Estimation of the optimum upward continuation height for chromite prospecting at Hormozgan province

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.The regional-residual gravity anomaly separation is one of the most important steps inprocessing and interpretation of potential field data. The regional and residual gravityanomalies have long and short wavelengths, respectively. On the other hand, upwardcontinuation is a mathematical transform that reduces the short-wavelength or shallow anomalies. Therefore, the upward continuation can be used to separate regional-residualgravity anomalies. In other words, upward continuation can be considered as a low-passfilter that attenuates the short-wavelength anomalies more than long-wavelength ones.Selection of the optimum height for upward continuation is very important. Bychoosing the upward height less than optimum value, remains the residual anomalies inthe result of upward continuation and choosing the upward height more than optimumvalue, reduces the amplitude of the regional anomalies in the data Qualitative comparingof results from the upward continuation with the main data for various heights of upwardcontinuation is a common method for detection of the optimum value of height.A good height to separate regional from residual gravity anomalies will have amaximum cross-correlation between the regional and the upward-continued data. Apossible method for estimating the optimum height for the synthetic model data can bederived using the cross-correlation between the regional anomaly at the observation leveland the upward continuation of the observed anomaly at different heights. We usedsynthetic model data as two regional bodies at the 1500-meter depth with various lateralboundaries and three local bodies at the 200-meter depth and various lateral boundaries.Because we do not know the real regional anomaly, we present a practical methodbased on the cross-correlations between the upward continuations at two successiveheights to derive an optimum upward continuation height for the regional-residual gravityseparatation. We calculate the cross-correlation versus height over a range, from zero to aheight where a change in the cross-correlation values has clearly passed a maximumdeflection from the chord joining the end point heights. The height of the maximumdeflection of these cross-correlation values yields the optimum height for the upwardcontinuation.We tested the efficiency of the method on synthetic data. Our results showed that themaximum cross-correlation between regional and the upward-continued data coincide onthe maximum deflection of the upward continuations at two successive heights.Furthermore, the method was applied to the real gravity anomaly in the HormozghanProvince in the south of Iran to prospect a chromite ore bodies. As we expected, theBouguer anomaly included both regional and residual anomalies related to the chromiteore body and other shallow sources, respectively. Therefore, we cannot distinguish thelocation of the mineral deposit. When we separate regional and residual gravity anomaliesby an upward continuation at the optimum height, we can see that the obtained residualanomaly clearly shows the location of mineral deposit.