The Differntiation And Application Of Gravity Anomalies Based On Wavelet Transform

With the rapid development of China's national economy in recent years, the privation of mineral resources has been increasingly felt as the bottleneck restricting its continuous and healthy progress. In purpose of exploring alternative mineral resources, a new round of exploration activities is now on with a considerable amount of investment. Now, after decades of exploration, the surface or near surface ore-beds have been discovered, what left for us are those buried ones mostly 500 meters deep, for which the geophysical methods and the gravity-magnetism method in particular prove effective and win wide application because of low cost, rapid performance, and favorable effect. The increased depth calls for new data processing techniques to extract the weak information of ores in the deep. To this end, the method of wavelet analysis, lauded as the"mathematic microscope", emerges as an effective tool.With the approach firstly from theories of the Fourier transform and the window Fourier transform, this paper points out their weakness, that is, the fixed value of the differentiating ratio of window W (x) within the phase space when analyzing the local features of time-space domain and frequency domain, and thus introduces the wavelet transform as a prerequisite means for information analysis. From basic mathematic theories, the paper deals with how the Fourier analysis advances to wavelet analysis, the basic forms and characters of continuous wavelet formula and wavelet transform, the binary discrete forms and characters of wavelet transform, etc., and further explains the spatial significance of binary wavelet from the perspective of multi-discrimination analysis and introduces the concepts of scale function and double-scale formula. In the application part, the paper firstly gives explanation to the various techniques involving in the binary wavelet arithmetic and continuous wavelet transform. It proceeds to introduce the effective application of wavelet analysis on several models: the elimination of shallow interference in gravity survey, the differentiation of small and weak overlapping fields from the tectonic background, and the extraction of fault reservoir anomalies.In the part of real examples, the paper, in line with the principle of increasing difficulty, starts firstly from the profile data, realizing the 1-D wavelet transform, extracting the tectonic gravity anomalies by wavelet analysis, and finding their compatibility with the tectonic profiles derived from other methods.In the part of real examples, we also make a point of highlighting the efficiency of 2-D wavelet transform in differentiating the high-grade wavelet's minute anomalies of real gravity field. Firstly, we describe the background of data collecting and explain the high accuracy of primary data rarely found in recent years, which demands fine processing method for detecting any significant local tectonic anomaly; then, based on calculation by the theoretical model, we provide detailed analysis about the relationship between the gravity anomaly value and the local tectonic parameters like magnitude, depth, and amplitude, clarifying the exploration ability limit of the real data, which, according to the results of gravity interpretation, can be any local structure of more than 2 km2 in area and 150m in amplitude; then, we explain the factors which might influence high-precision gravity exploration for ancient buried hills. They all provide basis for the interpretation of gravity data. What is more, we give a brief account of the geological background, past geophysical exploration activities, the overall geological and drilling and geophysical data collected thus far for comprehensive interpretation. According to the intensive analysis of the density, we point out that the interface between the pre-Neozoic base and the lower Paleozoic roof might influence the shape of gravity anomaly, its undulation as the major factor incurring the local gravity anomalies in the area. The research provides physical basis for the application of high-precision gravity survey in exploring the tectonics of ancient buried hills. Further, in accordance with the characteristics of Bouguer gravity anomalies in the area, we mark out the tectonic units before extracting local gravity anomalies with the technique of wavelet transform and thus detect 63 local gravity anomalies. Also, we give detailed explanation about the technical guideline and method of interpreting the local gravity anomalies which, after comprehensive interpretation, include 38 local ones theoretically caused by Mesozoic and Paleozoic buried hills. Their depth and tectonic amplitude have been derived through single inversion and theoretical model calculation.Through the interpretation of high-precision gravity data, we acquire abundant geological information. The high-precision and qualified primary data provide solid basis for further data processing and comprehensive interpretation, which are usually carried out by utilizing many methods, parameters and the latest techniques, among them the wavelet transform method capable of detecting the weak anomalies effectively solves the differentiation of gravity anomalies, and, in combination with the existent 2-D and 3-D seismic interpretation results, markedly enhances the credibility of anomaly interpretation. According to gravity field features, we mark out 27 fault tectonics, including 3 SN ones, 9 NE ones, and 15 EW ones, and we clarify the tectonic architecture which provides help for research about the tectonics of the area. The Gaoqing-Pingnan fault (F7) through its extended activities not only controls the development and distribution of strata in the area and also exerts restrictive influence over the genesis and migration and concentration of oil and gas in the area.Through further interpretation and deduction of the high-precision gravity data, we illuminate the distributive law of ancient buried hills in the area. The pre-Neozoic buried hills are mainly distributed in the tectonic positions like the northern slope of the Luxi rise, the Qingcheng doom, the Wangpan Town buried hill zone, the Linfanjia low doom, the Shahejie tectonic zone, the Pingfang buried hill zone, and the like. Specifically, we point out that the Wangpan Town buried hill zone, under the control of a series of NE faults, extends five roof-like files; that the Qingcheng and Pingfang and Wangpan Town and Linfanjia buried hill zones prove favorable for oil concentration. As a result, rich oil and gas resources have been discovered in the Qingcheng and Pingfang and Linfanjia buried hill zones. Of rather great depth, the Linfanjia and Wangpan Town and Shahejie buried hill zones, if further explored, will lend good promises. As a whole, our work has highlighted the vision of exploration for oil and gas in the area. The results of gravity interpretation in particular ascertain the existence of 5 files of buried hills in the Wangpan Town zone, correcting the previous knowledge of 3 ones, and provide important information for research in the ancient buried hill tectonics in the area.