Study On Gravity Data Processing Method And Apply It To Sylvite Mine Exploration

With the development of modern gravity exploration, nearly one hundred years, it is still a very important geophysical method. The data from gravity exploration provide us with a lot of underground geological information after data processing and interpretation. With the update of data processing method and interpretation technology, its capacity has been improved.In this paper, study on gravity data processing method. Mainly comparative analysis some routine method, and improve some of them. Take advantage of new method to deal with gravity data of sylvite mine in GanMeng Province of Lao People's Democratic Republic, and gain new understanding of the sylvite mine.The structure of this paper is the same as the flow of gravity data processing, details as follow:1. The method of extracting the target gravity anomalyGravity anomalies are the responses of all density distribution underground, and the results of extracting the target gravity anomaly effects latter interpretation directly. Base on the principle of dividing the Bouguer gravity anomaly into two parts, one is regional gravity anomaly and the other is local gravity anomaly, compare the trend analysis method, continuation method, finite element method, iterative difference method with nine points, cut interpolation, and wavelet multi-resolution analysis method. Begin with their basic principles, find out the conditions of each method and the implicit subjective factors. All of them can isolate the local anomalies and the regional anomalies well. Relatively, the finite elements method is the best while the continuation method is the worst. But so far, none is better than other method under any conditions. Every anomaly separation method has its assumptions. Once the given data satisfy the prerequisite of it, it will bring better result than any other method. So, when deal with gravity exploration data, we should analysis the data firstly. Then compare the best results of different methods, choose the result of the method, whose prerequisite is best met.2. Regional sub-block methodSub domain filtering play a role of compressing gravity gradient zone, which is used to divide research region into several sub blocks and to find deep fault in favorite conditions. Traditional sub domain filtering divides the moving windows into eight small domains, whose shapes are not the same. This paper proposes a new sub domain filtering, whose eight sub domains are the same. The size of new sub domain is only half of the size of traditional sub domain, so it is facilitate to calculation. Under the condition of with the same data and the same size of moving window, the time consuming of new method is only half of traditional method. More importantly, new method brings greater convergence at gravity gradient zone and less distortion at corners.3. Edge enhance, extraction and fracture interpretationThe borders of geological body or linear structures on Bouguer gravity anomaly map are usually at high frequency and low amplitudes. In order to obtain this information more accurately, it is necessary to enhance these wick anomalies. This paper compares the tilt method, the horizontal derivate of tilt method, normalized analytical signal amplitude method and normalized standard deviation of vertical derivate method, which are popular methods in recently years. Find out that all of them utilize the vertical derivate to enhance wick signal. So, it is important to calculate the vertical derivate correctly. This paper discusses the effect of different edge extension methods in frequency domain. Propose a new extension method by cycle expansion with spline interpolation, which enhance the accuracy of vertical derivate compared with regional extension method, cosine extension method, spline extension method. The effect of vertical derivate by Hilbert transform is also compared. Draw a conclusion that the favorable conditions for applying the new extension method by cycle expansion with spline interpolation are data is under sampling or both ends of data varies with small gradient.Set up several geological models buried with different depths, whose gravity anomaly is interference by regional gravity anomaly. All methods above can enhance weak anomaly aroused by geological edge. But the shape of anomaly is with distorted by interference. On tilt map, the zero contour line is related to the geology border, while on the other maps, the local maxim contour is correspond to the border. Both the zero contour and maxim contour deviate from the theoretical positon. It is difficault to locate the boder positon of geology model from these maps.Use the approximating edges method proposed by Blakely and Simpson to locate the local maixm points, but it dose not fit for zero contour detecting. This paper proposes applying the Canny edge detection oprator to locate the zero contour on tilt map at first time. Canny operator is one of the optimize operator, which has a low error rate and high accuracy of locating the marginal point. Besides, the probability of response to multiple margion is low. Model test proved that this method with strong ability of anti-noise and with high precision of extracting borders.In order to interprate the faults in details, we need to seek other methods. Through model test and application example, the gravity gradient method under favorable conditions can provide inclication and depth of fault.4. Density interface inversion and pseudo-density inversionCompared two kinds of density interface inversion method, one is widely used Parker-Oldenburg method, and the other is compressed density layer method. In the application example, the curst-mantle boundary shapes inversed by both methods are similar. The results depend on the gravity anomaly aroused by interface can be correctly extracted and both methods require the average depth of interface much deeper than the ups and downs. For 3D pseudo-density inversion, I adopt the method proposed by Xu Shizhe. Model test proved that the key technologies of this method– the use of iterative upward continuation to calculate downward continuation is feasible.Apply the new methods and techniques in gravity anomaly of sylvite mine in Laos, I gain new information:1. The local gravity anomaly map obtained from wavelet multresolution analysis is more in line with the geological condition than that from upward continuation method. At least it is more incordance with drill results and model results.2. The result of using Canny operator dealed with tilt map is similar with that from horizontal derivates at different angels and continuation method. But the front method provides more information. Appling the gravity gradient method to the main fault, calculate the inclication and depth.3. By pseudo-density inversion, I come to understand the relative density distributions at different depths.4. Reduce the boundarys of sylvite mine area, and find out a new favorable area for exploration.