Free-air Anomalies Research Articles

Improved Bathymetry Estimation Using Satellite Altimetry-Derived Gravity Anomalies and Machine Learning in the East Sea

This study aims to improve the accuracy of bathymetry predicted by gravity-geologic method (GGM) using the optimal machine learning model selected from machine learning techniques. In this study, several machine learning techniques were utilized to determine the optimal model from the performance of depth and gravity anomalies. In addition, a tuning density contrast calculated from satellite altimetry-derived free-air gravity anomalies (FAGAs) was applied to estimate enhanced bathymetry. By comparison with shipborne depth, the accuracy of the bathymetry estimated by using satellite altimetry-derived FAGAs and machine learning was evaluated. The findings reveal that the bathymetry predicted by the optimal machine learning using the Gaussian process regression and the GGM with a tuning density contrast can enhance the accuracy of 82.64 m, showing an improvement of 67.40% in the RMSE at shipborne depth measurements. Although the tuning density is larger than 1.67 g/cm3, bathymetry using satellite altimetry-derived FAGAs and machine learning can be effectively improved with higher accuracy.

Spatial variations in the effective elastic thickness of the Indian Ocean lithosphere

The Indian Ocean lithosphere is a complex assemblage of large igneous provinces, seamounts, plateaus and ridges of different loading ages and tectono-thermal evolution. As a proxy for the strength of tectonic plates, effective elastic thickness (Te) illustrates the relationship between surface deformation and lithospheric rheology of the diverse provinces in response to long-term tectonic processes. Mapping the spatial variations in lithospheric rheology can aid in understanding the detailed tectono-thermal history of the Indian Ocean. In this paper, we perform an assessment of the spatial variation of Te for the Indian Ocean from the inversion of the real free-air admittance between free-air gravity anomalies and bathymetry corrected for the effect of density variations within sediments using a continuous wavelet spectral analysis. Incorporating the effect of sediments substantially reduces Te estimates and better corresponds with the tectonic units in the study region. The results show low overall Te over the Indian Ocean attributed to magmatism and temperature during a multistage opening process. We further demonstrate that temperature controls the strength of warm and young oceanic lithosphere, evidenced by the positive correlation between Te and geothermal proxies. Finally, moderately low Te values at the Southwest Indian Ridge suggest a relatively cold ultraslow lithosphere with sparse magmatism compared to typical mid-ocean ridges.

Accuracy of marine gravimetric measurements in terms of geodetic coordinates of land reference benchmark

The article presents how the values of (3D) coordinates of land reference points affect the results of gravimetric measurements made from the ship in sea areas. These measurements are the basis for 3D maritime inertial navigation, improving ships' operational safety. The campaign verifying the network absolute point coordinates used as a reference point for relative marine gravity measurements was described. The obtained values were compared with catalogue values. In verification of network points 3D position the satellite data Global Satellite Navigation System (GNSS) and ground supporting systems (GBAS) was used. In this example, the height difference of the land reference point was 0.32 m. As a consequence, the offset budget of the marine campaign was affected in the range of up to 0.35 mGal. The influence on gravity free-air anomaly was not constant over the entire area covered by the campaign.

Contractional strains and maximum displacement-length ratios of lunar wrinkle ridges in four Maria of basalt

Compressional stresses from the basalt basins on the lunar are responsible for the formation of wrinkle ridges in lunar mare basalts. According to the wide angle multispectral camera (WAC) mosaic image, we selected 62, 75, 73, and 58 single wrinkle ridges in Mare Imbrium, Mare Serenitatis, Mare Fecunditatis, and Mare Tranquillitatis, respectively, for this paper. Several topographic profiles near the midpoint of each wrinkle ridge are generated to measure the maximum displacement (Dmax) and height of the wrinkle ridges using digital elevation model (LOLA) data. After that, we create 2D plots of displacement-length (L) for the ridge population in the four Maria and compare the results. A linear fit method derives the Dmax/L ratios (γ) from the D-L data. We calculated the contractional strains in each mare area based on the cap D sub m a. x over cap L data. Moreover, each mare's gravity pattern, mare thickness, and formation age are also presented. The ridges in Mare Imbrium and Mare Serenitatis have a higher γ value (1.83 × 10−2 and 1.98 × 10−2) than the ridges in Mare Fecunditatis and Mare Tranquillitatis, which have γ values of (1.67 × 10−2 and 1.75 × 10−2). Finally, the contractional strains (ε) in Mare Imbrium, Mare Serenitatis, Mare Fecunditatis, and Mare Tranquillitatis are estimated to be 0.23 %, 0.41 %, 0.39 %, and 0.18 % (considering 25° is the fault plane dip θ), respectively. The maximum values of the free-air gravity anomalies in Mare Fecunditatis range from −30 to 250 mGal, while minimum gravity anomalies in Mare Serenitatis range from −80 to 140 mGal. Mare Imbrium, Mare Serenitatis, Mare Fecunditatis, and Mare Tranquillitatis have an average thickness of 300 m, 910 m, 652 m, and 760 m, respectively. Furthermore, the Mare Imbrium ridge group is older than the Lunar Wrinkle Ridges in Mare Serenitatis. Mare Tranquillitatis ridge group formation takes longer than Mare Imbrium ridge group formation. Therefore, we believe that it has thicker basaltic units, a longer wrinkle ridge formation time, and higher gravity anomaly than the Mare Imbrium and Mare Serenitatis basins, even though the formation of the Mare Tranquillitatis and Mare Fecunditatis basins occurred earlier.

MEASUREMENT AND ENHANCEMENT OF GRAVITY DATA FOR GEOPHYSICAL INVESTIGATIONS IN ABUJA, NIGERIA

The availability of gravity data is useful to individuals, governments and organizations that wish to carry out a geophysical investigation in Abuja. The data provides an accurate model for environmental monitoring and disaster studies. This research was aimed at measurement and prediction of gravity data in Abuja, Nigeria. The aim was achieved by observation of terrestrial gravity data at forty (40) gravity stations around the federal capital territory Abuja, prediction of the gravity data at other points and the evaluation of the predicted data. Gravity reduction was carried out on the measured data to account for drift correction, free air correction, bouguer correction latitude correction and terrain correction. The data was further enhanced by interpolation using the Kriging method. A total of 59 points were predicted from the 40 measured stations. The predicted gravity anomalies were validated by comparing with their corresponding gravity anomalies obtained from the Earth Gravity Model 2008 (EGM08) because of their well-established reliability. For the number of observations, n = 59, the RMSE and SE were computed as 4.797mGal (4.787x10-5 ms-2) and 4.880mGal (4.880 x10-5 ms-2) respectively. Also, the correlation value obtained was 0.991 which is significant at 0.01 level. This shows a strong relationship signifying a perfect possible agreement in the values obtained. It was therefore concluded that Kriging is a good interpolator that provides an estimate with high reliability and dependency. In this regard, considering the high cost of geophysical data collection, gravity prediction can make geophysical data collection more efficient and cost-effective.

Refinement of microplate boundaries assisted by integrated gravity analysis: Application to the Caribbean Sea

Under the long-term influence of subduction, collision and strike-slip faults, the Caribbean Sea and its surroundings have evolved complex topography and terranes, making it an excellent site to study the tectonic of microplates. However, some microplate boundaries of this area are still controversial or unclear. Here we employed the widely-used satellite-derived gravity dataset in combination with previous findings to conduct a detailed re-interpretation of tectonic boundaries. A newly proposed product of normalized multi-edge-attributes (PNMEA) with upward-continued free-air gravity anomalies (FGA) is used to constrain deep large-scale boundaries (i.e., oceanic trenches, deformed belts and fault systems). The residual crustal gravity anomalies (RCGA) obtained based on wavelet multi-scale decomposition further reveal density variations within the crust domain. The Tilt angle (TA) of FGA highlights fault strikes and patterns in the near-surface. Consequently, we refine the possible boundaries of microplates within the northern Caribbean boundary zone and the Nicaraguan Rise, and analyze the contact relationships among microplates. The results suggest a triple junction among the Gonave, Hispaniola and Septentrional microplates exists in the Windward Passage, and a zigzag low RCGA in the Mona Passage separates the Hispaniola and Puerto Rico microplates. In addition, the north and south Nicaraguan rises are inclined to be unaffiliated implied by the differences in multiple physical properties. We thus conclude that an integrated analytical approach involving potential-field edge detection and RCGA can provide additional constraints for identification of tectonic or microplate boundaries. Deep-shallow coupling and lithosphere-scale cooling differences may provide new perspectives to explain the complexity of temporal-spatial configuration due to multiple microplate interactions.

Evaluating the Impact of the Recent Combined and Satellite-Only Global Geopotential Model on the Gravimetric Geoid Model

Geoid represents Earth’s surface, ocean, and gravitational field, which influence the elevations, shape, and mass distribution of the geopotential surface, a hypothetical surface that is perpendicular to the direction of gravity at every point. This geopotential surface serves as a reference for measuring elevations and is used as a fundamental reference surface for geodetic and surveying purposes. In this study, the Least Squares Modification of Stokes Formula (LSMS) with Additive Corrections (AC), also known as the KTH method, is used to generate a new gravimetric geoid model for Peninsular Malaysia. The KTH method was developed at the Royal Institute of Technology (KTH) in Stockholm-Sweden. The dataset used is the most recent global digital elevation model, Shuttle Radar Topography Mission (SRTM) 1 Arc-Second Global, generated by the National Aeronautics and Space Administration (NASA) and the National Imagery and Mapping Agency (NIMA). In addition to this elevation data, the dataset includes the Global Geopotential Model (GGM), which is composed of the XGM2016, XGM2019e, Tongji_GGMG2021S, and Tongji-Grace02k models. Furthermore, it incorporates sets of regional gravity data, including terrestrial gravity, airborne gravity, and marine gravity anomalies, all of which are derived from the Technical University of Denmark (DTU 21). The actual 45 Global Navigation Satellite System (GNSS)-levelling points data have been compared to the gravimetric geoid model developed in this study and the geoid acquired from Department of Survey and Mapping Malaysia (DSMM). According to the statistical results, NXGM2019e provides better accuracy, with the Root Mean Square Error (RMSE) geoid model errors of ±0.033 m, compared to the deviations in free-air anomalies, XGM2019e, which has the minimum RMSE of 10.291 mGal. Meanwhile, Tongji-GMMG2021S has the maximum RMSE of 14.792 mGal. The geoid is derived from the XGM2019e model and has maximum and minimum values of 0.032 m and 0.147 m, respectively, with mean residuals of 0.089 m. In conclusion, the XGM2019e has the potential to determine a precise local geoid model for Peninsular Malaysia

The comparison of different methods of determining the rock density from gravity data

We focus on two methods of determining the density of the rock environment based on gravimetric measurements. The first method, a combined underground-surface approach, relies on recent measurements made at pairs of points along the vertical. One point is located in accessible underground spaces, like tunnels, mine workings, or caves, while the other is on the Earth's surface. The second, surface method, is solely based on the analysis of the Gravimetric Database of the Slovak Republic. Both methods are based on the proportionality between free-air anomalies and the gravitational effect of topographic masses, calculated for unit density. We compare the respective outputs of both methods and attempt to understand the existing differences between them. We also confront the density estimates in question with one of the two existing density maps of Slovakia compiled from density measurements on rock samples. Across a total of 14 measured locations, we observe a relatively wide spectrum of values on the agreement-disagreement scale, ranging from close similarity to relatively significant differences.

Geological Structure Model for Recharge Area in Patuha Geothermal Field

Geothermal sustainability requires geological structure analysis using gravity methods for groundwater management. Geological structure analysis reviews fault data. Geological structure as a pathway for fluid movement for groundwater management in the research area, especially the Patuha Geothermal Field, West Java. This study uses global gravity model, terrestrial gravity, DEM, and geological data. The gravity method applied in this research involves calculations for latitude correction, free air correction, bouguer correction and terrain correction. The final results of this research are free air anomaly, complete Bouguer anomaly, and model evaluation (MRSE, MRE, and MAE). Free air anomaly results from free air correction of global gravity model data. DEM topography analyzes free air anomaly. The FAA data model evaluated RMSE at 1.27, MRE at 0.004, and MAE at 0.83. 138.02–280.26 mgal is the FAA range. The research area’s elevation range is 836–2431 m. Topography and free air anomaly correlate 0.75. Positive high connection. Mountains and plateaus may have positive gravity anomalies. Free air, bouguer, terrain, and CBA are applied to global gravity model data. The CBA model evaluation indicates quality. RMSE is 1.47, MRE is 0.012, and MAE is 1.11. CBA fluctuation implies fault spread. Faults serve as water flow conduits and channels, increasing the risk of infiltration. Meanwhile, cracks are being distributed as conduits and for infiltration. This research forms the basis for developing and increasing geothermal production so that further studies are needed regarding the influence of geological structures for reservoir in geothermal areas.

Integrated geophysical-petrological model of Damavand volcano, North Iran: Compositional structure of crust and upper mantle from seismic temperature profiling and gravity anomaly fitting

Our study employs well-constrained integrated geophysical-petrological modeling to investigate the compositional structure of Damavand volcano, the Alborz mountains in northern Iran. The thermochemical model utilizes high-resolution compressional velocity anomalies to determine temperature, while geochemical data from volcanic rocks further constrain mostly the lithospheric mantle model. By examining density and velocity, we deduce the upper mantle's compositional structure using observed free-air gravity anomalies and shear-wave seismic models. Our results reveal a lower crust density reduction of 150 kg/m3 due to a high-temperature regime (1080 °C) and the presence of melt indicated by a calculated Vp/Vs ≥ 1.8. The magma storage in the lithosphere is characterized by a harzburgite-melt mixture in agreement with shear-wave seismic profiles, and the lithospheric magma reservoir is positively buoyant with a density reduction of ∼75 kg/m3 compared to the ambient lithospheric mantle. The absence of a warm zone beneath the Alborz suggests that a transient mantle flow from the 660 km transition zone beneath Eurasia is responsible for the lateral heating (and possibly hydration) of the lithospheric mantle. This aligns with the high-temperature conditions observed in the Damavand (Alborz) region. Furthermore, the presence of elevated Nb (and high Nb/Yb ratio as well as radiogenic Nd isotope) concentrations in certain volcanic rocks suggests that a portion of the melt may have originated from the Earth's asthenosphere mantle. Moreover, the existence of water and/or volatiles (likely ≥0.64 wt%, as determined through a comparison of predicted and observed seismic velocities, along with previous laboratory analyses) promotes decompression melting of the lithospheric mantle at significantly lower temperatures (1230–1130 °C) in contrast to earlier estimations (1430–1180 °C). Finally, the reductions in velocity observed in the lower crust and uppermost lithospheric mantle beneath Damavand are likely attributed to thermal and compositional factors, respectively. The presence of a hydrated and melt-rich composition within the lithospheric mantle can account for the observed decrease in seismic velocity.

Seafloor and Ocean Crust Structure of the Kerguelen Plateau from Marine Geophysical and Satellite Altimetry Datasets

The volcanic Kerguelen Islands are formed on one of the world’s largest submarine plateaus. Located in the remote segment of the southern Indian Ocean close to Antarctica, the Kerguelen Plateau is notable for a complex tectonic origin and geologic formation related to the Cretaceous history of the continents. This is reflected in the varying age of the oceanic crust adjacent to the plateau and the highly heterogeneous bathymetry of the Kerguelen Plateau, with seafloor structure differing for the southern and northern segments. Remote sensing data derived from marine gravity and satellite radar altimetry surveys serve as an important source of information for mapping complex seafloor features. This study incorporates geospatial information from NOAA, EMAG2, WDMAM, ETOPO1, and EGM96 datasets to refine the extent and distribution of the extracted seafloor features. The cartographic joint analysis of topography, magnetic anomalies, tectonic and gravity grids is based on the integrated mapping performed using the Generic Mapping Tools (GMT) programming suite. Mapping of the submerged features (Broken Ridge, Crozet Islands, seafloor fabric, orientation, and frequency of magnetic anomalies) enables analysis of their correspondence with free-air gravity and magnetic anomalies, geodynamic setting, and seabed structure in the southwest Indian Ocean. The results show that integrating the datasets using advanced cartographic scripting language improves identification and visualization of the seabed objects. The results include 11 new maps of the region covering the Kerguelen Plateau and southwest Indian Ocean. This study contributes to increasing the knowledge of the seafloor structure in the French Southern and Antarctic Lands.

Evaluation of the precision of some new global Earth Gravitational Models in the East Vietnam Sea

This study is to evaluate the precision of some new global Earth Gravitational Models in the East Vietnam Sea, selecting the best model. The method and the program for calculating Free air gravity anomaly from the global earth gravitational model have been researched and developed. Evaluation of the precision of the models is done by comparing the models with ship-derived gravity anomalies. Data with anomalous signs are detected and removed when the deviation exceeds three times the root mean square deviation. The six global earth gravitational models were evaluated in the experimental section: EGM2008, EIGEN6-C4, GECO, SGG-UGM-1, XGM2019E, and SGG-UGM-2. The evaluation results show that: in the East Vietnam Sea, when compared with 35855 points of shipborne data, the above models respectively have standard deviations of: ±6.046 mGal, ±7.559 mGal, ±5.781 mGal, ±5.832 mGal, ±5.448 mGal, and ±5.236 mGal; all models have mean deviations from shipborne gravity anomalies of about +5 mGal; The correlation between the models and the shipborne data is quite good; The model SGG-UGM-2 has the highest precision. The Free air gravity anomalies from this model have also been calculated over the territorial waters of Vietnam in the form of a grid with a mesh size of 1' × 1'.

Correlations between the Topography-Induced Gravity, Terrain Structure and the Seismicity in the Gulf of Panama

This study presents new maps of the topographic and geophysical setting and seismicity in the region of the Gulf of Panama. The spatial analysis is based on the comparative analysis of the datasets on geoid, free-air gravity anomaly, topography and earthquakes. The cartographic framework is developed using the Generic Mapping Tools (GMT) scripting toolset. Seismic activity in the Central America is high due to the complex geologic setting, tectonic activity and lithosphere plate subduction. The data include the Earth Gravitational Model (EGM2008), the General Bathymetric Chart of the Oceans (GEBCO) and gravity grids. The seismicity data were collected from the Incorporated Research Institutions for Seismology (IRIS) catalogue on 1970–2021. The variations in data were compared to analyse correlations between the geophysical, seismic and topographic parameters. Free-air gravity, geoid and topographic data derived from the high-resolution datasets were used to investigate their effects on the main seismic sources in the region. The comparison of the maps showed that the distribution of the shallow earthquakes in the Pacific segment of Panama coincides with negative free-air anomalies and lower geoid values. The results revealed high values of geoid in the high mountainous regions of Panama (Cordilliera de Talamanca, southern coast of Peninsula de Azuero and eastern Panama, 77.5–78.5°W), which correspond to the topographic roughness in the highlands. Negative values of geoid are found over the Caribbean Sea basin (−4 to 0 m). The analyses of seismicity showed 1740 earthquake events varying by magnitudes from 2.9 to 7.8 at the depths up to 225 m (near the west coast of Colombia). A high concentration of the earthquakes is in the western region of the Panama’s shelf waters (~82–83.5°W), and on the border with Colombia (~77–78.5°W). High gravity anomalies (over 220 mGal) are found in the mountainous regions which match the geodynamic processes associated with the Earth structure and geodetic and geophysical effects. The regions of the high seismicity were defined in the Gulf of Chiriqui and eastern part of the Gulf of Panama.

A Set of Geophysical Fields for Modeling of the Lithosphere Structure and Dynamics in the Russian Arctic Zone

This paper presents a set of various geological and geophysical data for the Arctic zone, including some detailed models for the eastern part of the Russian Arctic zone. This hard-to-access territory has a complex geological structure, which is poorly studied by direct geophysical methods. Therefore, these data can be used in an integrative analysis for different purposes. These are the gravity field, heat flow, and various seismic tomography models. The gravity field data include several reductions calculated during our preceding studies, which are more appropriate for the study of the Earth’s interiors than the initial free air anomalies. Specifically, these are the Bouguer, isostatic, and decompensative gravity anomalies. A surface heat flow map included in the dataset is based on a joint inversion of multiple geophysical data constrained by the observations from the International Heat Flow Commission catalog. Available seismic tomography models were analyzed to select the best one for further investigation. We provide the models for the sedimentary cover and the Moho depth, which are significantly improved compared to the existing ones. The database provides a basis for qualitative and quantitative analysis of the region.

A review of the geophysical knowledge of the Borborema Province, NE-Brazil, and tectonic implications

This review summarizes the results of nearly 50 years of application of geophysical data to study tectonic problems in Borborema Province (BP), Northeastern Brazil. Much information has been obtained from the interpretation of magnetic, gamma-ray spectrometry, gravity, seismological, magnetotelluric, deep seismic and heat flow data. The resulting picture is that, in comparison with other Brazilian geological domains, BP has a thin crust, a lithosphere with small effective elastic thickness, the presence of predominantly positive free-air gravity anomalies, besides the most expressive geoid anomaly, in addition to relatively high heat flow and intraplate seismicity. These facts reinforce the hypothesis that the BP lithosphere suffered a strong impact in Gondwana fragmentation. Predominantly positive free-air gravity anomalies reveal that part of the province has not yet reached isostatic equilibrium. Gravity data established the boundaries of the BP crustal block with neighboring blocks; in particular, the crustal boundary with the São Francisco Craton is located in the innermost regions of the marginal fold belt, tens of kilometers at the north of the conventional limit. Magnetic data allowed to infer the continuity of basement structures beneath sedimentary basins, in particular that BP continues to the Transbrasiliano Lineament, underneath the sedimentary of the Parnaíba Basin. In addition, enhanced magnetic maps made it possible assessing the amount of deformation associated with the major shear zones and identified a continental-scale Cretaceous dike system crossing the BP. Gamma-ray spectrometry data allowed extensive geological mapping of metasedimentary sequences, granitic intrusions and basement complexes. Seismological data allowed to recognize that major shear zones in BP have lithospheric significance. Deep seismic data evidenced a well-defined contrast between the upper and lower crust in BP (Conrad discontinuity). Magnetotelluric data revealed several deep geoelectrical anomalies in BP, some of them interpreted as relics of subduction zones. Intraplate seismicity studies allowed inferring that the current stress state in the BP upper crust is predominantly transcurrent. The list of open problems in BP is also expressive. Notable examples are: a) it has not yet been possible to define which type of orogeny (accretionary or intracontinental?) was dominant in the Neoproterozoic; b) the cause of the expressive geoid anomaly is unknown; c) the tectonic significance of the Transbrasiliano Lineament (collisional suture? Just an intracontinental shear zone?) is also unknown. Regarding economic applications, the use of geophysical data to prospect new ore deposits is still relatively poor. We hope that this review can contribute to increase the familiarity and acceptance of geophysical results by the geological community.

Accuracy assessment of the SRTM2gravity high-resolution topographic gravity model in geoid computation

The accuracy of the SRTM2gravity high-resolution topographic gravity model is tested in this study. Complete Bouguer anomaly datasets calculated with classical formula in planar approximation and using the SRTM2gravity model in spherical approximation were compared in the study area. Two geoid models computed using free-air anomalies, which were restored from each Bouguer anomaly datasets, were compared to each other and they were validated at GPS/leveling points. In the mountainous part of the area, two geoid models differ by up to 4 cm. Following GPS/leveling validations, the geoid models provided nearly the same accuracy (∼4.2 cm). In conclusion, the accuracy of the SRTM2gravity model data was found sufficient to use in gravity reduction and it is recommended as a ready-to-use product in the area for gravity interpolation/gridding and geoid determination purposes.

Isostatic Anomaly and Isostatic Additional Force Analysis by Multiple Geodetic Observations in Qinling Area

Determination of the isostatic anomaly and the isostatic additional force plays a key role in understanding the deep tectonic features and dynamics in the Qinling area. At present, high-accuracy observation gravity data are one of the important means to obtain the isostatic anomaly and the isostatic additional force. Firstly, we calculate the free-air gravity anomalies and the Bouguer gravity anomalies by using hybrid gravity and GPS observation data. Then, we invert the isostatic anomaly and the isostatic additional force. The results show that the isostatic depth calculated by Airy isostatic theory is 40–49 km, and the Moho depth is 39–48 km. The Weihe Basin is in a non-isostatic state with an upward isostatic additional force that reached about 20 MPa. The isostatic anomaly and the isostatic additional force are approximately zero in the northern Sichuan Basin, which indicates that the crust is in isostatic state. The negative isostatic anomaly and isostatic additional force in Liupanshan Mountains, the southwest margin of the Ordos Basin, and the local areas of the Qinling Orogen and Dabashan indicate the existence of crustal movement. By combining the measurement of InSAR, we obtain the surface deformation information of the Weihe Basin, as well as an upward trend, which proves that the result is highly consistent with the gravity observation.

UTILIZATION OF SATELLITE GRAVITY ANOMALY DATA FOR TWO-DIMENSIONAL MODELING OF SUBSURFACE STRUCTURE OF SLAMET VOLCANO, CENTRAL JAVA, INDONESIA

Study using the satellite gravity method has been carried out in the Slamet Volcano area, Central Java, Indonesia. The gravimetric satellite has produced gravity anomalies data which have been corrected up to free air correction. This study purposes to determine the subsurface structure of Slamet Volcano by modeling residual gravity anomalies data. Data processing which has been conducted includes bouguer correction, terrain correction, data reduction to a horizontal surface, separation of regional and residual anomalies data, modeling, and interpretation. The residual gravity anomalies data obtained from the processing range from -40.62 – 66.22 mGal. The anomalies data are modeled in two dimensions (2D) to obtain the subsurface structure model of Slamet Volcano. The 2D-subsurface models on the AB and CD trajectories show the presence of magma chamber and several lava rocks which form the body of the volcano. The rocks are interpreted as andesitic lava with a density value of 2.72 g/cm3, andesitic-basaltic lava with a density value of 2.89 g/cm3, basaltic lava with a density value of 2.97 g/cm3, and magma chamber with a density value of 1.32 g/cm3. Such low value of magma chamber density relative to the surrounding rocks, indicates that Slamet Volcano is still active.

Seismic Multiple Attenuation in the Continent–Ocean Transition Zone of the Northern South China Sea

In this study, we process four new multichannel reflection seismic profiles acquired in 2015 and 2016 in the continent–ocean transition zone (COT) of the northern South China Sea (SCS). We apply a multi-domain, progressive, and seabed-controlled denoising technique and obtain a good denoising effect. Combining velocity analysis in the multi-round time domain and forward modeling, we analyze the types and characteristics of multiples in the study area and formulate an effective demultiple technique to attenuate strong seabed multiples, diffracted multiples from rough seafloor, and other multiples from deep reflectors. The processing results show that the sea surface-related multiple elimination technique predicts the sea surface-related multiples accurately by data convolution, and has a good effect in attenuating seabed multiples. Diffracted multiple attenuation method extracts high-frequency and high-energy diffracted multiples, and suppresses multiples by the energy ratios of multiples to primary events. To attenuate deep multiples, we select predictive deconvolution to attenuate periodic deep multiples after many trials and detailed analysis. The combination of these different techniques in sequence proves to be quite effective in attenuating different seismic multiples in the COT. The imaged crustal structures near the COT often show strong magmatism and/or basement uplifting. The faulted and thinned continental crust adjacent to the COT corresponds to the lowest free-air gravity anomalies. Gravity anomalies often increase from the COT to the oceanic crust. An exception is to the northeast of the SCS, where the relatively wide COT shows very high gravity anomalies, likely induced by mantle upwelling and serpentinization.

Satellite Altimetry and Gravimetry Data for Mapping Marine Geodetic and Geophysical Setting of the Seychelles and the Somali Sea, Indian Ocean

AbstractEvaluation of the representative cartographic techniques demonstrated that there are still considerable challenges facing the methods of marine geodetic, geophysical and bathymetric data visualisation. In an oceanic seafloor formation, the interaction between the geological structural elements and topographical relief can be analysed by advanced mapping. In present study, a correlation between geodesy, geophysics and topography has been examined including the following variables: geological structure, coastal topography and bathymetry, geophysical fields, free-air gravity anomalies and geoid undulation, sediment thickness, bathymetric patterns, and extension of the transform faults. The variables were visualised on the high-resolution raster grids using Generic Mapping Tools (GMT) scripting toolset. The study area is located in the Seychelles and the Somali Sea segment of the Indian Ocean. The data incorporates satellite-derived gravity grid, EGM-2008, geological structures, topography from GEBCO grid and GlobSed sediment thickness, processed by GMT scripts. The results demonstrated that western continental slope of Somalia is wide, gently declining to the seafloor at depths exceeding -5000 m. Kenya and Tanzania present a wide continental foot with depths ranging from -3500 to -5000 m. The Somali Sea basin shows low sedimentation lower than 500 m, while ridges and island chains have higher sediment influx (1,000-2,000 m). The Mozambique Channel has dominating values at 2,500-3,500 m. Higher values are noted near the Reunion and Mauritius islands until the Seychelles via the Mascarene Plateau (500-1,000 m) against the <500 m in the areas of the Mid-Indian Ridge, Carlsberg Ridge and open water.