استادیار دانشگاه شاهرود، شاهرود، ایران
عنوان مقاله [English]
Gravity anomaly data are usually interpreted on the initial acquired surface. In some situations, it is useful to move the data to another surface for interpretation or comparison with another data set. Moving the data to another surface is called upward or downward continuation.
Â Â Â The regional-residual gravity anomaly separation is one of the most important steps in processing and interpretation of potential field data. The regional and residual gravity anomalies have long and short wavelengths, respectively. On the other hand, upward continuation is a mathematical transform that reduces the short-wavelength or shallow anomalies. Therefore, the upward continuation can be used to separate regional-residual gravity anomalies. In other words, upward continuation can be considered as a low-pass filter that attenuates the short-wavelength anomalies more than long-wavelength ones.
Â Â Â Selection of the optimum height for upward continuation is very important. By choosing the upward heightless thanoptimum value,remains the residual anomalies in the result of upward continuation and choosing the upward heightmore thanoptimum value,reduces the amplitude of the regional anomalies in the data Qualitative comparing of results from the upward continuation with the main data for various heights of upward continuation is a common method for detection of the optimum value of height.
Â Â Â A good height to separate regional from residual gravity anomalies will have a maximum cross-correlation between the regional and the upward-continued data. A possible method for estimating the optimum height for the synthetic model data can be derived using the cross-correlation between the regional anomaly at the observation level and the upward continuation of the observed anomaly at different heights. We used synthetic model data as two regional bodies at the 1500-meter depth with various lateral boundaries and three local bodies at the 200-meter depth and various lateral boundaries.
Â Â Â Because we do not know the real regional anomaly, we present a practical method based on the cross-correlations between the upward continuations at two successive heights to derive an optimum upward continuation height for the regional-residual gravity separatation. We calculate the cross-correlation versus height over a range, from zero to a height where a change in the cross-correlation values has clearly passed a maximum deflection from the chord joining the end point heights. The height of the maximum deflection of these cross-correlation values yields the optimum height for the upward continuation.
Â Â Â We tested the efficiency of the method on synthetic data. Our results showed that the maximum cross-correlation between regional and the upward-continued data coincide on the maximum deflection of the upward continuations at two successive heights.
Â Â Â Furthermore, the method was applied to the real gravity anomaly in the Hormozghan Province in the south of Iran to prospect a chromite ore bodies. As we expected, the Bouguer anomaly included both regional and residual anomalies related to the chromite ore body and other shallow sources, respectively. Therefore, we cannot distinguish the location of the mineral deposit. When we separate regional and residual gravity anomalies by an upward continuation at the optimum height, we can see that the obtained residual anomaly clearly shows the location of mineral deposit.