Land Gravity Data Research Articles

Precise gravity-field modeling in the area of the Japanese Antarctic station Syowa and evaluation of recent EGMs

By combining a Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) Earth Gravity Model (EGM) and in situ gravity data obtained from the Japanese Antarctic Research Expedition (JARE) surveys, we estimated the regional gravity field in the area of Syowa Station, a Japanese research station located in Lützow-Holm Bay, East Antarctica. In situ data sets that were used consisted of land gravity data collected since 1967, shipborne data collected since 1985 and airborne gravity data collected in 2006. The GOCE direct (DIR) solution release 5 (R5) model was used as the long-wavelength reference of the gravity field. Using these data sets, we calculated gravity anomalies and geoid heights at 1-by-1′ grid by means of least-squares collocation.The resulting geoid height at Syowa Station was compared with a local height based on GPS, spirit leveling and tide gauge data. The result suggests that the sea surface height at Syowa Station is −1.57 m, which is consistent with a dynamic ocean topography model. During this investigation, we also evaluated GOCE EGMs and other recent EGMs by comparing them with the airborne gravity data. The results indicate that the GOCE DIR R5 produced the smallest RMS (Root Mean Square) differences and that the newer models performed nearly as well. These comparisons demonstrate the importance of using reliable in situ data when evaluating satellite-only EGMs.

Fractalness of land gravity data and residual isostatic anomalies map of Argentina, Chile and western Uruguay

Las anomalias de gravedad suelen estar fuertemente influenciadas por la topografia, cuyo efecto es atenuado a traves de adecuadas reducciones. Usualmente, ante la falta de informacion especifica, se usa un valor estandar de 2670 kg/m3 para la densidad de la topografia por encima del geoide. Hasta el presente varios metodos han sido implementados para obtener estimaciones de dicha densidad directamente a partir de los datos de gravedad mismos. Una tecnica especialmente conveniente para estudios de caracter regional se basa en analizar la naturaleza fractal de las anomalias de gravedad para una region en particular. En este trabajo se analiza la fractalidad de las anomalias de gravedad para Argentina, Chile y parte de Uruguay continentales utilizando una base de datos de gravedad actualizada. Se estimo la dimension fractal D de los datos topograficos y de anomalias de gravedad por medio de la tecnica de variograma en el dominio espacial. Como resultado de este analisis se encontro que la densidad optima para esta region tiene un valor de 2300 kg/m3 Esta densidad se uso para preparar un mapa de anomalias residuales isostaticas en el sistema de Airy-Heiskanen. Las principales anomalias reconocidas en este mapa se interpretaron con relacion al grado de balance isostatico y a las densidades de las rocas cercanas a la superficie

Fractalness of land gravity data and residual isostatic anomalies map of Argentina, Chile and western Uruguay

Gravity anomalies are often strongly influenced by topography, which effect is mitigated through appropriate reductions. Usually, a standard value of 2670 kg/m3 is used for the density of topography in the absence of specific information. However, various methods have been implemented to obtain density of topography estimates directly from the gravity data. A particularly suitable technique for regional scale studies is based on analyzing the fractal nature of gravity anomalies for a particular region.In this paper the fractalness of the onshore gravity anomalies for Argentina, Chile and part of Uruguay is analyzed by using an updated gravity database. The fractal dimensions of the topography and gravity anomaly data were estimated by means of the variogram technique in the spatial domain. As a result of this analysis, it was found that the optimum density for the studied region has a value of 2300 kg/m3. This density was used to prepare a map of isostatic residual anomalies using the Airy-Heiskanen model. The main anomalies recognized on this map were interpreted in relation to the degree of isostatic balance and the densities of rocks near to the surface.

High-resolution gravity and geoid models in Tahiti obtained from new airborne and land gravity observations: data fusion by spectral combination

Abstract For the first time, we carry out an airborne gravity survey and we collect new land gravity data over the islands of Tahiti and Moorea in French Polynesia located in the South Pacific Ocean. The new land gravity data are registered with GPS-derived coordinates, network-adjusted and outlier-edited, resulting in a mean standard error of 17 μGal. A crossover analysis of the airborne gravity data indicates a mean gravity accuracy of 1.7 mGal. New marine gravity around the two islands is derived from Geosat/GM, ERS-1/GM, Jason-1/GM, and Cryosat-2 altimeter data. A new 1-s digital topography model is constructed and is used to compute the topographic gravitational effects. To use EGM08 over Tahiti and Moorea, the optimal degree of spherical harmonic expansion is 1500. The fusion of the gravity datasets is made by the band-limited least-squares collocation, which best integrates datasets of different accuracies and spatial resolutions. The new high-resolution gravity and geoid grids are constructed on a 9-s grid. Assessments of the grids by measurements of ground gravity and geometric geoidal height result in RMS differences of 0.9 mGal and 0.4 cm, respectively. The geoid model allows 1-cm orthometric height determination by GPS and Lidar and yields a consistent height datum for Tahiti and Moorea. The new Bouguer anomalies show gravity highs and lows in the centers and land-sea zones of the two islands, allowing further studies of the density structure and volcanism in the region.

Preparation of the Free‐Air Gravity Anomaly Map in the Land and Seas of China and Adjacent Areas Using Multi‐Source Gravity Data and Interpretation of the Gravity Field

AbstractMulti‐source gravity data including measurements on land, shipboard surveys, satellite altimetry, Bouguer gravity anomalies and EGM2008 global gravity model data were collected and analyzed for compiling the free‐air gravity anomaly map on a scale of 1:5000000 in the land and seas of China and adjacent areas. The data merging and integration method was established based on analysis of the accuracy and resolution of different sources of gravity data. Evenly distributed land gravity data was gridded in flat land areas. A synthetic method using Bouguer gravity and high precision topographic data to calculate the free‐air gravity data was proposed to fill the land areas where the gravity measurement is hardly or impossible to implement due to complex terrain. Data comparison between the synthetic free‐air gravity data, measured data and EGM2008 data proves that using the synthetic data is a better choice for filling the areas lacking real data. This method was also applied to the continental and island areas outside mainland China where there are no real data collected. The STD analysis method was applied to the northern part of the South China Sea to compare the accuracy of three independent gravity data, the shipboard gravity data and two kinds of satellite altimetry derived gravity data, the SS series from Scripps, US and DNSC08GRA from DTU, Denmark. SSV18.1 and DNSC08GRA data, which show very good consistence in terms of accuracy and resolution. The standard deviation of the latest two satellite gravity data in the SCS is less than 2 mGal, which is a major improvement compared with the previous SS16.1. With the rectification of shipboard gravity data, the wide coverage of satellite gravity data could be used for small‐scale free‐air gravity mapping. In the coastal zone, the EGM2008 global potential model was used as a reference field providing a seamless gravity transition from land to ocean. Wavelet transform and multiscale analyses were applied to decompose the free‐air gravity field in China and its adjacent areas to help gravity field interpretation. Eight anomaly areas were demarcated by three horizontal and four vertical first‐order gravity gradient belts. Anomaly subareas and the second‐order gravity gradient belts were delineated in the gravity field to provide evidence for establishing the tectonic framework in the block tectonic scheme.

Intrusive dike complexes, cumulate cores, and the extrusive growth of Hawaiian volcanoes

The Hawaiian Islands are the most geologically studied hot‐spot islands in the world yet surprisingly, the only large‐scale compilation of marine and land gravity data is more than 45 years old. Early surveys served as reconnaissance studies only, and detailed analyses of the crustal‐density structure have been limited. Here we present a new chain‐wide gravity compilation that incorporates historical island surveys, recently published work on the islands of Hawai‘i, Kaua‘i, and Ni‘ihau, and >122,000 km of newly compiled marine gravity data. Positive residual gravity anomalies reflect dense intrusive bodies, allowing us to locate current and former volcanic centers, major rift zones, and a previously suggested volcano on Ka‘ena Ridge. By inverting the residual gravity data, we generate a 3‐D view of the dense, intrusive complexes and olivine‐rich cumulate cores within individual volcanoes and rift zones. We find that the Hāna and Ka‘ena ridges are underlain by particularly high‐density intrusive material (>2.85 g/cm3) not observed beneath other Hawaiian rift zones. Contrary to previous estimates, volcanoes along the chain are shown to be composed of a small proportion of intrusive material (<30% by volume), implying that the islands are predominately built extrusively.

우리나라 지오이드 모델 구축 및 정밀도 검증

2000년대 초반까지 우리나라 지오이드 모델의 정밀도는 지상 중력 자료 및 GPS/Leveling 자료의 낮은 분포 및 정밀도로 인해 약 14cm 수준에 머물렀다. 그러나 2007년 이후 높은 정밀도와 고른 분포를 나타내는 새로운 지상 및 항공 중력 자료, GPS/Leveling 자료가 획득됨에 따라 지오이드 모델의 정밀도를 향상할 수 있는 기반이 마련되었으며, 2011년 초반에 발표된 중력 지오이드 모델의 정밀도는 약 5.29cm로 기존 모델에 비해 약 27% 향상되었다. 2010년 이후에도 많은 중력 자료가 통합기준점, 수준점 및 삼각점에서 획득되었고, 특히 2010년 통합기준점 구축 사업이 완료됨에 따라 우리나라 전역에서 약 10km의 간격을 나타내는 GPS/Leveling 자료가 구축되어 보다 정밀한 지오이드 모델의 구축 및 검증이 가능하게 되었다. 본 연구에서는 2012년 현재 우리나라에서 가용한 지상, 항공, 위성고도계 중력 자료를 기반으로 하여 중력지오이드 모델을 구축하였다. 그 결과 우리나라 전역에서 중력 지오이드 모델은 18.05m부터 32.70m의 범위의 값을 나타내며, 정밀도는 약 5.76cm로 산출되었다. 또한, 중력 지오이드 모델과 GPS/Leveling 자료를 융합하여 구축한 합성 지오이드 모델의 적합도 및 정밀도는 각각 3.60cm와 4.06cm로 계산되었다. 마지막으로 건설현장 등에서 합성 지오이드 모델을 활용할 수 있는지 검증하기 위하여 15km 기선거리에 대한 상대 정밀도를 계산한 결과 약 3.35cm로 나타났다. 상대 정밀도가 낮게 나타나는 이유는 강원도 및 동해안, 서 남해안 등에서 우리나라 수직기준체계간의 불일치로 인해 지역적인 편이가 존재하며, 일부 내륙에서도 주변 성과와의 일관성 저하문제가 나타나는 점들이 있기 때문으로 향후 통합기준점 성과에 대한 신뢰도 검증이 필요한 것으로 사료된다. The previous geoid model developed in early 2000s shows 14cm level of precision due to the problems on distribution, and quality of the land gravity and GPS/Leveling data. From 2007, the new land and airborne gravity data as well as GPS/Leveling data having high quality and regular distribution has been obtained. In 2011, a new gravimetric geoid model has been constructed with precision of 5.29cm which was improved about 27% comparing to the previous model. However, much more land gravity data has been collected at the control point, bench marks and triangulation points since 2010. Also, GPS/Leveling data having 10km spacing over whole country has been obtained through the project which is for the construction of new control points. In this study, new gravimetric geoid has been calculated based on the all available gravity data up to present. The geoid height shows the range from 18.05m to 32.70m over whole country and its precision is 5.76cm. The degree of fit and precision of hybrid geoid model are 3.60cm and 4.06cm, respectively. At the end, 3.35cm of the relative precision in 15km baseline has been calculated to confirm its practical usage. Especially, it has been founded that regional bias occurred at the Kangwon and coastal area due to problems on the leveling data. Also, some inland points show inconsistent large difference which needs to be verified by analyzing the unified control points results.

Update of the precision geoid determination in Korea

ABSTRACTFrom the late 1990s, many studies on local geoid construction have been made in South Korea. However, the precision of the previous geoid has remained about 15 cm due to distribution and quality problems of gravity and GPS/levelling data. Since 2007, new land gravity data and GPS/levelling data have been obtained through many projects such as the Korean Land Spatilaization, Unified Control Point and Gravity survey on the Benchmark. The newly obtained data are regularly distributed to a certain degree and show much better improvement in their quality. In addition, an airborne gravity survey was conducted in 2008 to cover the Korean peninsula (South Korea only). Therefore, it is expected that the precision of the geoid could be improved. In this study, the new South Korean gravimetric geoid and hybrid geoid are presented based on land, airborne, ship‐borne, altimeter gravity data, geopotential model and topographic data. As for the methodology, the general remove‐restore approach was applied with the best chosen parameters in order to produce a precise local geoid. The global geopotential model EGM08 was used to remove the low‐frequency components using degree and order up to 360 and the short wavelength part of the gravity signal was dealt with by using the Shuttle Radar Topography Mission data. The parameters determined empirically in this study include for Stokes’ integral 0.5° and for Wong‐Gore kernel 110–120°, respectively and 10 km for both the Bjerhammar sphere depth and attenuation factor. The final gravimetric geoid in South Korea ranges from 20–31 m with a precision of 5.45 cm overall compared to 1096 GPS/levelling data. In addition, the South Korean hybrid geoid produces 3.46 cm and 3.92 cm for degrees of fitness and precision, respectively and a better statistic of 2.37 cm for plain and urban areas was achieved. The gravimetric and hybrid geoids are expected to improve further when the refined land gravity data are included in the near future.

육상 중력자료 기반의 전 지구 중력장 모델 분석

본 연구에서는 전 지구 중력장 모델 EGM2008, EGM96을 우리나라 육상에서 측정한 16,786 지점의 중력 자료와 비교 평가 하였다. 육상 중력자료가 가장 정확하다는 가정 하에 육상 중력자료와 EGM2008, EGM96 간의 상관계수(correlation coefficient), 차이, 차이의 평균제곱오차(MSE), 표고에 따른 차이의 변화 등을 통하여 분석하였다. 육상중력과 EGM2008 간의 상관계수는 0.824, EGM96 간의 상관계수는 0.538로 계산되었다. 육상중력과 EGM2008, EGM96 간의 차이에 대한 표준편차는 각각 13.196 mgal, 18.685 mgal로 계산되었으며, 차이의 변화폭 역시 EGM2008이 EGM96 보다 작은 것으로 나타났다. 또한 육상 중력과 전 지구 중력장 모델의 표고에 따른 고도이상 차이는 역시 EGM2008은 큰 변화가 없었으나 EGM96의 경우 고도 변화에 따라 고도이상의 차이가 큰 것을 확인하였다. 이러한 결과로 EGM2008이 EGM96에 비하여 우리나라의 중력장을 잘 표현하고 있는 것으로 평가된다. In this study, Global Gravitational Model EGM2008, EGM96 and 16,786 gravity points measured on land were compared and analyzed. On the assumption that land gravity data is most accurate, the correlation coefficient, differences, MSE and difference variance along the height were computed between land gravity data and EGM2008, EG96. The correlation coefficient, land gravity data and EGM2008, was computed most largely with 0.824 and the correlation coefficient with EGM96 was computed with 0.538. The standard deviation of differences between land gravity and EGM2008, EGM96 were 13.196 magl, 18.685 mgal respectively. Also the difference variance scope of EGM2008 was smaller than EGM96. There was no large variance of free-air anomaly differences between land gravity data and EGM2008 along the height however free-air anomaly differences with EGM96 varied along the height changes. Consequently EGM2008 nicely expresses Korea gravity field more than EGM96.

최신 중력 자료의 획득을 통한 우리나라 지오이드 모델 업데이트

기존의 우리나라의 지상중력 자료는 지역적으로 편향된 분포를 나타내며, 특히 산악지역에서는 그 분포가 현저히 떨어진다는 문제점이 존재하였다. 이러한 문제점은 GPS/Levelling 자료에서도 나타나며 이로 인하여 지오이드 정확도 향상에 한계가 있었다. 그러나 2008년부터 국토지리정보원에서 수행하고 있는 통합기준점 및 수준점 중력측량사업에서 새로운 중력 자료 및 GPS/Levelling 자료를 획득하였으며, 특히 이러한 자료들은 기존의 중력 및 GPS/Levelling 자료에 비해 월등한 분포와 정밀도를 나타내기 때문에 우리나라 지오이드 모델의 정밀도를 개선하는 기본 자료로 활용될 수 있다. 본 연구에서는 품질이 검증된 기존 자료와 새롭게 획득된 중력 자료를 포함하여 지오이드 모델을 구축하였다. 구축된 지오이드 모델의 정밀도는 새롭게 획득된 927점의 GPS/Levelling 자료와 비교한 결과 약 5.29cm로 나타났으며, 합성지오이드의 적합도 및 정밀도는 각각 약 2.99cm와 3.67cm로 산출되었다. 본 연구를 통하여 산출된 최신 지오이드 모델은 새로운 중력 자료를 포함하기 전에 구축된 모델에 비하여 우리나라 전역에서 약 27% 정도 정밀도가 향상되었으며, 특히 현저히 낮은 분포를 보이던 강원 및 경상지역에서는 중력자료의 업데이트에 의하여 약 42%의 정밀도 향상이 이루어 졌다. 2010년에 통합기준점 사업 및 수준점 측량사업을 통하여 약 4,000여점의 중력 자료와 300여점의 GPS/Levelling 자료가 획득되었기 때문에 향후 본 자료들을 함께 활용한다면 지오이드 모델 정밀도의 향상과 더불어 객관적으로 정밀도 검증이 가능할 것으로 판단된다. The previous land gravity data in Korea showed locally biased irregular distribution. Especially, this problem was more serious in the mountainous area where the data density was significantly low. The same problem appeared in GPS/Levelling data thus the precision of the geoid could not be improved. From 2008, new gravity and GPS/Levelling data has been collected by the unified control point and survey on the benchmark project which were funded by the national geographic information institute. The newly obtained data has much better distribution and precision so that it could be used for update precision of geoid model. In this study, the new precision geoid has been calculated based old and new gravity data and this model showed 5.29cm of precision compared to 927 points of GPS/Levelling data. And the degree of fit and precision of hybrid geoid has been calculated 2.99cm and 3.67cm. The new gravimetric geoid has been updated about 27% over whole country. And it showed 42% of precision update due to collection of new gravity data on the Kangwon/Kyeongsang area which showed quite low distribution. In 2010, about 4,000 points of gravity and 300 points of GPS/Levelling data has been obtained by unified control and survey on benchmark project. We expect that new data will contribute to updating geoid precision and veri tying precision more objectively.

Subsurface geometry and structural evolution of La Gomera island based on gravity data

We hereby present a new Bouguer gravity map of the La Gomera island (Canarian Archipelago), which is analysed and interpreted by means of a 3-dimensional inversion, in order to contribute to the knowledge of the structural setting of this volcanic island and its evolutionary history. A land gravity data set covering the whole island of La Gomera is used in combination with offshore measurements to achieve a better determination of the gravity field in areas near the coasts. The study of this map let us to shed some light on the hypothesis established about the volcanism of this island. Moreover, it shows the information that is hidden from a geological surface exploration, modelling deep sections of the crust in La Gomera, which have been unknown until now. A first interpretation of the Bouguer gravity anomaly is achieved from 1) the residual gravity map calculated by removing a regional component and 2) the total horizontal gradient of the gravity. These residual and derivative maps allow us to identify the horizontal location and borders of the shallowest gravity sources. This provides a useful tool to study the structures associated to the latest periods of the volcanism in the area. Moreover, the information so obtained supports the hypothesis about the migration of volcanic activity towards the south of the island. Subsequently, an inversion process is carried out looking for the 3D-modelisation of the sources of the observed gravity field, which provides a comprehensive view of the structures in volcanic environments. The inversion technique used is based on a genetic algorithm (GA) applied upon a prismatic partition of the subsoil volume, and adopting a priori values of density contrast (positive and negative). The main advantage of this method is that let us to model deep and shallow bodies which exhibit very different geometries and density contrasts. So, results indicate that this inversion strategy can be very effective for characterization of volcanic structures, improving the information from previous geologic and volcanologic studies. The inversion model obtained shows correlation between several sources of the gravity field and the volcanic units associated with the growth of La Gomera Island. The main gravity source of this model is associated with the oldest unit, called the Basal Complex. This unit corresponds to the first submarine growth stage and it is modelled as the most important and deepest high density structure. According to previous geological studies, the following edifice (Old Edifice) was also submarine in its initial phases, later being represented by a wide basaltic shield volcano. The original location and morphology of this Old Edifice is deduced from the distribution of positive density contrasts that appears in the model. Moreover, other gravity field sources are identified and associated to several feeding systems of this stage of the volcanism in La Gomera. The shallowest sections of the model let us recognise the distribution of light material inside the Vallehermoso caldera, surrounded by high density structures. This gives us some insight into the internal structure and morphology of the caldera, pointing to a vertical collapse origin followed by erosion and other destructive processes. Finally, other conclusions are obtained from the correlation found between the sources of the gravity field and the migration of the volcanic activity towards the southern area of the island.

Impact of the new GRACE Geopotential Model and SRTM data on the geoid modelling in Algeria

Abstract In Algeria and since 2000, two published geoid models have been produced by the Geodetic Laboratory of the National Centre of Space Techniques using different data sets and techniques. Although these results were satisfactory and internally consistent they do not have the required accuracy to be able to transform a GPS ellipsoidal height to an orthometric height. Recently, the quantity and quality of terrestrial gravity data increased slightly and, especially, several new geopotential models from the recent satellite missions (e.g., GRACE) were released. At the same time, the new high-resolution SRTM (Shuttle Radar Topography Mission) global DEM was constructed. The main purpose of this paper is to study the impact of the above new data on geoid heights in Algeria. For this reason, a new gravimetric geoid determination has been carried out including these new data. This solution was based on the land gravity data supplied by the BGI (Bureau Gravimetrique International), Digital Elevation model derived from SRTM for topographic correction and the optimal gravity field model GRACE (EIGEN-GL04C), which were combined using the remove–restore technique in connection with the Fast Fourier transformation technique. This gravimetric geoid and previous geoids existing for this study area are compared to the geoid undulations corresponding to 62 GPS/levelling points located in northern part of Algeria. The study shows that the new gravimetric geoid model agrees considerably better with GPS/levelling than any of the other local geoid models in the tested area. Its standard deviation fit with GPS/levelling data are ±27 and ±25.0 cm before and after fitting using the four-parametric model as corrector surface in minimizing the long-wavelength geoid errors and the datum inconsistencies between our height data and GPS. The new geoid model will be used in low accuracy scientific applications and in low-order levelling network densification with regard to the national levelling network coverage considered as good in the north and becomes poor in the south and west of the country. It is an important contribution to geophysics, because it can provide a constraint for density distribution, thermal state of the lithosphere and viscosity in the mantle.

Shallow‐water gravity anomalies from satellite altimetry: Case studies in the east china sea and Taiwan strait

The differences between two global satellite altimeter‐derived gravity anomaly grids over the East China Sea and the Taiwan Strait are investigated and the causes of the differences are discussed. Difference of gravity anomaly is correlated with tide model error, standard deviation of sea surface heights (SSHs) and ocean depth. We present two new SSH‐derived observations of altimetry (differenced height and height slope) for gravity derivation. An iterative method for removing altimeter data outliers is used, and differenced height is found to be most sensitive to this method. For altimeter‐gravity conversion, we used three altimeter‐derived observations, i.e., deflection of the vertical, differenced height and height slope, in two conversion methods, i.e., the least‐squares collocation (LSC) and the inverse Vening Meinesz method. Use of differenced heights in LSC produces the best result when comparing altimeter‐derived and shipborne gravity anomalies. Use of land gravity data in the vicinity of coasts enhances the accuracy of altimeter‐derived gravity anomalies.

Validation and fusion of different databases in preparation of high-resolution geoid determination

SUMMARY An up to date determination of a high-resolution geoid requires the use of best available databases concerning digital terrain model (DTM), bathymetry, global geopotential model and gravity field. The occasion to revisit methods to validate and merge different data sets has been created by a new project for the determination of a new European Geoid. Since the computation of the latest European geoid and quasi-geoid model (EGG97), significant new or improved data sets have become available, such as new global geopotential models from CHAMP and GRACE missions, new national and global DTMs and new or upgraded gravity data sets. In the context of the new European Gravity and Geoid Project (EGGP), within the IAG Commission 2, some data validation tests have been performed in the Italian zone. In the area 19 ◦ × 17 ◦ wide, covering Italy, three kinds of tests have been performed: comparison among different DTMs in order to choose the best one to be used; comparisons in terms of geoid computation in some coastal areas, to evaluate bathymetry effects, and the validation of the EIGEN-CG01C and EIGEN-CG03C new global models up to degree and order 360. These preliminary tests lead to the choice of SRTM DTM (integrated in no-data holes), with an added bathymetry derived by the Italian 1:25 000 official cartography near the coasts and the NOAA bathymetry in high seas. The validation of the new global models and the comparison with EGM96 model show that, in terms of geoid computation, the EGM96 yields better results. Moreover, the validation of new available land gravity data and the cross-validation of two sets of gravity data on sea have been completed.

Results of pre‐drilling potential field measurements at the Bosumtwi crater

Abstract— Gravity and magnetic measurements were carried out at the Bosumtwi crater to determine the geophysical signature of the crater. Land gravity data was acquired at 163 locations around the structure and on the shore of the lake. The separation between the gravity stations was 500 m for radial profiles, but 700–1000 m along roads and footpaths that ran parallel to the lake's shore. Additionally, a marine gravity survey was carried out along 14 north‐south and 15 east‐west profiles on the lake. Magnetic data was also acquired along 14 north‐south profiles on the lake. In all marine surveys, the line spacing was 800 m, and navigation was provided by a Garmin 235 Echo Sounder/GPS. The gravity signature of the crater is characterized by a negative Bouguer anomaly with an amplitude of about −18 mgal. Using the seismic results as constraints, the gravity model obtained indicates the central uplift at a depth of 250 m. The negative anomaly is the contribution of the gravity deficiencies due to fractured and brecciated rocks in the rim area and below the crater floor, the impact breccias within the crater, and the sedimentary and water infilling of the lake. Magnetic modeling yielded a model for the causative body, which is located north of the central uplift: the model has a magnetic susceptibility of 0.03 S.I. and extends from a depth of 250 to 610 m. The causative bodies have been interpreted as impactites.

Examination of consistency of marine gravity with land gravity in and around the Japanese Islands using a helicopter-borne gravimeter

Abstract This paper reports a finding that marine gravity data around the Japanese Islands are inconsistent with nearby land gravity data. The comparison between land and marine gravity was made possible by the gravity measurements from the air using a helicopter-borne gravimeter (SEGAWA Model) developed by the present authors. The ground/sea truth gravity anomaly can be checked against the gravity from the air, though it is not free air gravity anomaly but gravity disturbance. The newly-developed airborne gravimeter first manufactured in 1998 shows a good performance with a 1–2 mgal average repeatability of measurement under a 90-knot flight speed. Thus we have found disagreements between ground truth and sea truth gravity anomalies on the basis of airborne gravity data in the areas in Japan where we have so far made measurements. Among them we will report the case in the area from Saitama and Ibaraki offshore to the Kashima-Nada Sea, p]Japan. Our conclusion about this area is that the past marine gravity data obtained by surface ship gravimeters involve systematic errors of more than 10 mgals. This kind of inconsistency between marine and land gravity is expected to be found in a number of other areas around the Japanese Islands.

正標高の導入―もう一つの測地成果２０００―

The vertical datum of Japan, which is realized with official heights at the benchmarks, had been updated for the first time in about 30 years and the new heights were put in force in April 2002. The heights were computed from the latest leveling data throughout Japan and detailed land gravity data were taken into account, for the first time in Japan, for yielding Helmert orthomeitric heights. On this occasion, the paper tries to give readers a brief description of the height systems, their relationship with gravity and the computation method employed, and the current, more precise vertical datum of Japan updated from the previously-employed normal orthometric heights.

Simulation of the Indonesian land gravity data using a digital terrain model data

The Indonesian gravity field is neither accurately nor comprehensively determined, especially due to inadequacy of land gravity data. This study deals with determination of Indonesian land gravity and proposes the solution to data unavailability by means of a simulation technique. The simulation was carried out by combining short wavelength topographic effects from GTOPO30 and long wavelength information from EGM96. The simulated result was then compared with the observed gravity data. Over Java, Sumatra and Sulawesi islands, using three methods commonly used on the computation of topographic effect; topography, isostatic and RTM (Residual Terrain Model), it was estimated that error propagation by the GTOPO30 into the simulated gravity is about 4.5 to 11.7 mgal, with the RTM method was affected less than others. It was also shown that the simulated gravity from the RTM method gave the best agreement with STD (Standard Deviation) differences of 17 to 42 mgal compared to the observed data. This result was achieved after applying optimal RTM parameters over the Indonesian area: a reference field of 25′–27.5′ and density of 2–2.2 gr/cm3. Compared to STD differences between EGM96 and observed data, that between the simulated gravity and observed data improved by 2.5–7 mgal, and gave more detailed gravity features, especially over areas of high topography.

Augmenting satellite altimeter data with ship and land gravity data in the northwest shelf of Australia

High-resolution maps of the marine gravity field can be produced for oil and gas exploration. Ship and airborne gravity data collected by oil companies in the preliminary stages of a survey may be combined with existing public-domain satellite altimetry data, greatly increasing the resolution potential of the latter, and providing a regional gravity field for emplacement of the exploration data. The combination of altimeter data with land and ship gravity data is an iterative process, designed to seamlessly match the data sets. The method is quick and easy to implement, and avoids laborious crossover adjustments of the ship data.

日本の重力探査事情―地下構造とのかかわり

This article introduces the gravity methods for petroleum explorations. Gravitational methods are based on the sensitive measurement at the surface of small variations in the gravitational field, but the measurement technology of land gravimeter has already established. In the present, the new trends of gravity measurement are coming. Gravity measurements by ship and airplane are possible. Therefore, Bouguer anomaly map of Japan map was compiled from about 350, 000 land gravity data, about 1, 000, 000 ship-borne gravity data and altimeter data from satellite in area where observation data are sparse, and the gravity basement of Japan was obtained from these data.New technology, which is capable of accurately measuring the full gravity gradient tensor, has only recently become available in the exploration geophysics domain from military research projects (e. g. the Bell Geospace gradiometer). Gravity gradiometry data have superior characteristics to traditional gravity measurements. The components of the gravity gradient tensor hold much more information about the subsurface than do traditional potential field measurements. Such gravity data can constrain geology in areas where seismic interpretation is difficult, such as sub-basalt and sub-salt. These data are capable of resolving shallow structures, such as the limbs of salt domes, far more accurately than can seismic surveys.