated four first structural units and ten second structural units.Studying potential field data processing method and technique based on DCT have important significance at geology and physical geography. At first, in theory, we can develop and perfect current data processing theory and explanation technique, propose more actually theoretical simulation for gravity-magnetic exploration, and can has more solidly theoretic foundation for data processing inversion. Secondly, at practice, we can improve potential field anomaly data processing and explanation accuracy, which can let the topic more influence on physical geography exploration of my country. Therefore, it will has social economic benefit and realistic significance in the future.