### A Comparative Study on Two Different Methods for Calculating Gravity Effect of an Uneven Layer: Application to Computation of Bouguer Gravity Anomaly in the East Vietnam Sea and Adjacent Areas

## Main Article Content

## Abstract

Calculation of gravity anomaly caused by an uneven layer is essential for quantitative interpretation. By comparing calculated anomalies with observed anomalies, we may infer some parameters of subsurface structures. There are many different methods for computing gravity effect of an uneven layer. This paper presents a comparative study of two different forward methods such as the space domain method and the frequency domain method. The performance of each method was evaluated on two synthetic models. Finally, the more effective method was applied to calculate Bouguer gravity anomaly in the East Vietnam Sea and adjacent areas using the latest available dataset from the TOPEX mission.

*Keywords:*Forward method, space domain, frequency domain, Bouguer gravity anomaly, Vietnam.

## References

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[2] L.T. Pham, E. Oksum, T.D. Do, GCH_gravinv: A MATLAB-based program for inverting gravity anomalies over sedimentary basins, Computers & Geosciences 120 (2018) 40–47.

[3] E. Oksum, M.N. Dolmaz, L.T. Pham, Inverting gravity anomalies over the Burdur sedimentary basin, SW Turkey, Acta Geodaetica et Geophysica 54 (2019) 445–460.

[4] L.T. Pham, E. Oksum, T.D. Do, Edge enhancement of potential field data using the logistic function and the total horizontal gradient, Acta Geodaetica et Geophysica 54 (2019) 143-155.

[5] L.T. Pham, T.D. Do, E. Oksum, S.T. Le, Estimation of Curie point depths in the Southern Vietnam continental shelf using magnetic data, Vietnam Journal of Earth Sciences 41(2019) 216-228.

[6] A.M. Eldosouky, L.T. Pham, H. Mohammed, B. Pradhan, A comparative study of THG, AS, TA, Theta, TDX and LTHG techniques for improving source boundaries detection of magnetic data using synthetic models: a case study from G. Um Monqul, North Eastern Desert, Egypt, Journal of African Earth Sciences 170(2020) 103940.

[7] L.T. Pham, A comparative study on different filters for enhancing potential field source boundaries: synthetic examples and a case study from the Song Hong Trough (Vietnam), Arabian Journal of Geosciences 13(2020) 723.

[8] L.T. Pham, T.V. Vu, S. Le-Thi, P.T. Trinh, Enhancement of Potential Field Source Boundaries Using an Improved Logistic Filter, Pure and Applied Geophysics (2020). https://doi.org/10.1007/s00024-020-02542-9

[9] L.T. Pham, T.D. Do, Estimation of sedimentary basin depth using the hybrid technique for gravity data, VNU Journal of Science: Mathematics – Physics, 33 (2017), 48-52.

[10] L.T. Pham, E. Oksum, T.D. Do, M.D. Vu, Comparison of different approaches of computing the tilt angle of the total horizontal gradient and tilt angle of the analytic signal amplitude for detecting source edges, Bulletin of the Mineral Research and Exploration 16(2020). https://doi.org/10.19111/bulletinofmre.746858.

[11] M. Talwani, J. Worzel, M. Ladisman, Rapid gravity computations for two dimensional bodies with application to the Mendocino submarine fracture zone, Journal of Geophysical Research 64 (1959) 49–59.

[12] I.V.R. Murthy, D.B. Rao, Gravity anomalies of two-dimensional bodies of irregular cross-section with density contrast varying with depth, Geophysics 44 (1979) 1525–1530.

[13] I.V.R. Murthy, S.J. Rao, A Fortran 77 program for inverting gravity anomalies of two-dimensional basement structures, Computers & Geosciences 15 (1989) 1149–1156

[14] J.J. Pan, Gravity anomalies of irregularly shaped two-dimensional bodies with constant horizontal density gradient, Geophysics 54 (1989) 528–530.

[15] V.C. Rao, V. Chakravarthi, M.L. Raju, Forward modeling: Gravity anomalies of two-dimensional bodies of arbitrary shape with hyperbolic and parabolic density functions, Computers & Geosciences 20 (1994) 873-880.

[16] S.E. Oliva, C.L. Ravazzoli, Complex polynomials for the computation of 2D gravity anomalies, Geophysical Prospecting 45 (1997) 809–818.

[17] M. Talwani, M. Ewing, Rapid computation of gravitational attraction of three-dimensional bodies of arbitrary shape, Geophysics 25 (1960) 203-225.

[18] D. Nagy, The gravitational attraction of a right rectangular prism, Geophysics 31 (1966) 362–371.

[19] H.J. Goetze, B. Lahmerger, Application of threedimensional interactive modeling in gravity and magnetics, Geophysics 53 (1988) 1096–1108.

[20] D.B. Rao, M.J. Prakash, N. Ramesh Babu, 3-D and 2 1/2-D modeling of gravity anomalies with variable density contrast, Geophysical Prospecting 38 (1990) 411–422.

[21] H. Holstein, B. Ketteridge, Gravimetric analysis of uniform polyhedra, Geophysics 61 (1996) 357–364.

[22] B. Singh, D. Guptasarma, New method for fast computation of gravity and magnetic anomalies from arbitrary polyhedra, Geophysics 66 (2001) 521–526.

[23] K. Mallesh, V. Chakravarthi, B. Ramamma, 3D gravity analysis in the spatial domain: model simulation by multiple polygonal cross-sections coupled with exponential density contrast, Pure and Applied Geophysics 176 (2019) 2497–2511.

[24] R.L. Parker, The rapid calculation of potential anomalies, Geophysisical Journal of the Royal Astronomical Society 31 (1973) 447–455.

[25] H. Granser, Three-dimensional interpretation of gravity data from sedimentary basins using an exponential density-depth function, Geophysical Prospecting 35 (1987) 1030–1041.

[26] Y. Chai, W.J. Hinze, Gravity inversion of an interface above which the density contrast varies exponentially with depth, Geophysics 53 (1988) 837–845.

[27] J. Feng, X. Meng, Z. Chen, S. Zhang, Three-dimensional density interface inversion of gravity anomalies in the spectral domain, Journal of Geophysics and Engineering 11 (2014) 035001.

[28] W.H.F. Smith, D.T. Sandwell, Global seafloor topography from satellite altimetry and ship depth soundings, Science 277 (1997) 1957-1962.

[29] D.T. Sandwell, E. Garcia, K. Soofi, P. Wessel, and W.H.F. Smith, Towards 1 mGal Global Marine Gravity from CryoSat-2, Envisat, and Jason-1, The Leading Edge, 32 (2013) 892-899.

[30] D.T. Sandwell, R.D. Müller, W.H.F. Smith, E. Garcia, R. Francis, New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure, Science 46 (2014) 65-67.