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1. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December, pp. 145-159 © IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2013): 5.3277 (Calculated by GISI) www.jifactor.com IJCIET ©IAEME ELLIPSOIDAL APPROXIMATION FOR TOPOGRAPHIC-ISOSTATIC MASSES EFFECTS ON AIRBORNE AND SATELLITE GRAVITY GRADIOMETRY A.A. MAKHLOOF Civil Engineering Department, Faculty of Engineering-Minia University ABSTRACT The topographic-isostatic masses represent an important source of gravity field information especially in the high-frequency band of the gravity field spectrum, even if the detailed density function inside the topographic masses is unknown. If this information is used within a remove-restore procedure, then the instability problems related to the downward continuation of gradiometer from airplane or satellite altitude can be reduced. In this paper, integral formulae are derived for the determination of gravitational effects of topographic-isostatic masses of the second order derivatives of the gravitational potential for various topographic-isostatic models. The application of these formulae is useful especially for airborne gradiometry and satellite gravity gradiometry. The computation formulae are presented in ellipsoidal approximation by separating the three-dimensional integration in an analytical integration in ellipsoidal element direction and integration over the unit area. Therefore, in the numerical evaluation procedure the ellipsoidal volume elements can be considered as being approximated by mass-lines, located in the centre of the discretization compartments (the mass of this element is condensated mathematically along its ellipsoidal normal axis). The formulae are applied to various scenarios of satellite gradiometry measurement campaigns. The gravitational tensor in the ellipsoidal normal direction component at a satellite altitude of 230 km for ESA’s gravity satellite mission GOCE (Gravity Field and Steady-State Ocean-Circulation Explorer) has been computed. The numerical computations are based on digital elevation models with five arc-minute resolutions for gravity gradiometry effects at satellite altitude. Keywords: Topographic-Isostatic Models, Ellipsoidal Approximation, Mass-Lines, Satellite Gravity Gradiometry, Downward Continuation, Regularization, GOCE. 145
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME 1. INTRODUCTION The determination of the gravity field from observations at aircraft or satellite altitudes is an improperly posed problem in the sense that small changes in the observations at flight level produce large effects in the gravity field parameters on the Earth’s surface or geoid level. This holds especially for the high-frequency constituents of the observation spectrum. To prevent the results from unrealistic oscillations in the parameters, regularization techniques are usually applied in very poorly conditioned cases (e.g. Ilk 1993). Most of the regularization methods represent a filtering procedure and the filtering property can be controlled by a regularization parameter (e.g. Ilk 1998). This is critical in those cases where the signal shows similar spectral characteristics as the observation noise. The topographic-isostatic masses represent a gravity field information especially in the high frequency band of the gravity field spectrum which can be superposed with the measurement noise in aircraft or satellite altitude. Therefore, it is helpful if those signal parts are reduced before the downward continuation and restored afterwards. In this case, it can be assumed that the high-frequency part in the observations is mainly caused by the observation noise, which can be filtered without loosing gravity field information. This procedure is only a first step to process airborne or satellite gravity field information in an integrated computation environment by involving all available Earth system information as sketched in Ilk (2000). In global applications as envisaged here, the frequently applied planar approximations of the topographic-isostatic models cannot be used anymore (see Novák el al. 2001). Therefore, the very efficient fast Fourier transformation (FFT) techniques cannot be applied for the present computations as demonstrated by Schwarz et al. (1990) in case of airborne gravimetry applications. Also, spherical approximation can not give the to be applied, especially for global or large-scale regional applications. There are two principal possibilities for calculating the effects of the topographic-isostatic masses on gravitational functionals in ellipsoidal approximation: the representation of the topographic masses by any ellipsoidal discretisation in form of ellipsoidal compartments (e.g. defined by geographical coordinate lines) and a subsequent integration (Abd-Elmotaal 1995b, Smith et al. 2001; Tenzer et al. 2003; Heck 2003) or the representation of the Newton’s integral by a spherical harmonic expansion (e.g. Sünkel 1985; Rummel et al. 1988; Tsoulis 1999, 2001). Sjöberg (1998) implemented the formulae for the exterior Airy-Heiskanen topographic/isostatic gravitational potential and the corresponding gravity anomalies. Geoid determinations with density hypotheses from isostatic models based on geological information have been studied by Kuhn (2003). The investigations performed thus far are limited to the determination of the second derivatives of the gravitational potential of the topographic-isostatic masses, necessary for Satellite Gravity Gradiometry (SGG) are not treated for the general case. Only the topographic-isostatic effects on the vertical component of the gravitational tensor have been studied by Wild and Heck (2004a,b) and Heck and Wild (2005). The effects of topographic-isostatic masses on satellite-to-satellite tracking (SST) data and SGG functionals based on spherical harmonic series are investigated by Makhloof and Ilk (2004). This procedure is very efficient but limited to an upper spherical harmonic degree of about 2700 which corresponds to a 4 arc-minute resolution. Beyond this degree numerical computation problems concerning the stability of the recursive computation of Legendre’s polynomials occur (see e.g. Holmes and Featherstone 2002). In this paper integral formulae in ellipsoidal approximation and based on mass-lines, approximating the ellipsoidal volume elements of the topographic-isostatic masses are presented which can be evaluated numerically based on global digital elevation/bathymetry models over land and oceans. The formulae are derived for the gravitational potential of topographic-isostatic masses itself, as well as the first and second derivatives. Different topographic-isostatic models have been investigated such as the Airy-Heiskanen, Pratt-Hayford the formulae for Helmert’s first and second condensation method are derived as well. The computation formulae are applied to the gravitational ele146
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME ments at satellite altitude envisaged for European Space Agency’s (ESA’s) gravity satellite mission GOCE (Gravity Field and Steady-State Ocean-Circulation Explorer). The effect of the topographic and isostatic masses 2. THE EFFECT OF THE TOPOGRAPHIC AND COMPENSATING MASSES In the following, the geoid used as reference surface for the heights given by the DEMs is approximated by a geocentric reference ellipsoid of major radius (a=6378 km). The geocentric heights of the computation and the integration points are given from DEM heights, interpreted here as orthometric heights. Therefore, the Cartesian coordinates of the points can be determined from the ellipsoidal coordinates as follows (Fig. 1): x = [ N (ϕ ) + h (ϕ , λ ) ] cos ϕ cos λ y = [ N (ϕ ) + h (ϕ , λ ) ] cos ϕ sin λ (1) z = N (ϕ )(1 − E 2 ) + h (ϕ , λ ) cos ϕ , hP l1 l P` Surface of the earth Reference ellipsoid hQ hP` ξ Fig. 1: Geometry of the topographical masses in ellipsoidal approximation where h(ϕ , λ ) is the ellipsoidal height, refers the topographical surface to the surface of the geocentric biaxial ellipsoid used in geodesy as a reference body for geometric and the ellipsoidal prime vertical radius of curvature a N (ϕ ) = (2) 2 (1 − e sin 2 ϕ )1 2 where e is the first eccentricity of the reference ellipsoid. 147
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME The potential of the topographical masses can be computed from Newton integral in ellipsoidal coordinates as follows (Novák and Grafarend 2005, Fig. 1): V ( p) = G∫∫∫ T 2π ρ(ϕ,λ,ξ) dv = G ∫ l v π2 hQ ρ(ϕ, λ,ξ) ∫ ∫ ϕ π ξ l λ=0 =− 2 =0 (N(ϕQ) +ξ)(M(ϕQ) +ξ)dξdσ , (3) with dσ = cos ϕQ dϕQ d λQ , n is the geoid undulation and the ellipsoidal meridian of curvature is given by: M (ϕQ ) = a (1− E sin2 ϕQ )3 2 (4) 2 and the distance between the computation and the integrated point is 12 l = ξ 2 + 2uξ + l12 (5) with u =xQh=0 −xPcosϕξ cosλ +yQh=0 −yPcosϕξ sinλ +zQh=0 −zPsinϕξ ξ ξ (6) and { 2 2 12 2 l1 = xξ =0 − xP + yξ =0 − yP + zξ =0 − zP } . (7) Eq. (3) can be written as the sum of spherical effect and ellipsoidal correction to the spherical approximation (in case of constant density): V T ( p) = V Ts + V Te Where hQ (ϕ,λ) VTs = Gρcr ∫∫ a2k1 + 2ak2 + k3 dσ 0 σ VTe = Gρcr E2 ∫∫ a2k1 + 2k2 (2sin2 ϕ −1) σ and hQ (ϕ,λ ) 0 (8) dσ (9) with k1 = ln u + ξ + l , k 2 = l − uk1 , (10) k 3 = (ξ − 3u )l + (3u − l ) k1 . 1 2 2 2 1 148
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME The direct topographical effect on gravity (the first derivatives of the potential of the topographical masses with respect to the ellipsoidal surface normal) can be given by: ∂VT = Gρcr ∫∫ ∂h σ hQ (ϕ,λ) ∫ ξ =n ∂ (N(ϕQ ) +ξ )(M(ϕQ ) +ξ ) ∂h dξdσ . l (11) Also, the first derivative of potential can be separated into spherical and ellipsoidal correction to the spherical term as follow: ∂V T ∂V T ∂V T = + ∂h ∂h s ∂h e (12) Where hQ (ϕ , λ ) ∂V T = G ρ cr ∫∫ a 2 P1 + 2 aP2 + P3 dσ ∂h s σ 0 (13) hQ (ϕ , λ ) ∂V T = G ρcr ∫∫ a 2 P + 2 P2 (2sin 2 ϕ − 1) dσ 1 0 ∂h e σ (14) With P= 1 Ru − Sl12 R − Su , +ξ 2 2 2 (l1 − u )l (l1 − u 2 )l (15) P2 = Rl12 − Sl12u Sl 2 + Ru − 2 Su 2 −ξ 1 2 + Sk1 , (l12 − u 2 )l (l1 − u 2 )l (16) 2Sl14 + Rl12u − 3Sl12u2 2Ru2 + 5Sl12u − 6Su3 − Rl12 2 S P= +ξ + ξ + (R − 3Su)k1 , 3 (l12 − u2 )l (l12 − u2 )l l (17) and R = xQh=0 − xP cosϕP cos λP + yQh=0 − yP cosϕP sin λP + zQh=0 − zP sinϕP , (18) S =cosϕP cosλP cosϕQ cosλQ +cosϕP sinλP cosϕQ sinλQ +sinϕP sinϕQ . (19) 149
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME Then, the effect of the topographic masses at airborne or satellite altitude (the second derivatives of the potential of the topographic masses with respect to ellipsoidal height) can be given by: ∂ 2V T = G ρ cr ∫∫ ∂h 2 σ hQ (ϕ , λ ) ∫ 0 ∂ 2 ( N (ϕ ) + ξ )( M (ϕ ) + λ ) ∂h 2 d ξ dσ l (20) This equation can be transformed to the following equation: ∂ 2V T = G ρcr ∫∫ ∂h 2 σ hQ (ϕ ,λ ) ∫ 0 ∂ 2 ( N (ϕ ) + ξ )( M (ϕ ) + λ ) 2 d ξ dσ l ∂h hQ (ϕ ,λ ) hQ (ϕ ,λ ) hQ (ϕ , λ ) ∂2 1 ∂2 ξ ∂2 ξ 2 d ξ + ( N (ϕ ) + M (ϕ )) ∫ dξ + ∫ d ξ dσ . = G ρ cr ∫∫ N (ϕ ) M (ϕ ) ∫ ∂h 2 l ∂h 2 l ∂h 2 l 0 0 0 σ (21) Three integral of Eq. (21) can be estimated separately; the first term hQ (ϕ ,λ ) N (ϕ ) M (ϕ ) ∫ 0 hQ (ϕ , λ ) 1 ( R 2 + 2 RSξ + S 2ξ 2 ) − 3 +3 dξ , ∫ l l5 0 −(ξ + u ) = N (ϕ ) M (ϕ ) 2 + 3R 2 A + 6 RSB + 3S 2C 2 (l1 − u ) ∂2 1 d ξ = N (ϕ ) M (ϕ ) ∂h 2 l (22) := N (ϕ ) M (ϕ ) w1 Where hQ (ϕ ,λ ) A= ∫ 0 hQ (ϕ ,λ ) B= ∫ 0 hQ (ϕ ,λ ) C= ∫ 0 (ξ + u ) 1 2(ξ + u ) + 2 dξ = 2 5 2 3 2 l 3(l1 − u )l 3(l1 − u l (23) −1 d ξ = 3 + uA l 3l (24) −ξ u l2 d ξ = 3 − B + 1 A . l5 2 2l 2 (25) ξ 5 ξ2 The second integral is given by: hQ (ϕ , λ ) [ N (ϕ ) + M (ϕ )] ∫ 0 hQ (ϕ , λ ) 1 ( R 2ξ + 2 RS ξ 2 + S 2ξ 3 ) − 3 +3 dξ , ∫ l l5 0 2 (l + uξ ) = [ N (ϕ ) + M (ϕ ) ] 1 + 3 R 2 B + 6 RSC + 3S 2 D , 2 2 (l1 − u )l ∂2 ξ d ξ = [ N (ϕ ) + M (ϕ ) ] ∂h 2 l := [ N (ϕ ) + M (ϕ ) ] w2 (26) 150
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME where hQ (ϕ , λ ) ∫ D= o ξ 3 d ξ = 3 + uC + 2l12 B . 5 l l ξ3 (27) The third integral of Eq. ( 21 ) is given by: hQ (ϕ , λ ) ∫ 0 ∂2 ξ 2 dξ = ∂h 2 l hQ ( ϕ , λ ) ∫ n ξ2 ( R 2ξ 2 + 2 RS ξ 3 + S 2ξ 4 ) − 3 +3 dξ l5 l ξ (l12 − 2u 2 ) = 2 − k1 + 3 R 2 C + 6 RSD + 3 S 2 F , 2 (l1 − u )l := w3 (28) where hQ (ϕ ,λ ) F= ∫ 0 ξ3 ξ (l 2 − 2u2 ) dξ = 3 − uD + 2l12 B − 12 2 + k1 . l5 (l1 − u ) −3l ξ4 (29) Using the binomial expansion (1 − x) − 1 2 = 1+ 1 3 x + x 2 + .......... 2 8 (30) that can be successfully be truncated for x p 1 , both radii of curvature can be written in the following form: 1 N (ϕ ) = a 1 + E 2 sin 2 ϕ 2 (31) 3 M (ϕ ) = a 1 + E 2 sin 2 ϕ − E 2 2 (32) Then, Eq. (21) can be written in the following formula: ∂ 2V T 2 (1 − E 2 ) 2 − E 2 (1 + sin 2 ϕ ) = a ∫∫ w1dσ + a ∫∫ w2 dσ + ∫∫ w3 dσ . 2 2 2 2 2 32 ∂h σ (1 − E sin ϕ ) σ σ (1 − E sin ϕ ) (33) The effect of the topographic masses can be written also in two terms: one of the spherical effect and the other is the correction to spherical term. Eq. (33) are used to compute the effect of the topographic masses for the case of Bouguer model, Airy-Heiskanen model. For calculating the effect of condensation masses in case of Helmert 151
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME second method of condensation and generalized Helmert method of condensation (Heck 2003), the effect of the condensation masses can be given as follow: V CM ( p) = G ∫∫ σ k ( N − D1 )( M − D1 )dσ l2 (34) where k is he surface layer density and given by (Novak and Grafarend (2005), D1 is the Helmert condensation depth and it equals zero in Helmert second method of condensation and 32 km in case of generalized Helmert of condensation , l2 here denotes the distance between the computation point and the integrated point at the condensation surface. In case of constant mass density of the topographical masses, the surface mass density is given by (Novák and Grafarend 2005) 3 hQ NQ + M Q hQ 1 kQ (ϕ , λ ) = ρ hQ 1 + + 2 NQ * MQ 3 NQ * M Q (35) Eq. (34) can be transformed also in two terms: one for the spherical approximation and the other is the ellipsoidal correction to the spherical approximation. The first and second derivative of the condensated topography can be determined as follows: ∂V CM ( p) ∂ 1 = G ∫∫ k ( N − D1 )( M − D1 )dσ ∂h ∂h l2 σ (36) ∂ 2V CM ( p) ∂2 1 = G ∫∫ k 2 ( N − D1 )( M − D1 )dσ ∂h2 ∂h l2 σ In case of the Airy-Heiskanen model the topographic masses of constant density ρ float on a mantle of constant but larger density ρ m . An elevation column of height h is compensated by a corresponding root of thickness t. The higher the topographic features are, the deeper they sink. Thus the thickness of a root column under a mountain column with height h can be determined by the formula hQ −T ∫∫ ξ∫ ρ ( N +ξ )( M +ξ ) dξdσ = ∫∫ ξ ∫ σ σ =0 ∆ρ ( N + ξ )( M + ξ ) dξ dσ . (37) =−T −tQ After Performing the numerical integration for finding the root for height larger than 10 km and comparing the result with results from spherical formula and the error was lesser than 0.01%. Then the spherical formula for computing the roots is applied in the present investigation. The root is given by (Khun 2000) ( R + h)3 − R 3 ρ , t = ( R − T ) 1 − 3 1 − ( R − T ) 3 ∆ρ and the anti-roots of thickness t ′ in case of oceanic water columns of height h′ by, 152 (38)
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME R 3 − ( R − h′)3 ( ρ − ρ ) w 3 1+ t′ = (R − T ) − 1 , ( R − T )3 ∆ρ (39) with the water density ρ w and the mean radius of the Earth R (R=6371 km). The density difference ∆ρ = ρ m − ρ in the roots and anti-roots produces a buoyancy so that hydrostatic equilibrium is reached. The above formulae can be simplified if the heights of the Earth’s surface are expressed by the rock-equivalent topographic heights heq (Rummel et al. 1988). It can be expressed in planar approximation as follows: heq land: h eq = h = ρ − ρw , h′ ocean: h eq = ρ (40) The potential of the isostatic masses at the computation point P can be determined analogously as the potential of the topographic masses by Newton’s integral, 2π π2 −T ∆ρ(ϕ, λ,ξ) ∆ρ(ϕ, λ,ξ) V ( p) = G∫∫∫ dv = G ∫ ∫ ∫ (N(ϕQ ) +ξ)(M(ϕQ ) +ξ)dξdσ . l l v λ=0 ϕ=−π 2 ξ =−T −t I (41) This integration can be written as the sum V I ( p ) = V Is + V Ie (42) where −T V Is = G ρcr ∫∫ a 2 k1 + 2ak2 + k3 −T −tQ dσ (43) σ −T V Ie = G ρcr E 2 ∫∫ a 2 k1 + 2k2 (2sin 2 ϕ − 1) −T −tQ dσ (44) σ The combined effects of the topographic-isostatic masses on the different gravity functionals are the differences between the effect of the topographic masses and the effect of the isostatic compensation masses. It reads e.g., VTI h = V T P h=hP −V C (45) h=hP In case of the Pratt-Hayford model a certain adjustment surface is defined, in case of a ellipsoidal approximation an ellipse in a constant depths D2 ( D2 = 100 km ). At this ellipsoidal surface hydrostatic equilibrium is anticipated, i.e. the pressure of any topography column is identical at this 153
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME ellipse which requires constant mass but different density depending on the elevation of the surface of the Earth. The effect of the topographic masses according to the Pratt-Hayford model is calculated for two cases: one for the land areas and the other for the ocean areas. The same has to be done for the effects of the isostatic compensation masses. Then, the effect of the topographic masses is given by V P −T ( p ) = VTland + VTOcean (46) where land T V ( p) = G∫∫∫ ρL v VTOcean ( p) = G∫∫∫ v l 2π dv = G ∫ hQ π2 ρL (ϕ, λ,ξ) ∫ ∫ λ=0 ϕ=−π 2 ξ =0 2π π 2 l (N(ϕQ ) +ξ )(M(ϕQ ) +ξ )dξdσ , (47) 0 (ρ − ρw ) (ρ − ρw ) dv = G ∫ ∫ ∫ ( N (ϕQ ) + ξ )(M (ϕQ ) + ξ )dξ dσ l l λ =0 ϕ =−π 2 ξ =h′ (48) The effect of the compensation masses for land areas is given by 2π land C V π2 hQ (ρ − ρL ) (ρ − ρL ) ( p) = G∫∫∫ dv = G ∫ ∫ ∫ (N(ϕQ ) +ξ)(M(ϕQ ) +ξ)dξdσ , l l v λ=0 ϕ=−π 2 ξ =0 (49) and for ocean areas by: 2π Ocean C V π2 −hQ (ρ − ρO) (ρ − ρO) ( p) = G∫∫∫ dv = G ∫ ∫ ∫ (N(ϕQ) +ξ)(M(ϕQ) +ξ)dξdσ l l λ=0 ϕ=−π 2 ξ=−D v (50) where density ρ L in land areas and density under ocean ρO can be determined from Kuhn (2000). R 3 − ( R − D )3 ρL = ρ, 3 3 ( R + h) − ( R − D ) (51) R 3 − ( R − D )3 ρ − ρ w R 3 − ( R − h′)3 , ρO = 3 3 (52) ( R − h′) − ( R − D ) 154
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME 3. TEST REGIONS The numerical tests are based on the ETOPO5 with five arc-minute resolution. One typical test regions have been selected: This test area is covering the Himalaya region (Fig. 2), shall be used to visualize the damping of the topographic-isostatic effects in the component of the gravity gradiometry in the ellipsoidal normal direction at 230km above sea surface. In this case resolution for the elevation compartments has been used. The accuracy of this DEM is sufficient to demonstrate the topographic-isostatic effects at satellite altitude. Fig. 2: Topography of the test area (ETOPO5) The densities of crust and topography are considered to be constant and equal to 2670 kg/m3. As density of sea water a value of 1030 kg/m3 has been taken and as density of the mantle the frequently used value of 3270 kg/cm3 has been assumed. The Airy-Heiskanen depth of compensations is considered to be 30 km, The depth of the condensation surface of Helmert’s first condensation method is 21 km and 32 km for the generalized Helmert’s condensation model. 4. NUMERICAL ANALYSIS To give an impression of the size of the topographic-isostatic effects on the gravity gradiometry at satellite altitude, the topographic-isostatic effects on the of the gravitational tensor at a satellite altitude of 230 km are computed. It can be found that, the structure of the topographicisostatic effects at a satellite altitude of 230 km shows still steep changes with a gradient of 0.80 Eötvös per 100km in north-south direction and of 0.60 Eötvös per 100 km in east-west direction. If it is possible to remove this sort of roughness from the observations with a noise level of approximately 5 mE for GOCE the downward continuation can be considerably simplified. The GOCE mission is designed to derive the static part of the gravity field with an extremely high precision. Therefore, it is very important to filter the observations by the topographic-isostatic gravity field effects to ease the requirements for the downward continuation. An additional regularization might be avoided in that case; but this depends on the envisaged resolution of the gravity field model and on the error level of the observations. Fig. 3 gives an impression of the size of these effects for the hh-component of the gravitational tensor. 155
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME 5. CONCLUSIONS In this paper the formulae for the calculation of the first and second derivatives of the gravitational potential of the topographic-isostatic masses are derived for various frequently applied topographic-isostatic models in ellipsoidal approximation by approximating the ellipsoidal volume elements by mass-lines located in the middle of the compartments. Only the formula for the determination of the first derivatives in the ellipsoidal approximation has been derived by one author. The situation is different for the second derivatives of the potential; the second derivatives of the potential in the ellipsoidal normal direction have not been studied till now. The formulae can be used to determine the topographic-isostatic effects at aircraft altitudes for airborne applications or for satellite altitudes to reduce the observation functionals of airborne or satellite gravity gradiometry missions. The integral formulae presented here allow to use DEMs with an - in principle - arbitrary high resolution depending on the numerical integration method. Obviously such a high resolution is necessary for airborne altitudes. In these cases it might not be to apply formulae which are based on the expansion of Newton’s integral in spherical harmonics at least for global applications (see e.g., Makhloof 2007). As the higher resolution of the DEMs is indispensable and the computation of the corresponding spherical harmonic coefficients can be critical because of numerical problems (Holmes and Featherstone 2002). Therefore only a maximum degree of 2700 corresponding to a compartment size of 4 arc-minutes resolution can be selected. Due to this limitation a spherical harmonic expansion of the topographic-isostatic masses cannot be used for exact determinations of the geoid (see Kuhn and Seitz 2005). a) Bouguer method (topography) b) Generalized Helmert (D=32km) b) Helmert first method (D=21 km) b) Airy-Heiskanen method 156
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME f) Helmert first method Fig. 1: Effects of the topographic-isostatic masses on the tensor component Vzz at an altitude of 230 km for different topographic-isostatic models (Eötvös). Now the question arises which model should preferably be used for the filtering of satelliteborne observations such as gravity gradients as preparation for the subsequent downward continuation procedure? While Helmert’s second condensation method might be useful for geoid computations, its usefulness for the processing of satellite in-situ observations cannot be answered in such a simple generally valid way. Indeed, the task of filtering topographic-isostatic effects in the satellite observations is helpful only if these quantities have a significant magnitude larger than the observation noise. Obviously, this fact may depend on the validity of the topographic-isostatic hypothesis in specific geographic regions. This can be decided only after a careful analysis of the specific gravity field features within the various geographical regions of the Earth to find out which model describes the reality in these regions in the best possible way. It is well-known that the Earth is isostatically compensated by an amount of approximately 90% (Heiskanen and Moritz 1967), but it is difficult to decide which model fits best. Although seismic measurement results indicate the validity of an AiryHeiskanen type of topographic-isostatic compensation, but in some parts of the Earth the isostatic compensation seems to follow anther model (Heiskanen and Moritz 1967). The change of the condensation level by using a sort of a generalized Helmert model (Heck 2003) could be used to fit the topographic-isostatic model to the reality. Very promising seems to be to introduce geophysical models in coincidence with modern models of plate tectonics. If the specific topographic-isostatic model holds more or less uniformly for a larger region then this model can be used to filter the satellite observations before the application of the regional gravity field recovery procedure. The situation is more complicated in case of regionally varying deviations of the reality from a specific model; further investigations are necessary to consider these cases. Because of the varying effects of the topographic-isostatic models depending on the type of observables such as gravity vectors or tensor components the frequently expressed argument that a high resolution gravity field model might be sufficient to reduce observations at aircraft or satellite altitude is not valid. Therefore, additional investigations are necessary to demonstrate the benefit of a remove-restore procedure taking into account individually selected topographic-isostatic models for the processing of airborne measurements and SGG observables. Finally, the results computed here are computed with the results computed using spherical approximation and it is found that the ellipsoidal approximation gives exact results. 157
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. Abd-Elmotaal H (1995b), "Attraction of the topographic masses", Bull. Geod (69): 304-307 Heck B (2003), "On Helmert’s methods of condensation". J Geod 77 (3-4): 155-170 Heck B and Wild F (2005), Topographic-isostatic reductions in satellite gravity gradiometry based on a generalized condensation model. In: Sansó, F. (ed): A Window on the Future of Geodesy, Springer Verlag, IAG Symposia 128: 294-299 Heiskanen WA, Moritz H (1967) Physical Geodesy. WH Freeman, San Francisco Holmes SA and Featherstone WE (2002), "A unified approach to the Clenshaw summation and the recursive computation of very high degree and order normalized associated Legendre functions", J Geod 76(5): 279-299 Ilk KH (1993), "Regularization for high resolution gravity field recovery by future satellite techniques", in: G Anger el al. (eds.): Inv. Probl. Principles and Applications in Geophysics, Technology and Medicine, Mathematical Research, Vol. 74, Akademie Verlag GmbH, Berlin Ilk KH (1998), "A proposal for the determination of optimal regularization parameters in Tikhonov-type regularization methods", Proc. of the International Seminar on Model Optimization in Exploration Geopyhsics 2, Friedr. Vieweg und Söhne, Braunschweig-Wiesbaden Ilk KH (2000), "Envisaging a new area of gravity field research, in: Towards an Integrated Global Geodetic Observing System (IGGOS)", Rummel R, Drewes H, Bosch W, Hornik H (eds.): IAG Symposium 120, pp. 53-62, Springer-Berlin Heidelberg New York Kuhn M (2000) Geoidbestimmung unter Verwendung verschiedener Dichtehypothesen. Deutsche Geodätische Kommission, Rh. C,Heft Nr 250, Munich Kuhn M (2003), "Geoid determination with density hypotheses from isostatic models and geological information", J Geod 77 (1-2): 50-65 Kuhn M, Seitz K (2005), "Comparison of Newton’s integral in the space and frequency domain, In: Sansó", F. (ed): A Window on the Future of Geodesy, Springer Verlag, IAG Symposia 128: 386-391 Makhloof A. A, Ilk KH (2004), "The use of topographic-isostatic gravity field information in satellite-to-satellite tracking and satellite gravity gradiometry", Poster presented at the IAG International Symposium Gravity, Geoid and Space Missions, GGSM2004, Porto Makhloof, A. A. (2007)," The use of Topographic- isostatic mass information in geodetic applications", Ph. D. dissertation, Institute of geodesy and geoinformation, Bonn University, Bonn, Germany NOAA (1988), "Data Announcement 88-MGG-02, Digital relief of the surface of the Earth, National Geophysical Data Centre", Boulder Novák P, Vaniček P, Martinec Z, Véronneau M (2001), "Effect of the spherical terrain on gravity and the geoid", J Geod 75 (9-10): 491-504 Novák P, Grafarend EW (2005), "The ellipsoidal representation of the topographical potential and its vertical gradient", J Geod 78 (11-12): 691-706 Rummel R, Rapp R H, Sünkel H, Tscherning CC (1988), "Comparisons of global topographic-isostatic models to the Earth’s observed gravity field". Rep 388, Department of Geodetic Science and Surveying, The Ohio State University, Columbus Schwarz KP, Sideris MG, Forsberg R (1990), "The use of FFT technique in physical geodesy", Geophysical Journal International (100): 485-514 Smith DA, Robertson DS, Milbert DG (2001), "Gravitational attraction of local crustal masses in spherical coordinates". J Geod 74 (11-12): 783-795 Sjöberg LE (1998), "The exterior Airy/Heiskanen topographic-isostatic gravity potential, anomaly and the effect of analytical continuation in Stokes’ formula", J Geod (72): 654-662 158
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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 6, November – December (2013), © IAEME 21. Sünkel H (1985), "An isostatic Earth model", Report. 367, Department of Geodetic Science and Surveying, The Ohio State University, Colubus 22. Tenzer T, Vaníček P, Novák P (2003), "Far-zone contributions to topographical effects in the Stokes-Helmert method of the geoid determination", Studia Geophysica et geodaetica 47 (3): 467-480 23. Tsoulis D (1999), "Spherical harmonic computations with topographic/isostatic coefficients", Reports in the series IAPG / FESG (ISSN 1437-8280), Rep. No. 3 (ISBN 3-934205-02-X), Institute of Astronomical and Physical Geodesy, Technical University of Munich 24. Tsoulis D (2001), "A Comparison between the Airy-Heiskanen and the Pratt-Hayford isostatic models for the computation of potential harmonic coefficients", J Geod 74 (9): 637-643 25. Wild F, Heck B (2004a), "Comparison of different isostatic models applied to satellite gravity gradiometry". Jekeli, C. Bastos, L. Fernandes, J. (eds.): Gravity, Geoid and Space Missions, GGSM 2004, pp 230-235, Springer-Berlin Heidelberg New York 26. Wild F, Heck B (2004b), "Effects of topographic and isostatic masses in satellite gravity gradiometry". Proc. 2nd Int. GOCE User Workshop, Frascati Mach 2004 27. A.N.Satyanarayana, Dr Y.Venkatarami Reddy and B.C.S.Rao, “Remote Sensing Satellite Data Demodulation and Bit Synchronization”, International Journal of Advanced Research in Engineering & Technology (IJARET), Volume 4, Issue 3, 2013, pp. 1 - 12, ISSN Print: 0976-6480, ISSN Online: 0976-6499. 159
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