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A model study for estimating optimum upward-continuation ... for regional-residual gravity separation. Using this method we can calculate an optimal height for upward continuation

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    GEOPHYSICS, VOL. 72, NO. 4 �JULY-AUGUST 2007�; P. I45–I50, 9 FIGS., 1 TABLE. 10.1190/1.2719497

    model study for estimating optimum upward-continuation eight for gravity separation with application to a Bouguer gravity nomaly over a mineral deposit, Jilin province, northeast China

    ualin Zeng1, Deshu Xu2, and Handong Tan1

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    ABSTRACT

    Upward continuation can be used to separate a regional gravity anomaly resulting from deep sources from the ob- served gravity. We present a practical method, based on mod- el studies, to derive an optimum upward continuation height for regional-residual gravity separation. Using this method we can calculate an optimal height for upward continuation. Although mathematically there is no optimum height, this method provides an objective procedure to calculate a best height for upward continuation. We initially use a 2D model to calculate an optimum separation height, as given by the maximum crosscorrelation between the upward continuation of the observed gravity and a known regional anomaly. For an unknown regional field, we calculate a series of crosscorrela- tions between the upward continuations at two successive heights. The average height of the maximum deflection of these crosscorrelation values yields the optimum height for regional-residual separation. The method was applied to the Bouguer gravity anomaly over a mineral deposit in the Jilin province in northeast China. When we subtract the estimated regional anomaly obtained in this manner from the Bouguer anomaly, we can obtain a residual anomaly that clearly shows the location of two known iron bodies.

    INTRODUCTION

    Upward continuation is a method to separate a regional gravity nomaly resulting from deep sources from the observed gravity. The pward continuation operator �Jacobsen, 1987� is a numerically sta- le operation, and it forms a natural link between ground surveys and

    Manuscript received by the Editor June 13, 2006; revised manuscript recei 1China University of Geosciences, State Key Laboratory of Geological Pro

    gy, Beijing, China. E-mail: [email protected], [email protected] 2China University of Geosciences, State Key Laboratory of Geological Pro

    gy, Beijing, China; and Institute of Geophysical and Geochemical Explorati 2007 Society of Exploration Geophysicists.All rights reserved.

    I45

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    irborne surveys. There are two key problems with the method: �1� onventional upward continuation overattenuates the regional, and 2� the height must be known.

    First, let us consider the overattenuation problem. Pawlowski 1995� proposes a preferential continuation operator based on

    iener filtering and the Green’s theorem equivalent layer principle. he preferential continuation operator possesses a continuation re- ponse that acts upon a specific band of the observed potential field’s ourier amplitude spectrum. It is possible to use preferential upward ontinuation to attenuate short-wavelength anomalies from shallow ources while minimally attenuating long-wavelength signals from eep sources. Application of the preferential continuation to gravity nomalies in China has been effective in solving the overattenuation roblem �Xu and Zeng, 2000; Zeng and Xu, 2001�.

    Second, let us consider the requirement that upward continuation eight be known first. Gupta and Ramani �1980� point out that the ravity separation filter design is dictated, in the case of the upward ontinuation technique, by the choice of continuation height. The roblem is choosing a proper �optimum� height for upward continu- tion. Many people make this choice by inspection. Or they might hoose a height by comparing gravity anomalies upward continued o different heights. However, these two approaches lack an objec- ive criterion to give a proper �optimum� height for the upward con- inuation.

    We propose a method based on model studies to estimate an opti- al height for upward continuation to separate regional and residual

    nomalies. Using the method, we show a successful application on n anomaly over a mineral deposit in northeast China. Finally, we iscuss the conditions for applying the method.

    METHOD

    For separating regional gravity anomalies from residual ones, we ssume that a regional anomaly is caused by sources that �1� have

    uary 16, 2007; published online May 9, 2007. and Mineral Resources, and School of Geophysics and Information Technol-

    and Mineral Resources, and School of Geophysics and Information Technol- istry of Land and Resources, Hebei, China. E-mail: xu�[email protected]

    SEG license or copyright; see Terms of Use at http://segdl.org/

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    ide horizontal extent, �2� are deep in terms of the depth to the top of he sources, and �3� have similar depth. Likewise, we assume that a esidual anomaly is produced by sources that �1� are limited in hori- ontal extent, �2� are shallow, and �3� have depths different from the eep anomalies but similar to one another. The method proposed ere is based on these assumptions; it does not work for sources at hree or more depths.

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    igure 1. �a� Location of prisms producing the synthetic gravity nomaly. �b� The synthetic gravity anomaly, contour interval �CI� 1 mGal. �c� Gravity profile PP� along the east-west line of �a�; 1 synthetic gravity anomaly, 2 = regional anomaly resulting from

    odies A, 3 = local anomaly resulting from bodies B. �d� Vertical istributions of prisms along the profile.

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    odel

    We created gravity maps �see Figure 1b� using rectangular prisms t two different depths, A and B �Figure 1a and d�. The regional grav- ty anomaly �greg is assumed to be produced by the prisms at the reater depth �A�. The residual anomaly �gres is assumed to be pro- uced by bodies at a shallower depth �B� �Figure 1d�. Table 1 lists the odies’ depth to top, dimensions, width, and thickness. Bodies A1, 2, and A3 are the deep sources that produce the regional anomaly. odies B1, B2, B3, B4, and B5 are shallow and produce the residual nomaly. The upper part of Figure 1c shows three values: the total curve 1�, regional �curve 2�, and residual �curve 3�.

    A gravity anomaly profile PP� runs east to west and is located on igure 1a and b. The curves of Figure 2 show the profile at the ob- erved elevation, along with several upward-continued elevations. o carry out regional-residual separation using upward continua-

    ion, the key is to choose a height at which the continuation is most losely related to the known regional anomaly at observation level, s shown by anomaly curve 2 in Figure 1c.

    orrelation between continuation and regional anomaly

    The crosscorrelation r between the regional anomaly g1 ��greg� nd each continuation g2 is calculated by the following equation Abdelrahman et al., 1989�:

    rg1,g2 =

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    here M = 200 and N = 150. The crosscorrelation between the 2D regional anomaly and con-

    inuations to heights of 50,100, . . . ,1600 m are calculated �see Fig- re 3�. A good height to separate regional from residual will have a aximum crosscorrelation between regional and the upward-con-

    inued data. Figure 3 shows a correlation maximum at 600 m. ence, the continuation to 600 m is most similar to the regional

    nomaly, and 600 m is the optimum height for the upward continua- ion.

    Because we do not know the real regional anomaly, a possible ethod for estimating the optimum height for the model data can be

    erived using the crosscorrelation between the regional anomaly at he observation level and the upward continuation of the observed nomaly at different heights �see Figure 3�. When upward continua- ion height is smaller than optimal, the upward-continued anomaly learly consists of two components: the regional anomaly from the A odies and the anomaly from the B bodies. When the continuation eight is larger than optimal, the anomaly from the B bodies attenu- tes more and almost disappears. This situation results in a synthetic nomaly consisting mainly of

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