Retrieving Complete Spherical Bouguer and Isostatic Gravity Anomalies Using Global Gravity Forward Models

We discuss some theoretical aspects and practical consequences of using traditional versus “new”/rigorous formulations of the Bouguer and isostatic gravity anomalies/disturbances. In principle, the differences between these two concepts are in the definition of the so-called secondary indirect topographic effect (SITE) on the gravity data. Although we follow the tradition to call this effect SITE, we show that it is formally a direct topographic effect (DITE), needed to remove all topographic signal, but in practice not regarded as such. Consequently, there is a need for a no-topography gravity anomaly, which removes all topographic effects, leaving the below-crust Earth transparent for gravity inversion. Similarly, a rigorous isostatic gravity anomaly includes also a compensation effect for the SITE. By using a simple topographic model, we confirm a theoretically found ratio of 2/(n + 1) between the magnitudes of the SITE and DITE by wavelength (spherical harmonic degree n), both for the Bouguer and isostatic gravity anomalies. Finally, global gravity inversions are applied by utilizing the Vening Meinesz-Moritz isostatic model to determine the Moho geometry using the Bouguer gravity disturbances/anomalies and the no-topography gravity anomalies, and the results are compared. The numerical results confirm our theoretical findings that the Bouguer gravity disturbances and the no-topography gravity anomalies provide very similar results. A comparison of these gravimetrically computed Moho depths with the CRUST1.0 seismic model shows rms agreements of 4.3 and 4.5 km, respectively. This is a significant improvement when compared to the Moho result obtained by using the Bouguer gravity anomalies, yielding the rms difference of 7.3 km for the CRUST1.0 model. These results confirm a theoretical deficiency of the classical definition of the Bouguer and isostatic gravity anomalies, which do not take into consideration the SITE effects on the topography and its compensation.

SUMMARY Gravity anomalies from geological features in the upper crust are masked by large amplitude long wavelength gravity variations from isostatic roots. An isostatic residual (IR) gravity anomaly grid of onshore Australia has been produced by Geoscience Australia which has these long wavelength features removed. This gravity map reveals more clearly the density distributions of geological interest within the upper crust. The depth to mantle model and subsequent isostatic corrections were produced using a modified version of the USGS program AIRYROOT provided by Intrepid Geophysics. Geoscience Australia’s 2009 Bathymetry and Topography Grid was used to calculate the depth to crustal bottom following the Airy-Heiskanen crustal-root model. The isostatic corrections were then applied to the complete Bouguer anomalies to produce the Isostatic Residual Gravity Anomaly Grid of Australia.

Analysis of the subsurface structure of Mount Ijen, Banyuwangi was carried out based on anomaly data obtained from satellite image data. This research was conducted with the aim of identifying the subsurface structures around the research site. In this study, what is determined is the complete Bouguer anomaly (ABL), regional and residual anomalies based on the ABL, and their inversion modeling. The results of the representation of underground structures based on residual anomalies obtained from complete and regional Bouguer anomaly data. The complete bouguer anomaly values obtained in the Mount Ijen area range from 12.2 to 110.7 mGal. In the process of separating regional and residual anomalies, different anomaly values are produced. The regional anomaly value ranges from +12.2 to +110.7 mGal while the residual anomaly ranges from -4.2 to +2.4 mGal

The determination of the geoid, which is a real shape of the Earth, with an accuracy of 1 cm is one of the most important aim of the today’s geodetic community. The gravimetric geoid modeling is the most preferred technique in order to reach to this target. Nevertheless, the gravity values measured on the physical surface of the Earth can not be directly included to this process. First of all, surface gravity values should be reduced to gravity anomalies and, during this step they should not lose their topographic features of the Earth's surface where the data is collected. On the other hand, in order to generate gravity anomalies on a regular grid, it is expected that dependency of gravity anomalies (to be used asreference data in interpolation) to local topography should be minimum. While free-air anomalies are the basic data source for determining the geoid, Bouguer anomalies describing the smoother Earth’s shape, are convenient to the interpolation of gravity anomalies. Therefore, the dependence of Bouguer gravity anomalies, which is a main data in the interpolation procedure, should be reduced to minimum. Although the simple Bouguer anomalies are preferred in practice, it is known that they contain more or less the negative effects of irregular topography. In this study, the differences between simple and complete Bouguer anomalies are presented in a test area. It is seen that the differences between free-air anomalies derived from both approximations are numerically increased up to16 mGal, correlated with the topography. This result indicates that complete Bouguer anomalies should be used in interpolation process in regions posing irregular topography, such as Turkey.

The global geopotential models GO_CONS_GCF_2_DIR_R2, GOCO02S and EIGEN06C based on GOCE data, and EGM08, represent a new important contribution to medium and long wavelength components knowledge of the gravitational field. These models have impacts on the computation of the respective wavelength components of the geoid. They have been used as reference fields in the modified Stokes integral, generating different geoid models. The terrestrial gravity data in South America have been updated with the most recent measurements in Argentina, Brazil, Ecuador and Paraguay. The short wavelength components were estimated via FFT using Featherstone modified kernel. The complete Bouguer and Helmert gravity anomalies have been derived through the geoid modelling package SHGEO by University of New Brunswick, Canada. The GGMs and the geoid models have been evaluated using GPS observations on Bench Marks of the spirit levelling network (GPS/BM). The height anomaly derived from EGM08 (degree 2190 and order 2159) has also been checked out.KeywordsGeoid modelingGOCEGPSGRACEHeight

Comparison between simple and complete Bouguer approaches in interpolation of Mean Gravity Anomalies The determination of the geoid, which is a real shape of the Earth, with an accuracy of 1 cm is one of the most important aim of the today’s geodetic community. The gravimetric geoid modeling is the most preferred technique in order to reach to this target. Nevertheless, the gravity values measured on the physical surface of the Earth can not be directly included to this process. First of all, surface gravity values should be reduced to gravity anomalies and, during this step they should not lose their topographic features of the Earth's surface where the data is collected. On the other hand, in order to generate gravity anomalies on a regular grid, it is expected that dependency of gravity anomalies (to be used as reference data in interpolation) to local topography should be minimum. While free-air anomalies are the basic data source for determining the geoid, Bouguer anomalies describing the smoother Earth’s shape, are convenient to the interpolation of gravity anomalies. Therefore, the dependence of Bouguer gravity anomalies, which is a main data in the interpolation procedure, should be reduced to minimum. Although the simple Bouguer anomalies are preferred in practice, it is known that they contain more or less the negative effects of irregular topography. In this study, the differences between simple and complete Bouguer anomalies are presented in a test area. It is seen that the differences between free-air anomalies derived from both approximations are numerically increased up to 16 mGal, correlated with the topography. This result indicates that complete Bouguer anomalies should be used in interpolation process in regions posing irregular topography, such as Turkey.

Complete (or refined) spherical Bouguer gravity anomalies have been computed for all 1 095 065 land gravity observations in the June 2007 release of the Australian national gravity database. The spherical Bouguer shell contribution was computed using the supplied ground elevations of the gravity observations. The spherical terrain corrections, residual to each Bouguer shell, were computed on a 9 arc-second grid (∼250 m by ∼250 m spatial resolution) from a global Newtonian integration using heights from version 2.1 of the GEODATA digital elevation model (DEM) over Australia and the GLOBE and JGP95E global DEMs outside Australia. A constant topographic mass-density of 2670 kg/m3 was used for both the spherical Bouguer shell and spherical terrain correction terms. The difference between the complete spherical and complete planar Bouguer gravity anomaly exhibits an almost constant bias of about −18.7 mGal over areas with moderate elevation changes, thus verifying the planar model as a reasonable approximation in these areas. However, the results suggest that in mountainous areas with large elevation changes, the complete spherical Bouguer gravity anomaly should be selected in preference over the less-rigorous complete planar counterpart.

From abstract: This report contains the complete Bouger and isostatic residual gravity maps of the Anadarko basin, Wichita Mountains, and surrounding areas on parts of Oklahoma, Kansas, Texas and Colorado that were compiled using gravity data from 11,023 stations.

The Nevada Test Site (NTS) and vicinity includes portions of the Goldfield, Caliente, Death Valley, and Las Vegas. This report documents and consolidates previously published and recently compiled gravity data to establish a gravity data base of about 16,000 stations for the NTS and vicinity. While compiling data sets, redundant stations and stations having doubtful locations or gravity values were excluded. Details of compiling the gravity data sets are discussed in later sections. Where feasible, an accuracy code has been assigned to each station so that the accuracy or reliability of each station can be evaluated. This data base was used in preparing complete Bouguer and isostatic gravity maps of the NTS and vicinity. Since publication of the complete Bouguer gravity map, additional data were incorporated into the isostatic gravity map. Gravity data were compiled from five sources: 14,183 stations from the US Geological Survey (USGS), 326 stations from Exploration Data Consultants (EDCON) of Denver, Colorado, 906 stations from the Los Alamos National Laboratory (LANL), 212 stations from the University of Texas at Dallas (UTD), and 48 stations from the Defense Mapping Agency (DMA). Gravity stations established under YMP are shown. The objective of this gravity survey was to exploremore » for the presence of plutons. This volume contains only compiled data.« less

This study was about the gravity in the area of ​​Bendan Duwur, Gajah Mungkur, Semarang. The method used in this research was gravity. The following steps in this activity were: calculating the value of Reader Gravity (gobs), Free Air Correction (FAC), Free Air Anomaly (FAA), Bouguer Correction (BC), Simple Bouguer Anomaly (SBA), Terrain Correction (TC), and Complete Bouguer Anomaly (CBA). The results of this research were listed as the Duwur bendan area, which can be seen from the shape of the points contained in the CBA, which is the smallest mGal value, 2.78 and the largest was 14.14. This can be assessed as an anomaly value, blue was the lowest value while pink the highest value, and yellow was the medium category. Based on the results of gravity measurements in the Bendan Duwur area, Semarang, the smallest mGal value was recognized by the TC compared to the gobs, FAC, FAA, and BC values. TC was producing 0.24 and the highest value was 1.13. The largest correction value was the FAC, the lowest value of mGal was 16.14, while the highest value of mGal was 26.33. Adapters for the Bendan Duwur soil structure based on CBA including the underground category generated were directly related to tectonic activity, but were more related to geology that requires special sedimentation and anthropogenic activities such as water needed.

Gravity data assembled for a state gravity map to be published by the Nevada Bureau of Mines are available from the EROS data center on magnetic tape. The first 18 files of the tape contain principal facts for individual data points, organized by 1° x 2° quadrangle. The approximately 71,000 individual data points are from numerous sources including the Defense Mapping Agency data base and U.S. Geological Survey files. Each data record contains geographic position, observed gravity, terrain correction, Bouguer gravity anomaly (p = 2.67 g/crn3 ), and isostatic residual anomaly (Airy Heiskanen, T = 25 km, A/? = 0.4 g/cm3 ). The 19th and 20th data files are 2 km square grids of complete Bouguer and isostatic residual gravity values based on the data in the preceding files. The 9-track magnetic tape is written at 1,600 bytes-per-inch (BPI) in ASCII format with an 80-character record and 4,000-character (50 record) block size.

Alaska Peninsula

Characteristics of isostatic gravity anomaly in Sichuan-Yunnan region, China

Terrain correction is important in generating accurate Complete Bouguer Anomaly (CBA) maps and is essential for gravity studies. Traditional methods for terrain correction can be time-consuming, and many computerized programs are computationally intensive. To address these challenges, we introduce MAETEC, an optimized algorithm MATLAB-based approach designed to enhance the efficiency and accuracy of terrain correction in CBA computations. MAETEC uses an effective algorithm and MATLAB's parallel computing capabilities to speed up the terrain correction process. Our approach integrates simple gravity equations, e.g. Nagy prism and its simplification techniques, zonation strategy, and parallel processing to significantly reduce computation time without compromising accuracy. Validation of MAETEC against other terrain correction software and methods demonstrates its effectiveness in producing precise CBA maps with reduced processing times. We illustrate the application of MAETEC through a case study in a geothermal exploration area and also for a regional gravity study. The results show a marked improvement in the efficiency of terrain correction compared to existing available software, e.g. GTeC, Oasis, and TCDEMF, making MAETEC a valuable gravity processing tool. Overall, MAETEC offers a fast, accurate, and user-friendly solution for terrain correction, facilitating the generation of high-quality CBA maps and advancing our understanding of subsurface geological structures.

Delineation of geological structures of Arta geothermal prospect in Djibouti based on the gravity data analysis and interpretation