| The technique of filtering is an important method among geophysical prospecting data processing methods that have a great relation to the developmentof computer. With the development and perfect of computer technique and signal analysis theory, the methods of potential field data processing have improved the precision of gravity and magnetion data processing.This paper take the method and technique of filtering in potential field data processing that can determine geologic structure as central aim, take potential field spectrum analysis based on Fourier as foundation, we study and develop the method of filtering in potential field data processing based on Fourier transform.1. Study on the function of filtering windowWe get the next results by fixed quantity comparing three datas of window function in frequency:(1) The rectangle window has the narrowest main lobe B, but has the greatest edge lobe hump A and the slowest weakening speed D.(2) The width of trigonometric function window's main lobe is 8π/N, which is twice than rectangle, but its edge lobe is much less than rectangle at the same time.(3) Planning,Hamming and Blackman window can epress by an general format which called generalized cosin window. These windows are special case of generalized cosin window and adopt them can effectively reduce the hight of edge lobe, but will increase the width of main lobe. The value of the greatest edge lobe is less 43dB than its main lobe for Hamming, but which has the same width with Hanning. And the value of Blackman window's edge lobe is less than 58dB than its main lobe, but its width of main lobe is triple of rectangle, which is 12π/N.(4) Hanning window has a little greatly main lobe for other window, which has a less edge lobe and a greater weakening speed, we often use it to separate potential anomaly because of its better effect.2. Introduce Chebyshev approximate filter to implement potential separationAfter investigating into the design of classical FIR digital filter, a method of FIR filter design based on Chebyshev best uniform approximate which can be used to separate potential anomaly is proposed. In an experiment of theory model, the max error of regional anomaly separated by low pass filter of Hanning window is 1.449×10-6m/s2, mean square error is 0.452×10-6m/s2, max percentage error is 9.952% and the error is always greater than 1.8% while the computing points are beyond±9 km. On the contrary, regional and residual anomalies which are separated by best uniform approximation theory fitted better with theoretical anomaly, the curve almost matched together. The max error of regional anomaly separated is 0.526×10-6m/s2, mean square error is 0.188×10-6m/s2, the max percentage error is 7.76% which only occur for several data on boundary, the error of the other points is less than 0.8%. The method has been used to separate Bouguer gravity anomaly of Sunwu-Jiayin profile, and get a satisfactory results because the anomaly field has been completely separated.3. Development of Wiener filter based on Wiener principleBased on Wiener filtering principle, using some Green's equivalent layer model simulated radial log-power spectrum of measured gravity field, we got more satisfied result while calculating power spectrum density function by the method of improved gram-schmidt orthogonalization which guarantee the stability of the numeric value. When extracting deep filed can guarantee low-frequency component not weakening, and more accurately extracting region anomaly, which being testified by theory model experiment, then reached potential field separation in frequency domain. In this paper, the Bouguer anomaly of Xinjiang region is processed by using Wiener filtering method, and obtain region gravity anomaly of Xinjiang which based on optimum filter principle calculation, then according to the characteristic of Bouguer anomaly and region field of Xinjiang, nine deep and big faults and five first-order structural units being divided.4. The calculation of gravity anomaly about the model of three dimensional homogeneous and nonhomogeneous density sphere-coronal based on triangle element methodWe propose the triangle element method of calculating three-dimensional sphere-coronal model's gravity anomaly by using divergence theorem, changed volume integration into surface integral, appliance a series of triangle to fit surface, then adopted the Green's formula changed surface integral to line integral, at the same time give a reasonable calculation programs. By error analysis, the validity and the reliability of the above method has been proved. For other methods, take the methods of line element, surface element and body element for example, the pricision of fitting sphere-coronal by triangle is clearly better than them because there is no error of numerical integration.5. Three dimensional calculation of gravity normalize total gradientThere often have error of observation and interference of superficial part in gravity actual measurement data which will be enlarged differently while in course of down continuation and calculate derivative, GH field's false hood anomaly will be caused because of the gap at the end of line at the meantime. So, we put rounding factor into response factor of down continuation and calculate derivative separately but not rounding the anomaly of calculation which is called 'assembly filtering method'. And the method has been proved will increase the stability of GH field and the depth of down-continuation. For ball and density sphere-coronal modle, its gravity normalized total gradient will show three characterics mainly, first, no matter how much the barrier frequency is, the every depth plane's extremal centrum position which projecting on horizon plane is coincidenece with the two model's centrum projecting on horizon plane; second, we will get the depth of geologic body's centrum when get the greatest extremal's depth at different frequency; third, single-peaked anomaly has been expressed on the two model's slice plane contour at different depth. For non-density sphere-coronal, the GH field's counter of central cross section and slice on some depth will express the classical characteristic of "two higer clip one lower" which showing includes the oil gas pool only when we get suitable frequceny value, if we get nonsuitable frequency value, the characteristic of GH field's counter is similar to the model of ball and density sphere-coronal, and we also find the position of "two higer clip one lower" usually is the position and depth of the non-desity sphere-corconal.We think analyzing gravity normalized total gradient and its phase together can determine the geologic body's conformation more accurately by study on the gravity normalized total gradient and its phase characteristic of the infinite extends vertical stair, limitedly extends vertical stair, normal fault and reversed fault model.6. Data processingSunwu-Jiayin section plane has following characteristic generally by processing higher accuracy gravity anomaly data of Daqing exploration region peripheral basin and think of its electric data:(1) This region is an equalization region and has normal crust depth from gravity anomaly;(2) Form Sunwu to Jiayin, whole located on Sunwu-Jiayin basin, so, the surface is all covered by lowly resistance, the thick is about 0.5-1.5km, electrical resistivity is several dozensΩ·m;. the thick of highly resistance reduce to thin gradually form trumpet spot to large spot, for example, the thick of highly resistance exceed 20km between 1 to 9 spot, at medinum the thick is about 18km, but the thick is only 10 km between 31 to 41 spot.When we processing the Bouguer gravity anomaly of Xinjiang, thought that the Xinjiang area structure frameword basicly is "three mountain two basin", which is Kunlun mountain-Talimu basin-Tianshan-Zhunger basin-Artai mountain form south to north, form high to low. According to the fault mop situation as well as the structure essential feature, the Xinjiang area may divide into 5 first-level tectonic elements, respectively be the Altai fold system, Zhunger fold system, Tianshan fold system, Talimu platform and Kunlun fold system.Studying the filtering method and technique in potential field anomaly data processing has an important significance at geology and physical geography. At first, in theory, we can develop and perfect current data processing theory and explanationtechnique, propose more actually theoretical simulation for gravity-magnetic exploration, and can have 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 our country. Therefore, it will have socialeconomic benefit and realistic significance in the future.
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