Modelling Local Gravity Anomalies from Processed Observed Gravity Measurements for Geodetic Applications (original) (raw)
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Journal of Geodetic Science, 2018
Spherical harmonic expansion is a commonly applied mathematical representation of the earth’s gravity field. This representation is implied by the potential coeffcients determined by using elements/parameters of the field observed on the surface of the earth and/or in space outside the earth in the spherical harmonic expansion of the field. International Centre for Gravity Earth Models (ICGEM) publishes, from time to time, Global Gravity Field Models (GGMs) that have been developed. These GGMs need evaluation with terrestrial data of different locations to ascertain their accuracy for application in those locations. In this study, Bouguer gravity anomalies derived from a total of eleven (11) recent GGMs, using sixty sample points, were evaluated by means of Root-Mean-Square difference and correlation coeficient. The Root-Mean-Square differences of the computed Bouguer anomalies from ICGEMwebsite compared to their positionally corresponding terrestrial Bouguer anomalies range from 9....
Investigation of gravity anomalies in parts of Niger Delta Region in Nigeria using aerogravity data
International Journal of Physical Sciences
Gravity anomalies in parts of the Niger Delta region, Nigeria, were investigated through the interpretation of aerogarvity data with the objectives to determine the thickness of the sedimentary basin, establish the basement topography, density contrasts and the geological models which will give information about variation of geological structures. Four sheets of digital airborne gravity data were used for the study. Source parameter imaging (SPI), Standard Euler deconvolution and forward and inverse modeling techniques were employed in quantitative interpretation. The Bouguer anomaly of the study area varied from-20.0 to 37.7 mGal, while the residual Bouguer anomaly varied from-19.6 to 25.7 mGal. The SPI gave depth values ranging from-539.7 to-4276.0 m for shallow and deep lying gravity anomalous bodies. The windowed Euler-3D for Bouguer gravity result revealed the depth range of 1355.5 to-1518.1 m for structural index of one; 2384.5 to-3283.2 m for structural index of two and 2426.0 to-5011 m for structural index of three. The forward and inverse modeling gave the density values for the modeled profiles 1, 2, 3, 4 and 5 as 1.820, 2.410, 0.720, 2.310 and 2.100 gcm-3 , respectively, with their respective depths of 3872, 4228, 4880, 3560 and 2527 m. The results from this study have shown that the depth to basement and density contrast have influence on the petroleum/hydrocarbon accumulation.
The Evaluation of Gravity Anomalies Based on Global Models by Land Gravity Data
2018
The Earth system generates different phenomena that are observable at the surface of the Earth such as mass deformations and displacements leading to plate tectonics, earthquakes, and volcanism. The dynamic processes associated with the interior, surface, and atmosphere of the Earth affect the three pillars of geodesy: shape of the Earth, its gravity field, and its rotation. Geodesy establishes a characteristic structure in order to define, monitor, and predict of the whole Earth system. The traditional and new instruments, observables, and techniques in geodesy are related to the gravity field. Therefore, the geodesy monitors the gravity field and its temporal variability in order to transform the geodetic observations made on the physical surface of the Earth into the geometrical surface in which positions are mathematically defined. In this paper, the main components of the gravity field modeling, (Free-air and Bouguer) gravity anomalies are calculated via recent global models (E...
International Journal of Advanced Engineering Research and Science (IJAERS), 2019
The conversion of geometric as well as ellipsoidal heights from GNSS observations to practical heights for engineering constructions has necessitated the determination of the local geoid model of areas. Benin City is a developing area which requires a local geoid model for conversion of geometric heights to orthometric heights for physical developments in the area. This paper is on the best local geoid model of Benin City, Nigeria by comparing three gravimetric-geometric geoid models of the study area. GNSS and gravimetric observations were carried out on 49 points to respectively obtain their coordinates and absolute gravity values. The theoretical gravity values of the points were computed on the Clarke 1880 ellipsoid, subtracted from the absolute gravity values and corrected for the air (free air) to obtain the free air gravity anomalies of the points. The computed free air gravity anomalies were applied to compute the geoid heights of the points using the integration of the modified Stokes integral. Three geometric geoid surfaces (plane, second degree and third degree surfaces) were fitted to the computed gravimetric geoid heights using the least squares technique to obtain the gravimetric-geometric geoid models of the study area. The RMSE of the three gravimetric-geometric geoid models were computed to determine their (the models) accuracy. The three gravimetric-geometric geoid models were compared using their accuracy to obtain the most suitable geoid model of the study area. The results of the comparison showed that the third degree gravimetric-geometric geoid model is most suitable for application in the study area. It is recommended that ellipsoidal heights obtained from GNSS observation within Benin City, Nigeria should be converted to orthometric heights using the third degree geoid model.
Evaluation of the Recent African Gravity Databases V2.x
International Association of Geodesy Symposia
In the framework of the activities of the IAG Sub-Commission on the gravity and geoid in Africa, a recent set of gravity databases has been established. They are namely: AFRGDB_V2.0 and AFRGDB_V2.2. The AFRGDB_V2.0 has been created using the window remove-restore technique employing EGM2008 as geopotential Earth model complete to degree and order 1800. The AFRGDB_V2.2 has been established using the Residual Terrain Model (RTM) reduction technique employing GOCE DIR_R5 complete to degree and order 280, using the best RTM reference surface. The available gravity data set for Africa, used to establish the above mentioned two independently derived databases, consists of shipborne, altimetry derived gravity anomalies and of land point gravity data. In particular, the data set of point gravity values shows clear deficits with regard to a homogeneous data coverage over the completely African continent. The establishment of the gravity databases has been carried-out using the weighted least...
International Journal of Advanced Engineering, Management and Science (IJAEMS), 2019
The geometric heights obtained from GNSS observations cannot be used for engineering works as they are not reduced to the geoid. This study presents practical local geoid modelling from gravimetric observations using the modified Stokes integral for engineering applications in Benin City. A total of 52 points were observed with GNSS receivers and a gravimeter to respectively obtain their positions and absolute gravity values. The theoretical gravity values of the points were computed on the Clarke 1880 ellipsoid to obtain their local gravity anomalies. The modified Stokes integral was applied to compute the geoid heights of the points. The combined topographic effect was applied to the computed geoid heights of the points to obtain their precise geoid heights. The mean of the precise geoid heights of the points was computed to obtain the local gravimetric geoid model of the study area. The determined geoid model was validated for its reliability as well as the accuracy using the RMSE index. It is recommended that the use of assumed, as well as handheld GPS receiver heights for engineering works should be totally abolished as this study has established the local geoid model of Benin City.
Investigation of possible cause of gravity anomalies in parts of the Niger Delta Basin, Nigeria
2017
Possible cause of gravity anomalies in parts of the Niger Delta Basin, Nigeria was investigated using three methods of gravity data interpretation; Euler-3D deconvolution, source parameter imaging (SPI) and forward and inverse modeling. The estimated depths from the interpretation techniques used show much similarity. Quantitatively, the results obtained from the SPI method showed a minimum to maximum depth of 1264.11 to 9354.57 m to the anomalous body. The results obtained from Euler-3D deconvolution method showed a depth range of 2136.40 to 9167.57 m to the anomalous body using a structural index of 3. The results from the forward and inverse modeling revealed a depth range of 793 to 7586 m to the anomalous body. Qualitatively, the gravity signatures in profiles 1, 2 and 3 possibly depict antiformal geologic features whereas the gravity signatures in profiles 4 and 5 possibly depict synformal features/basin geometries. Key words: Gravity data, Niger Delta, source parameter imagi...
Ground Gravity Survey of 1:250,000 Sheet 61 (Akure), Southwestern Nigeria
Ground gravity survey was carried in part of southwestern Nigeria underlain mainly by the Basement Complex rocks to establish second order gravity base stations and produce the bouguer anomaly map of the area. Sixty (60) evenly distributed gravity base stations were established in the study area and were tied to IGSN 71 through the PGNN Base station at Akure. The processed data were presented as 2D maps with both contour and colour shaded maps. The Bouguer anomaly field values range from about-21.471 to 14.357mGal, the free-air anomaly values range from 3.428 to 68.813mGal/m while the residual Bouguer Anomaly values range from about-11.853 to 12.153mGal. There is mostly a general NE/SW and few N/S trends in all the anomaly fields. Four major bouguer anomalies with unique characteristic features were identified which maybe related to the contrasting lithological and structural features. These are: (1) Akure-Ikere-Igbaraoke High (2) Ikare-Ogbagi-Ajowa High (3) Ute-Uzebba High and (4) IjeshaIsu-Ikole-Ponyan Low. The relatively high oval shape bouguer anomaly "Ikare-Ogbagi-Ajowa high" at the NE part of the study area can be a new frontier for mineral exploration.
Determination of gravity anomalies in Java, Indonesia, from airborne gravity survey
Terrestrial, Atmospheric and Oceanic Sciences
The Geospatial Information Agency of Indonesia (BIG) recently carried out an airborne gravity survey mission to support a reliable Indonesian geoid model. The gravity observations covered all the main islands of Indonesia. This paper presents a state-of-the-art for gravity anomalies derivation using airborne gravity mission in Java, Indonesia. The common gravity corrections for deriving the scalar free-air gravity anomalies along the flight trajectory had been estimated using GNSS-derived positions. The corrected data were then filtered using the FIR method in which the cutoff frequency had been predetermined by considering aircraft altitude, geological setting, and instrument's accuracy. To assess the airborne gravity results, we compared them with the upward continued terrestrial gravity measurements. In addition, we performed crossover analysis and adjusted the estimated biases to the airborne gravity measurements. The accuracy of adjusted airborne gravity anomaly was estimated to 3.37 mGal. In conclusion, the airborne gravity mission provided valuable data needed for further geodesy and geophysics applications.
FITTING OF A TRANSFORMATION GEOID MODEL TO THE GRAVIMETRIC-GEOMETRIC GEOID MODEL OF BENIN CITY
FUDMA Journal of Sciences (FJS), 2021
The application of the transformation geoid model in Benin City has necessitated its fitting to the existing gravimetric-geometric geoid model of the study area. The transformation geoid model was determined using the Kotsakis (2008) model for the transformation of global geoid heights to local geoidal undulations. To obtain its accuracy, the root mean square error (RMSE) index was applied. The computed accuracy is 2.0172 m. To apply the determined geoid model in the study area, as well as improving on the computed accuracy, the model was fitted to the gravimetric-geometric geoid model of the study area. The fitting result shows that geoid heights can be computed using the determined geoid model with an accuracy of 1.1041 m in the study area.